Placodontia

(paleontology) A small order of Triassic marine reptiles of the subclass Euryapsida characterized by flat-crowned teeth in both the upper and lower jaws and on the palate.

A small but interesting order of marine reptiles of the subclass Euryapsida that are known only from deposits of Triassic age of Europe and the Near East (Israel). As the name implies, they are reptiles with grossly specialized dentitions—flat-crowned teeth are located in both the upper and lower jaws and on the palate—that probably functioned as crushing devices for hard-shelled prey.

The modification of the dentition had its effect on the entire skull, which became massive in relation to the body; and the coronoid region of the mandible, to which most of the masticatory muscles were attached, became greatly extended upward, giving it a superficial similarity to a mammalian jaw. The post-cranial skeleton shows a number of aquatic adaptations. The thorax was box-shaped and the digits of hand and foot were probably webbed; the joint surfaces of the limb bones suggest that the animals did little if any walking on land. The advanced placodonts resemble sea turtles in overall appearance and in the fact that their body is encased in an armor of dermal bones similar to that of the dermochelyid sea turtle Psephophorus. The more generalized genera lack such an armor and possess a long tail. See also Reptilia.