In the rocks of the Morrison formation at the quarry, both orders of dinosaurs are found—Saurischia and Ornithischia. Paleontologists have divided the dinosaurs into these two groups on the basis of important skeletal differences. These differences remain constant for the orders and vary within each order only in small details.
The important structural difference in dinosaurs is found in the pelvis. In all land vertebrates, the pelvis is made up of three pairs of bones called the ilium, pubis, and ischium. The paired ilium is joined to each side of the backbone and projects downward to meet the pubis and ischium at the socket for the head of the thigh bone. The pubis forms the front third and the ischium the rear third of this socket. In the order Saurischia the bones of the pelvis are arranged as in most reptiles and mammals. In the order Ornithischia the pubis extends backward along the ischium as it does in the birds.
Two types of saurischian dinosaurs are found in the quarry. Antrodemus, a flesh-eating type, was about the size of a horse, but was two-footed. It had strong sharp claws on its feet. Its teeth were about 2 inches long, flattened from side to side and with fine serrations on front and back edges. Actually it is not known whether Antrodemus overpowered and killed the large swamp-living dinosaurs, or merely fed on their carcasses after they had died from other causes. However, there has been found in the quarries at Como, Wyo., a partial skeleton of Apatosaurus with grooves on the bones which suggest tooth marks. The spacing of these grooves fit the spacing of the teeth of a specimen of Antrodemus found in the same quarry.
A. ORNITHISCHIAN PELVIS. B. SAURISCHIAN PELVIS.
KEY: IL—ILLIUM; IS—ISCHIUM; P—PUBIS.
The plant-eating dinosaurs of the order Saurischia which are found in the quarry were all four-footed. They had bodies about the size of an elephant or larger. The principal differences between the flesh- and plant-eating dinosaurs were the length of the neck and tail, the details of their skull structure, and other parts of their skeleton.
Apatosaurus is perhaps the most familiar dinosaur to most of us. Its hind legs were much longer than its front ones and gave the animal a high-hipped, stooped appearance. Apatosaurus was about 70 feet long and probably weighed close to 40 tons. Diplodocus was longer (one of them 75½ feet) but was slender and lightly built. Its neck was longer and it had a whiplash tail that looked much like the tail of the modern whiptailed lizard. Diplodocus also had long pencil-like teeth different from those of any other known dinosaur. Barosaurus has an extremely long neck with long individual neck bones. Two members of the genus Camarasaurus are similar to each other except for size; one was small, but the other was as large as Apatosaurus. Camarasaurus had longer front legs than Apatosaurus and was generally better proportioned.
Antrodemus, THE FEROCIOUS CARNIVORE OF MORRISON TIME. (FROM A DRAWING BY CHARLES R. KNIGHT. COURTESY, AMERICAN MUSEUM OF NATURAL HISTORY.)
