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The Mesozoic Era ( ), also called the Age of Reptiles and the Age of Conifers, is the second-to-last
era An era is a span of time defined for the purposes of chronology Chronology (from Latin ''chronologia'', from Ancient Greek , ''chrónos'', "time"; and , ''wikt:-logia, -logia'') is the science of arranging events in their order of occurrence ...
of Earth's
geological history
geological history
, lasting from about and comprising the
Triassic The Triassic ( ) is a geologic period The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in time. It is used by geologists, paleontology, paleontologists, and other earth ...

Triassic
,
Jurassic The Jurassic ( ) is a geologic period The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in time. It is used by geologists, paleontology, paleontologists, and other earth ...
and
Cretaceous The Cretaceous ( ) is a geological period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These periods form elements of a hierarchy of division ...

Cretaceous
Periods. It is characterized by the dominance of archosaurian
reptile Reptiles, as most commonly defined, are the animals in the class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or ...

reptile
s, like the
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 annum, million years ago, although the exact origin and timing of the evolution ...

dinosaur
s; 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 e ...
s and
fern A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plant Vascular plants (from Latin ''vasculum'': duct), also known as Tracheophyta (the tracheophytes , from Greek τραχεῖα ἀρτηρία ''trācheia art ...

fern
s; 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 material, such as glass, in which plant Plants are predominantly photosynthetic Photo ...
climate; and the
tectonic Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building A mountain is an elevated portion of the Earth's crust, gen ...
break-up of
Pangaea Pangaea or Pangea () was a supercontinent In geology Geology (from the Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí G ...

Pangaea
. The Mesozoic is the middle of three eras since complex life evolved: the
Paleozoic The Paleozoic (or Palaeozoic) Era ( ; from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Eu ...
, the Mesozoic, and the
Cenozoic The Cenozoic ( ; ) is Earth's current geological 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 ge ...

Cenozoic
. The era began in the wake of the
Permian–Triassic extinction event The Permian–Triassic (P–T, P–Tr) extinction event, also known as the End-Permian Extinction and colloquially as the Great Dying, formed the boundary between the Permian The Permian ( ) is a geologic period The geologic time scale (G ...
, the largest well-documented
mass extinction An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity Biodiversity is the biological variety and Genetic variability, variability of life, life on Earth. Biodiversity ...
in Earth's history, and ended with the
Cretaceous–Paleogene extinction event The Cretaceous–Paleogene (K–Pg) extinction event (also known as the Cretaceous–Tertiary extinction) was a sudden extinction event, mass extinction of three-quarters of the plant and animal species on Earth, approximately 66 millio ...
, another mass extinction whose victims included the non-avian dinosaurs. 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
bird Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...

bird
s appeared in the Jurassic, having
evolved Evolution is change in the Heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristics are the Gene expression, expressions of genes that are passed on from parent to offs ...
from a branch of
theropod Theropoda ( from Ancient Greek, Greek 'wild beast' and 'foot'), whose members are known as theropods, is a dinosaur clade that is characterized by hollow bones and three-toed limbs. Theropods are generally classed as a group of saurischian d ...

theropod
dinosaurs, then
true toothless birds
true toothless birds
appeared in the Cretaceous. The first
mammal Mammals (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be i ...
s also appeared during the Mesozoic, but would remain small—less than 15 kg (33 lb)—until the Cenozoic. The
flowering plants Flowering plants include multiple members of the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greec ...

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 Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene The Holocene ( ) is the current Geologic time scale, geological epoch. ...
Gideon Mantell Gideon Algernon Mantell MRCS FRS FRS may also refer to: Government and politics * Facility Registry System, a centrally managed Environmental Protection Agency database that identifies places of environmental interest in the United States * ...

Gideon Mantell
who viewed it as dominated by
diapsid Diapsids ("two arches") are a group of amniote tetrapods that developed two holes (temporal fenestra) in each side of their skulls about 300 million years ago during the late Carboniferous period. The diapsids are extremely diverse, and include all ...

diapsid
s such as ''
Iguanodon ''Iguanodon'' ( ; meaning 'iguana ''Iguana'' (, ) is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology Biology is the natural science that studie ...

Iguanodon
'', ''
Megalosaurus ''Megalosaurus'' (meaning "great lizard", from Ancient Greek, Greek , ', meaning 'big', 'tall' or 'great' and , ', meaning 'lizard') is an extinct genus of large carnivorous theropod dinosaurs of the Middle Jurassic Period (geology), period (Bat ...

Megalosaurus
'', ''
Plesiosaurus ''Plesiosaurus'' (Greek: ' ('), near to + ' ('), lizard) is a genus of extinct, large marine sauropterygia Sauropterygia ("lizard Lizards are a widespread group of Squamata, squamate reptiles, with over 6,000 species, ranging across all co ...

Plesiosaurus
'', and ''
Pterodactylus The Pterodactyl (Pterodactylus from Greek () meaning 'winged finger') is an extinct genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the sci ...

Pterodactylus
''. The current name was proposed in 1840 by the British geologist
John Phillips
John Phillips
(1800–1874). "Mesozoic" literally means 'middle life', deriving from the
Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
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 541million years ago. It is the most recent part of the Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest par ...
('earlier life') Eon. The Mesozoic Era was originally described as the "secondary" era, following the "primary" (
Paleozoic The Paleozoic (or Palaeozoic) Era ( ; from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Eu ...
), and preceding the
Tertiary Tertiary ( ) is a widely used but obsolete term for the geologic period The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in time. It is used by geologists, paleontology ...

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 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 ge ...

Cenozoic
Era began. This time frame is separated into three geologic periods. From oldest to youngest: *
Triassic The Triassic ( ) is a geologic period The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in time. It is used by geologists, paleontology, paleontologists, and other earth ...

Triassic
() *
Jurassic The Jurassic ( ) is a geologic period The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in time. It is used by geologists, paleontology, paleontologists, and other earth ...
() *
Cretaceous The Cretaceous ( ) is a geological period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These periods form elements of a hierarchy of division ...

Cretaceous
() The lower boundary of the Mesozoic is set by the
Permian–Triassic extinction event The Permian–Triassic (P–T, P–Tr) extinction event, also known as the End-Permian Extinction and colloquially as the Great Dying, formed the boundary between the Permian The Permian ( ) is a geologic period The geologic time scale (G ...
, 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 In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biol ...

extinct
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 The Cretaceous–Paleogene (K–Pg) extinction event (also known as the Cretaceous–Tertiary extinction) was a sudden extinction event, mass extinction of three-quarters of the plant and animal species on Earth, approximately 66 millio ...
(or K–Pg extinction event), which may have been caused by an asteroid impactor that created
Chicxulub Crater The Chicxulub crater () is an impact crater An impact crater is an approximately circular depression (geology), depression in the surface of a planet, natural satellite, moon, or other solid body in the Solar System or elsewhere, formed by t ...

Chicxulub Crater
on the
Yucatán Peninsula The Yucatán Peninsula (, also , ; es, Península de Yucatán ) is a large peninsula A peninsula ( la, paeninsula from 'almost' and 'island') is a landform A landform is a natural or artificial feature of the solid surface of the Earth ...

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
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 The Triassic–Jurassic (Tr-J) extinction event, sometimes called the end-Triassic extinction, marks the boundary between the Triassic The Triassic ( ) is a geologic period A geological period is one of the several subdivisions of geologic time ...
, two of the "
big fiveBig Five may refer to: Animals * the Big five game, Big Five, large African wild animals said to be most difficult to hunt: lion, leopard, rhinoceros, elephant and Cape buffalo * Big Five animals of the Kaziranga National Park, Assam, India: Ind ...
", 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 A herbivore is an animal Animals (also called Metazoa) ...

Lystrosaurus
'',
labyrinthodont Labyrinthodontia (Greek language, Greek, 'maze-toothed') is an Extinction, extinct amphibian subclass, which constituted some of the dominant animals of late Paleozoic and early Mesozoic eras (about 390 to 150 million years ago). The group evolve ...
s, and ''
Euparkeria ''Euparkeria'' (; meaning "Parker's good animal", named in honor of W.K. Parker) is an Extinction, extinct genus of archosauriform from the Middle Triassic of South Africa. It was a small reptile that lived between 245-230 million years ago, 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 evolved during this time and would be the dominant predator for much of the 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 The Tethys Ocean ( el, Τηθύς ''Tēthús''), also called the Tethys Sea or the Neo-Tethys, was an ocean during much of the Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles and the Age of Conifers, is the second-to-last era ...
. 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 Ancient Greek includes the forms of the used in and the from around 1500 BC to 300 BC. It is often roughly divided into the following periods: (), Dark Ages (), the period (), and the period (). Ancient ...
s and
nothosaur Nothosaurs (order Nothosauroidea) were Triassic marine sauropterygian reptiles that may have lived like pinniped, seals of today, catching food in water but coming ashore on rocks and beaches. They averaged about in length, with a long body and ...
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
pterosaur Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") were flying reptiles of the extinct clade or Order (biology), order Pterosauria. They existed during most of the Mesozoic: from the late Triassic to the end of the Cretac ...
s. 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 Annum, Ma), and extensively diversified after the Permian–Triassic extinction event. Cynodonts had a wide va ...

cynodont
s gave rise to the first
Mammaliaformes Mammaliaformes ("mammal-shaped") is a clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic – that is, composed of a common ancestor and all its lineage (evolution), ...

Mammaliaformes
. All this climatic change, however, resulted in a large die-out known as the Triassic–Jurassic extinction event, in which many
archosaurs Archosauria ("ruling reptiles") is a clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic – that is, composed of a common ancestor and all its lineage (evolution), l ...
(excluding pterosaurs, dinosaurs and
crocodylomorph Crocodylomorpha is a group of Pseudosuchia, pseudosuchian archosaurs that includes the crocodilians and their extinct relatives. They were the only members of Pseudosuchia to survive the Triassic–Jurassic extinction event, end-Triassic extinct ...

crocodylomorph
s), most
synapsid Synapsids are a group of animals that includes mammal Mammals (from Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome ...

synapsid
s, 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 eruptions at the
Central Atlantic magmatic province The Central Atlantic magmatic province (CAMP) is the Earth's largest continental large igneous province Image:Flood_Basalt_Map.jpg, 300px, Only a few of the largest large igneous provinces appear (coloured dark purple) on this geological map, wh ...
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 The Plesiosauria (; Ancient Greek, Greek: πλησίος, ''plesios'', meaning "near to" and Sauria, ''sauros'', meaning "lizard") or plesiosaurs are an Order (biology), order or clade of extinct Mesozoic marine reptiles, belonging to the Sauro ...
, ichthyosaurs and
ammonites Ammonoids are a group of Extinction, extinct marine mollusca, mollusc animals in the subclass Ammonoidea of the class (biology), class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids ( ...

ammonites
were abundant. On land, dinosaurs and other archosaurs staked their claim as the dominant race, with
theropods Theropoda ( from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is appro ...

theropods
such as ''
Dilophosaurus ''Dilophosaurus'' ( ) is a genus of theropod dinosaurs that lived in what is now North America during the Early Jurassic, about 193 million years ago. Three skeletons were discovered in northern Arizona in 1940, and the two best preserved ...

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, beaver Beavers are large, semiaquatic In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, ch ...
'', for example, had adaptations for swimming, digging and catching fish. ''
Fruitafossor ''Fruitafossor'' was a termite Termites are Eusociality, eusocial insects that are classified at the Taxonomy (biology), taxonomic rank of infraorder Isoptera, or as Taxonomic rank#All ranks, epifamily Termitoidae within the order Blattodea (a ...
'', 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
termite Termites are eusocial Eusociality (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its populati ...

termite
s, as ants had not yet appeared). The first
multituberculates Multituberculata (commonly known as multituberculates, named for the multiple tubercle Image:Mammillaria marksiana.jpg, 250px, This view of the cactus ''Mammillaria marksiana'' shows its pattern of prominent tubercles, with the spines emanating ...
like ''
Rugosodon ''Rugosodon'' is an extinct genus of multituberculata, multituberculate (rodent-like) mammals from eastern China that lived 160 million years ago during the Jurassic period. The discovery of its type species and currently only known species ''Rugo ...
'' evolved, while
volaticotheria Volaticotherini is a clade of eutriconodont mammals from the Mesozoic. In addition to the type genus ''Volaticotherium'', it includes the genera ''Argentoconodon'', ''Ichthyoconodon'', and potentially ''Triconolestes''. Since most remains are pr ...
ns took to the skies. The Middle Jurassic spans from 175 to 163 million years ago. During this epoch, dinosaurs flourished as huge herds of sauropods, such as ''
Brachiosaurus ''Brachiosaurus'' () is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxono ...

Brachiosaurus
'' and ''
Diplodocus ''Diplodocus'' (, , or ) is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (ta ...

Diplodocus
'', filled the fern prairies, chased by many new predators such as ''
Allosaurus ''Allosaurus'' () is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), ...

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 e ...
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
avialan 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 Deinonychosauria, deinonychosaurs, though alternative definitions are ...
s, like ''
Archaeopteryx ''Archaeopteryx'' ( "old wing"), sometimes referred to by its German name, ' ("original bird" or "first bird"), is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification ...

Archaeopteryx
'', evolved from small coelurosaurian 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 as a result, the decline and/or extinction of Laurasian sauropods. Some island-hopping dinosaurs, like ''Eustreptospondylus'', evolved to cope with the coastal shallows and small islands of ancient Europe. Other dinosaurs rose up to fill the empty space that the Jurassic-Cretaceous extinction left behind, such as ''Carcharodontosaurus'' and ''Spinosaurus''. 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''. Pterosaurs got larger as genera like ''Tapejara (pterosaur), 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 metatherians and eutherians, and cimolodont 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 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 ge ...

Cenozoic
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 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'' 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 Enantiornithes, enantiornithe and Ornithurae, ornithurine 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'' and ''Schowalteria''. Still, the dominant mammals were multituberculates, cimolodonts in the north and gondwanatheres in the south. At the end of the Cretaceous, the Deccan traps 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 Cretaceous–Paleogene extinction event, 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. Everything over 10 kilograms became extinct.


Paleogeography and tectonics

Compared to the vigorous convergent plate orogeny, 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 Brooks Range, the Verkhoyansk Range, Verkhoyansk and Cherskiy Ranges in Siberia, and the Khingan Mountains in Manchuria. This orogeny was related to the opening of the Arctic Ocean and subduction of the North China Craton, North China and Siberian cratons under the Pacific Ocean. In contrast, the era featured the dramatic rifting of the supercontinent Pangaea, which gradually split into a northern continent, Laurasia, and a southern continent, Gondwana. This created the passive continental margin that characterizes most of the Atlantic coastline (such as along the East Coast of the United States, U.S. East Coast) today. By the end of the era, the continents had rifted into nearly their present forms, though not their present positions. Laurasia became North America and Eurasia, while Gondwana split into South America, Africa, Australia, Antarctica and the South Asia, 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, 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 deserts. Abundant red beds 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 and one in the Rhaetian, 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 Earth's atmosphere, atmosphere are thought to have almost eliminated the north–south temperature gradient: temperatures were about the same across the planet, and about 10°Celsius, 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 deposition (sediment), deposited as "Oil shale, black shale". 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 is opposed to the earth's current flora, in which the dominant land plants in terms of number of species are angiosperms. 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 taxon, extant genus ''sequoia (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 (ecology), biomass was still dominated by cycads and
fern A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plant Vascular plants (from Latin ''vasculum'': duct), also known as Tracheophyta (the tracheophytes , from Greek τραχεῖα ἀρτηρία ''trācheia art ...

fern
s until after the Cretaceous–Paleogene extinction. Some plant species had distributions that were markedly different from succeeding periods; for example, the Schizeales, 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 adaptive radiation, radiation of many new lifeforms. In particular, the extinction of the large herbivore, herbivorous pareiasaurs and carnivore, carnivorous 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 Annum, Ma), and extensively diversified after the Permian–Triassic extinction event. Cynodonts had a wide va ...

cynodont
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:
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 annum, million years ago, although the exact origin and timing of the evolution ...

dinosaur
s, 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
bird Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...

bird
s and eutherian mammals also appeared. Some have argued that insects diversified in symbiosis 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 Mesozoic, Geological eras