Brontotheriidae

a family of extinct mammals of the order Perissodactyla. They existed in the Paleocene period (from the Lower Eocene to the Lower Oligocene). Outwardly similar to rhinoceroses, their skulls were long and low and concave in the middle portion (in most mammals this portion protrudes). Their size varied from that of a horse to a small elephant (height at the shoulders, up to 2.5 m). The most ancient Brontotheriidae were hornless. Later ones developed paired hornlike devices on the anterior part of the skull. The molars were short and nodular-pectinate. The extremities were short and massive with four digits on the forefoot and three on the hind foot. Brontotheriidae lived in open places near bodies of water and fed on soft vegetation. They are known to have existed in North America, Asia, and Europe. Small brain dimensions and the inadequacy of their teeth were the probable causes for their short span of existence.