Mounted specimen at the State Museum of Natural History Stuttgart, Germany
Reconstruction by Dan Varner.
When: Middle Triassic (~240 million years ago)
Where: Europe and China
What: Placodus is a marine reptile. At first glance it does not appear heavily adapted for aquatic life, as neither its tail nor its limbs are elongated and flattened, but specimens are routinely found in marine sediments. In lieu of being a fast swimmer Placodus was very robustly built, wide and flat, with knobby armor along its backbone and a very dense set ventral ribs. It did not need to swim fast, as its prey was hard bodied invertebrates which were mostly sedentary on the ocean floor. It plucked them out of their shallow burrows with its procumbent front teeth. In order to crunch down on the shells of mollusks and other hard-shelled animals, the rest of its teeth were very broad and flat. This is very distinctive among reptiles, so much so that that is what its name means: flat tooth.
Placodus is one of the most terrestrial placodonts, it would have moved awkwardly on land with its wide flat body and wide spread legs, but it is likely it spent a good deal of its time on the shore and only went into the ocean to feed. Later placodonts not only acquired more aquatic adaptations, but also became more and more heavily armored, some of them coming to superficially resemble turtles. At one point it was thought these animals were related to turtles, but current hypotheses have them far removed from turtles, and more closely related to the plesiosaurs, a group of obligatory aquatic reptiles.