There is a relative abundance of grazing species as opposed to browsers. Among the horse group there was one genus, Pliohippus, that was near the lineage of the modern horse. Other grazers were pronghorns, deer, rabbits, squirrels, and camels. Also, a fossil bear found in northern India has been shown to be virtually identical with Rattlesnake Formation specimens, tending to reinforce evidence from the Mascall Formation that an Asian land bridge did exist. Fossil beds that span even 5 million years are rare, yet in this valley the fossil record shows more than 40 million years of the diverse plant and animal life that existed here 54 million to 6 million years ago.
Fossils are common components of the earth's crust.
Typically they are tantalizingly incomplete, or may occasionally represent only a momentary environmental catastrophe. In only a few areas on the planet have earthly processes, events, and chance combined to produce such a generous array of information as is found in the John Day Fossil Basin. The fossils found here are generally very well preserved specimens. The Clarno Nutbeds yield fossilized material with cellular structure clearly visible. The John Day vertebrates are among the best preserved of their type.