Mesozoic

The Mesozoic era is one of three geologic eras of the Phanerozoic eon, being preceded by the Paleozoic era and followed by the Cenozoic. The Mesozoic extended from about 251 million years ago (mya) to 65 mya. The Phanerozoic eon, which is the fossil-rich time period that comes after the Precambrian, extends to the present day.

Mesozoic means "middle animals," derived from Greek prefix meso-/μεσο- for 'between' and zoon/ζωον meaning animal or 'living being'. The original name used by Giovanni Arduino, who divided time into eras in the 18th century, was "Secondary" (with the Paleozoic era labelled "Primitive" and the modern era the "Tertiary"). The prior Paleozoic was the era when most of the basic animal body plans came into existence.

The Mesozoic is separated into three geologic periods: Triassic, Jurassic, and Cretaceous. It is often called the "Age of the Dinosaurs," after the dominant fauna of the era. Many diverse animal species appeared during this time.

Just as in the development of a human, in the history of life on earth, later stages develop on the foundation of earlier stages. By the end of the Mesozoic era, the basis of modern life was in place.

Phanerozoic eon (542 mya - present)
Paleozoic era	Mesozoic era	Cenozoic era

Mesozoic era (251 - 65 mya)
Triassic	Jurassic	Cretaceous

Geologic periods

Following the Paleozoic, the Mesozoic extended roughly 180 million years: from 251 million years ago (mya) to when the Cenozoic era began 65 mya.

The three geologic periods into which the Mesozoic are separated, from oldest to youngest, are:

• Triassic (251.0 Ma to 199.6 Ma)
• Jurassic (199.6 Ma to 145.5 Ma)
• Cretaceous (145.5 Ma to 65.5 Ma)

The lower boundary of the Mesozoic (beginning of the Triassic) is set by the Permian-Triassic extinction, during which approximately 90% of marine species and 70% of terrestrial vertebrates became extinct. It is also known as the "Great Dying" because it is considered the largest mass extinction in history.

The upper boundary (end of the Cretaceous) is set at the Cretaceous-Tertiary (KT) extinction, which may have been caused by the meteor that created the Chicxulub Crater on the Yucatán Peninsula. Approximately 50% of all genera became extinct, including all of the non-avian dinosaurs.

The Mesozoic was a time of tectonic, climatic and evolutionary activity. The continents gradually shifted from a state of connectedness into their present configuration. The climate was exceptionally warm throughout the period,

Tectonics

Pangaea separation animation

After the vigorous convergent plate mountain-building of the late Paleozoic, Mesozoic tectonic deformation was comparatively mild. Nevertheless, the era featured the dramatic rifting of the supercontinent Pangea. Pangea 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 United States East Coast) today (Stanley 1999).

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.

Mesozoic climate

The Triassic was generally dry, a trend that began in the late Carboniferous. It was also highly seasonal, especially in the interior of Pangea. Low sea levels may have also exacerbated temperature extremes. Water acts as a temperature-stabilizing heatsink due to its high specific heat capacity, and land areas near large bodies of water, especially the oceans, experience less variation in temperature. Because much of the land that constituted Pangea was distant from the oceans, temperatures fluctuated enormously, and the interior of Pangea probably included expansive areas of desert. Abundant evidence of red beds and evaporites such as salt support these conclusions.

Sea levels began to rise during the Jurassic, probably due to an increase in seafloor spreading. The formation of new crust beneath the surface displaced ocean waters by as much as 200 m more than today, which flooded coastal areas. Furthermore, Pangea 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. Due in part to higher levels of carbon dioxide in the atmosphere, the world temperature gradient from North to South became almost flat: temperatures were about the same across the planet. Average temperatures were also higher than today, 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. For this reason, large volumes of organic matter accumulated, because they were unable to decompose and were eventually 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.

Mesozoic life

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 and carnivorous dinocephalia (a group of early, mammal-like reptiles) left those ecological niches empty. Some were filled by the suriving cynodonts and dicynodonts, the latter of which subsequently became extinct. Animal life during the Mesozoic was dominated, however, by large archosaurian reptiles that appeared a few million years after the Permian extinction: 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 placental 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 KT 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 the ancestor mammals to primates, evolved. The K-T extinction exacerbated this trend. The large archosaurs (dinosaurs, etc.) became extinct, while birds and mammals thrived, as they do today.

References

ISBN links support NWE through referral fees

• Buchdahl, J. 1999. Global Climate Change Student Information Guide: Mesozoic Climates. Manchester, United Kingdom: Atmosphere, Climate & Environment Information Programme.
• Doyle, J. A., and M. J. Donoghue. 1986. Seed plant phylogeny and the origin of angiosperms: An experimental cladistic approach. Botanical Review 52:321-431.
• Farabee, M. J. 2001. The Mesozoic, Age Of Cycads And Dinosaurs.
• Natural History Museum. 1983. British Mesozoic Fossils. The Natural History Museum, London.
• Palaeos. 2004. The Mesozoic.
• Stanley, S. M. 1999. Earth System History. New York: W.H. Freeman and Company. ISBN 0-7167-2882-6

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