December 12. Cenozoic mammals again

There are lots of “theres” among the Cenozoic mammals. Brontothere, Uintatherium, Chalicothere, Indricotherium – the “there” or “therium” suffix is just Greek for “beast,” so it gets attached to the scientific names of lots of critters, normally to mammals. 

image by DagdaMor, used under Creative Commons license.

Mammalian groups that began in the Paleocene or early Eocene diversified and thrived during the Eocene, Oligocene, and later epochs of the Cenozoic. Paraceratherium – which may be the same as Baluchitherium or Indrichotherium or both – was the largest land mammal that ever lived. It was related to rhinoceroses but had a long neck, giraffe-like in its proportions. There are no complete fossils, one reason we aren’t sure which name goes with it, but it’s estimated to have been 20 feet high and 27 feet long. It lived in Central Asia pretty much throughout Oligocene time, 34 to 23 million years ago. Paraceratherium means “near the hornless beast,” because it was similar to a contemporary hornless rhinoceros. The rhino family was much more diverse in the Oligocene than it is today. Even though the incomplete fossils mean we can’t really reconstruct it, there is no doubt that it was a huge animal that browsed on foliage in trees.

The name titanothere – titanic beast – has been pretty much discarded in favor of brontothere, the “thunder beast.” Brontotheres were hoofed animals that were descended from the ancestors of horses, but if you met a brontothere on the street you’d probably call it a rhinoceros. They typically had horns and other bony protuberances on their heads and faces, and unlike rhinos whose horns are made of keratin, the same compound that makes fingernails and hair, brontothere horns were bone. Brontotheres appear to have become extinct just about at the end of the Eocene. Their demise is speculated to be related to the rise of grasses – tough plants that they could not adequately graze on and process – and the related increase in arid conditions as the Oligocene began. 

Uintatherium skull (public domain)

Uintatherium, the Uinta Mountains Beast, lived in what is now Utah in middle to late Eocene time, and it had a cousin that lived in China. It was another critter that looked a lot like a rhino but wasn’t all that closely related to them. With big bony bumps and saber-like canine teeth, it must have looked intimidating, but it was a herbivore. Uintatherium belonged to the mammalian order Dinocerata, a group that is entirely extinct. 

We aren’t sure how the Dinocerata related to other mammals, but I think the favored position is that they branched from an ancestor that was also ancestral to the ungulates, the hoofed animals like horses. But that’s not certain. All the Dinocerata went extinct before the end of the Eocene. Changing conditions, together with competition from brontotheres may have eliminated them. But the brontotheres survived only a few more million years themselves before dying out about at the end of the Eocene.

The Chalicothere, whose name means “gravel beast,” appeared during the Eocene, and survived right up until about 4 million years ago, a run of 50 million years, unusual for the early Cenozoic mammals which typically evolved but in many cases did not survive as a genus or even at the family or order level. Chalicotheres had short hind legs and long front legs, so they were probably knuckle walkers, like modern gorillas. They were the size of a horse, and ranged across Europe and North America as well as parts of Asia and Africa. It is supposed that they used their strong but short hind legs to rear up and use their long forelegs to strip vegetation from trees. Unlike its hoofed relatives, the Chalicothere had three claws on its feet.

Gastornis (public domain)

Birds too were becoming huge, occupying ecological niches that their cousins, the non-avian dinosaurs held until the end of the Cretaceous. Gastornis giganteus, which lived in very late Paleocene and early Eocene time, stood nearly 7 feet high, comparable to ostriches and moas today. It was flightless, and various species lived in Europe, North America, and Asia. The giant “terror birds” of South America, the Phorusrhacids, were developing contemporaneously with Gastornis, but they were in completely different orders of birds. The Terror Birds reached their maximum sizes later in the Cenozoic.

—Richard I. Gibson

Links:
Paraceratherium 

Comparison image by DagdaMor, used under Creative Commons license.
Uintatherium skull photo by Jebulon, given to public domain.
Gastornis (public domain)