Mesozoic Era

The Mesozoic era is one of three geologic eras of the Phanerozoic eon. Lying between the Paleozoic and the Cenozoic, these three eras comprise the Phanerozoic eon. 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, also playing 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. The Mesozoic era extended roughly 180 million years, from 251 million years ago (mya) to when the Cenozoic era began 65 mya. This era is further separated into three geologic periods. From oldest to youngest: Triassic (251 to 199 mya), Jurassic (199 to 145 mya), Cretaceous (145 to 65 mya).

The lower (Triassic) boundary is set by the Permian-Triassic extinction, during which approximately 90–96 percent of marine species and 70 percent 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 percent of all genera became extinct, including all of the nonavian dinosaurs.

Dramatic Rifting

After the vigorous convergent plate mountainbuilding of the late Paleozoic, Mesozoic tectonic deformation was comparatively mild. Nevertheless, the era featured the dramatic rifting of the supercontinent, Pangaea. Pangaea gradually split into a northern continent, Laurasia, and a southern continent, Gondwana. 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. 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 also have exacerbated temperature extremes. 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 evidence of red beds and evaporites, such as salt, support these conclusions.

Sea levels began to rise during the Jurassic, probably caused by an increase in seafloor spreading. The formation of new crust beneath the surface displaced ocean waters by as much as 656 ft. (200 m) 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.

Widely Disputed

The climate of the Cretaceous is less certain and more widely disputed. Average temperatures were higher than today by about 18 degrees F (10 degrees C). In fact, by the middle Cretaceous, equatorial ocean waters 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. Large volumes of organic matter accumulated because they were unable to decompose and were eventually deposited as black shale.

The extinction of nearly all animal species by the end of the Permian period allowed for the radiation of many new life forms. Large archosaurian reptiles that appeared a few million years after the Permian extinction dominated animal life during the Mesozoic era. 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 period. 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 large archosaurs became extinct, while birds and mammals thrived.

During the early Mesozoic era, researchers do not conclusively predict high-latitude ice free environments during the Cretaceous period. Various explanations have been proposed for the discrepancy, and subsequently incorporated into further modelling studies. Two of these include ocean circulation changes and the role of CO2. Only an elevated atmospheric CO2 concentration could come close to reconciling the models with geological evidence. High levels of CO2 seem reasonable, considering the high global sea level and ensuing break-up of Pangea. In addition to increased outgasing of CO2, the reduced continental area would result in a decreased rate of weathering of silicates and removal of CO2 from the atmosphere.

Unfortunately, there is little reliable evidence to support the CO2 model. M. A. Kominz and colleagues have estimated that average rates of the velocities of major tectonic plates were higher in the late Cretaceous and ocean ridge volumes were greater. In addition, Cretaceous sea-beds were dominated by calcite minerals, implying higher aqueous, and consequently atmospheric, CO2 concentrations.