The Green River Formation represents one of the largest documented accumulations of sedimentary rock in the world and dates back 50 million years. It covers an area of more than 25,000 square miles and averages about 2000 feet in thickness. These lakes were formed by geologic events that uplifted the Rocky Mountains in early times. The climate was much different than the desert like climate of today. It was similar (subtropical) to the present climate of the Gulf Coast and southern Atlantic areas of the US.

TThe genus Phareodus is common in the Green River formation and is readily distinguished by the long pectoral fin (here prominantly displayed) and large pointed teeth. There are two spcies: encaustus and testis, of which encautus is the larger by nearly a factor of two. Other differences are to be found in the body outline, fine ray count, and scale row number. Phareodus sports teeth that are testament to the fish’s carnivorous habit. In fact, the genus name means "to have tooth", with a higher tooth count also found in P. encaustus.