MESOZOIC ERA

Table 4: Geologic time scale. To see more information about a period, select one from the chart. second of the Earth's three major geologic eras of Phanerozoic time. It began about 245 million years ago and ended 66.4 million years ago (see Table). The major divisions of the era, from oldest to youngest, are the Triassic, Jurassic, and Cretaceous periods. The Mesozoic was a time of heightened tectonic activity during which the supercontinent of Pangaea fragmented into separate continents that were gradually scattered across the Earth in a nearly modern geographic distribution. It also was a time marked by a distinct modernization of life-forms; the ancestors of the major plant and animal groups that exist today first made their appearance. The name Mesozoic is from the Greek for middle life. the second of the Earth's three major geologic eras of Phanerozoic time and the interval during which the continental landmasses as known today were separated from the supercontinents Laurasia and Gondwana by continental drift. The Mesozoic Era saw the evolution of widely diversified and advanced flora and fauna, quite different from those that had developed earlier during the Paleozoic Era and would develop later during the Cenozoic Era. It lasted from about 245 to 66.4 million years ago and included the Triassic Period, the Jurassic Period, and the Cretaceous Period (qq.v.). By mid-Mesozoic time, Laurasia, which included most of North America and Eurasia, was completely separated from Gondwana in the Southern Hemisphere by the Tethys Sea. Mesozoic rocks became widespread on every continent, and marine deposits were laid down by marine transgressions, which resulted in shallow epicontinental seas during periods of high sea level. Marine deposits were also laid down in geosynclines (large linear troughs) along the coasts of eastern North America and the Gulf of Mexico, on the margins of Eurasia and Gondwana, and around the rim of the present-day Pacific Ocean basin. Reptiles were the dominant terrestrial form of life during the Mesozoic Era. In the Triassic Period there were the labyrinthodonts, large, clumsy amphibians living mostly in water, but they died out suddenly, leaving the toads and frogs, which eventually flourished. The cotylosaurs, or stem reptiles, which had originated in the Late Carboniferous Epoch (320 to 286 million years ago), differentiated into the major reptile lines, leading to turtles, the major marine groups, the mammallike therapsid forms, and the thecodonts. It was from the thecodonts that the best-known reptiles of the Mesozoic Era evolvednamely, the dinosaurs. During the Jurassic Period, the dinosaurs developed into huge and impressive forms. These included the herbivorous Apatosaurus and Diplodocus, the armoured Stegosaurus, and the carnivorous, bipedal Allosaurus. Cretaceous dinosaurs were even more abundant and diversified than those of the Jurassic. They included large herbivores like Iguanodon, the armoured Ankylosaurus, and the horned Triceratops, together with the duckbill hadrosaurs and small, lightly built, bipedal forms such as Ornithomimus. The best-known carnivore of the Cretaceous was the formidable Tyrannosaurus. The dinosaurs flourished throughout the Cretaceous Period until, in one of the great mysteries of paleontology, they suddenly became extinct at the end of the period. The pterosaur, a creature with batlike wings, was the forerunner of the true fliers, the birds, and was one of the most unusually adapted reptile forms. Aerodynamically inefficient with their large, spreading, membranous wings, pterosaurs were little more than gliders that launched themselves from high places. That birds did, in fact, evolve from reptile forms is evidenced by one of the most spectacular fossils ever found, that of the Archaeopteryx, the earliest known bird, which clearly possesses not only reptilian features but also, in extraordinary definition, the feathers that birds developed from reptilian scales. The aquatic reptiles of the Mesozoic Era were widely diversified and thoroughly adapted to their marine environment. Among the most notable large-sized forms were the nothosaurs, placodonts, ichthyosaurs, plesiosaurs, and mosasaurs. Some forerunners of modern fish evolved during the Mesozoic. The most primitive form of the bony fishes was the Chondrosteiformes, which became extinct. They are represented in modern times by the sturgeon. The holost fishes replaced the chondrosts, but they, too, declined in numbers, and only a few modern forms, such as the Mississippi garpike, survive them. The teleosts, or bony fishes, were the most successful marine vertebrates; they became dominant in the Late Cretaceous Epoch and are well represented today. Probably the best-known survivors are the modern shark varieties that emerged during the Mesozoic. Some of the forms are little changed from their Jurassic and Cretaceous ancestors. Among marine invertebrates the most important were the ammonoids, Mesozoic relatives of the pearly nautilus, which flourished in great diversity. Their straight or coiled, many-chambered forms are a valuable index to Mesozoic rocks, since they often serve to identify widely separated but contemporaneous stratigraphic forms. The ammonoids were almost as much a Mesozoic phenomenon as the dinosaurs, and they, too, died out suddenly at the end of the Cretaceous Period. Additional reading For a survey of the era, see especially Carl K. Seyfert and Leslie A. Sirkin, Earth History and Plate Tectonics: An Introduction to Historical Geology, 2nd ed. (1979), which develops important aspects of Mesozoic plate divergence, mountain building, geosynclines, and life-forms and extinction events. John D. Cooper, Richard H. Miller, and Jacqueline Patterson, A Trip Through Time: Principles of Historical Geology, 2nd ed. (1990), incorporates a narrative review of Earth history by eras. Les Sirkin