Mesozoic

Mesozoic Era 252.17 - 66 million years ago

Key events in the Mesozoic view • discuss •  -260 — – -240 — – -220 — – -200 — – -180 — – -160 — – -140 — – -120 — – -100 — – -80 — – -60 — – Paleozoic Cenozoic Triassic Jurassic Cretaceous         M e s o z o i c  Phanerozoic An approximate timescale of key Mesozoic events. Axis scale: millions of years ago.

The Mesozoic Era (pronunciation: /ˌmɛsəˈzoʊɪk, ˌmiː-, -soʊ-/ or /ˌmɛzəˈzoʊɪk, ˌmiː-, -zoʊ-/^[1][2]) is an interval of geological time from about 252 to 66 million years ago. It is also called the Age of Reptiles, a phrase introduced by the 19th century paleontologist Gideon Mantell who viewed it as dominated by reptiles such as Iguanodon, Megalosaurus, Plesiosaurus and what are now called Pseudosuchia.^[3]

Mesozoic means "middle life", deriving from the Greek prefix meso-/μεσο- for "between" and zōon/ζῷον meaning "animal" or "living being".^[4] It is one of three geologic eras of the Phanerozoic Eon, preceded by the Paleozoic ("ancient life") and succeeded by the Cenozoic ("new life"). The era is subdivided into three major periods: the Triassic, Jurassic, and Cretaceous, which are further subdivided into a number of epochs and stages.

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 which is known for having killed off non-avian dinosaurs, as well as other plant and animal species. The Mesozoic was a time of significant tectonic, climate and evolutionary activity. The era witnessed the gradual rifting of the supercontinent Pangaea into separate landmasses that would eventually move into their current positions. The climate of the Mesozoic was varied, alternating between warming and cooling periods. Overall, however, the Earth was hotter than it is today. Non-avian dinosaurs appeared in the Late Triassic and became the dominant terrestrial vertebrates early in the Jurassic, occupying this position for about 135 million years until their demise at the end of the Cretaceous. Birds first appeared in the Jurassic, having evolved from a branch of theropod dinosaurs. The first mammals also appeared during the Mesozoic, but would remain small—less than 15 kg (33 lb)—until the Cenozoic.

Geologic periods

Following the Paleozoic, the Mesozoic extended roughly 186 million years, from 252.17 to 66 million years ago when the Cenozoic Era began. This time frame is separated into three geologic periods. From oldest to youngest:

• Triassic (252.17 to 201.3 million years ago)
• Jurassic (201.3 to 145 million years ago)
• Cretaceous (145 to 66 million years ago)

The lower (Triassic) boundary is set by the Permian–Triassic extinction event, during which approximately 90% to 96% of marine species and 70% of terrestrial vertebrates became extinct.^[5] It is also known as the "Great Dying" because it is considered the largest mass extinction in the Earth's history. The upper (Cretaceous) boundary is set at the Cretaceous–Tertiary (KT) extinction event (now more accurately called the Cretaceous–Paleogene (or K–Pg) extinction event^[6]), which may have been caused by the 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 dinosaurs.

Triassic

The Triassic ranges roughly from 252 million to 201 million years ago. The Triassic is a time in Earth's history bracketed between the Permian Extinction and the Triassic–Jurassic extinction event, two of the big five, and precedes the Jurassic Period. It has three major epochs: the Early Triassic, the Middle Triassic and the Late Triassic.^[7]

The Early Triassic was between about 252 million to 247 million years ago and was dominated by deserts as Pangaea had not yet broken up, thus the interior was nothing but arid. The Earth had just witnessed a massive die-off in which 95% of all life became extinct. The most common vertebrate life on earth were Lystrosaurus, Labyrinthodont, and Euparkeria along with many other creatures that managed to survive the Great Dying. Temnospondyli evolved during this time and would be the dominant predator for much of the Triassic.^[8]

Plateosaurus (a prosauropod)

The Middle Triassic spans roughly from 247 million to 237 million years ago. The Middle Triassic featured the beginnings of the breakup of Pangaea, and the opening of the Tethys Sea. The ecosystem had recovered from the devastation that was the Great Dying. Algae, sponge, corals, and crustaceans all had recovered, and the reptiles began to get bigger and bigger. New aquatic reptiles evolved such as Ichthyosaurs and Nothosaurs. Meanwhile, on land, Pine forests flourished, bringing along mosquitoes and fruit flies. The first ancient crocodilians evolved, which sparked competition with the large amphibians that had since ruled the freshwater world.^[9]

The Late Triassic spans roughly from 237 million to 201 million years ago. Following the bloom of the Middle Triassic, the Late Triassic featured frequent heat spells, as well as moderate precipitation (10-20 inches per year). The recent warming led to a boom of reptilian evolution on land as the first true dinosaurs evolve, as well as pterosaurs. All this climatic change, however, resulted in a large die-out known as the Triassic-Jurassic extinction event, in which all archosaurs (excluding ancient crocodiles), most synapsids, and almost all large amphibians became extinct, as well as 34% of marine life in the fourth mass extinction event of the world. The cause is debatable.^[10][11]

Jurassic

Rhamphorhynchus

The Jurassic ranges from 200 million years to 145 million years ago and features 3 major epochs: The Early Jurassic, the Middle Jurassic, and the Late Jurassic.^[12]

The Early Jurassic spans from 200 million years to 175 million years ago.^[12] The climate was much more humid than the Triassic, and as a result, the world was very tropical. In the oceans, Plesiosaurs, Ichthyosaurs and Ammonites fill waters as the dominant races of the seas. On land, dinosaurs and other reptiles stake their claim as the dominant race of the land, with species such as Dilophosaurus at the top. The first true crocodiles evolved, pushing out the large amphibians to near extinction. All-in-all, reptiles rise to rule the world. Meanwhile, the first true mammals evolve, but remained relatively small sized.

The Middle Jurassic spans from 175 million to 163 million years ago.^[12] During this epoch, reptiles flourished as huge herds of sauropods, such as Brachiosaurus and Diplodocus, filled the fern prairies of the Middle Jurassic. Many other predators rose as well, such as Allosaurus. Conifer forests made up a large portion of the forests. In the oceans, Plesiosaurs were quite common, and Ichthyosaurs were flourishing. This epoch was the peak of the reptiles.^[13]

Stegosaurus

The Late Jurassic spans from 163 million to 145 million years ago.^[12] The Late Jurassic featured a massive extinction of sauropods and Ichthyosaurs due to the separation of Pangaea into Laurasia and Gondwana in an extinction known as the Jurassic-Cretaceous extinction. Sea levels rose, destroying fern prairies and creating shallows in its wake. Ichthyosaurs became extinct whereas sauropods, as a whole, did not die out in the Jurassic; in fact, some species, like the Titanosaurus, lived up to the K-T extinction.^[14] The increase in sea-levels opened up the Atlantic sea way which would continue to get larger over time. The divided world would give opportunity for the diversification of new dinosaurs.

Cretaceous

The Cretaceous is the longest period in the Mesozoic, but has only two epochs: the Early Cretaceous, and the Late Cretaceous.^[15]

Tylosaurus (a mosasaur) hunting Xiphactinus

The Early Cretaceous spans from 145 million to 100 million years ago.^[15] The Early Cretaceous saw the expansion of seaways, and as a result, the decline and extinction of sauropods (except in South America). Many coastal shallows were created, and that caused Ichthyosaurs to die out. Mosasaurs evolved to replace them as head of the seas. 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. Of the most successful would be the Iguanodon which spread to every continent. Seasons came back into effect and the poles got seasonally colder, but dinosaurs still inhabited this area like the Leaellynasaura which inhabited the polar forests year-round, and many dinosaurs migrated there during summer like Muttaburrasaurus. Since it was too cold for crocodiles, it was the last stronghold for large amphibians, like Koolasuchus. Pterosaurs got larger as species like Tapejara and Ornithocheirus evolved.

The Late Cretaceous spans from 100 million to 65 million years ago.^[15] The Late Cretaceous featured a cooling trend that would continue on in the Cenozoic period. Eventually, tropics were restricted to the equator and areas beyond the tropic lines featured extreme seasonal changes in weather. Dinosaurs still thrived as new species such as Tyrannosaurus, Ankylosaurus, Triceratops and Hadrosaurs dominated the food web. In the oceans, Mosasaurs ruled the seas to fill the role of the Ichthyosaurs, and huge plesiosaurs, such as Elasmosaurus, evolved. Also, the first flowering plants evolved. At the end of the Cretaceous, the Deccan traps and other volcanic eruptions were poisoning the atmosphere. As this was continuing, it is thought that a large meteor smashed into earth, creating the Chicxulub Crater in an event known as the K-T Extinction, the fifth and most recent mass extinction event, in which 75% of life on earth became extinct, including all non-avian dinosaurs. Everything over 10 kilograms became extinct. The age of the dinosaurs was over.^[16][17]

Paleogeography and tectonics

Breakup of Pangaea

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 Brooks Range, the 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 and Siberian cratons under the Pacific Ocean.^[18] Nevertheless, the era featured the dramatic rifting of the supercontinent Pangaea. Pangaea 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 U.S. East Coast) today.^[19]

By the end of the era, the continents had rifted into nearly their present form. Laurasia became North America and Eurasia, while Gondwana split into South America, Africa, Australia, Antarctica and the Indian subcontinent, which collided with the Asian plate during the Cenozoic, the impact 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 the oceans—experience less variation in temperature. Because much of the land that constituted Pangaea was distant from the oceans, temperatures fluctuated greatly, and the interior of Pangaea probably included expansive areas of desert. Abundant red beds and evaporites such as halite support these conclusions, but evidence exists that the generally dry climate of the Triassic was punctuated by episodes of increased rainfall.^[20] Most important humid episodes were the Carnian Pluvial Event and one in the Rhaetian, few million years before the Triassic–Jurassic extinction event.

Sea levels began to rise during the Jurassic, which was probably caused by an increase in seafloor spreading. The formation of new crust beneath the surface displaced ocean waters by as much as 200 m (656 ft) more than today, which flooded coastal areas. Furthermore, Pangaea began to rift into smaller divisions, bringing more land area in contact with the ocean by forming the Tethys Sea. Temperatures continued to increase and 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. Higher levels of carbon dioxide in the atmosphere are thought to have caused the world temperature gradient from north to south to become almost flat: temperatures were about the same across the planet. Average temperatures were also higher than today by about 10°C. In fact, by the middle Cretaceous, equatorial ocean waters (perhaps as warm as 20 °C in the deep ocean) may have been too warm for sea life, and land areas near the equator may have been deserts despite their proximity to water. The circulation of oxygen to the deep ocean may also have been disrupted.^[21] For this reason, large volumes of organic matter that was unable to decompose accumulated, eventually being deposited as "black shale".

Not all of the data support these hypotheses, however. Even with the overall warmth, temperature fluctuations should have been sufficient for the presence of polar ice caps and glaciers, but there is no evidence of either. Quantitative models have also been unable to recreate the flatness of the Cretaceous temperature gradient.

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,^[22][23] some concluding they were lower in the Triassic and part of the Jurassic but higher in the Cretaceous,^[24][25][26] and some concluding they were higher throughout most or all of the Triassic, Jurassic and Cretaceous.^[27][28]

Life

Flora

Conifers were the dominant terrestrial plants for most of the Mesozoic. Flowering plants appeared late in the era but did not become widespread until the Cenozoic.

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. One particular plant genus, Ginkgo, is thought to have evolved at this time and is represented today by a single species, Ginkgo biloba. As well, the extant genus Sequoia is believed to have evolved in the Mesozoic.^[29]

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.^[30]

Fauna

Dinosaurs were the dominant terrestrial vertebrates throughout much of the Mesozoic.

The extinction of nearly all animal species at the end of the Permian Period allowed for the radiation of many new lifeforms. In particular, the extinction of the large herbivorous pareiasaurs and carnivorous gorgonopsians left those ecological niches empty. Some were filled by the surviving cynodonts and dicynodonts, the latter of which subsequently became extinct.

Recent research indicates that the specialized animals that formed complex ecosystems, with high biodiversity, complex food webs and a variety of niches, took much longer to reestablish, recovery did not begin until the start of the mid-Triassic, 4M to 6M years after the extinction^[31] and was not complete until 30M years after the Permian–Triassic extinction event.^[32] Animal life was then dominated by various archosaurian reptiles: dinosaurs, pterosaurs, and aquatic reptiles such as ichthyosaurs, plesiosaurs, and mosasaurs.

The climatic changes of the late Jurassic and Cretaceous provided for further adaptive radiation. The Jurassic was the height of archosaur diversity, and the first birds and eutherian mammals also appeared. Angiosperms radiated sometime in 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 was still dominated by cycad and ferns until after the Cretaceous–Paleogene extinction.

Some have argued that insects diversified 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.

As the temperatures in the seas increased, the larger animals of the early Mesozoic gradually began to disappear while smaller animals of all kinds, including lizards, snakes, and perhaps primates, evolved. The Cretaceous–Paleogene extinction event exacerbated this trend. The large archosaurs became extinct, while birds and mammals thrived, as they do today.

See also

• Cenozoic
• Paleozoic
• Phanerozoic Eon

References

1. Jones, Daniel (2003) [1917], Peter Roach, James Hartmann and Jane Setter, eds., English Pronouncing Dictionary, Cambridge: Cambridge University Press, ISBN 3-12-539683-2
2. "Mesozoic". Dictionary.com Unabridged. Random House.
3. Dean, Dennis R. (1999). Gideon Mantell and the Discovery of Dinosaurs. Cambridge University Press. pp. 97–98. ISBN 0521420482.
4. "Mesozoic". Online Etymology Dictionary.
5. Benton M J (2005). When life nearly died: the greatest mass extinction of all time. London: Thames & Hudson. ISBN 0-500-28573-X.
6. Gradstein F, Ogg J, Smith A. A Geologic Time Scale 2004.
7. Alan Logan. "Triassic". University of New Brunswick.
8. Alan Kazlev. "Early Triassic". unknown.
9. Rubidge. "Middle Triassic". unknown.
10. Graham Ryder, David Fastovsky, and Stefan Gartner. "Late Triassic Extinction". Geological Society of America.
11. Enchanted Learning. "Late Triassic life". Enchanted Learning.
12. Carol Marie Tang. "Jurassic Era". California Academy of Sciences.
13. Enchanted Learning. "Middle Jurassic". Enchanted Learning.
14. Bob Strauss. "Cretaceous sauropods". author.
15. Carl Fred Koch. "Cretaceous". Old Dominion University.
16. University of California. "Cretaceous". University of California.
17. Elizabeth Howell. "K-T Extinction event". Universe Today.
18. See Hughes, T.; “ The case for creation of the North Pacific Ocean during the Mesozoic Era” in Palaeogeography, Palaeoclimatology, Palaeoecology; Volume 18, Issue 1, August 1975, Pages 1-43
19. Stanley, Steven M. Earth System History. New York: W.H. Freeman and Company, 1999. ISBN 0-7167-2882-6
20. Preto, N.; Kustatscher, E.; Wignall, P.B. (2010). "Triassic climates — State of the art and perspectives". Palaeogeography, Palaeoclimatology, Palaeoecology 290: 1–10. doi:10.1016/j.palaeo.2010.03.015.
21. http://www.britannica.com/EBchecked/topic/142729/Cretaceous-Period/69972/Paleoclimate
22. Robert A. Berner, John M. VandenBrooks and Peter D. Ward, 2007, Oxygen and Evolution. Science 27 April 2007, Vol. 316 no. 5824 pp. 557-558 . A graph showing the reconstruction from this paper can be found here, from the webpage Paleoclimate - The History of Climate Change.
23. Berner R. A. 2006 GEOCARBSULF: a combined model for Phanerozoic atmospheric O2 and CO2. Geochim. Cosmochim. Acta 70, 5653–5664. See the dotted line in Fig. 1 of Atmospheric oxygen level and the evolution of insect body size by Jon F. Harrison, Alexander Kaiser and John M. VandenBrooks
24. Berner, Robert A., 2009, Phanerozoic atmospheric oxygen: New results using the GEOCARBSULF model. Am. J. Sci. 309 no. 7, 603-606. A graph showing the reconstructed levels in this paper can be found on p. 31 of the book Living Dinosaurs by Gareth Dyke and Gary Kaiser.
25. Berner R. A., Canfield D. E. 1989 A new model for atmospheric oxygen over phanerozoic time. Am. J. Sci. 289, 333–361. See the solid line in Fig. 1 of Atmospheric oxygen level and the evolution of insect body size by Jon F. Harrison, Alexander Kaiser and John M. VandenBrooks
26. Berner, R, et al., 2003, Phanerozoic atmospheric oxygen, Annu. Rev. Earth Planet. Sci., V, 31, p. 105-134. See the graph near the bottom of the webpage Phanerozoic Eon
27. Glasspool, I.J., Scott, A.C., 2010, Phanerozoic concentrations of atmospheric oxygen reconstructed from sedimentary charcoal, Nature Geosciences, 3, 627-630
28. 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 of Atmospheric oxygen level and the evolution of insect body size by Jon F. Harrison, Alexander Kaiser and John M. VandenBrooks
29. Stan Baducci. Mesozoic Plants..
30. C.Michael Hogan. 2010. Fern. Encyclopedia of Earth. National council for Science and the Environment. Washington, DC
31. 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): 1053–1056. Bibcode:2006Geo....34.1053L. doi:10.1130/G22827A.1.
32. Sahney, S. and Benton, M.J. (2008). "Recovery from the most profound mass extinction of all time" (PDF). Proceedings of the Royal Society: Biological 275 (1636): 759–65. doi:10.1098/rspb.2007.1370. PMC 2596898. PMID 18198148.

• British Mesozoic Fossils, 1983, The Natural History Museum, London.

Preceded by Proterozoic Eon	Phanerozoic Eon
	Paleozoic Era						Mesozoic Era			Cenozoic Era
	Cambrian	Ordovician	Silurian	Devonian	Carboniferous	Permian	Triassic	Jurassic	Cretaceous	Paleogene	Neogene	4ry