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The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last
era An era is a span of time defined for the purposes of chronology or historiography, as in the regnal eras in the history of a given monarchy, a calendar era used for a given calendar, or the geological eras defined for the history of Earth. Compa ...
of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Periods. It is characterized by the dominance of
archosaurian Archosauria () is a clade of diapsids, with birds and crocodilians as the only living representatives. Archosaurs are broadly classified as reptiles, in the cladistic sense of the term which includes birds. Extinct archosaurs include non-avi ...
reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...
s, like the dinosaurs; an abundance of
conifer Conifers are a group of conifer cone, cone-bearing Spermatophyte, seed plants, a subset of gymnosperms. Scientifically, they make up the phylum, division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a single ...
s and ferns; a hot
greenhouse A greenhouse (also called a glasshouse, or, if with sufficient heating, a hothouse) is a structure with walls and roof made chiefly of Transparent ceramics, transparent material, such as glass, in which plants requiring regulated climatic condit ...
climate; and the tectonic break-up of Pangaea. The Mesozoic is the middle of the three eras since complex life evolved: the Paleozoic, the Mesozoic, and the
Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configura ...
. The era began in the wake of the Permian–Triassic extinction event, the largest well-documented mass extinction in Earth's history, and ended with the Cretaceous–Paleogene extinction event, another mass extinction whose victims included the non-avian dinosaurs,
pterosaurs 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 to ...
,
mosasaurs Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Greek ' meaning 'lizard') comprise a group of extinct, large marine reptiles from the Late Cretaceous. Their first fossil remains were discovered in a limestone quarry at Maastricht on the ...
, and plesiosaurs. The Mesozoic was a time of significant tectonic, climatic, and evolutionary activity. The era witnessed the gradual rifting of the supercontinent Pangaea into separate landmasses that would move into their current positions during the next era. The climate of the Mesozoic was varied, alternating between warming and cooling periods. Overall, however, the Earth was hotter than it is today. Dinosaurs first appeared in the Mid-Triassic, and became the dominant terrestrial vertebrates in the Late Triassic or Early Jurassic, occupying this position for about 150 or 135 million years until their demise at the end of the Cretaceous. Archaic birds appeared in the Jurassic, having evolved from a branch of theropod dinosaurs, then true toothless birds appeared in the Cretaceous. The first
mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
s also appeared during the Mesozoic, but would remain small—less than 15 kg (33 lb)—until the Cenozoic. The flowering plants appeared in the early Cretaceous Period and would rapidly diversify throughout the end of the era, replacing conifers and other gymnosperms as the dominant group of plants.


Naming

The phrase "Age of Reptiles" was introduced by the 19th century paleontologist Gideon Mantell who viewed it as dominated by diapsids such as '' Iguanodon'', '' Megalosaurus'', '' Plesiosaurus'', and '' Pterodactylus''. The current name was proposed in 1840 by the British geologist John Phillips (1800–1874). "Mesozoic" literally means 'middle life', deriving from the Greek prefix ( 'between') and ( 'animal, living being'). In this way, the Mesozoic is comparable to the Cenozoic () and Paleozoic ('old life') Eras as well as the
Proterozoic The Proterozoic () is a geological eon spanning the time interval from 2500 to 538.8million years ago. It is the most recent part of the Precambrian "supereon". It is also the longest eon of the Earth's geologic time scale, and it is subdivided ...
('earlier life') Eon. The Mesozoic Era was originally described as the "secondary" era, following the "primary" ( Paleozoic), and preceding the Tertiary.


Geologic periods

Following the Paleozoic, the Mesozoic extended roughly 186 million years, from when the
Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configura ...
Era began. This time frame is separated into three geologic periods. From oldest to youngest: * Triassic () * Jurassic () * Cretaceous () The lower boundary of the Mesozoic is set by the Permian–Triassic extinction event, during which it has been estimated that up to 90-96% of marine species became
extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
although those approximations have been brought into question with some paleontologists estimating the actual numbers as low as 81%. It is also known as the "Great Dying" because it is considered the largest mass extinction in the Earth's history. The upper boundary of the Mesozoic is set at the Cretaceous–Paleogene extinction event (or K–Pg extinction event), which may have been caused by an asteroid impactor that created Chicxulub Crater on the Yucatán Peninsula. Towards the Late Cretaceous, large volcanic eruptions are also believed to have contributed to the Cretaceous–Paleogene extinction event. Approximately 50% of all genera became extinct, including all of the non-
avian Avian may refer to: *Birds or Aves, winged animals *Avian (given name) (russian: Авиа́н, link=no), a male forename Aviation *Avro Avian, a series of light aircraft made by Avro in the 1920s and 1930s *Avian Limited, a hang glider manufacture ...
dinosaurs.


Triassic

The Triassic ranges roughly from 252 million to 201 million years ago, preceding the Jurassic Period. The period is bracketed between the Permian–Triassic extinction event and the Triassic–Jurassic extinction event, two of the " big five", and it is divided into three major epochs: Early, Middle, and Late Triassic. The Early Triassic, about 252 to 247 million years ago, was dominated by deserts in the interior of the Pangaea supercontinent. The Earth had just witnessed a massive die-off in which 95% of all life became extinct, and the most common vertebrate life on land were ''
Lystrosaurus ''Lystrosaurus'' (; 'shovel lizard'; proper Greek is λίστρον ''lístron'' ‘tool for leveling or smoothing, shovel, spade, hoe’) is an extinct genus of herbivorous dicynodont therapsids from the late Permian and Early Triassic epochs (a ...
'',
labyrinthodont "Labyrinthodontia" (Greek, 'maze-toothed') is an informal grouping of extinct predatory amphibians which were major components of ecosystems in the late Paleozoic and early Mesozoic eras (about 390 to 150 million years ago). Traditionally consid ...
s, and '' Euparkeria'' along with many other creatures that managed to survive the Permian extinction.
Temnospondyl Temnospondyli (from Greek language, Greek τέμνειν, ''temnein'' 'to cut' and σπόνδυλος, ''spondylos'' 'vertebra') is a diverse order (biology), order of small to giant tetrapods—often considered Labyrinthodontia, primitive amphi ...
s reached peak diversity during the early Triassic. The Middle Triassic, from 247 to 237 million years ago, featured the beginnings of the breakup of Pangaea and the opening of the Tethys Ocean. Ecosystems had recovered from the Permian extinction. Algae, sponge, corals, and crustaceans all had recovered, and new aquatic reptiles evolved, such as
ichthyosaur Ichthyosaurs (Ancient Greek for "fish lizard" – and ) are large extinct marine reptiles. Ichthyosaurs belong to the order known as Ichthyosauria or Ichthyopterygia ('fish flippers' – a designation introduced by Sir Richard Owen in 1842, altho ...
s and
nothosaur Nothosaurs (order Nothosauroidea) were Triassic marine sauropterygian reptiles that may have lived like seals of today, catching food in water but coming ashore on rocks and beaches. They averaged about in length, with a long body and tail.F. v. ...
s. On land, pine forests flourished, as did groups of insects like mosquitoes and fruit flies. Reptiles began to get bigger and bigger, and the first crocodilians and dinosaurs evolved, which sparked competition with the large amphibians that had previously ruled the freshwater world, respectively mammal-like reptiles on land. Following the bloom of the Middle Triassic, the Late Triassic, from 237 to 201 million years ago, featured frequent heat spells and moderate precipitation (10–20 inches per year). The recent warming led to a boom of dinosaurian evolution on land as those one began to separate from each other (Nyasasaurus from 243 to 210 million years ago, approximately 235–30 ma, some of them separated into Sauropodomorphs, Theropods and Herrerasaurids), as well as the first pterosaurs. During the Late Triassic, some advanced
cynodont The cynodonts () (clade Cynodontia) are a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 mya), and extensively diversified after the Permian–Triassic extinction event. Cynodonts had a wide variety ...
s gave rise to the first
Mammaliaformes Mammaliaformes ("mammalian forms") is a clade that contains the crown group mammals and their closest Extinction, extinct relatives; the group adaptive radiation, radiated from earlier probainognathian cynodonts. It is defined as the clade origin ...
. All this climatic change, however, resulted in a large die-out known as the Triassic–Jurassic extinction event, in which many archosaurs (excluding pterosaurs, dinosaurs and
crocodylomorph Crocodylomorpha is a group of pseudosuchian archosaurs that includes the crocodilians and their extinct relatives. They were the only members of Pseudosuchia to survive the end-Triassic extinction. During Mesozoic and early Cenozoic times, cro ...
s), most synapsids, and almost all large amphibians became extinct, as well as 34% of marine life, in the Earth's fourth mass extinction event. The cause is debatable;
flood basalt eruption 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 reach ...
s at the Central Atlantic magmatic province is cited as one possible cause.


Jurassic

The Jurassic ranges from 200 million years to 145 million years ago and features three major epochs: The Early Jurassic, the Middle Jurassic, and the Late Jurassic. The Early Jurassic spans from 200 to 175 million years ago. The climate was tropical and much more humid than the Triassic, as a result of the large seas appearing between the land masses. In the oceans, plesiosaurs, ichthyosaurs and ammonites were abundant. On land, dinosaurs and other archosaurs staked their claim as the dominant race, with theropods such as '' Dilophosaurus'' at the top of the food chain. The first true crocodiles evolved, pushing the large amphibians to near extinction. All-in-all, archosaurs rose to rule the world. Meanwhile, the first true mammals evolved, remaining relatively small but spreading widely; the Jurassic ''
Castorocauda ''Castorocauda'' is an extinct, semi-aquatic, superficially otter-like genus of docodont mammaliaforms with one species, ''C. lutrasimilis''. It is part of the Yanliao Biota, found in the Daohugou Beds of Inner Mongolia, China dating to the M ...
'', for example, had adaptations for swimming, digging and catching fish. ''
Fruitafossor ''Fruitafossor'' was a termite-eating mammal endemic to North America during the Late Jurassic epoch (around 150 mya). The description is based on a complete skeleton of a chipmunk-sized animal. It was discovered on March 31, 2005, in Fruita, ...
'', from the late Jurassic Period about 150 million years ago, was about the size of a chipmunk, and its teeth, forelimbs and back suggest that it dug open the nests of social insects (probably termites, as ants had not yet appeared) ; '' Volaticotherium'' was able to glide for short distances, like modern
flying squirrel Flying squirrels (scientifically known as Pteromyini or Petauristini) are a tribe of 50 species of squirrels in the family Sciuridae. Despite their name, they are not in fact capable of full flight in the same way as birds or bats, but they ar ...
s. The first multituberculates like '' Rugosodon'' evolved. The Middle Jurassic spans from 175 to 163 million years ago. During this epoch, dinosaurs flourished as huge herds of sauropods, such as '' Brachiosaurus'' and '' Diplodocus'', filled the fern prairies, chased by many new predators such as '' Allosaurus''.
Conifer Conifers are a group of conifer cone, cone-bearing Spermatophyte, seed plants, a subset of gymnosperms. Scientifically, they make up the phylum, division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a single ...
forests made up a large portion of the forests. In the oceans, plesiosaurs were quite common, and ichthyosaurs flourished. This epoch was the peak of the reptiles. The Late Jurassic spans from 163 to 145 million years ago. During this epoch, the first avialans, like ''
Archaeopteryx ''Archaeopteryx'' (; ), sometimes referred to by its German name, "" ( ''Primeval Bird''), is a genus of bird-like dinosaurs. The name derives from the ancient Greek (''archaīos''), meaning "ancient", and (''ptéryx''), meaning "feather" ...
'', evolved from small
coelurosaurian Coelurosauria (; from Greek, meaning "hollow tailed lizards") is the clade containing all theropod dinosaurs more closely related to birds than to carnosaurs. Coelurosauria is a subgroup of theropod dinosaurs that includes compsognathids, tyrann ...
dinosaurs. The increase in sea levels opened up the Atlantic seaway, which has grown continually larger until today. The further separation of the continents gave opportunity for the diversification of new dinosaurs.


Cretaceous

The Cretaceous is the longest period of the Mesozoic, but has only two epochs: Early and Late Cretaceous. The Early Cretaceous spans from 145 to 100 million years ago. The Early Cretaceous saw the expansion of seaways and a decline in diversity of sauropods, stegosaurs, and other high-browsing groups, with sauropods particularly scarce in North America. Seasons came back into effect and the poles got seasonally colder, but some dinosaurs still inhabited the polar forests year round, such as '' Leaellynasaura'' and '' Muttaburrasaurus''. The poles were too cold for crocodiles, and became the last stronghold for large amphibians like ''
Koolasuchus ''Koolasuchus'' is an extinct genus of brachyopoid temnospondyl in the family Chigutisauridae. Fossils have been found from Victoria, Australia and date back 120 Ma to the Aptian stage of the Early Cretaceous. ''Koolasuchus'' is the youngest kno ...
''. Pterosaurs got larger as genera like '' Tapejara'' and '' Ornithocheirus'' evolved. Mammals continued to expand their range: eutriconodonts produced fairly large, wolverine-like predators like '' Repenomamus'' and '' Gobiconodon'', early therians began to expand into
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 well ...
ns and eutherians, and
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 (biology), order Multituberculata. They probably lived something of a rodent-like existence u ...
multituberculates went on to become common in the fossil record. The Late Cretaceous spans from 100 to 66 million years ago. The Late Cretaceous featured a cooling trend that would continue in the
Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configura ...
Era. Eventually, tropics were restricted to the equator and areas beyond the tropic lines experienced extreme seasonal changes in weather. Dinosaurs still thrived, as new taxa such as '' Tyrannosaurus'', '' Ankylosaurus'', '' Triceratops'' and hadrosaurs dominated the food web. In the oceans,
mosasaurs Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Greek ' meaning 'lizard') comprise a group of extinct, large marine reptiles from the Late Cretaceous. Their first fossil remains were discovered in a limestone quarry at Maastricht on the ...
ruled, filling the role of the ichthyosaurs, which, after declining, had disappeared in the Cenomanian-Turonian boundary event. Though pliosaurs had gone extinct in the same event, long-necked plesiosaurs such as ''
Elasmosaurus ''Elasmosaurus'' (;) is a genus of plesiosaur that lived in North America during the Campanian stage of the Late Cretaceous period, about 80.5million years ago. The first specimen was discovered in 1867 near Fort Wallace, Kansas, US, and was se ...
'' continued to thrive. Flowering plants, possibly appearing as far back as the Triassic, became truly dominant for the first time. Pterosaurs in the Late Cretaceous declined for poorly understood reasons, though this might be due to tendencies of the fossil record, as their diversity seems to be much higher than previously thought. Birds became increasingly common and diversified into a variety of
enantiornithe The Enantiornithes, also known as enantiornithines or enantiornitheans in literature, are a group of extinct avialans ("birds" in the broad sense), the most abundant and diverse group known from the Mesozoic era. Almost all retained teeth and cl ...
and
ornithurine Ornithurae (meaning "bird tails" in Greek) is a natural group which includes the common ancestor of ''Ichthyornis'', ''Hesperornis'', and all modern birds as well as all other descendants of that common ancestor. Classification Ernst Haeckel coi ...
forms. Though mostly small, marine hesperornithes became relatively large and flightless, adapted to life in the open sea. Metatherians and primitive eutherian also became common and even produced large and specialised genera like ''
Didelphodon ''Didelphodon'' (from ''is''/nowiki>.html" ;"title="/nowiki>''is''/nowiki>">/nowiki>''is''/nowiki> "opossum" plus "tooth") is a genus of stagodont metatherians from the Late Cretaceous of North America. Description Although perhaps little la ...
'' and ''
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 ...
''. Still, the dominant mammals were multituberculates,
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 (biology), order Multituberculata. They probably lived something of a rodent-like existence u ...
s in the north and gondwanatheres in the south. At the end of the Cretaceous, the
Deccan traps The Deccan Traps is a large igneous province of west-central India (17–24°N, 73–74°E). It is one of the largest volcanic features on Earth, taking the form of a large shield volcano. It consists of numerous layers of solidified flood ...
and other volcanic eruptions were poisoning the atmosphere. As this continued, it is thought that a large meteor smashed into earth 66 million years ago, creating the Chicxulub Crater in an event known as the K-Pg Extinction (formerly K-T), the fifth and most recent mass extinction event, in which 75% of life became extinct, including all non-avian dinosaurs.


Paleogeography and tectonics

Compared to the vigorous convergent plate mountain-building of the late Paleozoic, Mesozoic tectonic deformation was comparatively mild. The sole major Mesozoic orogeny occurred in what is now the Arctic, creating the
Innuitian orogeny The Innuitian orogeny, sometimes called the Ellesmere orogeny, was a major tectonic (mountain building) episode responsible for the formation of a series of mountain ranges in the Canadian Arctic and Northernmost Greenland. The episode started wit ...
, the Brooks Range, the Verkhoyansk and Cherskiy Ranges in Siberia, and the
Khingan Mountains The Greater Khingan Range or Da Hinggan Range (; IPA: ), is a -long volcanic mountain range in the Inner Mongolia region of Northeast China. It was originally called the Xianbei Mountains, which later became the name of the northern branch of th ...
in Manchuria. This orogeny was related to the opening of the Arctic Ocean and
suturing A surgical suture, also known as a stitch or stitches, is a medical device used to hold body tissues together and approximate wound edges after an injury or surgery. Application generally involves using a needle with an attached length of threa ...
of the
North China North China, or Huabei () is a List of regions of China, geographical region of China, consisting of the provinces of Beijing, Tianjin, Hebei, Shanxi and Inner Mongolia. Part of the larger region of Northern China (''Beifang''), it lies north ...
and Siberian cratons to Asia. In contrast, the era featured the dramatic rifting of the supercontinent Pangaea, which gradually split into a northern continent,
Laurasia Laurasia () was the more northern of two large landmasses that formed part of the Pangaea supercontinent from around ( Mya), the other being Gondwana. It separated from Gondwana (beginning in the late Triassic period) during the breakup of Pan ...
, and a southern continent,
Gondwana Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final stages ...
. This created the
passive continental margin A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The continental margin ...
that characterizes most of the Atlantic coastline (such as along the
U.S. East Coast The East Coast of the United States, also known as the Eastern Seaboard, the Atlantic Coast, and the Atlantic Seaboard, is the coastline along which the Eastern United States meets the Atlantic Ocean, North Atlantic Ocean. The eastern seaboard ...
) today. By the end of the era, the continents had rifted into nearly their present forms, though not their present positions.
Laurasia Laurasia () was the more northern of two large landmasses that formed part of the Pangaea supercontinent from around ( Mya), the other being Gondwana. It separated from Gondwana (beginning in the late Triassic period) during the breakup of Pan ...
became
North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...
and Eurasia, while
Gondwana Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final stages ...
split into South America, Africa,
Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...
, Antarctica and the Indian subcontinent, which collided with the Asian plate during the Cenozoic, giving rise to the Himalayas.


Climate

The Triassic was generally dry, a trend that began in the late
Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...
, and highly seasonal, especially in the interior of Pangaea. Low sea levels may have also exacerbated temperature extremes. With its high specific heat capacity, water acts as a temperature-stabilizing heat reservoir, and land areas near large bodies of water—especially oceans—experience less variation in temperature. Because much of Pangaea's land was distant from its shores, temperatures fluctuated greatly, and the interior probably included expansive
desert A desert is a barren area of landscape where little precipitation occurs and, consequently, living conditions are hostile for plant and animal life. The lack of vegetation exposes the unprotected surface of the ground to denudation. About on ...
s. Abundant
red beds Red beds (or redbeds) are sedimentary rocks, typically consisting of sandstone, siltstone, and shale, that are predominantly red in color due to the presence of ferric oxides. Frequently, these red-colored sedimentary strata locally contain ...
and evaporites such as halite support these conclusions, but some evidence suggests the generally dry climate of was punctuated by episodes of increased rainfall. The most important humid episodes were the
Carnian Pluvial Event The Carnian pluvial episode (CPE), often called the Carnian pluvial event, was an interval of major change in global climate synchronous with significant changes in Earth's biota both in the sea and on land. It occurred during the latter part of ...
and one in the
Rhaetian The Rhaetian is the latest age of the Triassic Period (in geochronology) or the uppermost stage of the Triassic System (in chronostratigraphy). It was preceded by the Norian and succeeded by the Hettangian (the lowermost stage or earliest age ...
, a few million years before the Triassic–Jurassic extinction event. Sea levels began to rise during the Jurassic, probably caused by an increase in seafloor spreading. The formation of new crust beneath the surface displaced ocean waters by as much as above today's sea level, flooding coastal areas. Furthermore, Pangaea began to rift into smaller divisions, creating new shoreline around the Tethys Ocean. Temperatures continued to increase, then began to stabilize. Humidity also increased with the proximity of water, and deserts retreated. The climate of the Cretaceous is less certain and more widely disputed. Probably, higher levels of carbon dioxide in the
atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
are thought to have almost eliminated the north–south
temperature gradient A temperature gradient is a physical quantity that describes in which direction and at what rate the temperature changes the most rapidly around a particular location. The temperature gradient is a dimensional quantity expressed in units of degree ...
: temperatures were about the same across the planet, and about 10° C higher than today. The circulation of oxygen to the deep ocean may also have been disrupted, preventing the decomposition of large volumes of organic matter, which was eventually deposited as "
black shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especiall ...
". Different studies have come to different conclusions about the amount of oxygen in the atmosphere during different parts of the Mesozoic, with some concluding oxygen levels were lower than the current level (about 21%) throughout the Mesozoic, some concluding they were lower in the Triassic and part of the Jurassic but higher in the Cretaceous, and some concluding they were higher throughout most or all of the Triassic, Jurassic and Cretaceous.


Life


Flora

The dominant land plant species of the time were gymnosperms, which are vascular, cone-bearing, non-flowering plants such as conifers that produce seeds without a coating. This contrasts with the earth's current flora, in which the dominant land plants in terms of number of species are
angiosperm 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 th ...
s. The earliest members of the genus '' Ginkgo'' first appeared during the Middle Jurassic. This genus is represented today by a single species, '' Ginkgo biloba''. The
extant Extant is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Extant taxon, a taxon which is not extinct, ...
genus '' Sequoia'' is believed to have evolved in the Mesozoic. Bennettitales, an extinct group of gymnosperms with foliage superficially resembling that of cycads gained a global distribution during the Late Triassic, and represented one of the most common groups of Mesozoic seed plants. Flowering plants radiated during the early Cretaceous, first in the tropics, but the even temperature gradient allowed them to spread toward the poles throughout the period. By the end of the Cretaceous, angiosperms dominated tree floras in many areas, although some evidence suggests that
biomass Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms bi ...
was still dominated by cycads and ferns until after the Cretaceous–Paleogene extinction. Some plant species had distributions that were markedly different from succeeding periods; for example, the
Schizeales Schizaeales is an order of ferns (class Polypodiopsida). Description While the three clades of Schizaeales are all well-distinguished from one another by numerous morphological characters, members of the order all have dimorphic fertile and s ...
, a fern order, were skewed to the Northern Hemisphere in the Mesozoic, but are now better represented in the Southern Hemisphere.C.Michael Hogan. 2010
''Fern''. Encyclopedia of Earth. National council for Science and the Environment
. Washington, DC


Fauna

The extinction of nearly all animal species at the end of the Permian Period allowed for the
radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: * ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...
of many new lifeforms. In particular, the extinction of the large herbivorous
pareiasaur Pareiasaurs (meaning "cheek lizards") are an extinct clade of large, herbivorous parareptiles. Members of the group were armoured with scutes which covered large areas of the body. They first appeared in southern Pangea during the Middle Permian, ...
s and
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 food and energy requirements derive from animal tissues (mainly muscle, fat and other sof ...
gorgonopsians left those ecological niches empty. Some were filled by the surviving
cynodont The cynodonts () (clade Cynodontia) are a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 mya), and extensively diversified after the Permian–Triassic extinction event. Cynodonts had a wide variety ...
s and dicynodonts, the latter of which subsequently became extinct. Recent research indicates that it took much longer for the reestablishment of complex ecosystems with high biodiversity, complex food webs, and specialized animals in a variety of niches, beginning in the mid-Triassic 4 million to 6 million years after the extinction, and not fully proliferated until 30 million years after the extinction. Animal life was then dominated by various archosaurs: dinosaurs, pterosaurs, and aquatic reptiles such as ichthyosaurs, plesiosaurs, and mosasaurs. The climatic changes of the late Jurassic and Cretaceous favored further adaptive radiation. The Jurassic was the height of archosaur diversity, and the first birds and eutherian mammals also appeared. Some have argued that insects diversified in
symbiosis Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasit ...
with angiosperms, because insect anatomy, especially the mouth parts, seems particularly well-suited for flowering plants. However, all major insect mouth parts preceded angiosperms, and insect diversification actually slowed when they arrived, so their anatomy originally must have been suited for some other purpose.


See also

*


References

*''British Mesozoic Fossils'', 1983, The Natural History Museum, London.


External links


Mesozoic (chronostratigraphy scale)
{{Authority control Geological eras