Other Triassic Diapsids

While the crurotarsans and ornithodirans had great success as part of the radiation of ruling archosaurian reptiles, several other kinds of diapsid reptiles became widespread during the Triassic Period.

A group of reptiles known as rhynchosaurs was one of the most common herbivore groups of the Late Triassic, particularly in South America and Africa. Neither lepidosauromorph nor mammal-l ike reptiles, the rhynchosaurs were distant kin of the archosaurs. The rhynchosaurs evolved independently of the true "ruling reptiles," however. Hyperodapedon was a small to medium-sized rhynchosaur that thrived on the tough vegetation of the Mesozoic woodlands. It walked on four squat legs and had a beaklike cutting surface as part of its upper front jaw. When viewed from above, the head of Hypero-dapedon was triangular and showed the ample space devoted to batteries of grinding teeth in its expanded cheek regions. The success of this 4-foot (1.3-m) plant eater was due to its teeth and powerful beak and jaws, which could easily grind the tough, stringy vegetation of the time, including seed ferns. Fossils of Hyperodapedon are found in many widespread sites, including Brazil, Argentina, India, Madagascar, and Scotland. Unfortunately for the rhynchosaurs, however, climate changes by the end of the Triassic greatly diminished the abundance of seed ferns, and it appears that this line of diapsids became extinct because the animals were so specially adapted for



Hyperodapedon fossil

this kind of food. During their time, however, rhynchosaurs such as Hyperodapedon were among the most populous vertebrates found in their habitats.

Another form of medium-sized herbivore of the Middle to Late Triassic was the trilophosaurids. The best known member of this taxon is Trilophosaurus, from the Late Triassic of Texas. Trilophosau-rus had a lifestyle similar to that of the rhynchosaurs. Trilophosaurus measured about 6.5 feet (2 m) long and had a tall skull with small, rounded teeth lining the jaw in the cheek area; it lacked teeth in the front of the jaw, however. As in Hyperodapedon, the sturdy skull and jaw of Trilophosaurus were adapted to cut and chew the tough kinds of vegetation that were common in the animal's habitat. Curiously, Trilophosaurus did not have a lower temporal opening in its skull like other diapsids; this condition is believed to have evolved secondarily from an earlier skull structure that included the opening. Trilophosaurus had a sprawling posture, long digits reminiscent of modern reptiles, and a long, bony tail with stout chevrons along its caudal vertebrae.

One additional line of diapsids were the protorosaurs, a group with roots in the Permian that persisted into the Triassic. With long necks and lizardlike bodies, the group is best represented by the strange Tanystropheus, from the Middle Triassic of Israel, Italy, Germany, and Switzerland. Tanystropheus had a short body and a neck so long that it seems impractical. The animal was about ten feet (3 m) long, with six feet (2 m) of that length consisting of a long, stiff neck. The neck of Tanystropheus itself is a puzzle, being composed of only 10 elongate vertebrae and accounting for the meaning of its name: "long vertebrae." Because Tanystropheus was equipped with small, carnivorous teeth, many paleontologists assume that it lived in a near-shore environment, where it probably could take to the water to support its gawky body. One scenario pictures Tanystropheus perched on a rock, dipping into the water below to catch fish and other small sea creatures. In fact, possible stomach contents discovered near the belly region of some specimens of Tanystropheus include fish scales and pieces of invertebrates. This suggests an aquatic or near-shore lifestyle. The feet of Tanystropheus were not webbed like those of most aquatic reptiles, however—a fact that adds to the puzzle of which environment is most appropriate for this enigmatic creature.

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