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, havingevolved from a branch oftheropod dinosaurs, thentrue toothless birds appeared in the Cretaceous. The firstmammals also appeared during the Mesozoic, but would remain small—less than 15 kg (33 lb)—until the Cenozoic.Flowering plants appeared in the Early Cretaceous and would rapidly diversify through the end of the era, replacingconifers and other gymnosperms (sensu lato), such asginkgoales, cycads andbennettitales as the dominant group of plants.
Following the Paleozoic, the Mesozoic extended roughly 186 million years, from251.902 to 66 million years ago when theCenozoic Era began. This time frame is separated into three geologicperiods. From oldest to youngest:
The lower boundary of the Mesozoic is set by thePermian–Triassic extinction event, during which it has been estimated that up to 90–96% of marine species becameextinct[7] although those approximations have been brought into question with some paleontologists estimating the actual numbers as low as 81%.[8] 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 theCretaceous–Paleogene extinction event (or K–Pg extinction event[9]), which may have been caused by an asteroid impactor that createdChicxulub Crater on theYucatán Peninsula. Towards the Late Cretaceous, large volcanic eruptions are also believed to have contributed to the Cretaceous–Paleogene extinction event.[10] Approximately 50% of all genera became extinct, including all of the non-avian dinosaurs.
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 theTriassic–Jurassic extinction event, two of the "big five", and it is divided into three major epochs: Early, Middle, and Late Triassic.[11]
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 wereLystrosaurus,labyrinthodonts, andEuparkeria along with many other creatures that managed to survive the Permian extinction.Temnospondyls reached peak diversity during the early Triassic.[12]
The Middle Triassic, from 247 to 237 million years ago, featured the beginnings of the breakup of Pangaea and the opening of theTethys Ocean. Ecosystems had recovered from the Permian extinction. Algae, sponge, corals, and crustaceans all had recovered, and new aquatic reptiles evolved, such asichthyosaurs andnothosaurs. On land, pine forests flourished, as did groups of insects such as 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.[13]
Approximate map of Earth in the Early Triassic Period.
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 the continents 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 firstpterosaurs. During the Late Triassic, some advancedcynodonts gave rise to the firstMammaliaformes. All this climatic change, however, resulted in a large die-out known as the Triassic–Jurassic extinction event, in which manyarchosaurs (excluding pterosaurs, dinosaurs andcrocodylomorphs), mostsynapsids, 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;[14][15]flood basalt eruptions at theCentral Atlantic magmatic province is cited as one possible cause.[16][17][18]
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.[19]
The Early Jurassic spans from 200 to 175 million years ago.[19] 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 andammonites were abundant. On land, dinosaurs and other archosaurs staked their claim as the dominant race, withtheropods such asDilophosaurus 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 JurassicCastorocauda, for example, had adaptations for swimming, digging and catching fish.Fruitafossor, 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 (probablytermites, as ants had not yet appeared) ;Volaticotherium was able to glide for short distances, such as modernflying squirrels. The firstmultituberculates such asRugosodon evolved.[citation needed]
The Middle Jurassic spans from 175 to 163 million years ago.[19] During this epoch, dinosaurs flourished as huge herds of sauropods, such asBrachiosaurus andDiplodocus, filled the fern prairies, chased by many new predators such asAllosaurus.Conifer 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.[20][failed verification][self-published source]
The Late Jurassic spans from 163 to 145 million years ago.[19] During this epoch, the firstavialans, such asArchaeopteryx, evolved from smallcoelurosaurian 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.
The Early Cretaceous spans from 145 to 100 million years ago.[21] 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.[22]Some island-hopping dinosaurs, such asEustreptospondylus, 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 asCarcharodontosaurus andSpinosaurus.[citation needed] Seasons came back into effect and the poles got seasonally colder, but some dinosaurs still inhabited the polar forests year round, such asLeaellynasaura andMuttaburrasaurus. The poles were too cold for crocodiles, and became the last stronghold for large amphibians such asKoolasuchus. Pterosaurs got larger as genera such asTapejara andOrnithocheirus evolved. Mammals continued to expand their range:eutriconodonts produced fairly large,wolverine-like predators such asRepenomamus andGobiconodon, earlytherians began to expand intometatherians andeutherians, andcimolodontmultituberculates 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 theCenozoic 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 asTyrannosaurus,Ankylosaurus,Triceratops andhadrosaurs dominated the food web. In the oceans,mosasaurs ruled, filling the role of the ichthyosaurs, which, after declining, had disappeared in theCenomanian-Turonian boundary event. Thoughpliosaurs had gone extinct in the same event, long-necked plesiosaurs such asElasmosaurus 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 ofenantiornithe andornithurine forms. Though mostly small, marinehesperornithes 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 such asDidelphodon andSchowalteria. Still, the dominant mammals were multituberculates,cimolodonts in the north andgondwanatheres in the south. At the end of the Cretaceous, theDeccan 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 theK-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.[23][24][25]
The Triassic was generally dry, a trend that began in the lateCarboniferous, and highly seasonal, especially in the interior of Pangaea. Low sea levels may have also exacerbated temperature extremes. With its highspecific 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 expansivedeserts. Abundantred beds and evaporites such ashalite support these conclusions, but some evidence suggests the generally dry climate of the Triassic was punctuated by episodes of increased rainfall.[28] The most important humid episodes were theCarnian Pluvial Event and one in theRhaetian, a few million years before the Triassic–Jurassic extinction event.
Sea levels began to rise during the Jurassic, probably caused by an increase inseafloor spreading. The formation of new crust beneath the surface displaced ocean waters by as much as 200 m (656 ft) 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.[29]
The climate of the Cretaceous is less certain and more widely disputed. Probably, higher levels ofcarbon dioxide in theatmosphere are thought to have almost eliminated the north–southtemperature gradient: temperatures were about the same across the planet, and about 10°C higher than today. The circulation ofoxygen to the deep ocean may also have been disrupted, preventing thedecomposition of large volumes of organic matter, which was eventuallydeposited as "black shale".[30][31]
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,[32][33] some concluding they were lower in the Triassic and part of the Jurassic but higher in the Cretaceous,[34][35][36] and some concluding they were higher throughout most or all of the Triassic, Jurassic and Cretaceous.[37][38]
Conifers were the dominant terrestrial plants for most of the Mesozoic, withgrasses becoming widespread in theLate Cretaceous.Flowering plants appeared late in the era but did not become widespread until theCenozoic.
The dominant land plant species of the time weregymnosperms, 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 areangiosperms. The earliest members of the genusGinkgo first appeared during the Middle Jurassic. This genus is represented today by a single species,Ginkgo biloba.[39] Modern conifer groups began to radiate during the Jurassic.[40]Bennettitales, an extinct group of gymnosperms with foliage superficially resembling that ofcycads gained a global distribution during the Late Triassic, and represented one of the most common groups of Mesozoic seed plants.[41]
Flowering plants radiated during the early Cretaceous, first in thetropics, 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 thatbiomass was still dominated by cycads andferns until after the Cretaceous–Paleogene extinction. Some plant species had distributions that were markedly different from succeeding periods; for example, theSchizeales, a fern order, were skewed to the Northern Hemisphere in the Mesozoic, but are now better represented in the Southern Hemisphere.[42]
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,[43] and not fully proliferated until 30 million years after the extinction.[44] During the Triassic, terrestrial herbivores avoided competition through significant niche partitioning by generally occupying highly distinct guilds.[45] Animal life was then dominated by various archosaurs:dinosaurs, pterosaurs, and aquatic reptiles such as ichthyosaurs, plesiosaurs, andmosasaurs.
The climatic changes of the late Jurassic and Cretaceous favored further adaptive radiation. The Jurassic was the height of archosaur diversity, and the firstbirds andeutherian mammals also appeared. Some have argued thatinsects diversified insymbiosis with angiosperms, because insectanatomy, 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.[citation needed]
At the dawn of the Mesozoic, ocean plankton communities transitioned from ones dominated by greenarchaeplastidans to ones dominated by endosymbiotic algae with red-algal-derived plastids. This transition is speculated to have been caused by an increasing paucity of many trace metals in the Mesozoic ocean.[46]
^Jones, Daniel (2003) [1917], Peter Roach; James Hartmann; Jane Setter (eds.),English Pronouncing Dictionary, Cambridge: Cambridge University Press,ISBN978-3-12-539683-8
Phillips, John (1840)."Palæozoic series".Penny Cyclopaedia of the Society for the Diffusion of Useful Knowledge. Vol. 17. London: Charles Knight and Co. pp. 153–54. "As many systems or combinations of organic forms as are clearly traceable in the stratified crust of the globe, so many corresponding terms (as Palæozoic, Mesozoic, Kainozoic, &c.) may be made, … "
^Tang, Carol Marie."Mesozoic Era".Encyclopædia Britannica. Retrieved5 September 2019.
^Benton M J (2005). "Chapter 8: Life's Biggest Challenge".When life nearly died: the greatest mass extinction of all time. London: Thames & Hudson.ISBN978-0-500-28573-2.[page needed]
^Berner, R, et al., 2003,Phanerozoic atmospheric oxygen, Annu. Rev. Earth Planet. Sci., V, 31, p. 105–34. See the graph near the bottom of the webpagePhanerozoic EonArchived 27 April 2013 at theWayback Machine
^Glasspool, I.J., Scott, A.C., 2010,Phanerozoic concentrations of atmospheric oxygen reconstructed from sedimentary charcoal,Nature Geoscience, 3, 627–30
^Bergman N. M., Lenton T. M., Watson A. J. 2004 COPSE: a new model of biogeochemical cycling over Phanaerozoic time.Am. J. Sci. 304, 397–437. See the dashed line in Fig. 1 ofAtmospheric oxygen level and the evolution of insect body size by Jon F. Harrison, Alexander Kaiser and John M. VandenBrooks
^Balducci, Stan (2000)."Mesozoic Plants". fossilnews.com. Archived fromthe original on 23 January 2013. Retrieved28 July 2023.
^Lehrmann, D. J.; Ramezan, J.; Bowring, S.A.; et al. (December 2006). "Timing of recovery from the end-Permian extinction: Geochronologic and biostratigraphic constraints from south China".Geology.34 (12):1053–56.Bibcode:2006Geo....34.1053L.doi:10.1130/G22827A.1.
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