This group of odd-toed mammals appeared first in the Eocene, at which
time they were about the size of a sheep. By Middle Oligocene time they
had increased to gigantic proportions but still had a small and
primitive brain. _Brontotherium_ (Pl. 48) was slightly rhinoceros-like
in appearance and is believed to be the largest land animal that ever
inhabited the North American continent. This animal was about 8 feet
tall at the shoulders; a large bony growth protruded from the skull and
this was extended into a flattened horn, which was divided at the top.
[Illustration: Plate 48
TERTIARY MAMMALS]
UINTATHERIUM × ¹/₄₅
BRONTOTHERIUM × ¹/₃₅
Although the titanotheres underwent rapid development during the early
Tertiary, these huge beasts became extinct during the middle of the
Oligocene epoch. Titanothere remains have been reported from the
Trans-Pecos region of Texas.