TITANOTHERIIDAE (also known as Menodontidae and 3rontotheriidae), 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 n the lower teeth the crown is formed of crescentic ridges, of which there are three in the last and two each in the other eeth. There is generally little gap between the canines and the premolars.
Titanotherium, of the Oligocene of the Dakotas and neighbourng districts, was a huge beast, with the hinder upper premolars imilar in character to the molars, a pair of horn-cores, arising rom the maxilla, overhanging the nose-cavity, four front and hree hind toes, only twenty dorso-lumbar vertebrae, and an almost continuous and unbroken series of teeth, in which the anines are short; the dental formula being i. f , c. \, p. -J-, m. f . The muzzle probably formed a snout in life; and there is ^resumptive evidence that these animals were very long-lived.
Brontops seem scarcely separated from the type genus; but the iame Brontotherium is applied to species with two pairs of incisor eeth in both jaws. The length of the largest species was about 4 ft.; and the height about 8 ft. The alleged occurrence of emains of members of the group in the Balkans apparently ests on insufficient evidence.
A second group is typified by Palaeosyops, of the Bridger Eocene of North America; P. paludosus being an animal bout the size of a tapir. The skull, which has a longer ace than in Titanotherium, lacks horn-cores, while all the pper premolars are simpler than the molars, and the full eries of 44 teeth was present. The limbs were relatively lender, and the brain was small. In the lower, or Wasatch, Eocene the group was represented by the still more primitive .ambdotherium. On the other hand, Palaeosyops is connected >rith 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 Tilanotherium. 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 Tilanotherium. These intermediate forms render the reference of the group to a distinct family Palaeosyopidae unnecessary.
Professor H. F. Osborn, who recognises four genera, Tilanotherium, Megacerops, Symborodon and Bronlolherium, in the typical section of the family, considers that each of these represents a distinct line of descent from the Palaeosypps-\ik.e 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. cil. (1903), xvi. 01 ; C. H. Earle, " A Memoir on the Genus Palaeosyops and its Allies," Journ. Acad. Philadelphia (1892), ix. 267. (R. L.*)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)