Placodonts were a group of Marine Reptiles from the Triassic. Unlike their neighbours, they didn't feed on fish or cephalopods, but on shellfish. Consequently, they avoided competition with such reptiles like Cymbospondylus or Nothosaurus, but it also made them relatively poor swimmers, and so they usually swam close to the shore, in shallower waters, where they avoided predators.
There were two main groups of placodonts - those like Placodus, who looked like giant lizards or newts (but weren't related to either), and those like Henodus, who actually looked like turtles instead. (Placodonts are not closely related to turtles either.) Regardless of differences in appearance, all placodonts died out by Jurassic.
In Walking with... series[]
Sea Monsters: Prehistoric Predators of the Deep featured placodonts as background charactes.