Brontotheriidae

Brontotheriidae (or Titanotheriidae), is a family of extinct mammals belonging to the order Perissodactyla, the order that includes horses, rhinoceroses, and tapirs from the Eocene epoch. Brontotheres had a Holarctic distribution, with the exception of Western Europe: their fossils have been found in North America and Asia, with a few also known from Eastern Europe. In larger and often better-known genera of the group, a paired or battering-ram-like horn was present on the snout above the eye socket, made of bone, unlike the horns of rhinoceroses. However, this feature is not present in all members of the family. Brontotheres lived in dense forests and were all herbivores with a broad specialization toward foliage. Their evolutionary history spanned nearly 20 million years and most likely began in North America around 53 million years ago with still relatively small, tapir-sized representatives and were some of the earliest mammals to have evolved large body sizes of several tonnes. In biological systematics, brontotheres are frequently placed near horses based on dental morphology, though the overall relationships among the large extinct Perissodactyla groups remain incompletely resolved. They were the first fossilized mammals to be discovered west of the Mississippi, and were first discovered in South Dakota.

== Characteristics and evolution ==