POSITION OF DINOSAURS AMONG REPTILES
It seems there are several orders of reptiles similar to and closely related to the dinosaurs. Remains of these reptiles are found in the sedimentary rocks which contain the earliest known dinosaurs. A number of them resembled the dinosaurs but do not quite meet the requirements as far as details of the skeleton are concerned. In the scheme of classification these orders of reptiles are grouped together into the subclass Archosauria. This subclass includes the dinosaurs, crocodiles, and the flying reptiles. The lizards, snakes, turtles, and the tuatera of New Zealand belong to other subclasses of reptiles which have been distinct from that of the dinosaurs as far back in geologic time as we can trace them. The kinship between the dinosaurs and the small lizards living in the monument today lies only in that both are reptiles. The only living relatives of the dinosaurs are the alligator and the crocodile.
The dinosaurs were so numerous, and so dominated the whole of the Mesozoic Era, that this period of earth history is frequently referred to as the Age of Reptiles.
GEOLOGIC HISTORY
The Mesozoic Era began some 200 million years ago and ended some 60 million years ago. Although many other animals lived during that era, the dinosaurs were the dominant forms of animal life on land. The 140 million years of the Mesozoic are divided into geologic periods named Triassic (the oldest), Jurassic, and Cretaceous (the most recent). Continental deposits representing each of these periods have been found in all parts of the world and on all continents. Dinosaur bones have been found in these deposits—even in such far-away places as Australia and the southern tip of South America. Only Jurassic dinosaurs have been found at Dinosaur National Monument.
Coelophysis—SMALL TRIASSIC DINOSAURS, FORERUNNERS OF THE HUGE DINOSAURS OF JURASSIC PERIOD. (DRAWN BY MARGARET M. COLBERT. COURTESY, AMERICAN MUSEUM OF NATURAL HISTORY.)
The oldest known dinosaurs are found in rocks of the Triassic Period. The smaller of these were chicken-size and the largest were about as big as kangaroos. All of these Triassic dinosaurs were two-footed. They can be divided into flesh-eaters and plant-eaters, although none are believed to have been particularly specialized in their food habits. In general the flesh-eaters were small, agile, and had sharp teeth for seizing and overpowering active prey. The plant-eaters were larger with rather long front legs and small blunt teeth suited only to cropping vegetation. These plant-eaters are believed to be the Triassic ancestors of the giant marsh-dwelling dinosaurs of the Jurassic and Cretaceous Periods.
A greater variety of dinosaurs lived during the Jurassic Period than in the Triassic. Both two- and four-footed types were present. The flesh-eaters remained two-footed but increased in size. Antrodemus, perhaps the best known, was much bigger than a kangaroo. The larger plant-eaters weighed from 30 to 40 tons and all were four-footed. The largest land animals, they lived on dry land and in the swamps that formed an important part of the Jurassic landscape. The first of the armored plant-eating dinosaurs, Stegosaurus, inhabited the dry plains. There were also some smaller, kangaroo-size plant-eaters that were two-footed.
A wide variety of dinosaur fossils has been found in the rocks of the Cretaceous Period, the last of the Mesozoic Era. The huge swamp dwellers still thrived. The flesh-eaters had evolved much larger types and included 40-foot Tyrannosaurus, the largest that ever lived. All the flesh-eaters walked on their hind legs as did their predecessors of the Jurassic and Triassic Periods.