The Cretaceous ( ) is a
geological period that lasted from about 145 to 66
million years ago
The abbreviation Myr, "million years", is a unit of a quantity of (i.e. ) years, or 31.556926 teraseconds.
Usage
Myr (million years) is in common use in fields such as Earth science and cosmology. Myr is also used with Mya (million years ago ...
(Mya). It is the third and final period of the
Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of the entire
Phanerozoic. The name is derived from the Latin
''creta'', "
chalk
Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor. C ...
", which is abundant in the latter half of the period. It is usually abbreviated K, for its German translation ''Kreide''.
The Cretaceous was a period with a relatively warm
climate, resulting in high
eustatic sea levels that created numerous shallow
inland seas. These oceans and seas were populated with now-
extinct marine reptiles,
ammonites
Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefis ...
, and
rudists
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 ...
, while
dinosaurs continued to dominate on land. The world was ice free, and forests extended to the poles. During this time, new groups of
mammals and
birds appeared. During the Early Cretaceous,
flowering plant
Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants t ...
s appeared and began to rapidly diversify, becoming the dominant group of plants across the Earth by the end of the Cretaceous, coincident with the decline and extinction of previously widespread
gymnosperm groups.
The Cretaceous (along with the Mesozoic) ended with the
Cretaceous–Paleogene extinction event, a large mass extinction in which many groups, including non-avian dinosaurs,
pterosaur
Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is 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 ...
s, and large marine reptiles, died out. The end of the Cretaceous is defined by the abrupt
Cretaceous–Paleogene boundary (K–Pg boundary), a geologic signature associated with the mass extinction that lies between the Mesozoic and
Cenozoic Eras.
Etymology and history
The Cretaceous as a separate period was first defined by Belgian geologist
Jean d'Omalius d'Halloy in 1822 as the ''Terrain Crétacé'', using
strata in the
Paris Basin and named for the extensive beds of
chalk
Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor. C ...
(
calcium carbonate deposited by the shells of marine
invertebrates, principally
coccoliths), found in the upper Cretaceous of
Western Europe. The name Cretaceous was derived from
Latin
Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...
''creta'', meaning ''chalk''. The twofold division of the Cretaceous was implemented by
Conybeare and Phillips in 1822.
Alcide d'Orbigny in 1840 divided the French Cretaceous into five ''étages'' (stages): the
Neocomian, Aptian, Albian, Turonian, and Senonian, later adding the ''Urgonian'' between Neocomian and Aptian and the Cenomanian between the Albian and Turonian.
Geology
Subdivisions
The Cretaceous is divided into
Early
Early may refer to:
History
* The beginning or oldest part of a defined historical period, as opposed to middle or late periods, e.g.:
** Early Christianity
** Early modern Europe
Places in the United States
* Early, Iowa
* Early, Texas
* Early ...
and
Late Cretaceous epochs, or Lower and Upper Cretaceous
series. In older literature, the Cretaceous is sometimes divided into three series:
Neocomian (lower/early),
Gallic (middle) and
Senonian
The Late Cretaceous (100.5–66 Ma) is the younger 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 ''creta'', t ...
(upper/late). A subdivision into 12
stages, all originating from European stratigraphy, is now used worldwide. In many parts of the world, alternative local subdivisions are still in use.
From youngest to oldest, the subdivisions of the Cretaceous period are:
Boundaries
The lower boundary of the Cretaceous is currently undefined, and the Jurassic–Cretaceous boundary is currently the only system boundary to lack a defined
Global Boundary Stratotype Section and Point
A Global Boundary Stratotype Section and Point (GSSP) is an internationally agreed upon reference point on a stratigraphic section which defines the lower boundary of a stage on the geologic time scale. The effort to define GSSPs is conducted ...
(GSSP). Placing a GSSP for this boundary has been difficult because of the strong regionality of most biostratigraphic markers, and the lack of any
chemostratigraphic events, such as
isotope
Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers ( mass num ...
excursions (large sudden changes in
ratios of isotopes) that could be used to define or correlate a boundary.
Calpionellid
Calpionellids are an extinct group of eukaryotic single celled organisms of uncertain affinities. Their fossils are found in marine rocks of Upper Jurassic to Lower Cretaceous age. They were planktic organisms with urn-shaped, calcitic tests tha ...
s, an enigmatic group of
planktonic
protists with urn-shaped calcitic
tests
Test(s), testing, or TEST may refer to:
* Test (assessment), an educational assessment intended to measure the respondents' knowledge or other abilities
Arts and entertainment
* ''Test'' (2013 film), an American film
* ''Test'' (2014 film), ...
briefly abundant during the latest Jurassic to earliest Cretaceous, have been suggested as the most promising candidates for fixing the Jurassic–Cretaceous boundary. In particular, the first appearance ''
Calpionella alpina'', coinciding with the base of the eponymous Alpina subzone, has been proposed as the definition of the base of the Cretaceous. The working definition for the boundary has often been placed as the first appearance of the ammonite ''
Strambergella jacobi'', formerly placed in the genus ''
Berriasella
''Berriasella'' is a discoidal evolute perisphinctacean ammonite, and type genus for the neocomitid subfamily Berriasellinae. Its ribbing is distinct, consisting of both simple and bifurcated ribs that extend from the umbilical seam across th ...
'', but its use as a stratigraphic indicator has been questioned, as its first appearance does not correlate with that of ''C. alpina''. The boundary is officially considered by the
International Commission on Stratigraphy to be approximately 145 million years ago, but other estimates have been proposed based on U-Pb geochronology, ranging as young as 140 million years ago.
The upper boundary of the Cretaceous is sharply defined, being placed at an
iridium-rich layer found worldwide that is believed to be associated with the
Chicxulub impact crater, with its boundaries circumscribing parts of the
Yucatán Peninsula and extending into the
Gulf of Mexico. This layer has been dated at 66.043 Mya.
At the end of the Cretaceous, the impact of a large
body with the Earth may have been the punctuation mark at the end of a progressive decline in
biodiversity during the Maastrichtian age. The result was the extinction of three-quarters of Earth's plant and animal species. The impact created the sharp break known as the
K–Pg boundary (formerly known as the K–T boundary). Earth's biodiversity required substantial time to recover from this event, despite the probable existence of an abundance of vacant
ecological niches.
Despite the severity of the K-Pg extinction event, there were significant variations in the rate of extinction between and within different
clades. Species that depended on
photosynthesis declined or became extinct as atmospheric particles blocked
solar energy. As is the case today, photosynthesizing organisms, such as
phytoplankton and land
plants, formed the primary part of the
food chain in the late Cretaceous, and all else that depended on them suffered, as well.
Herbivorous animals, which depended on plants and plankton as their food, died out as their food sources became scarce; consequently, the top
predators, such as ''
Tyrannosaurus rex'', also perished. Yet only three major groups of
tetrapods disappeared completely; the nonavian
dinosaurs, the
plesiosaurs and the
pterosaur
Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is 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 ...
s. The other Cretaceous groups that did not survive into the Cenozoic the
ichthyosaurs, last remaining
temnospondyls (
Koolasuchus), and nonmammalian were already extinct millions of years before the event occurred.
Coccolithophorids
Coccolithophores, or coccolithophorids, are single celled organisms which are part of the phytoplankton, the autotrophic (self-feeding) component of the plankton community. They form a group of about 200 species, and belong either to the kingdo ...
and
molluscs, including
ammonites,
rudists,
freshwater snails, and
mussels, as well as organisms whose food chain included these shell builders, became extinct or suffered heavy losses. For example,
ammonites
Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefis ...
are thought to have been the principal food of
mosasaurs, a group of giant marine lizards related to snakes that became extinct at the boundary.
Omnivores,
insectivores, and
carrion-eaters survived the extinction event, perhaps because of the increased availability of their food sources. At the end of the Cretaceous, there seem to have been no purely herbivorous or
carnivorous mammals. Mammals and birds that survived the extinction fed on
insects,
larvae,
worms, and snails, which in turn fed on dead plant and animal matter. Scientists theorise that these organisms survived the collapse of plant-based food chains because they fed on
detritus.
In
stream communities, few groups of animals became extinct. Stream communities rely less on food from living plants and more on detritus that washes in from land. This particular ecological niche buffered them from extinction. Similar, but more complex patterns have been found in the oceans. Extinction was more severe among animals living in the
water column than among animals living on or in the seafloor. Animals in the water column are almost entirely dependent on
primary production from living phytoplankton, while animals living on or in the
ocean floor feed on detritus or can switch to detritus feeding.
The largest air-breathing survivors of the event,
crocodilians and
champsosaurs, were semiaquatic and had access to detritus. Modern crocodilians can live as scavengers and can survive for months without food and go into hibernation when conditions are unfavorable, and their young are small, grow slowly, and feed largely on invertebrates and dead organisms or fragments of organisms for their first few years. These characteristics have been linked to crocodilian survival at the end of the Cretaceous.
Geologic formations
The high sea level and warm climate of the Cretaceous meant large areas of the continents were covered by warm, shallow seas, providing habitat for many marine organisms. The Cretaceous was named for the extensive chalk deposits of this age in Europe, but in many parts of the world, the deposits from the Cretaceous are of
marine
Marine is an adjective meaning of or pertaining to the sea or ocean.
Marine or marines may refer to:
Ocean
* Maritime (disambiguation)
* Marine art
* Marine biology
* Marine debris
* Marine habitats
* Marine life
* Marine pollution
Military ...
limestone
Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms w ...
, a rock type that is formed under warm, shallow marine conditions. Due to the high sea level, there was extensive
space for such
sedimentation. Because of the relatively young age and great thickness of the system, Cretaceous rocks are evident in many areas worldwide.
Chalk
Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor. C ...
is a rock type characteristic for (but not restricted to) the Cretaceous. It consists of
coccoliths, microscopically small
calcite skeletons of
coccolithophores, a type of
algae that prospered in the Cretaceous seas.
Stagnation of deep sea currents in middle Cretaceous times caused anoxic conditions in the sea water leaving the deposited organic matter undecomposed. Half of the world's petroleum reserves were laid down at this time in the anoxic conditions of what would become the Persian Gulf and the Gulf of Mexico. In many places around the world, dark anoxic
shales were formed during this interval, such as the
Mancos Shale of western North America. These shales are an important
source rock
In petroleum geology, source rock is rock which has generated hydrocarbons or which could generate hydrocarbons. Source rocks are one of the necessary elements of a working petroleum system. They are organic-rich sediments that may have been depo ...
for
oil and gas
A fossil fuel is a hydrocarbon-containing material formed naturally in the Earth's crust from the remains of dead plants and animals that is extracted and burned as a fuel. The main fossil fuels are coal, oil, and natural gas. Fossil fuels ...
, for example in the subsurface of the North Sea.
Europe
In northwestern Europe, chalk deposits from the Upper Cretaceous are characteristic for the
Chalk Group
The Chalk Group (often just called the Chalk) is the lithostratigraphic unit (a certain number of rock strata) which contains the Upper Cretaceous limestone succession in southern and eastern England. The same or similar rock sequences occur acro ...
, which forms the
white cliffs of Dover on the south coast of
England
England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe ...
and similar cliffs on the
French
French (french: français(e), link=no) may refer to:
* Something of, from, or related to France
** French language, which originated in France, and its various dialects and accents
** French people, a nation and ethnic group identified with Franc ...
Normandian coast. The
group is found in England, northern France, the
low countries, northern
Germany
Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwee ...
,
Denmark
)
, song = ( en, "King Christian stood by the lofty mast")
, song_type = National and royal anthem
, image_map = EU-Denmark.svg
, map_caption =
, subdivision_type = Sovereign state
, subdivision_name = Kingdom of Denmark
, establish ...
and in the subsurface of the southern part of the
North Sea. Chalk is not easily
consolidated and the Chalk Group still consists of loose sediments in many places. The group also has other
limestone
Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms w ...
s and
arenites. Among the fossils it contains are
sea urchins,
belemnite
Belemnitida (or the belemnite) is an extinct order of squid-like cephalopods that existed from the Late Triassic to Late Cretaceous. Unlike squid, belemnites had an internal skeleton that made up the cone. The parts are, from the arms-most ...
s,
ammonites and sea reptiles such as ''
Mosasaurus''.
In southern Europe, the Cretaceous is usually a marine system consisting of
competent
Competence may refer to:
*Competence (geology), the resistance of a rock against deformation or plastic flow.
*Competence (human resources), a standardized requirement for an individual to properly perform a specific job
*Competence (law), the me ...
limestone beds or incompetent
marls. Because the
Alpine mountain chains did not yet exist in the Cretaceous, these deposits formed on the southern edge of the European
continental shelf, at the margin of the
Tethys Ocean.
North America
During the Cretaceous, the present North American continent was isolated from the other continents. In the Jurassic, the North Atlantic already opened, leaving a proto-ocean between Europe and North America. From north to south across the continent, the
Western Interior Seaway started forming. This inland sea separated the elevated areas of
Laramidia in the west and
Appalachia in the east. Three dinosaur clades found in Laramidia (troodontids, therizinosaurids and oviraptorosaurs) are absent from Appalachia from the Coniacian through the Maastrichtian.
Paleogeography
During the Cretaceous, the late-
Paleozoic-to-early-Mesozoic
supercontinent of
Pangaea completed its
tectonic breakup into the present-day
continents, although their positions were substantially different at the time. As the
Atlantic Ocean widened, the convergent-margin mountain building (
orogenies
Orogeny is a mountain building process. An orogeny is an event that takes place at a convergent plate margin when plate motion compresses the margin. An ''orogenic belt'' or ''orogen'' develops as the compressed plate crumples and is uplifted t ...
) that had begun during the
Jurassic continued in the
North American Cordillera, as the
Nevadan orogeny was followed by the
Sevier and
Laramide orogenies.
Gondwana had begun to break up during the Jurassic Period, but its fragmentation accelerated during the Cretaceous and was largely complete by the end of the period.
South America,
Antarctica, and
Australia
Australia, officially the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands. With an area of , Australia is the largest country by ...
rifted away from
Africa (though
India
India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the List of countries and dependencies by area, seventh-largest country by area, the List of countries and dependencies by population, second-most populous ...
and
Madagascar remained attached to each other until around 80 million years ago); thus, the South Atlantic and
Indian Ocean
The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by ...
s were newly formed. Such active rifting lifted great undersea mountain chains along the welts, raising
eustatic sea levels worldwide. To the north of Africa the
Tethys Sea continued to narrow. During the most of the Late Cretaceous, North America would be divided in two by the
Western Interior Seaway, a large interior sea, separating
Laramidia to the west and
Appalachia to the east, then receded late in the period, leaving thick marine deposits sandwiched between
coal beds. At the peak of the Cretaceous
transgression, one-third of Earth's present land area was submerged.
The Cretaceous is justly famous for its
chalk
Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor. C ...
; indeed, more chalk formed in the Cretaceous than in any other period in the
Phanerozoic.
Mid-ocean ridge activity—or rather, the circulation of seawater through the enlarged ridges—enriched the oceans in
calcium
Calcium is a chemical element with the 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 ...
; this made the oceans more saturated, as well as increased the bioavailability of the element for
calcareous nanoplankton. These widespread
carbonates and other
sedimentary deposits make the Cretaceous rock record especially fine. Famous
formations from North America include the rich marine fossils of
Kansas
Kansas () is a U.S. state, state in the Midwestern United States, Midwestern United States. Its Capital city, capital is Topeka, Kansas, Topeka, and its largest city is Wichita, Kansas, Wichita. Kansas is a landlocked state bordered by Nebras ...
's
Smoky Hill Chalk Member and the terrestrial fauna of the late Cretaceous
Hell Creek Formation. Other important Cretaceous exposures occur in
Europe
Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...
(e.g., the
Weald) and
China
China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's List of countries and dependencies by population, most populous country, with a Population of China, population exceeding 1.4 billion, slig ...
(the
Yixian Formation). In the area that is now India, massive lava beds called the
Deccan Traps were erupted in the very late Cretaceous and early Paleocene.
Climate
Palynological evidence indicates the Cretaceous climate had three broad phases: a Berriasian–Barremian warm-dry phase, a Aptian–Santonian warm-wet phase, and a Campanian–Maastrichtian cool-dry phase. The cooling trend of the last epoch of the Jurassic continued into the Berriasian, the first age of the Cretaceous. There is evidence that snowfalls were common in the higher latitudes during this age, and the tropics became wetter than during the Triassic and Jurassic.
Glaciation was however restricted to high-
latitude mountains, though seasonal snow may have existed farther from the poles. Rafting by ice of stones into marine environments occurred during much of the Cretaceous, but evidence of deposition directly from glaciers is limited to the Early Cretaceous of the
Eromanga Basin in southern
Australia
Australia, officially the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands. With an area of , Australia is the largest country by ...
.
After the end of the first age, however, temperatures increased again, and these conditions were almost constant until the end of the period.
The warming may have been due to intense
volcanic activity which produced large quantities of
carbon dioxide. Between 70 and 69 Ma and 66–65 Ma, isotopic ratios indicate elevated atmospheric
CO2 pressures with levels of 1000–1400 ppmV and mean annual temperatures in
west Texas between . Atmospheric CO
2 and temperature relations indicate a doubling of pCO
2 was accompanied by a ~0.6 °C increase in temperature. The production of large quantities of magma, variously attributed to
mantle plumes or to extensional tectonics,
further pushed sea levels up, so that large areas of the continental crust were covered with shallow seas. The
Tethys Sea connecting the tropical oceans east to west also helped to warm the global climate. Warm-adapted
plant fossils are known from localities as far north as
Alaska
Alaska ( ; russian: Аляска, Alyaska; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Yup'ik: ''Alaskaq''; tli, Anáaski) is a state located in the Western United States on the northwest extremity of North America. A semi-exclave of the U ...
and
Greenland, while
dinosaur fossils have been found within 15 degrees of the Cretaceous
south pole. It was suggested that there was
Antarctic marine glaciation in the
Turonian Age, based on isotopic evidence. However, this has subsequently been suggested to be the result of inconsistent isotopic proxies, with evidence of polar rainforests during this time interval at 82° S.
A very gentle
temperature gradient from the
equator to the poles meant weaker global winds, which drive the ocean currents, resulted in less
upwelling and more stagnant
oceans than today. This is evidenced by widespread black
shale deposition and frequent
anoxic events. Sediment cores show that tropical
sea surface temperatures may have briefly been as warm as , warmer than at present, and that they averaged around . Meanwhile, deep ocean temperatures were as much as warmer than today's.
Flora
Flowering plant
Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants t ...
s (angiosperms) make up around 90% of living plant species today. Prior to the rise of angiosperms, during the Jurassic and the Early Cretaceous, the higher flora was dominated by
gymnosperm groups, including
cycads,
conifers,
ginkgophyte
Ginkgoales are a gymnosperm order containing only one extant species: '' Ginkgo biloba'', the ginkgo tree. It is monotypic, (the only taxon) within the class Ginkgoopsida, which itself is monotypic within the division Ginkgophyta . The order inc ...
s,
gnetophytes
Gnetophyta () is a division of plants (alternatively considered the subclass Gnetidae or order Gnetales), grouped within the gymnosperms (which also includes conifers, cycads, and ginkgos), that consists of some 70 species across the three reli ...
and close relatives, as well as the extinct
Bennettitales. Other groups of plants included
pteridosperms
The term Pteridospermatophyta (or "seed ferns" or "Pteridospermatopsida") is a polyphyletic group of extinct seed-bearing plants (spermatophytes). The earliest fossil evidence for plants of this type is the genus ''Elkinsia'' of the late Devonian ...
or "seed ferns", a collective term that refers to disparate groups of extinct seed plants with fern-like foliage, including groups such as
Corystospermaceae and
Caytoniales
The Caytoniales (Figs. 1-2) are an extinct order of seed plants known from fossils collected throughout the Mesozoic Era, around . They are regarded as seed ferns because they are seed-bearing plants with fern-like leaves. Although at one time c ...
. The exact origins of angiosperms are uncertain, although molecular evidence suggests that they are not closely related to any living group of gymnosperms.
The earliest widely accepted evidence of flowering plants are monosulcate (single-grooved)
pollen grains from the late
Valanginian
In the geologic timescale, the Valanginian is an age or stage of the Early or Lower Cretaceous. It spans between 139.8 ± 3.0 Ma and 132.9 ± 2.0 Ma (million years ago). The Valanginian Stage succeeds the Berriasian Stage of the Lower Cretace ...
(~ 134 million years ago) found in Israel and Italy, initially at low abundance.
Molecular clock estimates conflict with fossil estimates, suggesting the diversification of
crown-group angiosperms during the Upper Triassic or Jurassic, but such estimates are difficult to reconcile with the heavily sampled pollen record and the distinctive tricolpate to tricolporoidate (triple grooved) pollen of
eudicot angiosperms.
Among the oldest records of Angiosperm
macrofossils are ''
Montsechia
''Montsechia'' is an extinct genus of aquatic plants containing the species ''Montsechia vidalii'', discovered in Spain. ''Montsechia vidalii'' lived about 130 million years ago, during the Barremian age, and appears to be the earliest known flo ...
'' from the
Barremian
The Barremian is an age in the geologic timescale (or a chronostratigraphic stage) between 129.4 ± 1.5 Ma (million years ago) and 121.4 ± 1.0 Ma). It is a subdivision of the Early Cretaceous Epoch (or Lower Cretaceous Series). It is preceded ...
aged
Las Hoyas beds of Spain and ''
Archaefructus'' from the Barremian-Aptian boundary
Yixian Formation in China. Tricolpate pollen distinctive of eudicots first appears in the Late Barremian, while the earliest remains of
monocots are known from the Aptian.
Flowering plants underwent a rapid radiation beginning during the middle Cretaceous, becoming the dominant group of land plants by the end of the period, coincident with the decline of previously dominant groups such as conifers. The oldest known fossils of
grasses are from the
Albian, with the family having diversified into modern groups by the end of the Cretaceous. The oldest large angiosperm trees are known from the Turonian (c. 90 Mya) of New Jersey, with the trunk having a preserved diameter of and an estimated height of .
During the Cretaceous,
Polypodiales ferns, which make up 80% of living fern species, would also begin to diversify.
Terrestrial fauna
On land,
mammals were generally small sized, but a very relevant component of the
fauna
Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is '' flora'', and for fungi, it is '' funga''. Flora, fauna, funga and other forms of life are collectively referred to as '' biota''. ...
, with
cimolodont
Cimolodonta is a taxon of extinct mammals that lived from the Cretaceous to the Eocene. They were some of the more derived members of the extinct order Multituberculata. They probably lived something of a rodent-like existence until their ecolo ...
multituberculates outnumbering dinosaurs in some sites. Neither true
marsupials nor
placentals existed until the very end, but a variety of non-marsupial
metatherians and non-placental
eutherians had already begun to diversify greatly, ranging as carnivores (
Deltatheroida), aquatic foragers (
Stagodontidae) and herbivores (''
Schowalteria
''Schowalteria'' is a genus of extinct mammal from the Cretaceous of Canada. It is the earliest known representative of Taeniodonta, a specialised lineage of non-placental eutherian mammals otherwise found in Paleocene and Eocene deposits. It is ...
'', Zhelestidae). Various "archaic" groups like
eutriconodonts were common in the Early Cretaceous, but by the Late Cretaceous northern mammalian faunas were dominated by multituberculates and
therians, with
dryolestoids dominating
South America.
The apex predators were
archosaurian
reptiles, especially
dinosaurs, which were at their most diverse stage. Avians such as the ancestors of modern-day
birds also diversified. They inhabited every continent, and were even found in cold polar latitudes.
Pterosaur
Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is 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 ...
s were common in the early and middle Cretaceous, but as the Cretaceous proceeded they declined for poorly understood reasons (once thought to be due to competition with early
birds, but now it is understood avian
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 in ...
is not consistent with pterosaur decline). By the end of the period only three highly specialized
families remained;
Pteranodontidae,
Nyctosauridae, and
Azhdarchidae.
The
Liaoning lagerstätte
A Lagerstätte (, from '' Lager'' 'storage, lair' '' Stätte'' 'place'; plural ''Lagerstätten'') is a sedimentary deposit that exhibits extraordinary fossils with exceptional preservation—sometimes including preserved soft tissues. These f ...
(
Yixian Formation) in China is an important site, full of preserved remains of numerous types of small dinosaurs, birds and mammals, that provides a glimpse of life in the Early Cretaceous. The
coelurosaur dinosaurs found there represent types of the group
Maniraptora, which includes modern birds and their closest non-avian relatives, such as
dromaeosaurs,
oviraptorosaurs,
therizinosaurs
Therizinosaurs (once called segnosaurs) were large herbivorous theropod dinosaurs whose fossils have been found across the Early to Late Cretaceous deposits in Asia and North America. Various features of the forelimbs, skull and pelvis unite thes ...
,
troodontids along with other
avialans
Avialae ("bird wings") is a clade containing the only living dinosaurs, the birds. It is usually defined as all theropod dinosaurs more closely related to birds (Aves) than to deinonychosaurs, though alternative definitions are occasionally used ...
. Fossils of these dinosaurs from the
Liaoning lagerstätte
A Lagerstätte (, from '' Lager'' 'storage, lair' '' Stätte'' 'place'; plural ''Lagerstätten'') is a sedimentary deposit that exhibits extraordinary fossils with exceptional preservation—sometimes including preserved soft tissues. These f ...
are notable for the presence of hair-like
feathers.
Insects diversified during the Cretaceous, and the oldest known
ants,
termites and some
lepidopterans, akin to
butterflies and
moths, appeared.
Aphids,
grasshoppers and
gall wasp
Gall wasps, also incorrectly called gallflies, are hymenopterans of the family Cynipidae in the wasp superfamily Cynipoidea. Their common name comes from the galls they induce on plants for larval development. About 1,300 species of this gener ...
s appeared.
File:Tyrannosaurus-rex-Profile-steveoc86.png, '' Tyrannosaurus rex'', one of the largest land predators of all time, lived during the Late Cretaceous
File: Velociraptor Restoration.png, Up to 2 m long and 0.5 m high at the hip, ''Velociraptor
''Velociraptor'' (; ) is a genus of small dromaeosaurid dinosaur that lived in Asia during the Late Cretaceous epoch, about 75 million to 71 million years ago. Two species are currently recognized, although others have been assigned in th ...
'' was feathered and roamed the Late Cretaceous
File: Triceratops by Tom Patker.png, '' Triceratops'', one of the most recognizable genera of the Cretaceous
File:Quetzalcoatlus07.jpg, The azhdarchid '' Quetzalcoatlus'', one of the largest animals to ever fly, lived during the Late Cretaceous
File:Confuciusornis sanctus mmartyniuk.png, '' Confuciusornis'', a genus of crow-sized birds from the Early Cretaceous
File:Ichthyornis restoration.jpeg, '' Ichthyornis'' was a toothed, seabird-like ornithuran from the Late Cretaceous
Rhynchocephalians
Rhynchocephalians (which today only includes the
Tuatara) disappeared from North America and Europe after the
Early Cretaceous,
and were absent from North Africa
and northern South America by the early
Late Cretaceous. The cause of the decline of Rhynchocephalia remains unclear, but has often been suggested to be due to competition with advanced lizards and mammals.
They appear to have remained diverse in high-latitude southern South America during the Late Cretaceous, where lizards remained rare, with their remains outnumbering terrestrial lizards 200:1.
Choristodera
Choristoderes, a group of freshwater aquatic reptiles that first appeared during the preceding Jurassic, underwent a major
evolutionary radiation in Asia during the Early Cretaceous, which represents the high point of choristoderan diversity, including long necked forms such as ''
Hyphalosaurus'' and the first records of the gharial-like
Neochoristodera, which appear to have evolved in the regional absence of aquatic
neosuchian crocodyliformes. During the Late Cretaceous the neochoristodere ''
Champsosaurus
''Champsosaurus'' is an extinct genus of crocodile-like choristodere reptile, known from the Late Cretaceous and early Paleogene periods of North America and Europe (Campanian-Paleocene). The name ''Champsosaurus'' is thought to come from , () s ...
'' was widely distributed across western North America.
Marine fauna
In the seas,
rays, modern
shark
Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...
s and
teleosts became common.
Marine reptiles included
ichthyosaurs in the early and mid-Cretaceous (becoming extinct during the late Cretaceous
Cenomanian-Turonian anoxic event
The Cenomanian-Turonian boundary event, also known as the Cenomanian-Turonian extinction, Cenomanian-Turonian oceanic anoxic event ( OAE 2), and referred to also as the Bonarelli event, was one of two anoxic extinction events in the Cretaceous p ...
),
plesiosaurs throughout the entire period, and
mosasaurs appearing in the Late Cretaceous. Sea turtles in the form of
Cheloniidae
Cheloniidae is a family of typically large marine turtles that are characterised by their common traits such as, having a flat streamlined wide and rounded shell and almost paddle-like flippers for their forelimbs. They are the only sea turtles t ...
and
Panchelonioidea
Panchelonioidea is a clade of marine turtles that includes the sea turtle
Sea turtles (superfamily Chelonioidea), sometimes called marine turtles, are reptiles of the order Testudines and of the suborder Cryptodira. The seven existing species ...
lived during the period and survived the extinction event. Panchelonioidea is today represented by a single species; the
leatherback sea turtle.
''
Baculites'', an
ammonite genus with a straight shell, flourished in the seas along with reef-building
rudist clams. The
Hesperornithiformes were flightless, marine diving birds that swam like
grebe
Grebes () are aquatic diving birds in the order Podicipediformes . Grebes are widely distributed freshwater birds, with some species also found in marine habitats during migration and winter. Some flightless species exist as well, most notably ...
s. Globotruncanid
Foraminifera and
echinoderms such as sea urchins and
starfish (sea stars) thrived.
Thylacocephala, a class of crustaceans, went extinct in the Late Cretaceous. The first radiation of the
diatoms (generally
siliceous shelled, rather than
calcareous) in the oceans occurred during the Cretaceous; freshwater diatoms did not appear until the
Miocene.
The Cretaceous was also an important interval in the evolution of
bioerosion
Bioerosion describes the breakdown of hard ocean substrates – and less often terrestrial substrates – by living organisms. Marine bioerosion can be caused by mollusks, polychaete worms, phoronids, sponges, crustaceans, echinoids, and ...
, the production of borings and scrapings in rocks,
hardgrounds and shells.
File:Kronosaurus hunt1DB.jpg, A scene from the early Cretaceous: a ''Woolungasaurus
''Woolungasaurus'' ('Woolunga lizard', named after an Aboriginal mythical reptile, Persson 1960) is a plesiosaur, an extinct marine reptile, belonging to the Elasmosauridae.
The type species, ''Woolungasaurus glendowerensis'', was named after ...
'' is attacked by a ''Kronosaurus
''Kronosaurus'' ( ; meaning "lizard of Kronos") is a potentially dubious genus of extinct short-necked pliosaur. With an estimated length of , it was among the largest pliosaurs, and is named after the leader of the Greek Titans, Kronos. It ...
''.
File:Tylosaurus pembinensis 1DB.jpg, '' Tylosaurus'' was a large mosasaur, carnivorous marine reptiles that emerged in the late Cretaceous.
File:Hesperornis BW (white background).jpg, Strong-swimming and toothed predatory waterbird '' Hesperornis'' roamed late Cretacean oceans.
File:DiscoscaphitesirisCretaceous.jpg, The ammonite '' Discoscaphites iris'', Owl Creek Formation (Upper Cretaceous), Ripley, Mississippi
File:The fossils from Cretaceous age found in Lebanon.jpg, A plate with ''Nematonotus
''Nematonotus'' is an extinct genus of prehistoric bony fish that lived during the Lower Cenomanian.
See also
* Prehistoric fish
The evolution of fish began about 530 million years ago during the Cambrian explosion. It was during this tim ...
sp.'', ''Pseudostacus sp.'' and a partial ''Dercetis triqueter'', found in Hakel, Lebanon
File:Cretoxyrhina attacking Pteranodon.png, '' Cretoxyrhina'', one of the largest Cretaceous shark
Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...
s, attacking a '' Pteranodon'' in the Western Interior Seaway
See also
*
Mesozoic Era
*
Cretaceous-Paleogene extinction
*
Chalk Formation
*
Cretaceous Thermal Maximum
The Cretaceous Thermal Maximum (CTM), also known as Cretaceous Thermal Optimum, was a period of climatic warming that reached its peak approximately 90 million years ago (90 Ma) during the Turonian age of the Late Cretaceous epoch. The CTM is not ...
*
List of fossil sites (with link directory)
*
South Polar region of the Cretaceous
References
Citations
Bibliography
*
*
*
*
* —detailed coverage of various aspects of the evolutionary history of the insects.
*
*
*
External links
UCMP Berkeley Cretaceous pageCretaceous Microfossils: 180+ images of Foraminifera*
{{Authority control
Geological periods