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.

[citation needed] 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.

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).

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.

[1][2] 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.