Toxotherium is an extinct genus of perissodactyl of uncertain affinities that lived in North America during the Late Eocene, in the Chadronian land mammal age. Two species of Toxotherium are recognized, T. hunteri from the Flagstaff Rim and T. woodi from the White River Formation and Cypress Hills Formation. Both species are known only from partial lower jaws.
Research history Toxotherium is known only from anterior fragments of mandibles, found in Early Oligocene and possibly Late Eocene strata. The type species, Toxotherium hunteri, was described by Horace Elmer Wood in 1961, based on a fragmentary mandible from the Cypress Hills Formation in Saskatchewan, Canada. Since 1961, similar fragmentary lower jaws referred to T. hunteri have also been found in United States, in the White River Formation in Wyoming.
A second species of Toxotherium, T. woodi, was described by Shirley M. Skinner and Raymond J. Gooris in 1966, based on a mandible fragment found in 1958 in Natrona County, Wyoming.
Description Toxotherium was a very small perissodactyl. Toxotherium had a rhinocerotoid molar pattern, a shortened series of premolars, and greatly enlarged and procumbent tusk-like canine teeth. Based on the reduced premolars and the relatively deep posterior ramus, Toxotherium must have had a very brachycephalic (relatively short and broad) face.
Classification The affinities of Toxotherium are unclear. Different paleontologists have variously classified Toxotherium as a tapiroid, incertae sedis within Rhinocerotoidea, a hyracodont, an amynodont. The brachycephalic face, and procumbent tusk, suggest amynodont affinities. The tusk of Toxotherium is relatively heavier and more pronounced than the tusks of all known amynodonts. The ossified external auditory meatus in Toxotherium has been suggested to rule out classification in Amynodontidae, and to more closely resemble Triplopus.
In 1966, Skinner and Gooris suggested that Toxotherium could be placed in a new subfamily, or perhaps even family, of rhinocerotoids. In 1986, Donald Prothero, Earl Manning, and Bruce Hanson suggested that Toxotherium could represent a lophiodont, a group predominantly known from Europe, though this suggestion was partially based on a controversial placement of the lophiodonts inside Tapiroidea. Koenigswald, Holbrook & Rose (2011) classified Toxotherium as incertae sedis within Perissodactyla.
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