Lepidosaurs

Lepidosaurs are by far the most diverse of modern reptiles, consisting of lizards, snakes, and spheno-dontids, but their fossil record is poorly documented in the Triassic. There are approximately 6,000 different species of lizards and snakes living today, chiefly in tropical regions. Unlike other reptilian groups, the snakes are still a rapidly evolving assemblage; their earliest records are known from the Cretaceous Period.

The earliest lizard, Paliguana, is known from the Permo-Triassic boundary of South Africa. However, the fossil record of lizards remains blank for most of the Triassic. By the Late Jurassic, modern lizards appeared in many parts of the world, and they continue to be important elements in the world fauna. In the Late Triassic, some lizards, such as kuehneosaurids, developed gliding abilities. They were characterized by the elongation of the ribs to support a membrane that allowed this reptile to sail from tree to tree, much as does the modern lizard, Draco, of the Orient. The "wingspan" was about 12 inches. When the rib wings were folded, they formed a crest over the animal's back.

Sphenodontids, which appeared in the Late Tri-assic, still survive in New Zealand as an endangered species, the Sphenodon or tuatara. The modern form shows very little change from its Triassic ancestor, and is called a living fossil.

PRIMITIVE ARCHOSAUROMORPHS

Some Triassic reptiles, such as protorosaurs and rhyn-chosaurs, were traditionally grouped with lepido-saurs. Recent work shows that these reptiles are more closely related to archosaurs and may be included in a larger assemblage called the archosauromorphs.

The protorosaurs appeared in the Late Permian, and were very successful during the Triassic Period, being recorded from all continents except South America. They were small, gracile, long-necked animals, similar to lizards in size, proportions, and inferred activities. They show a skull configuration similar to lizards. The similarity of protorosaurs to lizards is superficial, and is an example of convergent evolution. Some of the protorosaurs adapted to marine life. Others became bipeds like the modern basilisk lizard, and could run effectively.

Rhynchosaurs were widely distributed reptiles during the Triassic, and have been recorded from Europe, North and South America, India, and East Africa. They were small quadrupeds in the Early Triassic, but became progressively bigger and more heavily built in the Middle and Late Triassic. They were then about the size of a large pig, with sprawling limbs and large claws, while the skull was short and broad with a pair of bony beaks in front. Rhyn-chosaurs have highly specialized dentition with multiple tooth rows in the upper jaws, separated by a central groove. The lower jaw forms a sharp cutting edge that fits into the groove of the upper jaw like a chopper. The diet of rhynchosaurs probably included various fruits, seeds, rhizomes, or even mollusks. The teeth have long roots and were fused to the bone. I have named this unusual tooth implantation in rhyn-chosaurs "ankylothecodont." The most remarkable feature of the teeth is that they were not regularly replaced as in most reptiles, but were continually added backward as the jaws grew.

Another peculiar Late Triassic archosauromorph is the trilophosaur, known mainly from the American Southwest. The skull has a toothless beak, presumably horn-covered, but the cheek teeth are transversely widened and three-cusped, well suited to munching thick leaves, soft stems, and fruits. The teeth suggest an affinity with rhynchosaurs. Unlike other primitive archosauromorphs, trilophosaurs had a solid cheek.

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  • caramella
    What dinosaur has a toothlest beack and large grooved cheek teeth not sharp?
    8 years ago

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