Fossil Vertebrate of the Month: Diceratherium

Diceratherium
John Day Fossil Beds National Monument

This is the last FVOTM I'll be publishing in Oregon, so I thought I'd focus it on the most impressive fossil vertebrate I've found during my time in that state (I would have put the spotlight on Metasequoia, the state fossil, but as a plant it falls outside the scope of a feature on fossil vertebrates).  In the summer of 2010, I was TA-ing the UO Geology Department's field camp in eastern Oregon.  We were prospecting for fossils in the gullies of the Turtle Cove Member of the John Day Formation when I almost literally stumbled across what turned out to be a tibia of the rhinoceros Diceratherium.  One of the things I've discovered during my time here is that I have a terrible eye for fossils in the field, which made finding a rhino leg fairly exciting for me.  However, this excitement was tempered by the fact that Diceratherium was an extremely common member of the John Day ecosystem.  This may come as something of a surprise to many people, as rhinos are, of course, not members of the North American megafauna today.  However, the oldest fossil rhinos are from the Eocene of North America.  The two-horned Diceratherium first appeared in the Oligocene, a period of time during which rhinos had begun to spread across the world.  Diceratherium was an especially successful disperser, having spread into Asia and Europe by the Miocene.  By the Pliocene, rhinos had disappeared from North America, and they survive today only in the Old World.  There are a number of unusual features of rhinos, including their horns and broad teeth, but perhaps one of the strangest things about them is a product of their evolutionary history.  The earliest rhinos were small, running animals; enormous size did not evolve until later in the group's history, but even the very large rhinos of today still run frequently (charging being one of their main forms of defense and intimidation).  The stress this puts on their legs is enormous, and as a result most adult rhinos have arthritis.  Research by former UO undergrad Kelsey Stilson has shown that this trend extends far back into rhino evolution and would have occurred even in mid-sized members of the group such as Diceratherium.