Pycnodontiforms, order of extinct fishes of the class Actinopterygii, containing the genus Pycnodus, were common in the Jurassic seas of 200 million to 146 million years ago. They were small to middle sized fish, with laterally compressed bodies, and an almost circular outline. Pycnodontiform fishes lived mostly in shallow-water seas. They had special jaws with round and flattened teeth, well adapted to crush food items. Some species lived in rivers and possibly fed on mollusks and crustaceans.