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PROSAUROPODS

prosauropod

The relationship of prosauropods to sauropods is not yet clear. It is possible that the smallest and earliest prosauropods were ancestors of both the prosauropods and sauropods. Of the larger prosauropods, at least some seem to be a sister group of the sauropods rather than ancestors. Some of the prosauropods, such as Riojasaurus, seem to be closer to the sauropod lineages than others. Although the existing evidence suggests that the known prosauropods were a separate clade related to sauropods rather than their ancestors, the evidence only weakly supports this conclusion (Benton, 2000).The mid-Triassic dinosaur Spondylosoma is known from South America and may have been related to prosauropods (Czerkas, 1990).

The worldwide spread of prosauropods was the first radiation of herbivorous dinosaurs. There is some debate over what exactly composed their diet since their jaws not well adapted to herbivory. There was less room jaw adductor muscles and no transverse movement of jaw was possible. Their teeth don’t contact each other except in Yunanosaurus. The only anatomical modification for eating plants was their increased size which gave them more room for intestines. (Weishampel, p. 342)

Thecodontosaurus antiquus was the most primitive prosauropod and possessed a small, gracile body. It was the 6^th dinosaur named and the first dinosaur known from Triassic. It was first recognized as a dinosaur by Huxley. Its vertebral centrum was virtually hollow. There were 2 small phalanges on fourth finger and no known phalanges on its fifth, giving it a phalangeal formula of 1-2-3-1-0. Thecodontosaurus possessed the most slender prosauropod hand known. Later prosauropods possessed more robust hands as the hands began to bear more of the body’s weight (Benton, 2000).

The prosauropods possessed leaf-shaped, serrated teeth like the earliest ornithopods (Lesothosaurus) and like the front teeth of the earliest known theropod, Eoraptor. Azendohsaurus was a prosauropod related in the same family with Thecodontosaurus and one of its teeth closely resembles teeth of the basal ornithopod Lesothosaurus. Over time, the teeth of prosauropods became larger and more concentrated in the front of the mouth. By the evolution of the melanosaurs, their teeth had begun to resemble those of sauropods (Czerkas, 1990).

Prosauropods are known from all continents and include some of the earliest known dinosaurs. They range in size from 2-10 meters. Two prosauropod fossils from

Madagascar

are among the earliest known fossil dinosaurs (Flynn, 1999). They possessed large thumb claws for defense. Prosauropods became extinct at the end of the Early Jurassic, at the same time the herbivorous sauropods and ornithischians are becoming common. This was the first significant extinction of a group of dinosaurs (Lambert, 1990, Fastovsky, 1996).

Species of the Anchisauridae could use bipedal and quadrupedal locomotion and could rear on their hind legs to reach the upper branches of trees. They possessed five digits on their hands and feet. They lived from the Mid Triassic to Early Jurassic and ranged in size from 2-3 meters. In Anchisaurus, “near lizard”, 2 fingers are small and vestigial. Anchisaurus was the first American dinosaur found in 1818, at first its remains were thought to be human. Ammosaurus was once thought a coelurosauran theropod (Galton, 1971).

Species of the family Plateosauridae were similar to anchisaurids but were larger, had broader feet and hands, a stronger skull, and a stronger bite because of a modified jaw hinge. In Plateosaurus, the nasal bones were more than half the length of the skull and the eyes were surrounded by a sclerotic ring of bones (Weishampel). They lived from the Late Triassic to the Early Jurassic and ranged in size from 1.5 to 8 meters. Plateosaurus, “flat lizard”, had forelimbs that were 2/3 the length of the hindlimb and thus were better adapted to being quadrupedal. Its 5^th toe a simply a vestigial splint and two of its five fingers were very small. Originally they were thought to be carnivorous because of theropod teeth found in a fossil carcass. The femur may still have been angled a bit in a stance that was slightly sprawling like their archosaur ancestors (Lambert, 1990, Fastovsky, 1996).

Riojasaurus possessed reduced digits 4 and 5 digits in its hand (Bonaparte, 1996; Bonaparte, 1995).

The family Melanosauridae represent the largest prosauropods and could reach 6-12 meters in length. They were once thought to be the ancestors of sauropods but ankle differences makes this unlikely (Lambert, 1990).