We do not know the ancestors or the origin of the placodonts. The latest understanding is that they may be diapsids (a group of reptiles with two pairs of holes in the skull behind the eyes) and may be related to the ancestors of the plesiosaurs, although their limbs were not so highly adapted for swimming.
Helveticosaurus from the Anisian stage of the Triassic in Switzerland was once thought to have been a primitive, possibly ancestral, placodont. It resembles Placodus in appearance. However it now seems to be totally unrelated.
Psephoderma from several localities in Europe resembled Cyamodus with its two-part carapace. Its shell has three distinct ridges running longitudinally along it. Animatronic Dinosaur
Placochelys from the Landinian stage of the Triassic in Hungary has jaws that were pointed at the front, and no front teeth. It is possible that it possessed a horny beak to peck the shellfish from the reef.Protenodontosaurus appears to be intermediate between Cyamodus and the more advanced turtle-like placochelyids.