The Mesozoic Era, covering an interval of Earth history from about 230 million to 65 million years ago, comprises three geologic time periods: Triassic, Jurassic, and Cretaceous. The term, meaning "middle life," was introduced in 1841 by the English geologist John Phillips. In most places the rock layers deposited during the three eras are separated from one another by unconformities, breaks in the sequence of deposition of the geologic record.


Paleogeography and Tectonics
The Mesozoic was a time of transition in the history of life and in the evolution of the Earth. By the close of the Paleozoic Era, geosynclines were confined to the Tethys Seas (modern Mediterranean Sea and Middle East) and circum-Pacific region, the others having undergone the final phases of mountain-building orogenies that transformed them into ranges. According to the theory of plate tectonics, the supercontinent of Pangaea, created by the merging of the ancestral continents during the Paleozoic Era, was slowly torn apart during the Mesozoic. In the Jurassic, shallow seas spread northward out of the Tethys and southward from the Arctic onto western Europe. This also happened in North America, where an Arctic sea spread over the present-day Rocky Mountains south to Utah; the area adjacent to the Gulf of Mexico and Atlantic coastal plain was also inundated. These seas retreated by the end of the Jurassic but returned in the Early Cretaceous to roughly their former extent: one reached from the Gulf of Mexico to the Arctic, another covered much of western Europe, and a broad channel spread across the Sahara, joining the Gulf of Guinea and the Tethys. At the close of the Cretaceous Period the seas retreated from the continents. Deformation of the Earth's crust, minor during the Triassic, intensified in the Jurassic and reached a peak in the Cretaceous, during which time our present Alpine-Himalayan chain, Rocky Mountains, and Andes began forming.

Carbonates are the major sedimentary rock of the Mesozoic in the Tethyan belt; outside this region, detrital rocks predominate. Desert deposits and red bed facies are characteristic of the Triassic, as is chalk of the Cretaceous.

Marine invertebrate life during the Mesozoic was dominated by the Mollusca, of which the ammonites were the most important. Other important invertebrate groups were the bivalves, brachiopods, crinoids, and corals.

The vertebrates of this era were dominated by reptiles, especially the dinosaurs. All the major groups were established by the Triassic, and this period marked the spread of these reptiles into almost every major habitat. The dinosaurs underwent extinction at the end of the Cretaceous. Mammals evolved from therapsid reptiles during the Triassic, and the first birds appeared during the Jurassic.

Mesozoic plants consisted of the ferns and the gymnosperm orders of cycads, ginkgos, and conifers. Angiosperms, which may have first appeared in the Triassic Period, became well established by the end of the Mesozoic.

Bibliography: Hsu, K., ed., Mesozoic and Cenozoic Oceans (1986); Long, J., Dinosaurs of Australia and New Zealand and Other Animals of the Mesozoic Era (1998); Wicander, R., and Monroe, J., Historical Geology, 2d ed. (1993).