Mesozoic

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Mesozoic Era
252.17 - 66 million years ago
Key events in the Mesozoic
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An approximate timescale of key Mesozoic events.
Axis scale: millions of years ago.

The Mesozoic Era /mɛzɵˈzɪk/ 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.[1]

Mesozoic means "middle life", deriving from the Greek prefix meso-/μεσο- for "between" and zōon/ζῷον meaning "animal" or "living being".[2] 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[edit]

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:

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.[3] 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[4]), 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.

Paleogeography and tectonics[edit]

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.[5] 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.[6]

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[edit]

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.[7] 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,[dubious ][citation needed] 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.[dubious ][citation needed] 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.[citation needed]

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,[8][9] some concluding they were lower in the Triassic and part of the Jurassic but higher in the Cretaceous,[10][11][12] and some concluding they were higher throughout most or all of the Triassic, Jurassic and Cretaceous.[13][14]

Life[edit]

Flora[edit]

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, Ginko biloba. As well, the extant genus Sequoia is believed to have evolved in the Mesozoic.[15]

Fauna[edit]

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. 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.[16]

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[17] and was not complete until 30M years after the Permian–Triassic extinction event.[18] 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[edit]

References[edit]

  1. ^ Dean, Dennis R. (1999). Gideon Mantell and the Discovery of Dinosaurs. Cambridge University Press. pp. 97–98. ISBN 0521420482. 
  2. ^ "Mesozoic". Online Etymology Dictionary. 
  3. ^ Benton M J (2005). When life nearly died: the greatest mass extinction of all time. London: Thames & Hudson. ISBN 0-500-28573-X. [page needed]
  4. ^ Gradstein F, Ogg J, Smith A. A Geologic Time Scale 2004. 
  5. ^ 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
  6. ^ Stanley, Steven M. Earth System History. New York: W.H. Freeman and Company, 1999. ISBN 0-7167-2882-6
  7. ^ 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. 
  8. ^ 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.
  9. ^ 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
  10. ^ 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.
  11. ^ 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
  12. ^ Berner, R, et al., 2003, Phanerozoic atmospheric oxygen, Ann. Rev. Earth Planet. Sci., V, 31, p. 105-134. See the graph near the bottom of the webpage Phanerozoic Eon
  13. ^ Glasspool, I.J., Scott, A.C., 2010, Phanerozoic concentrations of atmospheric oxygen reconstructed from sedimentary charcoal, Nature Geosciences, 3, 627-630
  14. ^ 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
  15. ^ Stan Baducci. Mesozoic Plants..
  16. ^ C.Michael Hogan. 2010. Fern. Encyclopedia of Earth. National council for Science and the Environment. Washington, DC
  17. ^ 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. 
  18. ^ 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