''
skull The earliest forms, like
Placodus, which lived in the early to middle
Triassic, resembled barrel-bodied lizards superficially similar to the
marine iguana of today, but larger. In contrast to the marine iguana, which feeds on
algae, the placodonts ate
molluscs and so their teeth were flat and tough to crush shells. In the earliest periods, their size was probably enough to keep away the top sea predators of the time: the
sharks. However, as time passed, other kinds of carnivorous reptiles began to colonize the seas, such as
ichthyosaurs and
nothosaurs, and later placodonts developed bony plates on their backs to protect their bodies while feeding. By the Late Triassic, these plates had grown so much that placodonts of the time, such as
Henodus and
Placochelys, resembled the
sea turtles of the modern day more than their ancestors without bony plates. Other placodonts, like
Psephoderma, developed plates as well, but in a different articulated manner that resembled the carapace of
horseshoe crabs more than those of sea turtles. All these adaptations can be counted as perfect examples of
convergent evolution, as placodonts were not related to any of these animals. Because of their dense bone and heavy armour plating, these creatures would have been too heavy to float in the ocean and would have used a lot of energy to reach the water surface. For this reason, and because of the type of sediment found accompanying their
fossils, it is suggested that they lived in shallow waters and not in deep oceans. The diet of placodonts consisted of marine
bivalves,
brachiopods, and other hard-shelled invertebrates. They were notable for their large, flat, often protruding
teeth, which they used to crush the molluscs and brachiopods that they hunted on the sea bed (another way in which they were similar to
walruses). The palate teeth were adapted for this
durophagous diet, being extremely thick and large enough to crush thick shell.
Henodus, however, differs from other placodonts in having developed unique
baleen-like denticles, which alongside features of the hyoid and jaw musculature suggest that it was a
filter feeder. Recent comparisons to
Atopodentatus suggest that it was a
herbivore as well, bearing a similar broad jaw shape, albeit it obtained plant matter through filter-feeding it from the substrates. The group was once believed to be restricted to the western
Tethys, but the discovery of
Sinocyamodus xinpuensis in China overturned this view. '' '' '' ==Classification==