Your Daily Fossil

Hyopsodus

Reconstruction by M. R. Long 

When: Eocene (~55 to 35 million years ago)

Where: North America

What: Hyopsodus is a small herbivorous animal common in the fossil deposits of the American west. This animal was about a foot and a half (~45 cm) long at most, with a very long body with relatively short legs. It would have appeared rather ‘weasel like’, but it has no close relation to modern weasels. It is a condylarth - which means its in a giant waste basket group that nobody believes is real. You find this a lot with Paleocene and Eocene mammals. More and more animals formerly grouped under Condylarthra (for lack of any other place to put them) are being referred to as Archaic Ungulates of Uncertain Affinities. Modern ungulates are the hoofed animals (elephants, sheep, horses, cows, etc) and do not form a monophyletic group either, they just all have hooves and thus walk on the very tips of their digits. Hyopsodus itself has been variably placed with respect to living ungulates; sometimes it is found nearer to perissodactyls (horses, rhinos, and tapirs) and artiodactyls (hippos, cows, sheep, camels, etc) but other studies have found it to fall closer to the afrotherians - the clade that includes elephants, tenrecs, and elephant shrews. Personally I suspect a perissodactyl/artiodactyl link is more likely. 

Whatever the true phylogenetic placement of Hyopsodus, it is one of the most common fossils found in the american west. Thousands of specimens are held in museum collections (triva: AMNH Fossil Mammal specimen #1? A Hyopsodus jawbone!) , and I myself have found dozens of Hyopsodus teeth while doing my own field work. The overwhelming vast majority of these specimens are just teeth, sometimes isolated teeth but commonly still held in the jawbone or maxilla. Why just teeth? Well, teeth preserve more frequently than bone in the first place, but on top of that, poor little Hyopsodus  was the bottom of the food chain in the american jungles of the Paleocene and Eocene. The lack of any skeletons is partially due to the fact most of them probably when down a predators gullet. A few complete skeletons have been found, from sites that show evidence of extremely rapid burial. These skeletons show us that Hyopsodus  was a generalized, not specialized for any one locomotion style, and most likely capable of some level of digging as well as climbing. The mounds of teeth clearly show this was a herbivorous animal, probably also a generalist, eating whatever it could find. 

Despite the abundance of Hyopsodus material (albiet mostly dental) there is still a lot of work to be done on this little animal and its kin, not only for evolutionary relationships, but exactly how Hyopsodus lived as a common prey item in the jungles of Wyoming.  I will also note that the reconstruction above is overall OK, but it does err in making the tail too long.