TITANOTHERIIDAE (also known as Menodontidae and Brontotheriidae), a family of large rhinoceros-like perissodactyl ungulate mammals from the Oligocene and Eocene strata of North America. The cheek-teeth are low-crowned, with the external cones of the upper molars fused into a W-like outer wall, and the inner ones retaining a regular conical form; while in the lower teeth the crown is formed of crescentic ridges, of which there are three in the last and two each in the other teeth. There is generally little gap between the canines and the premolars.
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. 21, c. 11, p. 44, m. 33. The muzzle probably formed a snout in life; and there is presumptive evidence that these animals were very long-lived. Brontops seem scarcely separated from the type genus; but the name Brontotherium is applied to species with two pairs of incisor teeth in both jaws. The length of the largest species was about 14 ft.; and the height about 8 ft. The alleged occurrence of remains of members of the group in the Balkans apparently rests on insufficient evidence.
A second group is typified by Palaeosyops, of the Bridger Eocene of North America; P. paludosus being an animal about the size of a tapir. 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. The limbs were relatively slender, and the brain was small. In the lower, or Wasatch, Eocene the group was represented by the still more primitive Lambdotherium. 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. Another of these titanotheroid forms is Diplacodon, from the Upper or Uinta Eocene; an animal the size of a rhinoceros, with the last two upper premolars molar-like. It was probably off the direct ancestral line of Titanotherium. These intermediate forms render the reference of the group to a distinct family—Palaeosyopidae—unnecessary.
Professor H. F. 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.
See H. F. Osborn, “The Cranial Evolution of Titanotherium,” Bull. Amer. Mus. (1896), viii., 137, and the “Four Phyla of Oligocene Titanotheres,” op. cit. (1903), xvi. 91; C. H. Earle, “A Memoir on the Genus Palaeosyops and its Allies,” Journ. Acad. Philadelphia (1892), ix. 267. (R. L.*)