Titanotherium, of the Oligocene of the Dakotas and neighbouring districts, was a huge beast, with the hinder upper premolars similar in character to the molars, a pair of horn-cores, arising from the maxilla, overhanging the nose-cavity, four front and three hind toes, only twenty dorso-lumbar vertebrae, and an almost continuous and unbroken series of teeth, in which the canines are short; the dental formula being i.
The skull, which has a longer face than in Titanotherium, lacks horn-cores, while all the upper premolars are simpler than the molars, and the full series of 44 teeth was present.
On the other hand, Palaeosyops is connected with Titanotherium by means of Telmatotherium of the upper Bridger and Washakia Eocene, a larger animal, with a longer and flatter skull, showing rudiments of horn-cores, only two pairs of lower incisors, and a general approximation in dental character to Titanotherium.
Osborn, who recognises four genera, Titanotherium, Megacerops, Symborodon and Brontotherium, in the typical section of the family, considers that each of these represents a distinct line of descent from the Palaeosyops-like group. The whole assemblage forms one of the four main sections of the Perissodactyla, namely the Titanotheroidea.
Osborn, "The Cranial Evolution of Titanotherium," Bull.