Placodont

Placodonts were a group of Triassic marine reptiles which had heavy flat teeth. These teeth were used for crushing shellfish. Placodonts are in the Order Sauropterygia, the group that includes Plesiosaurs. Placodonts were mostly between in length, up to a maximum long. They are found in marine sediments of the Middle and Upper Triassic, 235–250 million years ago.
The first specimen was discovered in 1830, and the group was very widely distributed. Shellfish were common throughout the Mesozoic period, but this group of shellfish-eaters went extinct at the end of the Triassic. They were clearly a victim of some event which disturbed their lifestyle. The end-Triassic had several extinction events which are not well understood.
Palaeobiology.
The earliest forms like Placodus, in the early to middle Triassic, looked rather barrel-shaped. Later placodonts had bony plates on their backs which protected their bodies while they were feeding.
By the Upper Triassic, these plates had grown so that some placodonts of the time looked like the sea turtles of modern day. Other placodonts developed plates that resembled the shells of horseshoe crabs and trilobites rather than those of sea turtles. These adaptations are examples of convergent evolution. 
Because of their dense bone and heavy armour plating, these creatures would have been too heavy to float in the ocean. They would have used a lot of energy to reach the water surface. For this reason, and because of the type of sediment found with the fossils, it is suggested placodonts lived in shallow waters and not in deep oceans.
Feeding.
The placodonts ate shellfish. Their front teeth stuck out: they would be used to dig up the shellfish, and pull them from any attachment to the ground. Inside their mouth, the teeth were large and flat: they were used to crush the shells.
Their diet consisted of marine bivalves, brachiopods, and other invertebrates.