Pycnodontiformes, order of extinct fishes of the class Actinopterygii, containing the genus Pycnodus, common in the Jurassic seas of 200 million to 146 million years ago. Pycnodus is typical of pycnodonts, which were characterized by deep, narrow bodies that were very circular in outline in side view. The pycnodonts had a downturned beak and small mouth with an abundance of bulblike, rounded teeth with thick enamel surfaces; these structures enabled pycnodonts to crush their prey, such as the shelled invertebrates of coral reefs.