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