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The Mesoic was a time of Tectonic , Climatic , and Evolution ary activity. The continents gradually shifted from a state of connectedness into their present configuration; this rifting provided for Speciation and other important evolutionary developments. The climate was exceptionally warm throughout the period and also played an important role in the evolution and diversification of new Animal species. By the end of the era, the basis of modern life was in place. GEOLOGIC PERIODS Following the ) to when the Cenozoic era began 65 Ma. This time frame is separated into three geologic periods. From oldest to youngest:
The lower (Triassic) boundary 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 (Cretaceous) boundary 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. TECTONICS In general, the Mesozoic era was one of hightened tectonic activity. It began with all of the world's continents collected into a Supercontinent called Pangea . Pangea gradually split into a northern continent, Laurasia , and a southern continent, Gondwana . By the end of the era, these continents had rifted further into 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 Asia to form the Himalaya s. MESOZOIC CLIMATE The Triassic was generally dry, a trend that began in the late Carboniferous , and highly seasonal, especially in the interor 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 Ocean s, experience less variation in temperature. Because much of the land that constituted Pangea was distant from the oceans, temperatures fluctated 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 . 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 Cap s and Glacier s, but there is no evidence of either. Quantitive 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 s, Pterosaur s, and aquatic reptiles such as Ichthyosaur s, Plesiosaur s, and Mosasaur s. 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 Bird s and Placental Mammal s 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 Fern s until after the KT extinction. Some have argued that Insect s diversied 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 Lizard s, Snake s, and perhaps the ancestor Mammal s to Primate s, evolved. The KT extinction exacerbated this trend. The large archosaurs became extinct, while birds and mammals thrived, as they do today. REFERENCES AND FURTHER READING
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