The Dead Zoo: Uintatherium

This fossil beast has baffled paleontologists for decades.

Uintatherium in the traveling Extreme Mammal exhibit. Credit: Brian Switek
Uintatherium in the traveling Extreme Mammal exhibit. Credit: Brian Switek


No one knows what Uintatherium is. Not entirely. There are plenty of terms that can help us feel around the outline of this long-deceased beast. Mammal. Eocene. Fossil. Extinct. Massive. But despite being known to paleontologists for nearly a century and a half, this most charismatic of dawn beasts remains about as puzzling now as when it was first uncovered among the badlands of the American west.

The famous “Beast of the Uinta Mountains” was one of the many offspring spun off by the Bone Wars. Although the name itself was coined by Philadelphia polymath Joseph Leidy in 1872, it was the perpetually cantankerous rivals Edward Drinker Cope and Othniel Charles Marsh who tried to make this peculiar fossil mammal their own. Both considered their respective fossils of this mammal to be among their prize finds, and it’s not difficult to see why. Uintatherium was a very impressive quadruped, its stocky frame tipped by a striking skull decorated by four protrusions of bone and long saberteeth. (These were not for shearing flesh, the rest of the animal's herbivorous dentition shows, but instead were weapons of intimidation when rivals met each other.) Leidy's choice for a name was the one that stuck - following the arcane rules of how fossil species are named - but I can't help but think of Uintatherium as being a little chuffed that so many scientists were fussing over its bones.

There's no modern equivalent for Uintatherium. It's the chief member of a totally-extinct group of mammals called the Dinocerata. But even in pop culture terms, Uintatherium is so far from our modern experience that the beast has often been thrown in with cheap plastic dinosaurs you’ll find hanging from peg bags among the aisles of grocery stores. If that's not enough, Uintatherium even had a cameo in the 1977 schlock flick The Last Dinosaur as the monster who welcomes humans to a lost world. On aesthetic grounds alone, Uintatherium simply looks prehistoric.

Yet Uintatherium is familiar only as an icon of the early days of American paleontology. Multiple fossil finds from the 19th century on have given us the outline of this beast. But how did it evolve, and how did such a creature live? Here, the picture gets fuzzier.

Uintatherium was a creature of Eocene time, when mammalian life was finally free of dinosaurian oppression and was able to flourish in ways never seen before. This was the beginning of the Age of Mammals. But many of these early mammalian spinoffs left no descendants and were so out of the ordinary that trying to classify them is harder than excavating the rock they were buried in. (Not to mention that fossil mammals have faded to the background after the Dinosaur Renaissance of the 1970s and 80s, still actively researched but not enjoying nearly as much interest and adulation as the “terrible lizards.”) Various placements for Uintatherium and its relatives have been proposed over the years - related to pachyderms, hoofed mammals, and even rabbit-like mammals - but this beast's place in the tree of life is still disputed. The habits and lifestyle of the beast are also obscure. The clues are held in the mammal's bones, but they haven't been fully teased out yet.

I hope that this ancient mammal can enjoy a revival in paleontological attention. There's so much we don't understand about this celebrity species. How Uintatherium moved, ate, and interacted with its own species… how it looked and sounded and mated and scared off predators… all these things are unknown, and some of them may remain so. But that's what keeps paleontologists going back to the fossil record. We know the bones. What we wish to find out is the nature of the beast.

More profiles in this series:
Tyrannosaurus | Stegosaurus

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