An order of very specialized deep-bodied fishes near the holostean level of organization that are known only from the fossil record. They were a widespread group that first appeared in the Upper Triassic of Europe with the genus Eomesodon, flourished during the Jurassic and Cretaceous (Gyrodus, Coelodus, Gyronchus, Proscinetes, Mesturus, and others), and persisted to the Upper Eocene (Pycnodus and Palaeobalistum). Pycnodontiforms are a closely interrelated group most commonly found preserved in marine limestone and associated with coraliferous facies.