Placodont

Placodonts were a group of Triassic marine reptiles which had heavy flat teeth. These teeth were used for crushing shellfish.Benton M.J. 1990. The reign of the reptiles. Crescent, New York. 118–122 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.Benton M.J. 2015. Vertebrate palaeontology, 4th ed p150, 152/3.

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.

"Spade-like incisors at the front, for snipping and scraping oysters from the rocks; and broad circular back teeth that acted as a pounding board to crush the shells, before the flesh was swallowed". Benton.p119

Their diet consisted of marine bivalves, brachiopods, and other invertebrates.

 References 

Category:Sauropterygia
Category:Marine reptiles