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The Cretaceous–Paleogene (K–Pg) extinction event, also known as the K–T extinction, was the
mass extinction An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity on Earth. Such an event is identified by a sharp fall in the diversity and abundance of multicellular organisms. It occ ...
of three-quarters of the plant and animal
species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
on Earth approximately 66 million years ago. The event caused the extinction of all non-avian
dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...
s. Most other
tetrapods A tetrapod (; from Ancient Greek τετρα- ''(tetra-)'' 'four' and πούς ''(poús)'' 'foot') is any four- limbed vertebrate animal of the clade Tetrapoda (). Tetrapods include all extant and extinct amphibians and amniotes, with the lat ...
weighing more than also became extinct, with the exception of some ectothermic species such as sea turtles and
crocodilians Crocodilia () is an order of semiaquatic, predatory reptiles that are known as crocodilians. They first appeared during the Late Cretaceous and are the closest living relatives of birds. Crocodilians are a type of crocodylomorph pseudosuchi ...
. It marked the end of the
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...
period, and with it the
Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...
era, while heralding the beginning of the current era, the
Cenozoic The Cenozoic Era ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterized by the dominance of mammals, insects, birds and angiosperms (flowering plants). It is the latest of three g ...
. In the
geologic record The geologic record in stratigraphy, paleontology and other natural sciences refers to the entirety of the layers of rock strata. That is, deposits laid down by volcanism or by deposition of sediment derived from weathering detritus (clays, sa ...
, the K–Pg event is marked by a thin layer of
sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...
called the K–Pg boundary or K–T boundary, which can be found throughout the world in marine and terrestrial rocks. The boundary clay shows unusually high levels of the metal
iridium Iridium is a chemical element; it has the symbol Ir and atomic number 77. This very hard, brittle, silvery-white transition metal of the platinum group, is considered the second-densest naturally occurring metal (after osmium) with a density ...
, which is more common in
asteroid An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...
s than in the
Earth's crust Earth's crust is its thick outer shell of rock, referring to less than one percent of the planet's radius and volume. It is the top component of the lithosphere, a solidified division of Earth's layers that includes the crust and the upper ...
. As originally proposed in 1980 by a team of scientists led by Luis Alvarez and his son
Walter Walter may refer to: People and fictional characters * Walter (name), including a list of people and fictional and mythical characters with the given name or surname * Little Walter, American blues harmonica player Marion Walter Jacobs (1930–19 ...
, it is now generally thought that the K–Pg extinction was caused by the impact of a massive asteroid wide, 66 million years ago causing the
Chicxulub crater The Chicxulub crater is an impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is offshore, but the crater is named after the onshore community of Chicxulub Pueblo (not the larger coastal town of Chicxulub Puerto). I ...
, which devastated the global environment, mainly through a lingering impact winter which halted
photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...
in
plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...
s and
plankton Plankton are the diverse collection of organisms that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...
. The impact hypothesis, also known as the
Alvarez hypothesis The Alvarez hypothesis posits that the Extinction event, mass extinction of the non-avian dinosaurs and many other living things during the Cretaceous–Paleogene extinction event was caused by the Impact event, impact of a large asteroid on the Ea ...
, was bolstered by the discovery of the
Chicxulub crater The Chicxulub crater is an impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is offshore, but the crater is named after the onshore community of Chicxulub Pueblo (not the larger coastal town of Chicxulub Puerto). I ...
in the
Gulf of Mexico The Gulf of Mexico () is an oceanic basin and a marginal sea of the Atlantic Ocean, mostly surrounded by the North American continent. It is bounded on the northeast, north, and northwest by the Gulf Coast of the United States; on the southw ...
's
Yucatán Peninsula The Yucatán Peninsula ( , ; ) is a large peninsula in southeast Mexico and adjacent portions of Belize and Guatemala. The peninsula extends towards the northeast, separating the Gulf of Mexico to the north and west of the peninsula from the C ...
in the early 1990s, which provided conclusive evidence that the K–Pg boundary clay represented debris from an
asteroid impact An impact event is a collision between astronomical objects causing measurable effects. Impact events have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or meteoroids and have minimal effe ...
. The fact that the extinctions occurred simultaneously provides strong evidence that they were caused by the asteroid. A 2016 drilling project into the Chicxulub peak ring confirmed that the peak ring comprised
granite Granite ( ) is a coarse-grained (phanerite, phaneritic) intrusive rock, intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly coo ...
ejected within minutes from deep in the earth, but contained hardly any
gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate Hydrate, dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, drywall and blackboard or sidewalk ...
, the usual sulfate-containing sea floor rock in the region: the gypsum would have vaporized and dispersed as an
aerosol An aerosol is a suspension (chemistry), suspension of fine solid particles or liquid Drop (liquid), droplets in air or another gas. Aerosols can be generated from natural or Human impact on the environment, human causes. The term ''aerosol'' co ...
into the atmosphere, causing longer-term effects on the climate and
food chain A food chain is a linear network of links in a food web, often starting with an autotroph (such as grass or algae), also called a producer, and typically ending at an apex predator (such as grizzly bears or killer whales), detritivore (such as ...
. In October 2019, researchers asserted that the event rapidly acidified the oceans and produced long-lasting effects on the climate, detailing the mechanisms of the mass extinction. Other causal or contributing factors to the extinction may have been the
Deccan Traps The Deccan Traps are a large igneous province of west-central India (17–24°N, 73–74°E). They are one of the largest volcanic features on Earth, taking the form of a large shield volcano. They consist of many layers of solidified flood ...
and other volcanic eruptions,
climate change Present-day climate change includes both global warming—the ongoing increase in Global surface temperature, global average temperature—and its wider effects on Earth's climate system. Climate variability and change, Climate change in ...
, and sea level change. However, in January 2020, scientists reported that climate-modeling of the extinction event favored the asteroid impact and not
volcanism Volcanism, vulcanism, volcanicity, or volcanic activity is the phenomenon where solids, liquids, gases, and their mixtures erupt to the surface of a solid-surface astronomical body such as a planet or a moon. It is caused by the presence of a he ...
. A wide range of terrestrial species perished in the K–Pg extinction, the best-known being the non-avian dinosaurs, along with many mammals, birds, lizards,
insect Insects (from Latin ') are Hexapoda, hexapod invertebrates of the class (biology), class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (Insect morphology#Head, head, ...
s, plants, and all the
pterosaur Pterosaurs are an extinct clade of flying reptiles in the order Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 million to 66 million years ago). Pterosaurs are the earli ...
s. In the oceans, the K–Pg extinction killed off
plesiosaurs The Plesiosauria or plesiosaurs are an Order (biology), order or clade of extinct Mesozoic marine reptiles, belonging to the Sauropterygia. Plesiosaurs first appeared in the latest Triassic Period (geology), Period, possibly in the Rhaetian st ...
and
mosasaur Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Ancient Greek, Greek ' meaning 'lizard') are an extinct group of large aquatic reptiles within the family Mosasauridae that lived during the Late Cretaceous. Their first fossil remains wer ...
s and devastated
teleost Teleostei (; Ancient Greek, Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts (), is, by far, the largest group of ray-finned fishes (class Actinopterygii), with 96% of all neontology, extant species of f ...
fish,
shark Sharks are a group of elasmobranch cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the ...
s,
mollusk Mollusca is a phylum of protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000  extant species of molluscs are recognized, making it the second-largest animal phylum after Arthropoda. The ...
s (especially
ammonites Ammonoids are extinct, (typically) coiled-shelled cephalopods comprising the subclass Ammonoidea. They are more closely related to living octopuses, squid, and cuttlefish (which comprise the clade Coleoidea) than they are to nautiluses (family N ...
, which became extinct), and many species of plankton. It is estimated that 75% or more of all species on Earth vanished. However, the extinction also provided
evolution Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...
ary opportunities: in its wake, many groups underwent remarkable
adaptive radiation In evolutionary biology, adaptive radiation is a process in which organisms diversify rapidly from an ancestral species into a multitude of new forms, particularly when a change in the environment makes new resources available, alters biotic int ...
—sudden and prolific divergence into new forms and species within the disrupted and emptied ecological niches. Mammals in particular diversified in the
Paleogene The Paleogene Period ( ; also spelled Palaeogene or Palæogene) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period Ma (million years ago) to the beginning of the Neogene Period Ma. It is the fir ...
, evolving new forms such as
horse The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 mi ...
s,
whale Whales are a widely distributed and diverse group of fully Aquatic animal, aquatic placental mammal, placental marine mammals. As an informal and Colloquialism, colloquial grouping, they correspond to large members of the infraorder Cetacea ...
s,
bat Bats are flying mammals of the order Chiroptera (). With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in flight than most birds, flying with their very long spread-out ...
s, and
primate Primates is an order (biology), order of mammals, which is further divided into the Strepsirrhini, strepsirrhines, which include lemurs, galagos, and Lorisidae, lorisids; and the Haplorhini, haplorhines, which include Tarsiiformes, tarsiers a ...
s. The surviving group of dinosaurs were avians, a few species of ground and water fowl, which radiated into all modern species of birds. Among other groups, teleost fish and perhaps lizards also radiated.


Extinction patterns

The K–Pg extinction event was severe, global, rapid, and selective, eliminating a vast number of species. Based on marine fossils, it is estimated that 75% or more of all species became extinct. The event appears to have affected all continents at the same time. Non-avian
dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...
s, for example, are known from the
Maastrichtian The Maastrichtian ( ) is, in the International Commission on Stratigraphy (ICS) geologic timescale, the latest age (geology), age (uppermost stage (stratigraphy), stage) of the Late Cretaceous epoch (geology), Epoch or Upper Cretaceous series (s ...
of North America,
Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...
, Asia,
Africa Africa is the world's second-largest and second-most populous continent after Asia. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 20% of Earth's land area and 6% of its total surfac ...
, South America, and
Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean (also known as the Antarctic Ocean), it contains the geographic South Pole. ...
, but are unknown from the Cenozoic anywhere in the world. Similarly, fossil pollen shows devastation of the plant communities in areas as far apart as
New Mexico New Mexico is a state in the Southwestern United States, Southwestern region of the United States. It is one of the Mountain States of the southern Rocky Mountains, sharing the Four Corners region with Utah, Colorado, and Arizona. It also ...
,
Alaska Alaska ( ) is a non-contiguous U.S. state on the northwest extremity of North America. Part of the Western United States region, it is one of the two non-contiguous U.S. states, alongside Hawaii. Alaska is also considered to be the north ...
,
China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...
, and
New Zealand New Zealand () is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and List of islands of New Zealand, over 600 smaller islands. It is the List of isla ...
. Nevertheless, high latitudes appear to have been less strongly affected than low latitudes. Despite the event's severity, there was significant variability in the rate of extinction between and within different
clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
s. Species that depended on
photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...
declined or became extinct as atmospheric particles blocked sunlight and reduced the
solar energy Solar energy is the radiant energy from the Sun's sunlight, light and heat, which can be harnessed using a range of technologies such as solar electricity, solar thermal energy (including solar water heating) and solar architecture. It is a ...
reaching the ground. This plant extinction caused a major reshuffling of the dominant plant groups.
Omnivores An omnivore () is an animal that regularly consumes significant quantities of both plant and animal matter. Obtaining energy and nutrients from plant and animal matter, omnivores digest carbohydrates, protein, fat, and fiber, and metabolize t ...
,
insectivores A robber fly eating a hoverfly An insectivore is a carnivorous animal or plant which eats insects. An alternative term is entomophage, which can also refer to the human practice of eating insects. The first vertebrate insectivores wer ...
, and
carrion Carrion (), also known as a carcass, is the decaying flesh of dead animals. Overview Carrion is an important food source for large carnivores and omnivores in most ecosystems. Examples of carrion-eaters (or scavengers) include crows, vultures ...
-eaters survived the extinction event, perhaps because of the increased availability of their food sources. Neither strictly
herbivorous A herbivore is an animal anatomically and physiologically evolved to feed on plants, especially upon vascular tissues such as foliage, fruits or seeds, as the main component of its diet. These more broadly also encompass animals that eat n ...
nor strictly
carnivorous A carnivore , or meat-eater (Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant whose nutrition and energy requirements are met by consumption of animal tissues (mainly mu ...
mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...
s seem to have survived. Rather, the surviving mammals and birds fed on
insect Insects (from Latin ') are Hexapoda, hexapod invertebrates of the class (biology), class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (Insect morphology#Head, head, ...
s,
worm Worms are many different distantly related bilateria, bilateral animals that typically have a long cylindrical tube-like body, no limb (anatomy), limbs, and usually no eyes. Worms vary in size from microscopic to over in length for marine ...
s, and
snail A snail is a shelled gastropod. The name is most often applied to land snails, terrestrial molluscs, terrestrial pulmonate gastropod molluscs. However, the common name ''snail'' is also used for most of the members of the molluscan class Gas ...
s, which in turn fed on
detritus In biology, detritus ( or ) is organic matter made up of the decomposition, decomposing remains of organisms and plants, and also of feces. Detritus usually hosts communities of microorganisms that colonize and decomposition, decompose (Reminera ...
(dead plant and animal matter). In
stream A stream is a continuous body of water, body of surface water Current (stream), flowing within the stream bed, bed and bank (geography), banks of a channel (geography), channel. Depending on its location or certain characteristics, a strea ...
communities A community is a Level of analysis, social unit (a group of people) with a shared socially-significant characteristic, such as place (geography), place, set of Norm (social), norms, culture, religion, values, Convention (norm), customs, or Ide ...
and lake ecosystems, few animal groups became extinct, including large forms like
crocodyliform Crocodyliformes is a clade of Crurotarsi, crurotarsan archosaurs, the group often traditionally referred to as "crocodilians". They are the first members of Crocodylomorpha to possess many of the features that define later relatives. They are the ...
s and champsosaurs, because such communities rely less directly on food from living plants, and more on detritus washed in from the land, protecting them from extinction. Modern crocodilians can live as scavengers and survive for months without food, and their young are small, grow slowly, and feed largely on invertebrates and dead organisms for their first few years. These characteristics have been linked to crocodilian survival at the end of the Cretaceous. Similar, but more complex patterns have been found in the oceans. Extinction was more severe among animals living in the
water column The (oceanic) water column is a concept used in oceanography to describe the physical (temperature, salinity, light penetration) and chemical ( pH, dissolved oxygen, nutrient salts) characteristics of seawater at different depths for a defined ...
than among animals living on or in the sea floor. Animals in the water column are almost entirely dependent on
primary production In ecology, primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through ...
from living
phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater Aquatic ecosystem, ecosystems. The name comes from the Greek language, Greek words (), meaning 'plant', and (), mea ...
, while animals on the
ocean floor The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as seabeds. The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...
always or sometimes feed on detritus. Coccolithophorids and
mollusk Mollusca is a phylum of protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000  extant species of molluscs are recognized, making it the second-largest animal phylum after Arthropoda. The ...
s (including
ammonite Ammonoids are extinct, (typically) coiled-shelled cephalopods comprising the subclass Ammonoidea. They are more closely related to living octopuses, squid, and cuttlefish (which comprise the clade Coleoidea) than they are to nautiluses (family N ...
s,
rudist Rudists are a group of extinct box-, tube- or ring-shaped marine heterodont bivalves belonging to the order Hippuritida that arose during the Late Jurassic and became so diverse during the Cretaceous that they were major reef-building organis ...
s,
freshwater snail Freshwater snails are gastropod mollusks that live in fresh water. There are many different families. They are found throughout the world in various habitats, ranging from ephemeral pools to the largest lakes, and from small seeps and springs t ...
s, and
mussel Mussel () is the common name used for members of several families of bivalve molluscs, from saltwater and Freshwater bivalve, freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other ...
s), and those organisms whose
food chain A food chain is a linear network of links in a food web, often starting with an autotroph (such as grass or algae), also called a producer, and typically ending at an apex predator (such as grizzly bears or killer whales), detritivore (such as ...
included these shell builders, became extinct or suffered heavy losses. For example, it is thought that
ammonites Ammonoids are extinct, (typically) coiled-shelled cephalopods comprising the subclass Ammonoidea. They are more closely related to living octopuses, squid, and cuttlefish (which comprise the clade Coleoidea) than they are to nautiluses (family N ...
were the principal food of
mosasaur Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Ancient Greek, Greek ' meaning 'lizard') are an extinct group of large aquatic reptiles within the family Mosasauridae that lived during the Late Cretaceous. Their first fossil remains wer ...
s, a group of giant marine
reptiles Reptiles, as commonly defined, are a group of tetrapods with an ectothermic metabolism and Amniotic egg, amniotic development. Living traditional reptiles comprise four Order (biology), orders: Testudines, Crocodilia, Squamata, and Rhynchocepha ...
that became extinct at the boundary. The K–Pg extinction had a profound effect on the evolution of life on Earth. The elimination of dominant Cretaceous groups allowed other organisms to take their place, causing a remarkable amount of species diversification during the Paleogene Period. After the K–Pg extinction event, biodiversity required substantial time to recover, despite the existence of abundant vacant
ecological niche In ecology, a niche is the match of a species to a specific environmental condition. Three variants of ecological niche are described by It describes how an organism or population responds to the distribution of Resource (biology), resources an ...
s. Evidence from the
Salamanca Formation The Salamanca Formation is a geologic formation in the Golfo San Jorge Basin of central Patagonia that yields well-preserved, well-dated fossils from the early Paleocene. Studies of these fossils are providing new data on plant and animal diversit ...
suggests that biotic recovery was more rapid in the Southern Hemisphere than in the Northern Hemisphere. Despite the massive loss of life inferred to have occurred during the extinction, and a number of geologic formations worldwide that span the boundary, only a few fossil sites contain direct evidence of the mass mortality that occurred exactly at the K-Pg boundary. These include the
Tanis Tanis ( ; ; ) or San al-Hagar (; ; ; or or ; ) is the Greek name for ancient Egyptian ''ḏꜥn.t'', an important archaeological site in the northeastern Nile Delta of ancient Egypt, Egypt, and the location of a city of the same name. Tanis ...
site of the
Hell Creek Formation The Hell Creek Formation is an intensively studied division of mostly Upper Cretaceous and some lower Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana. The Formation (stratigraphy), formation s ...
in
North Dakota North Dakota ( ) is a U.S. state in the Upper Midwest, named after the indigenous Dakota people, Dakota and Sioux peoples. It is bordered by the Canadian provinces of Saskatchewan and Manitoba to the north and by the U.S. states of Minneso ...
, USA, which contains a high number of well-preserved fossils that appear to have been buried in a catastrophic flood event that was likely caused by the impact. Another important site is the Hornerstown Formation in
New Jersey New Jersey is a U.S. state, state located in both the Mid-Atlantic States, Mid-Atlantic and Northeastern United States, Northeastern regions of the United States. Located at the geographic hub of the urban area, heavily urbanized Northeas ...
, USA, which has prominent layer at the K-Pg boundary known as the Main Fossiliferous Layer (MFL) containing a thanatocoenosis of disarticulated vertebrate fossils, which was likely also caused by a catastrophic flood from the impact.


Microbiota

The K–Pg boundary represents one of the most dramatic turnovers in the
fossil record A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
for various
calcareous Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime (mineral), lime or being chalky. The term is used in a wide variety of Science, scientific disciplines. In zoology ''Calcare ...
nannoplankton that formed the
calcium Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...
deposits for which the Cretaceous is named. The turnover in this group is clearly marked at the species level. Statistical analysis of marine losses at this time suggests that the decrease in diversity was caused more by a sharp increase in extinctions than by a decrease in
speciation Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within ...
. Major spatial differences existed in calcareous nannoplankton diversity patterns; in the Southern Hemisphere, the extinction was less severe and recovery occurred much faster than in the Northern Hemisphere. Following the extinction, survivor communities dominated for several hundred thousand years. The North Pacific acted as a diversity hotspot from which later nannoplankton communities radiated as they replaced survivor faunas across the globe. The K–Pg boundary record of
dinoflagellate The Dinoflagellates (), also called Dinophytes, are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered protists. Dinoflagellates are mostly marine plankton, but they are also commo ...
s is not so well understood, mainly because only
microbial cyst A microbial cyst is a resting or dormant stage of a microorganism, that can be thought of as a state of suspended animation in which the metabolic processes of the cell are slowed and the cell ceases all activities like feeding and locomotion. ...
s provide a fossil record, and not all dinoflagellate species have cyst-forming stages, which likely causes diversity to be underestimated. Recent studies indicate that there were no major shifts in dinoflagellates through the boundary layer. There were blooms of the taxa ''Thoracosphaera operculata'' and ''Braarudosphaera bigelowii'' at the boundary.
Radiolaria The Radiolaria, also called Radiozoa, are unicellular eukaryotes of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell into the inner and outer portions of endoplasm and ect ...
have left a geological record since at least the
Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...
times, and their mineral fossil skeletons can be tracked across the K–Pg boundary. There is no evidence of mass extinction of these organisms, and there is support for high productivity of these species in southern high latitudes as a result of cooling temperatures in the early
Paleocene The Paleocene ( ), or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), ...
. Approximately 46% of
diatom A diatom (Neo-Latin ''diatoma'') is any member of a large group comprising several Genus, genera of algae, specifically microalgae, found in the oceans, waterways and soils of the world. Living diatoms make up a significant portion of Earth's B ...
species survived the transition from the
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...
to the Upper Paleocene, a significant turnover in species but not a catastrophic extinction. The occurrence of
plankton Plankton are the diverse collection of organisms that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...
ic
foraminifera Foraminifera ( ; Latin for "hole bearers"; informally called "forams") are unicellular organism, single-celled organisms, members of a phylum or class (biology), class of Rhizarian protists characterized by streaming granular Ectoplasm (cell bio ...
across the K–Pg boundary has been studied since the 1930s. Research spurred by the possibility of an impact event at the K–Pg boundary resulted in numerous publications detailing planktonic foraminiferal extinction at the boundary; there is ongoing debate between groups which think the evidence indicates substantial extinction of these species at the K–Pg boundary, and those who think the evidence supports a gradual extinction through the boundary. There is strong evidence that local conditions heavily influenced diversity changes in planktonic foraminifera. Low and mid-latitude communities of planktonic foraminifera experienced high extinction rates, while high latitude faunas were relatively unaffected. Numerous species of
benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...
foraminifera became extinct during the event, presumably because they depend on organic debris for nutrients, while
biomass Biomass is a term used in several contexts: in the context of ecology it means living organisms, and in the context of bioenergy it means matter from recently living (but now dead) organisms. In the latter context, there are variations in how ...
in the ocean is thought to have decreased. As the marine microbiota recovered, it is thought that increased speciation of benthic foraminifera resulted from the increase in food sources. In some areas, such as Texas, benthic foraminifera show no sign of any major extinction event, however. Phytoplankton recovery in the early Paleocene provided the food source to support large benthic foraminiferal assemblages, which are mainly detritus-feeding. Ultimate recovery of the benthic populations occurred over several stages lasting several hundred thousand years into the early Paleocene.


Marine invertebrates

There is significant variation in the fossil record as to the extinction rate of
marine invertebrates Marine invertebrates are invertebrate animals that live in marine habitats, and make up most of the macroscopic life in the oceans. It is a polyphyletic blanket term that contains all marine animals except the marine vertebrates, including the ...
across the K–Pg boundary. The apparent rate is influenced by a lack of fossil records, rather than extinctions.
Ostracod Ostracods, or ostracodes, are a Class (biology), class of the crustacean, Crustacea (class Ostracoda), sometimes known as seed shrimp. Some 33,000 species (only 13,000 of which are extant taxon, extant) have been identified,Brandão, S.N.; Antoni ...
s, a class of small
crustacean Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...
s that were prevalent in the upper Maastrichtian, left fossil deposits in a variety of locations. A review of these fossils shows that ostracod diversity was lower in the Paleocene than any other time in the
Cenozoic The Cenozoic Era ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterized by the dominance of mammals, insects, birds and angiosperms (flowering plants). It is the latest of three g ...
. Current research cannot ascertain whether the extinctions occurred prior to, or during, the boundary interval. Ostracods that were heavily sexually selected were more vulnerable to extinction, and ostracod sexual dimorphism was significantly rarer following the mass extinction. Among
decapods The Decapoda or decapods, from Ancient Greek δεκάς (''dekás''), meaning "ten", and πούς (''poús''), meaning "foot", is a large order (biology), order of crustaceans within the class Malacostraca, and includes crabs, lobsters, crayfis ...
, extinction patterns were highly heterogeneous and cannot be neatly attributed to any particular factor. Decapods that inhabited the Western Interior Seaway were especially hard-hit, while other regions of the world's oceans were refugia that increased chances of survival into the Palaeocene. Among retroplumid crabs, the genus ''Costacopluma'' was a notable survivor. Approximately 60% of late-Cretaceous scleractinian
coral Corals are colonial marine invertebrates within the subphylum Anthozoa of the phylum Cnidaria. They typically form compact Colony (biology), colonies of many identical individual polyp (zoology), polyps. Coral species include the important Coral ...
genera failed to cross the K–Pg boundary into the Paleocene. Further analysis of the coral extinctions shows that approximately 98% of colonial species, ones that inhabit warm, shallow
tropical The tropics are the regions of Earth surrounding the equator, where the sun may shine directly overhead. This contrasts with the temperate or polar regions of Earth, where the Sun can never be directly overhead. This is because of Earth's ax ...
waters, became extinct. The solitary corals, which generally do not form reefs and inhabit colder and deeper (below the
photic zone The photic zone (or euphotic zone, epipelagic zone, or sunlight zone) is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological ...
) areas of the ocean were less impacted by the K–Pg boundary. Colonial coral species rely upon
symbiosis Symbiosis (Ancient Greek : living with, companionship < : together; and ''bíōsis'': living) is any type of a close and long-term biological interaction, between two organisms of different species. The two organisms, termed symbionts, can fo ...
with photosynthetic
algae Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...
, which collapsed due to the events surrounding the K–Pg boundary, but the use of data from coral fossils to support K–Pg extinction and subsequent Paleocene recovery, must be weighed against the changes that occurred in coral ecosystems through the K–Pg boundary. Most species of
brachiopods Brachiopods (), phylum Brachiopoda, are a phylum of animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the fron ...
, a small
phylum In biology, a phylum (; : phyla) is a level of classification, or taxonomic rank, that is below Kingdom (biology), kingdom and above Class (biology), class. Traditionally, in botany the term division (taxonomy), division has been used instead ...
of marine invertebrates, survived the K–Pg extinction event and diversified during the early Paleocene. The numbers of
bivalve Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of aquatic animal, aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed b ...
genera exhibited significant diminution after the K–Pg boundary. Entire groups of bivalves, including
rudists Rudists are a group of extinct box-, tube- or ring-shaped Marine (ocean), marine Heterodonta, heterodont bivalves belonging to the order Hippuritida that arose during the Late Jurassic and became so diverse during the Cretaceous that they were m ...
(reef-building clams) and inoceramids (giant relatives of modern
scallop Scallop () is a common name that encompasses various species of marine bivalve molluscs in the taxonomic family Pectinidae, the scallops. However, the common name "scallop" is also sometimes applied to species in other closely related famili ...
s), became extinct at the K–Pg boundary, with the gradual extinction of most inoceramid bivalves beginning well before the K–Pg boundary. Deposit feeders were the most common bivalves in the catastrophe's aftermath. Abundance was not a factor that affected whether a bivalve taxon went extinct, according to evidence from North America. Veneroid bivalves developed deeper burrowing habitats as the recovery from the crisis ensued. Except for
nautiloid Nautiloids are a group of cephalopods (Mollusca) which originated in the Late Cambrian and are represented today by the living ''Nautilus'' and ''Allonautilus''. Fossil nautiloids are diverse and species rich, with over 2,500 recorded species. Th ...
s (represented by the modern order
Nautilida The Nautilida constitute a large and diverse order of generally coiled nautiloid cephalopods that began in the mid Paleozoic and continues to the present with a single family, the Nautilidae which includes two genera, ''Nautilus'' and ''Allonauti ...
) and coleoids (which had already diverged into modern octopodes,
squid A squid (: squid) is a mollusc with an elongated soft body, large eyes, eight cephalopod limb, arms, and two tentacles in the orders Myopsida, Oegopsida, and Bathyteuthida (though many other molluscs within the broader Neocoleoidea are also ...
s, and
cuttlefish Cuttlefish, or cuttles, are Marine (ocean), marine Mollusca, molluscs of the order (biology), suborder Sepiina. They belong to the class (biology), class Cephalopoda which also includes squid, octopuses, and nautiluses. Cuttlefish have a unique ...
) all other species of the
mollusca Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...
n class Cephalopoda became extinct at the K–Pg boundary. These included the ecologically significant
belemnoids Belemnoids are an extinct group of marine cephalopod, very similar in many ways to the modern squid. Like them, the belemnoids possessed an ink sac, but, unlike the squid, they possessed ten Cephalopod arm, arms of roughly equal length, and no t ...
, as well as the ammonoids, a group of highly diverse, numerous, and widely distributed shelled cephalopods. The extinction of belemnites enabled surviving cephalopod clades to fill their niches. Ammonite genera became extinct at or near the K–Pg boundary; there was a smaller and slower extinction of ammonite genera prior to the boundary associated with a late Cretaceous marine regression, and a small, gradual reduction in ammonite diversity occurred throughout the very late Cretaceous. Researchers have pointed out that the reproductive strategy of the surviving nautiloids, which rely upon few and larger eggs, played a role in outsurviving their ammonoid counterparts through the extinction event. The ammonoids utilized a planktonic strategy of reproduction (numerous eggs and planktonic larvae), which would have been devastated by the K–Pg extinction event. Additional research has shown that subsequent to this elimination of ammonoids from the global biota, nautiloids began an evolutionary radiation into shell shapes and complexities theretofore known only from ammonoids. Approximately 35% of echinoderm genera became extinct at the K–Pg boundary, although
taxa In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...
that thrived in low-latitude, shallow-water environments during the late Cretaceous had the highest extinction rate. Mid-latitude, deep-water echinoderms were much less affected at the K–Pg boundary. The pattern of extinction points to habitat loss, specifically the drowning of
carbonate platform A carbonate platform is a Sedimentary rock, sedimentary body which possesses topographic relief, and is composed of Autochthon (geology), autochthonic calcareous deposits. Platform growth is mediated by Sessility (zoology), sessile organisms whose ...
s, the shallow-water reefs in existence at that time, by the extinction event. Atelostomatans were affected by the Lilliput effect.


Terrestrial invertebrates

Insect damage to the fossilized leaves of
flowering plant Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (). The term angiosperm is derived from the Ancient Greek, Greek words (; 'container, vessel') and (; 'seed'), meaning that the seeds are enclosed with ...
s from fourteen sites in North America was used as a proxy for insect diversity across the K–Pg boundary and analyzed to determine the rate of extinction. Researchers found that Cretaceous sites, prior to the extinction event, had rich plant and insect-feeding diversity. During the early Paleocene, flora were relatively diverse with little predation from insects, even 1.7 million years after the extinction event. Studies of the size of the
ichnotaxon An ichnotaxon (plural ichnotaxa) is "a taxon based on the fossilized work of an organism", i.e. the non-human equivalent of an artifact. ''Ichnotaxon'' comes from the Ancient Greek (''íchnos'') meaning "track" and English , itself derived from ...
''Naktodemasis bowni'', produced by either cicada nymphs or beetle larvae, over the course of the K-Pg transition show that the Lilliput effect occurred in terrestrial invertebrates thanks to the extinction event. The extinction event produced major changes in Paleogene insect communities. Many groups of ants were present in the Cretaceous, but in the Eocene ants became dominant and diverse, with larger colonies. Butterflies diversified as well, perhaps to take the place of leaf-eating insects wiped out by the extinction. The advanced mound-building termites,
Termitidae Termitidae is the largest family of termites consisting of 2,125 described species of which are commonly known as the higher termites. They are evolutionarily the most specialised termite group, with their highly compartmentalized hindgut lackin ...
, also appear to have risen in importance.


Fish

There are fossil records of jawed fishes across the K–Pg boundary, which provide good evidence of extinction patterns of these classes of marine vertebrates. While the deep-sea realm was able to remain seemingly unaffected, there was an equal loss between the open marine
apex predator An apex predator, also known as a top predator or superpredator, is a predator at the top of a food chain, without natural predators of its own. Apex predators are usually defined in terms of trophic dynamics, meaning that they occupy the hig ...
s and the
durophagous Durophagy is the eating behavior of animals that consume hard-shelled or exoskeleton-bearing organisms, such as corals, shelled mollusks, or crabs. It is mostly used to describe fish, but is also used when describing reptiles, including fossil t ...
demersal The demersal zone is the part of the sea or ocean (or deep lake) consisting of the part of the water column near to (and significantly affected by) the seabed and the benthos. The demersal zone is just above the benthic zone and forms a layer o ...
feeders on the continental shelf. Within
cartilaginous fish Chondrichthyes (; ) is a class of jawed fish that contains the cartilaginous fish or chondrichthyans, which all have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fish'', which have skeleto ...
, approximately 7 out of the 41 families of neoselachians (modern
sharks Sharks are a group of elasmobranch cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the ...
, skates, and rays) disappeared after this event and
batoids Batomorphi is a division of cartilaginous fishes, commonly known as rays, this taxon is also known as the superorder Batoidea, but the 5th edition of ''Fishes of the World'' classifies it as the division Batomorphi. They and their close relati ...
(skates and rays) lost nearly all the identifiable species, while more than 90% of teleost fish (bony fish) families survived. In the Maastrichtian age, 28 
shark Sharks are a group of elasmobranch cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the ...
families and 13 batoid families thrived, of which 25 and 9, respectively, survived the K–T boundary event. Forty-seven of all neoselachian genera cross the K–T boundary, with 85% being sharks. Batoids display with 15%, a comparably low survival rate. Among elasmobranchs, those species that inhabited higher latitudes and lived pelagic lifestyles were more likely to survive, whereas epibenthic lifestyles and durophagy were strongly associated with the likelihood of perishing during the extinction event. There is evidence of a mass extinction of bony fishes at a fossil site immediately above the K–Pg boundary layer on Seymour Island near
Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean (also known as the Antarctic Ocean), it contains the geographic South Pole. ...
, apparently precipitated by the K–Pg extinction event; the marine and freshwater environments of fishes mitigated the environmental effects of the extinction event. The result was Patterson's Gap, a period in the earliest part of the Cenozoic of decreased acanthomorph diversity, although acanthomorphs diversified rapidly after the extinction. Teleost fish diversified explosively after the mass extinction, filling the niches left vacant by the extinction. Groups appearing in the Paleocene and Eocene epochs include billfish, tunas, eels, and flatfish.


Amphibians

There is limited evidence for extinction of amphibians at the K–Pg boundary. A study of fossil vertebrates across the K–Pg boundary in
Montana Montana ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Idaho to the west, North Dakota to the east, South Dakota to the southeast, Wyoming to the south, an ...
concluded that no species of amphibian became extinct. Yet there are several species of Maastrichtian amphibian, not included as part of this study, which are unknown from the Paleocene. These include the frog '' Theatonius lancensis'' and the albanerpetontid '' Albanerpeton galaktion''; therefore, some amphibians do seem to have become extinct at the boundary. The relatively low levels of extinction seen among amphibians probably reflect the low extinction rates seen in freshwater animals. Following the mass extinction, frogs radiated substantially, with 88% of modern anuran diversity being traced back to three lineages of frogs that evolved after the cataclysm.


Reptiles


Choristoderes

The choristoderes (a group of semi-aquatic diapsids of uncertain position) survived across the K–Pg boundary subsequently becoming extinct in the
Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...
. The gharial-like choristodere genus '' Champsosaurus'' palatal teeth suggest that there were dietary changes among the various species across the K–Pg event.


Turtles

More than 80% of Cretaceous
turtle Turtles are reptiles of the order (biology), order Testudines, characterized by a special turtle shell, shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Crypt ...
species passed through the K–Pg boundary. All six turtle families in existence at the end of the Cretaceous survived into the
Paleogene The Paleogene Period ( ; also spelled Palaeogene or Palæogene) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period Ma (million years ago) to the beginning of the Neogene Period Ma. It is the fir ...
and are represented by living species. Analysis of turtle survivorship in the Hell Creek Formation shows a minimum of 75% of turtle species survived. Following the extinction event, turtle diversity exceeded pre-extinction levels in the Danian of North America, although in South America it remained diminished. European turtles likewise recovered rapidly following the mass extinction.


Lepidosauria

The
rhynchocephalia Rhynchocephalia (; ) is an order of lizard-like reptiles that includes only one living species, the tuatara (''Sphenodon punctatus'') of New Zealand. Despite its current lack of diversity, during the Mesozoic rhynchocephalians were a speciose g ...
ns, which were a globally distributed and diverse group of lepidosaurians during the early
Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...
, had begun to decline by the mid-Cretaceous, although they remained successful in the Late Cretaceous of southern
South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...
. They are represented today by a single species, the
tuatara The tuatara (''Sphenodon punctatus'') is a species of reptile endemic to New Zealand. Despite its close resemblance to lizards, it is actually the only extant member of a distinct lineage, the previously highly diverse order Rhynchocephal ...
(''Sphenodon punctatus'') found in
New Zealand New Zealand () is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and List of islands of New Zealand, over 600 smaller islands. It is the List of isla ...
. Outside of New Zealand, one rhynchocephalian is known to have crossed the K-Pg boundary, '' Kawasphenodon peligrensis,'' known from the earliest Paleocene (Danian) of Patagonia. The order
Squamata Squamata (, Latin ''squamatus'', 'scaly, having scales') is the largest Order (biology), order of reptiles; most members of which are commonly known as Lizard, lizards, with the group also including Snake, snakes. With over 11,991 species, it i ...
comprising lizards and snakes first diversified during the Jurassic and continued to diversify throughout the Cretaceous. They are currently the most successful and diverse group of living reptiles, with more than 10,000 extant species. The only major group of terrestrial lizards to go extinct at the end of the Cretaceous were the polyglyphanodontians, a diverse group of mainly herbivorous lizards known predominantly from the Northern Hemisphere. The
mosasaur Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Ancient Greek, Greek ' meaning 'lizard') are an extinct group of large aquatic reptiles within the family Mosasauridae that lived during the Late Cretaceous. Their first fossil remains wer ...
s, a diverse group of large predatory marine reptiles, also became extinct. Fossil evidence indicates that squamates generally suffered very heavy losses in the K–Pg event, only recovering 10 million years after it. The extinction of Cretaceous lizards and snakes may have led to the evolution of modern groups such as iguanas, monitor lizards, and boas. The diversification of crown group snakes has been linked to the biotic recovery in the aftermath of the K-Pg extinction event. Pan-Gekkotans weathered the extinction event well, with multiple lineages likely surviving.


Marine reptiles

44/42Ca values indicate that prior to the mass extinction, marine reptiles at the top of food webs were feeding on only one source of calcium, suggesting their populations exhibited heightened vulnerability to extinctions at the terminus of the Cretaceous. Along with the aforementioned mosasaurs,
plesiosaurs The Plesiosauria or plesiosaurs are an Order (biology), order or clade of extinct Mesozoic marine reptiles, belonging to the Sauropterygia. Plesiosaurs first appeared in the latest Triassic Period (geology), Period, possibly in the Rhaetian st ...
, represented by the families
Elasmosauridae Elasmosauridae, often called elasmosaurs or elasmosaurids, is an extinct family of plesiosaurs that lived from the Hauterivian stage of the Early Cretaceous to the Maastrichtian stage of the Late Cretaceous period (c. 130 to 66 mya). The taxo ...
and Polycotylidae, became extinct during the event. The
ichthyosaur Ichthyosauria is an order of large extinct marine reptiles sometimes referred to as "ichthyosaurs", although the term is also used for wider clades in which the order resides. Ichthyosaurians thrived during much of the Mesozoic era; based on fo ...
s had disappeared from fossil record tens of millions of years prior to the K-Pg extinction event.


Crocodyliforms

Ten families of crocodilians or their close relatives are represented in the Maastrichtian fossil records, of which five died out prior to the K–Pg boundary. Five families have both Maastrichtian and Paleocene fossil representatives. All of the surviving families of crocodyliforms inhabited freshwater and terrestrial environments—except for the Dyrosauridae, which lived in freshwater and marine locations. Approximately 50% of crocodyliform representatives survived across the K–Pg boundary, the only apparent trend being that no large crocodiles survived. Crocodyliform survivability across the boundary may have resulted from their aquatic niche and ability to burrow, which reduced susceptibility to negative environmental effects at the boundary. Jouve and colleagues suggested in 2008 that juvenile marine crocodyliforms lived in freshwater environments as do modern marine
crocodile Crocodiles (family (biology), family Crocodylidae) or true crocodiles are large, semiaquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. The term "crocodile" is sometimes used more loosely to include ...
juveniles, which would have helped them survive where other
marine reptile Marine reptiles are reptiles which have become secondarily adapted for an aquatic or semiaquatic life in a marine environment. Only about 100 of the 12,000 extant reptile species and subspecies are classed as marine reptiles, including mari ...
s became extinct; freshwater environments were not so strongly affected by the K–Pg extinction event as marine environments were. Among the terrestrial clade
Notosuchia Notosuchia is a clade of primarily Gondwanan mesoeucrocodylian Crocodylomorpha, crocodylomorphs that lived during the Jurassic and Cretaceous. Some phylogeny, phylogenies recover Sebecosuchia as a clade within Notosuchia, others as a sister group ...
, only the family Sebecidae survived; the exact reasons for this pattern are not known. Sebecids were large terrestrial predators, are known from the Eocene of Europe, and would survive in South America into the Miocene. Tethysuchians radiated explosively after the extinction event.


Pterosaurs

Two families of pterosaurs,
Azhdarchidae Azhdarchidae (from the Persian word , , a dragon-like creature in Persian mythology) is a family of pterosaurs known primarily from the Late Cretaceous Period, though an isolated vertebra apparently from an azhdarchid is known from the Early Cre ...
and
Nyctosauridae Nyctosauridae (meaning "night lizards" or "bat lizards") is a family of specialized soaring pterosaurs of the late Cretaceous Period of North America, Africa, and possibly other continents including South America. It was named in 1889 by Henry ...
, were definitely present in the Maastrichtian, and they likely became extinct at the K–Pg boundary. Several other pterosaur lineages may have been present during the Maastrichtian, such as the ornithocheirids, pteranodontids, a possible tapejarid, a possible thalassodromid and a basal toothed taxon of uncertain affinities, though they are represented by fragmentary remains that are difficult to assign to any given group. While this was occurring, modern birds were undergoing diversification; traditionally it was thought that they replaced archaic birds and pterosaur groups, possibly due to direct competition, or they simply filled empty niches, but there is no correlation between pterosaur and avian diversities that are conclusive to a competition hypothesis, and small pterosaurs were present in the Late Cretaceous. At least some niches previously held by birds were reclaimed by pterosaurs prior to the K–Pg event.


Non-avian dinosaurs

Scientists agree that all non-avian dinosaurs became extinct at the K–Pg boundary. The dinosaur fossil record has been interpreted to show both a decline in diversity and no decline in diversity during the last few million years of the Cretaceous, and it may be that the quality of the dinosaur fossil record is simply not good enough to permit researchers to distinguish between the options. There is no evidence that late Maastrichtian non-avian dinosaurs could burrow, swim, or dive, which suggests they were unable to shelter themselves from the worst parts of any environmental stress that occurred at the K–Pg boundary. It is possible that small dinosaurs (other than birds) did survive, but they would have been deprived of food, as herbivorous dinosaurs would have found plant material scarce and carnivores would have quickly found prey in short supply. The growing consensus about the endothermy of dinosaurs (see dinosaur physiology) helps to understand their full extinction in contrast with their close relatives, the crocodilians. Ectothermic ("cold-blooded") crocodiles have very limited needs for food (they can survive several months without eating), while endothermic ("warm-blooded") animals of similar size need much more food to sustain their faster metabolism. Thus, under the circumstances of food chain disruption previously mentioned, non-avian dinosaurs died out, while some crocodiles survived. In this context, the survival of other endothermic animals, such as some birds and mammals, could be due, among other reasons, to their smaller needs for food, related to their small size at the extinction epoch. Prolonged cold is unlikely to have been a reason for the extinction of non-avian dinosaurs given the adaptations of many dinosaurs to cold environments. Whether the extinction occurred gradually or suddenly has been debated, as both views have support from the fossil record. A highly informative sequence of dinosaur-bearing rocks from the K–Pg boundary is found in western North America, particularly the late Maastrichtian-age
Hell Creek Formation The Hell Creek Formation is an intensively studied division of mostly Upper Cretaceous and some lower Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana. The Formation (stratigraphy), formation s ...
of
Montana Montana ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Idaho to the west, North Dakota to the east, South Dakota to the southeast, Wyoming to the south, an ...
. Comparison with the older Judith River Formation (Montana) and
Dinosaur Park Formation The Dinosaur Park Formation is the uppermost member of the Belly River Group (also known as the Judith River Group), a major geologic unit in southern Alberta. It was deposited during the Campanian stage of the Late Cretaceous, between about 7 ...
(
Alberta Alberta is a Provinces and territories of Canada, province in Canada. It is a part of Western Canada and is one of the three Canadian Prairies, prairie provinces. Alberta is bordered by British Columbia to its west, Saskatchewan to its east, t ...
), which both date from approximately 75 Ma, provides information on the changes in dinosaur populations over the last 10 million years of the Cretaceous. These fossil beds are geographically limited, covering only part of one continent. The middle–late Campanian formations show a greater diversity of dinosaurs than any other single group of rocks. The late Maastrichtian rocks contain the largest members of several major clades: ''
Tyrannosaurus ''Tyrannosaurus'' () is a genus of large theropod dinosaur. The type species ''Tyrannosaurus rex'' ( meaning 'king' in Latin), often shortened to ''T. rex'' or colloquially t-rex, is one of the best represented theropods. It lived througho ...
'', ''
Ankylosaurus ''Ankylosaurus'' is a genus of Thyreophora, armored dinosaur. Its fossils have been found in geological formations dating to the very end of the Cretaceous Period (geology), Period, about 68–66 million years ago, in western North America, m ...
'', ''
Pachycephalosaurus ''Pachycephalosaurus'' (; meaning "thick-headed lizard", from Greek ''pachys-/'' "thickness", ''kephalon/'' "head" and ''sauros/'' "lizard") is a genus of pachycephalosaurid ornithischian dinosaur. The type species, ''P. wyomingensis'', ...
'', ''
Triceratops ''Triceratops'' ( ; ) is a genus of Chasmosaurinae, chasmosaurine Ceratopsia, ceratopsian dinosaur that lived during the late Maastrichtian age of the Late Cretaceous Period (geology), period, about 68 to 66 million years ago on the island ...
'', and ''
Torosaurus ''Torosaurus'' (meaning "perforated lizard", in reference to the large openings in its frill) is a genus of herbivorous Chasmosaurinae, chasmosaurine Ceratopsia, ceratopsian dinosaur that lived during the late Maastrichtian age of the Late Cret ...
'', which suggests food was plentiful immediately prior to the extinction. A study of 29 fossil sites in Catalan
Pyrenees The Pyrenees are a mountain range straddling the border of France and Spain. They extend nearly from their union with the Cantabrian Mountains to Cap de Creus on the Mediterranean coast, reaching a maximum elevation of at the peak of Aneto. ...
of Europe in 2010 supports the view that dinosaurs there had great diversity until the asteroid impact, with more than 100 living species. More recent research indicates that this figure is obscured by taphonomic biases and the sparsity of the continental fossil record. The results of this study, which were based on estimated real global biodiversity, showed that between 628 and 1,078 non-avian dinosaur species were alive at the end of the Cretaceous and underwent sudden extinction after the Cretaceous–Paleogene extinction event. Alternatively, interpretation based on the fossil-bearing rocks along the
Red Deer River The Red Deer River is a river in Alberta and a small portion of Saskatchewan, Canada. It is a major tributary of the South Saskatchewan River and is part of the larger Saskatchewan / Nelson River, Nelson system that empties into Hudson Bay. T ...
in Alberta, Canada, supports the gradual extinction of non-avian dinosaurs; during the last 10 million years of the Cretaceous layers there, the number of dinosaur species seems to have decreased from about 45 to approximately 12. Other scientists have made the same assessment following their research. Several researchers support the existence of Paleocene non-avian dinosaurs. Evidence of this existence is based on the discovery of dinosaur remains in the
Hell Creek Formation The Hell Creek Formation is an intensively studied division of mostly Upper Cretaceous and some lower Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana. The Formation (stratigraphy), formation s ...
up to above and 40,000 years later than the K–Pg boundary. Pollen samples recovered near a fossilized
hadrosaur Hadrosaurids (), also hadrosaurs or duck-billed dinosaurs, are members of the ornithischian family Hadrosauridae. This group is known as the duck-billed dinosaurs for the flat duck-bill appearance of the bones in their snouts. The ornithopod fami ...
femur The femur (; : femurs or femora ), or thigh bone is the only long bone, bone in the thigh — the region of the lower limb between the hip and the knee. In many quadrupeds, four-legged animals the femur is the upper bone of the hindleg. The Femo ...
recovered in the Ojo Alamo Sandstone at the San Juan River in Colorado, indicate that the animal lived during the Cenozoic, approximately (about 1 million years after the K–Pg extinction event). If their existence past the K–Pg boundary can be confirmed, these hadrosaurids would be considered a dead clade walking. The scientific consensus is that these fossils were eroded from their original locations and then re-buried in much later sediments (also known as reworked fossils).


Birds

Most paleontologists regard birds as the only surviving dinosaurs (see
Origin of birds The scientific question of which larger group of animals birds evolved within has traditionally been called the "origin of birds". The present scientific consensus is that birds are a group of maniraptoran theropod dinosaurs that originated du ...
). It is thought that all non-avian
theropods Theropoda (; from ancient Greek , (''therion'') "wild beast"; , (''pous, podos'') "foot"">wiktionary:ποδός"> (''pous, podos'') "foot" is one of the three major groups (clades) of dinosaurs, alongside Ornithischia and Sauropodom ...
became extinct, including then-flourishing groups such as enantiornithines and hesperornithiforms. Several analyses of bird fossils show divergence of species prior to the K–Pg boundary, and that duck, chicken, and
ratite Ratites () are a polyphyletic group consisting of all birds within the infraclass Palaeognathae that lack keels and cannot fly. They are mostly large, long-necked, and long-legged, the exception being the kiwi, which is also the only nocturnal ...
bird relatives coexisted with non-avian dinosaurs. Large collections of bird fossils representing a range of different species provide definitive evidence for the persistence of archaic birds to within 300,000 years of the K–Pg boundary. The absence of these birds in the Paleogene is evidence that a mass extinction of archaic birds took place there, although '' Qinornis'' from China has been suggested to be a more basal member of
Ornithurae Ornithurae (meaning "bird tails" in Greek) is a natural group that includes modern birds and their very close relatives such as the ichthyornithines and the hesperornithines. This clade is defined in the ''PhyloCode'' by Juan Benito and collea ...
which survived into the Paleocene.Mayr G., (2007). "The birds from the Paleocene fissure filling of Walbeck (Germany)." ''Journal of Vertebrate Paleontology'', 27(2): 394-408
PDF fulltext
Only a small fraction of ground and water-dwelling Cretaceous bird species survived the impact, giving rise to today's birds. The only bird group known for certain to have survived the K–Pg boundary is the
Aves Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight ...
. Avians may have been able to survive the extinction as a result of their abilities to dive, swim, or seek shelter in water and marshlands. Many species of avians can build burrows, or nest in tree holes, or termite nests, all of which provided shelter from the environmental effects at the K–Pg boundary. Long-term survival past the boundary was assured as a result of filling ecological niches left empty by extinction of non-avian dinosaurs. Based on molecular sequencing and fossil dating, many species of birds (the
Neoaves Neoaves is a clade that consists of all modern bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard ...
group in particular) appeared to radiate after the K–Pg boundary. The open niche space and relative scarcity of predators following the K-Pg extinction allowed for adaptive radiation of various avian groups.
Ratite Ratites () are a polyphyletic group consisting of all birds within the infraclass Palaeognathae that lack keels and cannot fly. They are mostly large, long-necked, and long-legged, the exception being the kiwi, which is also the only nocturnal ...
s, for example, rapidly diversified in the early Paleogene and are believed to have convergently developed flightlessness at least three to six times, often fulfilling the niche space for large herbivores once occupied by non-avian dinosaurs.


Mammals

Mammalian species began diversifying approximately 30 million years prior to the K–Pg boundary. Diversification of mammals stalled across the boundary. All major Late Cretaceous mammalian lineages, including
monotreme Monotremes () are mammals of the order Monotremata. They are the only group of living mammals that lay eggs, rather than bearing live young. The extant monotreme species are the platypus and the four species of echidnas. Monotremes are typified ...
s (egg-laying mammals), multituberculates,
metatheria Metatheria is a mammalian clade that includes all mammals more closely related to marsupials than to placentals. First proposed by Thomas Henry Huxley in 1880, it is a more inclusive group than the marsupials; it contains all marsupials as wel ...
ns (which includes modern marsupials),
eutheria Eutheria (from Greek , 'good, right' and , 'beast'; ), also called Pan-Placentalia, is the clade consisting of Placentalia, placental mammals and all therian mammals that are more closely related to placentals than to marsupials. Eutherians ...
ns (which includes modern placentals), meridiolestidans, and gondwanatheres survived the K–Pg extinction event, although they suffered losses. In particular, metatherians largely disappeared from North America, and the Asian
deltatheroida Deltatheroida is an extinct group of basal metatherians that were distantly related to modern marsupials. The majority of known members of the group lived in the Cretaceous; one species, '' Gurbanodelta kara'', is known from the late Paleocene ( ...
ns became extinct (aside from the lineage leading to '' Gurbanodelta''). In the Hell Creek beds of North America, at least half of the ten known multituberculate species and all eleven metatherians species are not found above the boundary. Multituberculates in Europe and North America survived relatively unscathed and quickly bounced back in the Paleocene, but Asian forms were devastated, never again to represent a significant component of mammalian fauna. A recent study indicates that metatherians suffered the heaviest losses at the K–Pg event, followed by multituberculates, while eutherians recovered the quickest. K–Pg boundary mammalian species were generally small, comparable in size to
rat Rats are various medium-sized, long-tailed rodents. Species of rats are found throughout the order Rodentia, but stereotypical rats are found in the genus ''Rattus''. Other rat genera include '' Neotoma'' (pack rats), '' Bandicota'' (bandicoo ...
s; this small size would have helped them find shelter in protected environments. It is postulated that some early monotremes, marsupials, and placentals were semiaquatic or burrowing, as there are multiple mammalian lineages with such habits today. Any burrowing or semiaquatic mammal would have had additional protection from K–Pg boundary environmental stresses. After the K–Pg extinction, mammals evolved to fill the niches left vacant by the dinosaurs. Some research indicates that mammals did not explosively diversify across the K–Pg boundary, despite the ecological niches made available by the extinction of dinosaurs. Several mammalian orders have been interpreted as diversifying immediately after the K–Pg boundary, including Chiroptera (
bat Bats are flying mammals of the order Chiroptera (). With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in flight than most birds, flying with their very long spread-out ...
s) and Cetartiodactyla (a diverse group that today includes whales and dolphins and
even-toed ungulate Artiodactyls are placental mammals belonging to the order Artiodactyla ( , ). Typically, they are ungulates which bear weight equally on two (an even number) of their five toes (the third and fourth, often in the form of a hoof). The other t ...
s), although recent research concludes that only
marsupial Marsupials are a diverse group of mammals belonging to the infraclass Marsupialia. They are natively found in Australasia, Wallacea, and the Americas. One of marsupials' unique features is their reproductive strategy: the young are born in a r ...
orders diversified soon after the K–Pg boundary. However, morphological diversification rates among eutherians after the extinction event were thrice those of before it. Also significant, within the mammalian genera, new species were approximately 9.1% larger after the K–Pg boundary. After about 700,000 years, some mammals had reached 50 kilos (110 pounds), a 100-fold increase over the weight of those which survived the extinction. It is thought that body sizes of placental mammalian survivors evolutionarily increased first, allowing them to fill niches after the extinctions, with
brain size The size of the brain is a frequent topic of study within the fields of anatomy, biological anthropology, animal science and evolution. Measuring brain size and cranial capacity is relevant both to humans and other animals, and can be done by wei ...
s increasing later in the
Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...
.


Terrestrial plants

Plant fossils illustrate the reduction in plant species across the K–Pg boundary. There is overwhelming evidence of global disruption of plant communities at the K–Pg boundary. Extinctions are seen both in studies of fossil pollen, and fossil leaves. In North America, the data suggests massive devastation and mass extinction of plants at the K–Pg boundary sections, although there were substantial megafloral changes before the boundary. In North America, approximately 57% of plant species became extinct. In high southern hemisphere latitudes, such as New Zealand and Antarctica, the mass die-off of flora caused no significant turnover in species, but dramatic and short-term changes in the relative abundance of plant groups. European flora was also less affected, most likely due to its distance from the site of the Chicxulub impact. In northern Alaska and the Anadyr-Koryak region of Russia, the flora was minimally impacted. Another line of evidence of a major floral extinction is that the divergence rate of subviral pathogens (viroids) of angiosperms sharply decreased, which indicates an enormous reduction in the number of flowering plants. However, phylogenetic evidence shows no mass angiosperm extinction. Due to the wholesale destruction of plants at the K–Pg boundary, there was a proliferation of saprotrophic organisms, such as
fungi A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...
, that do not require photosynthesis and use nutrients from decaying vegetation. The dominance of fungal species lasted only a few years while the atmosphere cleared and plenty of organic matter to feed on was present. Once the atmosphere cleared photosynthetic organisms returned – initially ferns and other ground-level plants. In some regions, the Paleocene recovery of plants began with recolonizations by fern species, represented as a
fern spike In paleontology, a fern spike is the occurrence of unusually high spore abundance of ferns in the fossil record, usually immediately (in a geological sense) after an extinction event. The spikes are believed to represent a large, temporary incre ...
in the geologic record; this same pattern of fern recolonization was observed after the 1980 Mount St. Helens eruption. Just two species of fern appear to have dominated the landscape for centuries after the event. In the sediments below the K–Pg boundary the dominant plant remains are
angiosperm Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (). The term angiosperm is derived from the Greek words (; 'container, vessel') and (; 'seed'), meaning that the seeds are enclosed within a fruit ...
pollen grains, but the boundary layer contains little pollen and is dominated by fern spores. More usual pollen levels gradually resume above the boundary layer. This is reminiscent of areas blighted by modern volcanic eruptions, where the recovery is led by ferns, which are later replaced by larger angiosperm plants. In North American terrestrial sequences, the extinction event is best represented by the marked discrepancy between the rich and relatively abundant late-Maastrichtian
pollen Pollen is a powdery substance produced by most types of flowers of seed plants for the purpose of sexual reproduction. It consists of pollen grains (highly reduced Gametophyte#Heterospory, microgametophytes), which produce male gametes (sperm ...
record and the post-boundary fern spike.
Polyploidy Polyploidy is a condition in which the cells of an organism have more than two paired sets of ( homologous) chromosomes. Most species whose cells have nuclei (eukaryotes) are diploid, meaning they have two complete sets of chromosomes, one fro ...
appears to have enhanced the ability of flowering plants to survive the extinction, probably because the additional copies of the genome such plants possessed allowed them to more readily adapt to the rapidly changing environmental conditions that followed the impact. Beyond extinction impacts, the event also caused more general changes of flora such as giving rise to
neotropical The Neotropical realm is one of the eight biogeographic realms constituting Earth's land surface. Physically, it includes the tropical terrestrial ecoregions of the Americas and the entire South American temperate zone. Definition In biogeogra ...
rainforest Rainforests are forests characterized by a closed and continuous tree Canopy (biology), canopy, moisture-dependent vegetation, the presence of epiphytes and lianas and the absence of wildfire. Rainforests can be generally classified as tropi ...
biome A biome () is a distinct geographical region with specific climate, vegetation, and animal life. It consists of a biological community that has formed in response to its physical environment and regional climate. In 1935, Tansley added the ...
s like the Amazonia, replacing species composition and structure of local forests during ~6 million years of recovery to former levels of plant
diversity Diversity, diversify, or diverse may refer to: Business *Diversity (business), the inclusion of people of different identities (ethnicity, gender, age) in the workforce *Diversity marketing, marketing communication targeting diverse customers * ...
.


Fungi

While it appears that many fungi were wiped out at the K-Pg boundary, there is some evidence that some fungal species thrived in the years after the extinction event. Microfossils from that period indicate a great increase in fungal spores, long before the resumption of plentiful fern spores in the recovery after the impact. Monoporisporites and
hypha A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium. Structure A hypha consists of one o ...
are almost exclusive microfossils for a short span during and after the iridium boundary. These saprophytes would not need sunlight, allowing them to survive during a period when the atmosphere was likely clogged with dust and sulfur aerosols. The proliferation of fungi has occurred after several extinction events, including the
Permian–Triassic extinction event The Permian–Triassic extinction event (also known as the P–T extinction event, the Late Permian extinction event, the Latest Permian extinction event, the End-Permian extinction event, and colloquially as the Great Dying,) was an extinction ...
, the largest known mass extinction in Earth's history, with up to 96% of all species suffering extinction.


Dating

A 1991 study of fossil leaves dated the extinction-associated freezing to early June. A later study shifted the dating to spring season, based on the osteological evidence and
stable isotope Stable nuclides are Isotope, isotopes of a chemical element whose Nucleon, nucleons are in a configuration that does not permit them the surplus energy required to produce a radioactive emission. The Atomic nucleus, nuclei of such isotopes are no ...
records of well-preserved bones of acipenseriform fishes. The study noted that "the palaeobotanical identities, taphonomic inferences and
stratigraphic Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithost ...
assumptions" for the June dating have since all been refuted. Depalma et al. (2021) opted for the spring–summer range, but During et al. (2024) reevaluated and criticized this study based on its lack of primary data, unidentified laboratory for the analyses, insufficient methods for accurate replication and problematic isotopic graphs with irregular data and error bars. A study of fossilized fish bones found at
Tanis Tanis ( ; ; ) or San al-Hagar (; ; ; or or ; ) is the Greek name for ancient Egyptian ''ḏꜥn.t'', an important archaeological site in the northeastern Nile Delta of ancient Egypt, Egypt, and the location of a city of the same name. Tanis ...
in
North Dakota North Dakota ( ) is a U.S. state in the Upper Midwest, named after the indigenous Dakota people, Dakota and Sioux peoples. It is bordered by the Canadian provinces of Saskatchewan and Manitoba to the north and by the U.S. states of Minneso ...
suggests that the Cretaceous-Paleogene mass extinction happened during the Northern Hemisphere spring.


Duration

The extinction's rapidity is a controversial issue because some researchers think the extinction was the result of a sudden event, while others argue that it took place over a long period. The exact length of time is difficult to determine because of the Signor–Lipps effect, where the fossil record is so incomplete that most extinct
species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
probably died out long after the most recent fossil that has been found. Scientists have also found very few continuous beds of fossil-bearing rock that cover a time range from several million years before the K–Pg extinction to several million years after it. The sedimentation rate and thickness of K–Pg clay from three sites suggest rapid extinction, perhaps over a period of less than 10,000 years. At one site in the
Denver Basin The Denver Basin, variously referred to as the Julesburg Basin, Denver-Julesburg Basin (after Julesburg, Colorado), or the D-J Basin, is a geology, geologic structural basin centered in eastern Colorado in the United States, but extending into sou ...
of
Colorado Colorado is a U.S. state, state in the Western United States. It is one of the Mountain states, sharing the Four Corners region with Arizona, New Mexico, and Utah. It is also bordered by Wyoming to the north, Nebraska to the northeast, Kansas ...
, after the K–Pg boundary layer was deposited, the
fern spike In paleontology, a fern spike is the occurrence of unusually high spore abundance of ferns in the fossil record, usually immediately (in a geological sense) after an extinction event. The spikes are believed to represent a large, temporary incre ...
lasted approximately 1,000 years, and no more than 71,000 years; at the same location, the earliest appearance of
Cenozoic The Cenozoic Era ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterized by the dominance of mammals, insects, birds and angiosperms (flowering plants). It is the latest of three g ...
mammals occurred after approximately 185,000 years, and no more than 570,000 years, "indicating rapid rates of biotic extinction and initial recovery in the Denver Basin during this event." Analysis of the carbon cycle disruptions caused by the impact constrains them to an interval of just 5,000 years. Models presented at the annual meeting of the
American Geophysical Union The American Geophysical Union (AGU) is a 501(c)(3) nonprofit organization of Earth, Atmospheric science, atmospheric, Oceanography, ocean, Hydrology, hydrologic, Astronomy, space, and Planetary science, planetary scientists and enthusiasts that ...
demonstrated that the period of global darkness following the
Chicxulub impact The Chicxulub crater is an impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is offshore, but the crater is named after the onshore community of Chicxulub Pueblo (not the larger coastal town of Chicxulub Puerto). I ...
would have persisted in the
Hell Creek Formation The Hell Creek Formation is an intensively studied division of mostly Upper Cretaceous and some lower Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana. The Formation (stratigraphy), formation s ...
nearly 2 years.


Causes


Chicxulub impact


Evidence for impact

In 1980, a team of researchers consisting of
Nobel Prize The Nobel Prizes ( ; ; ) are awards administered by the Nobel Foundation and granted in accordance with the principle of "for the greatest benefit to humankind". The prizes were first awarded in 1901, marking the fifth anniversary of Alfred N ...
-winning physicist Luis Alvarez, his son, geologist Walter Alvarez, and chemists
Frank Asaro Frank Asaro (born Francesco Asaro, July 31, 1927 – June 10, 2014) was an Emeritus Senior Scientist at the Lawrence Berkeley National Laboratory associated with the University of California at Berkeley. He is best known as the chemist who discove ...
and Helen Michel discovered that
sedimentary Sedimentary rocks are types of rock formed by the cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or deposited at Earth's surface. Sedime ...
layers found all over the world at the Cretaceous–Paleogene boundary contain a
concentration In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', '' molar concentration'', '' number concentration'', ...
of
iridium Iridium is a chemical element; it has the symbol Ir and atomic number 77. This very hard, brittle, silvery-white transition metal of the platinum group, is considered the second-densest naturally occurring metal (after osmium) with a density ...
many times greater than normal (30, 160, and 20 times in three sections originally studied). Iridium is extremely rare in
Earth's crust Earth's crust is its thick outer shell of rock, referring to less than one percent of the planet's radius and volume. It is the top component of the lithosphere, a solidified division of Earth's layers that includes the crust and the upper ...
because it is a siderophile element which mostly sank along with
iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...
into
Earth's core The internal structure of Earth are the layers of the Earth, excluding its atmosphere and hydrosphere. The structure consists of an outer silicate solid crust, a highly viscous asthenosphere, and solid mantle, a liquid outer core whose flow g ...
during
planetary differentiation In planetary science, planetary differentiation is the process by which the chemical elements of a planetary body accumulate in different areas of that body, due to their physical or chemical behavior (e.g. density and chemical affinities). The pr ...
. Instead, iridium is more common in
comet A comet is an icy, small Solar System body that warms and begins to release gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or Coma (cometary), coma surrounding ...
s and
asteroid An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...
s. Because of this, the Alvarez team suggested that an asteroid struck the Earth at the time of the K–Pg boundary. There were earlier speculations on the possibility of an
impact event An impact event is a collision between astronomical objects causing measurable effects. Impact events have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or meteoroids and have minimal effe ...
, but this was the first hard evidence, and since then, studies have continued to demonstrate elevated iridium levels in association with the K-Pg boundary. This hypothesis was viewed as radical when first proposed, but additional evidence soon emerged. The boundary clay was found to be full of minute spherules of rock, crystallized from droplets of molten rock formed by the impact. Shocked quartzShocked minerals have their internal structure deformed, and are created by intense pressures as in nuclear blasts and meteorite impacts. and other minerals were also identified in the K–Pg boundary. The identification of giant tsunami beds along the Gulf Coast and the Caribbean provided more evidence, and suggested that the impact might have occurred nearby, as did the discovery that the K–Pg boundary became thicker in the southern United States, with meter-thick beds of debris occurring in northern New Mexico. A K-Pg boundary "cocktail" of microfossils, lithic fragments, and impact-derived material deposited by gigantic sediment gravity flows was discovered in the Caribbean that served to demarcate the impact. Further research identified the giant
Chicxulub crater The Chicxulub crater is an impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is offshore, but the crater is named after the onshore community of Chicxulub Pueblo (not the larger coastal town of Chicxulub Puerto). I ...
, buried under
Chicxulub The Chicxulub crater is an impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is offshore, but the crater is named after the onshore community of Chicxulub Pueblo (not the larger coastal town of Chicxulub Puerto). I ...
on the coast of
Yucatán Yucatán, officially the Free and Sovereign State of Yucatán, is one of the 31 states which, along with Mexico City, constitute the 32 federal entities of Mexico. It comprises 106 separate municipalities, and its capital city is Mérida. ...
, as the source of the K–Pg boundary clay. Identified in 1990 based on work by geophysicist Glen Penfield in 1978, the crater is oval, with an average diameter of roughly , about the size calculated by the Alvarez team. In March 2010, an international panel of 41 scientists reviewed 20 years of scientific literature and endorsed the asteroid hypothesis, specifically the Chicxulub impact, as the cause of the extinction, ruling out other theories such as massive
volcanism Volcanism, vulcanism, volcanicity, or volcanic activity is the phenomenon where solids, liquids, gases, and their mixtures erupt to the surface of a solid-surface astronomical body such as a planet or a moon. It is caused by the presence of a he ...
. They had determined that a asteroid hurtled into Earth at Chicxulub on Mexico's Yucatán Peninsula. Additional evidence for the impact event is found at the Tanis site in southwestern
North Dakota North Dakota ( ) is a U.S. state in the Upper Midwest, named after the indigenous Dakota people, Dakota and Sioux peoples. It is bordered by the Canadian provinces of Saskatchewan and Manitoba to the north and by the U.S. states of Minneso ...
,
United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
. Tanis is part of the heavily studied
Hell Creek Formation The Hell Creek Formation is an intensively studied division of mostly Upper Cretaceous and some lower Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana. The Formation (stratigraphy), formation s ...
, a group of rocks spanning four states in North America renowned for many significant
fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...
discoveries from the
Upper Cretaceous The Late Cretaceous (100.5–66 Ma) is the more recent of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''cret ...
and lower
Paleocene The Paleocene ( ), or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), ...
. Tanis is an extraordinary and unique site because it appears to record the events from the first minutes until a few hours after the impact of the giant Chicxulub asteroid in extreme detail.Smit, J., ''et al.'' (2017
Tanis, a mixed marine-continental event deposit at the KPG Boundary in North Dakota caused by a seiche triggered by seismic waves of the Chicxulub Impact
Paper No. 113–15, presented 23 October 2017 at the GSA Annual Meeting, Seattle, Washington, USA.
DePalma, R. ''et al.'' (2017

Paper No. 113–16, presented 23 October 2017 at the GSA Annual Meeting, Seattle, Washington, USA.
Amber from the site has been reported to contain microtektites matching those of the Chicxulub impact event. Some researchers question the interpretation of the findings at the site or are skeptical of the team leader, Robert DePalma, who had not yet received his Ph.D. in geology at the time of the discovery and whose commercial activities have been regarded with suspicion. Furthermore, indirect evidence of an asteroid impact as the cause of the mass extinction comes from patterns of turnover in marine plankton. Some critics of the impact theory have put forward that the impact precedes the mass extinction by about 300,000 years and thus was not its cause. However, in a 2013 paper,
Paul Renne Paul R. Renne (born 1957 in San Antonio, Texas) is the director of the Berkeley Geochronology Center and also Professor in Residence of geology in the Department of Earth & Planetary Science, University of California, Berkeley (UC Berkeley). Ren ...
of the
Berkeley Geochronology Center The Berkeley Geochronology Center (BGC) is a non-profit geochronology research institute in Berkeley, California. It was originally a research group in the laboratory of geochronologist Garniss Curtis at the University of California, Berkeley. Th ...
dated the impact at years ago, based on
argon–argon dating Argon–argon (or 40Ar/39Ar) dating is a radiometric dating method invented to supersede Potassium-argon dating, potassiumargon (K/Ar) dating in accuracy. The older method required splitting samples into two for separate potassium and argon measur ...
. He further posits that the mass extinction occurred within 32,000 years of this date. The dating of hydrothermally altered structures around the crater is consistent with this timeline. In 2007, it was proposed that the impactor belonged to the Baptistina family of asteroids. This link has been doubted, though not disproved, in part because of a lack of observations of the asteroid and its family. It was reported in 2009 that 298 Baptistina does not share the chemical signature of the K–Pg impactor. Further, a 2011
Wide-field Infrared Survey Explorer Wide-field Infrared Survey Explorer (WISE, List of observatory codes, observatory code C51, Explorer 92 and MIDEX-6) was a NASA infrared astronomy Space observatory, space telescope in the Explorers Program launched in December 2009.. . WISE L ...
(WISE) study of reflected light from the asteroids of the family estimated their break-up at 80 Ma, giving them insufficient time to shift orbits and impact Earth by 66 Ma.


Effects of impact

The collision would have released the same energy as —more than a billion times the energy of the
atomic bombings of Hiroshima and Nagasaki On 6 and 9 August 1945, the United States detonated two atomic bombs over the Japanese cities of Hiroshima and Nagasaki, respectively, during World War II. The aerial bombings killed between 150,000 and 246,000 people, most of whom were civili ...
. The Chicxulub impact caused a global catastrophe. Some of the phenomena were brief occurrences immediately following the impact, but there were also long-term geochemical and climatic disruptions that devastated the ecology. The scientific consensus is that the asteroid impact at the K–Pg boundary left
megatsunami A megatsunami is an incredibly large wave created by a substantial and sudden displacement of material into a body of water. Megatsunamis have different features from ordinary tsunamis. Ordinary tsunamis are caused by underwater tectonic activi ...
deposits and sediments around the area of the Caribbean Sea and Gulf of Mexico, from the colossal waves created by the impact. These deposits have been identified in the La Popa basin in northeastern Mexico, platform carbonates in northeastern Brazil, in
Atlantic The Atlantic Ocean is the second largest of the world's five oceanic divisions, with an area of about . It covers approximately 17% of Earth's surface and about 24% of its water surface area. During the Age of Discovery, it was known for se ...
deep-sea sediments, and in the form of the thickest-known layer of graded sand deposits, around , in the Chicxulub crater itself, directly above the shocked granite ejecta. The
megatsunami A megatsunami is an incredibly large wave created by a substantial and sudden displacement of material into a body of water. Megatsunamis have different features from ordinary tsunamis. Ordinary tsunamis are caused by underwater tectonic activi ...
has been estimated at more than tall, as the asteroid fell into relatively shallow seas; in deep seas it would have been tall. Fossiliferous sedimentary rocks deposited during the K–Pg impact have been found in the Gulf of Mexico area, including tsunami wash deposits carrying remains of a
mangrove A mangrove is a shrub or tree that grows mainly in coastal saline water, saline or brackish water. Mangroves grow in an equatorial climate, typically along coastlines and tidal rivers. They have particular adaptations to take in extra oxygen a ...
-type ecosystem, indicating that water in the Gulf of Mexico sloshed back and forth repeatedly after the impact; dead fish left in these shallow waters were not disturbed by scavengers. The re-entry of ejecta into Earth's atmosphere included a brief (hours-long) but intense pulse of
infrared radiation Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...
, cooking exposed organisms. This is debated, with opponents arguing that local ferocious fires, probably limited to North America, fall short of global
firestorm A firestorm is a conflagration which attains such intensity that it creates and sustains its own wind system. It is most commonly a natural phenomenon, created during some of the largest bushfires and wildfires. Although the term has been used ...
s. This is the " Cretaceous–Paleogene firestorm debate". A paper in 2013 by a prominent modeler of
nuclear winter Nuclear winter is a severe and prolonged anti-greenhouse effect, global climatic cooling effect that is hypothesized to occur after widespread firestorms following a large-scale Nuclear warfare, nuclear war. The hypothesis is based on the fact ...
suggested that, based on the amount of soot in the global debris layer, the entire terrestrial
biosphere The biosphere (), also called the ecosphere (), is the worldwide sum of all ecosystems. It can also be termed the zone of life on the Earth. The biosphere (which is technically a spherical shell) is virtually a closed system with regard to mat ...
might have burned, implying a global soot-cloud blocking out the sun and creating an impact winter effect. If widespread fires occurred this would have exterminated the most vulnerable organisms that survived the period immediately after the impact. Experimental analysis suggests that any impact-induced wildfires were insufficient on their own to cause plant extinctions, and much of the thermal radiation generated by the impact would have been absorbed by the atmosphere and ejecta in the lower atmosphere. Aside from the hypothesized fire effects on reduction of insolation, the impact would have created a dust cloud that blocked sunlight for up to a year, inhibiting photosynthesis. The asteroid hit an area of
gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate Hydrate, dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, drywall and blackboard or sidewalk ...
and
anhydrite Anhydrite, or anhydrous calcium sulfate, is a mineral with the chemical formula CaSO4. It is in the orthorhombic crystal system, with three directions of perfect cleavage parallel to the three planes of symmetry. It is not isomorphous with the ...
rock containing a large amount of combustible hydrocarbons and sulfur, much of which was vaporized, thereby injecting
sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...
aerosol An aerosol is a suspension (chemistry), suspension of fine solid particles or liquid Drop (liquid), droplets in air or another gas. Aerosols can be generated from natural or Human impact on the environment, human causes. The term ''aerosol'' co ...
s into the
stratosphere The stratosphere () is the second-lowest layer of the atmosphere of Earth, located above the troposphere and below the mesosphere. The stratosphere is composed of stratified temperature zones, with the warmer layers of air located higher ...
, which might have reduced sunlight reaching the Earth's surface by more than 50%. Fine silicate dust also contributed to the intense impact winter, as did soot from wildfires. The climatic forcing of this impact winter was about 100 times more potent than that of the
1991 eruption of Mount Pinatubo The 1991 eruption of Mount Pinatubo in the Philippines' Luzon Volcanic Arc was the second-largest volcanic eruption of the 20th century, behind only the 1912 eruption of Novarupta in Alaska. Eruptive activity began on April 2 as a series of p ...
. According to models of the
Hell Creek Formation The Hell Creek Formation is an intensively studied division of mostly Upper Cretaceous and some lower Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana. The Formation (stratigraphy), formation s ...
, the onset of global darkness would have reached its maximum in only a few weeks and likely lasted upwards of 2 years. Freezing temperatures probably lasted for at least three years. At Brazos section, the
sea surface temperature Sea surface temperature (or ocean surface temperature) is the ocean temperature, temperature of ocean water close to the surface. The exact meaning of ''surface'' varies in the literature and in practice. It is usually between and below the sea ...
dropped as much as for decades after the impact. It would take at least ten years for such aerosols to dissipate, and would account for the extinction of plants and phytoplankton, and subsequently herbivores and their
predators Predation is a biological interaction in which one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill ...
. Creatures whose food chains were based on
detritus In biology, detritus ( or ) is organic matter made up of the decomposition, decomposing remains of organisms and plants, and also of feces. Detritus usually hosts communities of microorganisms that colonize and decomposition, decompose (Reminera ...
would have a reasonable chance of survival. In 2016, a scientific drilling project obtained deep rock-
core sample A core sample is a cylindrical section of (usually) a naturally-occurring substance. Most core samples are obtained by drilling with special drills into the substance, such as sediment or rock, with a hollow steel tube, called a core drill. The ...
s from the peak ring around the Chicxulub impact crater. The discoveries confirmed that the rock comprising the peak ring had been shocked by immense pressure and melted in just minutes from its usual state into its present form. Unlike sea-floor deposits, the peak ring was made of granite originating much deeper in the earth, which had been ejected to the surface by the impact.
Gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate Hydrate, dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, drywall and blackboard or sidewalk ...
is a
sulfate The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ...
-containing rock usually present in the shallow seabed of the region; it had been almost entirely removed, vaporized into the atmosphere. The impactor was large enough to create a peak ring, to melt, shock, and eject deep granite, to create colossal water movements, and to eject an immense quantity of vaporized rock and sulfates into the atmosphere, where they would have persisted for several years. This worldwide dispersal of dust and sulfates would have affected climate catastrophically, led to large temperature drops, and devastated the food chain. The release of large quantities of sulphur aerosols into the atmosphere as a consequence of the impact would also have caused acid rain. Oceans acidified as a result. This decrease in ocean pH would kill many organisms that grow shells of
calcium carbonate Calcium carbonate is a chemical compound with the chemical formula . It is a common substance found in Rock (geology), rocks as the minerals calcite and aragonite, most notably in chalk and limestone, eggshells, gastropod shells, shellfish skel ...
. The heating of the atmosphere during the impact itself may have also generated
nitric acid Nitric acid is an inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but samples tend to acquire a yellow cast over time due to decomposition into nitrogen oxide, oxides of nitrogen. Most com ...
rain through the production of nitrogen oxides and their subsequent reaction with water vapour. After the impact winter, the Earth entered a period of global warming as a result of the vapourisation of carbonates into carbon dioxide, whose long residence time in the atmosphere ensured significant warming would occur after more short-lived cooling gases dissipated. Carbon monoxide concentrations also increased and caused particularly devastating global warming because of the consequent increases in tropospheric ozone and methane concentrations. The impact's injection of water vapour into the atmosphere also produced major climatic perturbations. The end-Cretaceous event is the only
mass extinction An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity on Earth. Such an event is identified by a sharp fall in the diversity and abundance of multicellular organisms. It occ ...
definitively known to be associated with an impact, and other large extraterrestrial impacts, such as the
Manicouagan Reservoir Manicouagan Reservoir (also Lake Manicouagan ; ) is an annular lake in central Quebec, Canada, covering an area of . The lake island in its centre is known as René-Levasseur Island, and its highest point is Mount Babel. The structure was c ...
impact, do not coincide with any noticeable extinction events.


Multiple impact event

Other crater-like topographic features have also been proposed as impact craters formed in connection with Cretaceous–Paleogene extinction. This suggests the possibility of near-simultaneous multiple impacts, perhaps from a fragmented asteroidal object similar to the Shoemaker–Levy 9 impact with
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a Jupiter mass, mass more than 2.5 times that of all the other planets in the Solar System combined a ...
. In addition to the
Chicxulub crater The Chicxulub crater is an impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is offshore, but the crater is named after the onshore community of Chicxulub Pueblo (not the larger coastal town of Chicxulub Puerto). I ...
, there is the Boltysh crater in
Ukraine Ukraine is a country in Eastern Europe. It is the List of European countries by area, second-largest country in Europe after Russia, which Russia–Ukraine border, borders it to the east and northeast. Ukraine also borders Belarus to the nor ...
(), the Silverpit crater in the
North Sea The North Sea lies between Great Britain, Denmark, Norway, Germany, the Netherlands, Belgium, and France. A sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian Se ...
() possibly formed by
bolide A bolide is normally taken to mean an exceptionally bright meteor, but the term is subject to more than one definition, according to context. It may refer to any large Impact crater, crater-forming body, or to one that explodes in the atmosphere. ...
impact, and the controversial and much larger Shiva crater. Any other craters that might have formed in the
Tethys Ocean The Tethys Ocean ( ; ), also called the Tethys Sea or the Neo-Tethys, was a prehistoric ocean during much of the Mesozoic Era and early-mid Cenozoic Era. It was the predecessor to the modern Indian Ocean, the Mediterranean Sea, and the Eurasia ...
would since have been obscured by the northward tectonic drift of Africa and India.


Deccan Traps

The Deccan Traps, which erupted close to the boundary between the Mesozoic and Cenozoic, have been cited as an alternate explanation for the mass extinction. Before 2000, arguments that the
Deccan Traps The Deccan Traps are a large igneous province of west-central India (17–24°N, 73–74°E). They are one of the largest volcanic features on Earth, taking the form of a large shield volcano. They consist of many layers of solidified flood ...
flood basalt A flood basalt (or plateau basalt) is the result of a giant volcanic eruption or series of eruptions that covers large stretches of land or the ocean floor with basalt lava. Many flood basalts have been attributed to the onset of a hotspot (geolo ...
s caused the extinction were usually linked to the view that the extinction was gradual, as the flood basalt events were thought to have started around 68 Mya and lasted more than 2 million years. The most recent evidence shows that the traps erupted over a period of only 800,000 years spanning the K–Pg boundary, and therefore may be responsible for the extinction and the delayed biotic recovery thereafter. The Deccan Traps could have caused extinction through several mechanisms, including the release of dust and sulfuric aerosols into the air, which might have blocked sunlight and thereby reduced photosynthesis in plants. In addition, the latest Cretaceous saw a rise in global temperatures; Deccan Traps volcanism resulted in carbon dioxide emissions that increased the greenhouse effect when the dust and aerosols cleared from the atmosphere. Plant fossils record a 250 ppm increase in carbon dioxide concentrations across the K-Pg boundary likely attributable to Deccan Traps activity. The increased carbon dioxide emissions also caused acid rain, evidenced by increased mercury deposition due to increased solubility of mercury compounds in more acidic water. Some geophysical models suggest that the Chicxulub impact could have triggered some of the largest Deccan eruptions, as well as eruptions at active volcano sites anywhere on Earth. Evidence for extinctions caused by the Deccan Traps includes the reduction in diversity of marine life when the climate near the K–Pg boundary increased in temperature. The temperature increased about three to four degrees very rapidly between 65.4 and 65.2 million years ago, which is very near the time of the extinction event. Not only did the climate temperature increase, but the water temperature decreased, causing a drastic decrease in marine diversity. Evidence from Tunisia indicates that marine life was deleteriously affected by a major period of increased warmth and humidity linked to a pulse of intense Deccan Traps activity, and that marine extinctions there began before the impact event. Charophyte declines in the Songliao Basin, China before the asteroid impact have been concluded to be connected to climate changes caused by Deccan Traps activity. In the years when the Deccan Traps hypothesis was linked to a slower extinction, Luis Alvarez (d. 1988) replied that paleontologists were being misled by sparse data. While his assertion was not initially well-received, later intensive field studies of fossil beds lent weight to his claim. Eventually, most paleontologists began to accept the idea that the mass extinctions at the end of the Cretaceous were largely or at least partly due to a massive Earth impact. Even Walter Alvarez acknowledged that other major changes might have contributed to the extinctions. More recent arguments against the Deccan Traps as an extinction cause include that the timeline of Deccan Traps activity and pulses of climate change has been found by some studies to be asynchronous, that palynological changes do not coincide with intervals of volcanism, and that many sites show climatic stability during the latest Maastrichtian and no sign of major disruptions caused by volcanism. A study published in 2020 dated the peak of the Deccan Traps eruptions, and consequent global heating, as 200,000 years before the extinction, and the conclusion that the Deccan Traps did not cause the extinction is accepted by the paleontologist
Michael Benton Michael James Benton (born 8 April 1956) is a British palaeontologist, and professor of vertebrate paleontology, vertebrate palaeontology in the School of Earth Sciences at the University of Bristol. His published work has mostly concentrated on ...
. Another study published in the same year argued that the long-term warming caused by the increased volcanism would have increased habitat suitability, not caused an extinction.


Maastrichtian sea-level regression

There is clear evidence that sea levels fell in the final stage of the Cretaceous by more than at any other time in the Mesozoic era. In some Maastrichtian
stage Stage, stages, or staging may refer to: Arts and media Acting * Stage (theatre), a space for the performance of theatrical productions * Theatre, a branch of the performing arts, often referred to as "the stage" * ''The Stage'', a weekly Brit ...
rock layers from various parts of the world, the later layers are terrestrial; earlier layers represent shorelines and the earliest layers represent seabeds. These layers do not show the tilting and distortion associated with
mountain building Mountain formation occurs due to a variety of geological processes associated with large-scale movements of the Earth's crust (List of tectonic plates, tectonic plates). Fold (geology), Folding, Fault (geology), faulting, Volcano, volcanic acti ...
, therefore the likeliest explanation is a ''regression'', a drop in sea level. There is no direct evidence for the cause of the regression, but the currently accepted explanation is that the mid-ocean ridges became less active and sank under their own weight. A severe regression would have greatly reduced the
continental shelf A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an islan ...
area, the most species-rich part of the sea, and therefore could have been enough to cause a marine mass extinction. This change would not have caused the extinction of the ammonites. The regression would also have caused climate changes, partly by disrupting winds and ocean currents and partly by reducing the Earth's
albedo Albedo ( ; ) is the fraction of sunlight that is Diffuse reflection, diffusely reflected by a body. It is measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects ...
and increasing global temperatures. Marine regression also resulted in the loss of epeiric seas, such as the
Western Interior Seaway The Western Interior Seaway (also called the Cretaceous Seaway, the Niobraran Sea, the North American Inland Sea, or the Western Interior Sea) was a large inland sea (geology), inland sea that existed roughly over the present-day Great Plains of ...
of North America. The loss of these seas greatly altered habitats, removing
coastal plain A coastal plain (also coastal plains, coastal lowland, coastal lowlands) is an area of flat, low-lying land adjacent to a sea coast. A fall line commonly marks the border between a coastal plain and an upland area. Formation Coastal plains can f ...
s that ten million years before had been host to diverse communities such as are found in rocks of the Dinosaur Park Formation. Another consequence was an expansion of
freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include non-salty mi ...
environments, since continental runoff now had longer distances to travel before reaching
ocean The ocean is the body of salt water that covers approximately 70.8% of Earth. The ocean is conventionally divided into large bodies of water, which are also referred to as ''oceans'' (the Pacific, Atlantic, Indian Ocean, Indian, Southern Ocean ...
s. While this change was favorable to freshwater
vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...
s, those that prefer marine environments, such as sharks, suffered. However, sea level fall as a cause of the extinction event is contradicted by other evidence, namely that sections which show no sign of marine regression still show evidence of a major drop in diversity.


Multiple causes

Proponents of multiple causation view the suggested single causes as either too small to produce the vast scale of the extinction, or not likely to produce its observed taxonomic pattern. In a review article, J. David Archibald and David E. Fastovsky discussed a scenario combining three major postulated causes: volcanism,
marine regression A marine regression is a geological process occurring when areas of submerged seafloor are exposed during a drop in sea level. The opposite event, marine transgression, occurs when flooding from the sea covers previously-exposed land. Descript ...
, and extraterrestrial impact. In this scenario, terrestrial and marine communities were stressed by the changes in, and loss of, habitats. Dinosaurs, as the largest vertebrates, were the first affected by environmental changes, and their diversity declined. At the same time,
particulate Particulate matter (PM) or particulates are microscopic particles of solid or liquid matter suspended in the air. An ''aerosol'' is a mixture of particulates and air, as opposed to the particulate matter alone, though it is sometimes define ...
materials from volcanism cooled and dried areas of the globe. Then an impact event occurred, causing collapses in photosynthesis-based food chains, both in the already-stressed terrestrial food chains and in the marine food chains. Based on studies at Seymour Island in
Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean (also known as the Antarctic Ocean), it contains the geographic South Pole. ...
, Sierra Petersen and colleagues argue that there were two separate extinction events near the Cretaceous–Paleogene boundary, with one correlating to Deccan Trap volcanism and one correlated with the Chicxulub impact. The team analyzed combined extinction patterns using a new clumped isotope temperature record from a hiatus-free, expanded K–Pg boundary section. They documented a 7.8±3.3 °C warming synchronous with the onset of Deccan Traps volcanism and a second, smaller warming at the time of meteorite impact. They suggested that local warming had been amplified due to the simultaneous disappearance of continental or sea ice. Intra-shell variability indicates a possible reduction in seasonality after Deccan eruptions began, continuing through the meteorite event. Species extinction at Seymour Island occurred in two pulses that coincide with the two observed warming events, directly linking the end-Cretaceous extinction at this site to both volcanic and meteorite events via climate change.


See also

* * * * * * *


Explanatory notes


Citations


Further reading

* * *
Papers and presentations resulting from the 2016 Chicxulub drilling project
��The
Geological Society of America The Geological Society of America (GSA) is a nonprofit organization dedicated to the advancement of the geosciences. History The society was founded in Ithaca, New York, in 1888 by Alexander Winchell, John J. Stevenson, Charles H. Hi ...
, GSA Annual Meeting in Seattle, Washington, USA – 2017, Session No. 192 * *


External links


What killed the dinosaurs?
��University of California Museum of Paleontology (1995)
The Great Chicxulub Debate 2004
��
Geological Society of London The Geological Society of London, known commonly as the Geological Society, is a learned society based in the United Kingdom. It is the oldest national geological society in the world and the largest in Europe, with more than 12,000 Fellows. Fe ...
{{DEFAULTSORT:Cretaceous-Tertiary Extinction Event * * Cenozoic volcanism Events in the geological history of Earth Events that forced the climate Evolution of mammals Hypothetical impact events Megatsunamis Mesozoic volcanism Meteorological hypotheses Dinosaurs