MESOZOIC ERA, in geology, the mid period of life in geologic time, also called the Secondary Period. Paleozoic precedes and Cen ozoic follows this era, these two terms refer ring respectively to "older" and "newer," or "earlier" and "later," forms of prehistoric life. The subdivisions of the Mesozoic are the Trias sic, Jurassic and Cretaceous periods (qq.v.).
The Life of the life history of the era can be best understood, by first re ferring to the life of the Paleozoic (q.v.). That era was closed under conditions (aridity, gla ciation and mountain making) unfavorable to plant and animal life, and as a result many unfit genera failed to live from Paleozoic on into Mesozoic. The long-hinged brachiopods, so characteristic of the earlier era, became ,ex tinct early in the latter. A few of the more primitive types persisted, but brachiopods be came relatively unimportant. The old straight cephalopods practically became extinct with the passing of the Paleozoic. The nautilus type lived on through, even to the present time, but the era was characterized particularly by the dominance of a type called the ammonites, which appeared in late Paleozoic and died out at the close of the Cretaceous. They evolved rapidly and are valuable time markers. The Mesozoic has often been called the age of rep tiles. This group of vertebrates, which ap peared toward the close of the Paleozoic, in primitive form, developed rapidly into a num ber of highly specialized types. The Dinosaurs (terrible reptiles) developed along several lines. The carnivorous group walked on two legs, the two front limbs being very short. It was this reptile that made the famous three-toed tracks of the Connecticut Valley. Some of these Dinosaurs got to he as long as 40 feet. Her bivorous types also developed, great awkward beasts, some of them 80 feet long, 16 feet high and weighing probably 30 tons or more. This type were the largest land animals that ever lived. Late in the era peculiar armored Dino saurs also appeared, with great horns on their heads and spikes on their tails. The Ichthyo
saurs (fish reptiles) took to the sea and de veloped fins, much after the manner of modern fish. The Pterosaur (wing reptiles) developed the power of flight, some of them having a wing spread of 20 feet. These unusual reptiles all died off suddenly at the close of the Cre taceous, and in the following era reptiles took on a modern aspect. Peculiar birds with teeth, the first known representatives of the feathrse. world, are found fossil in Jurassic and CI taceous rocks. A few primitive mamrii mostly of very small size, are known in ra Mesozoic, but the group did not develop ripe:i until after the close of this era. Among Fix the common groups of flowering plane did L' appear until Cretaceous times.
Paleogeography of the Mesozoic in Unite: States.-- During Triassic and Jurassic practically all of the United States east of Mississippi was land. During Cretaceous It• sea encroached on the coastal plain but did reach the eastern interior region. The his: 7 of the west was more complex. During :E. Triassic a considerable part of the Pacific region was under water but most of the it tenor was land. Similar conditions prevn-', during early Jurassic, but late in the
great arm of the sea encroached from ±, Arctic, reaching as far south as northern Cc; rado. The great differences in fossils