Toxotherium
| Toxotherium | |
|---|---|
 | |
| Mandible of T. hunteri, National Museum of Natural History | |
Scientific classification  | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Perissodactyla |
| Family: | incertae sedis |
| Genus: | †Toxotherium Wood, 1961 |
| Type species | |
| †Toxotherium hunteri Wood, 1961 | |
| Other species | |
| |
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.[1] Two species of Toxotherium are recognized, T. hunteri from the Flagstaff Rim[1] and T. woodi from the White River Formation and Cypress Hills Formation.[1][2] Both species are known only from partial lower jaws.
Research history
[edit]Toxotherium is known only from anterior fragments of mandibles, found in Early Oligocene and possibly Late Eocene strata.[3] 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.[2] Since 1961, similar fragmentary lower jaws referred to T. hunteri have also been found in United States, in the White River Formation in Wyoming.[1]
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.[2]
Description
[edit]Toxotherium was a very small perissodactyl.[3] Toxotherium had a rhinocerotoid molar pattern, a shortened series of premolars, and greatly enlarged and procumbent tusk-like canine teeth.[3] Based on the reduced premolars and the relatively deep posterior ramus, Toxotherium must have had a very brachycephalic (relatively short and broad) face.[4]
Classification
[edit]The affinities of Toxotherium are unclear. Different paleontologists have variously classified Toxotherium as a tapiroid,[4] incertae sedis within Rhinocerotoidea,[2] a hyracodont, an amynodont.[4] The brachycephalic face, and procumbent tusk, suggest amynodont affinities.[4] The tusk of Toxotherium is relatively heavier and more pronounced than the tusks of all known amynodonts.[2] The ossified external auditory meatus in Toxotherium has been suggested to rule out classification in Amynodontidae, and to more closely resemble Triplopus.[4]
In 1966, Skinner and Gooris suggested that Toxotherium could be placed in a new subfamily, or perhaps even family, of rhinocerotoids.[2] 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.[4] Koenigswald, Holbrook & Rose (2011) classified Toxotherium as incertae sedis within Perissodactyla.[1]
References
[edit]- 1 2 3 4 5 Koenigswald, Wighart Von; Holbrook, Luke T.; Rose, Kenneth D. (2011). "Diversity and Evolution of Hunter-Schreger Band Configuration in Tooth Enamel of Perissodactyl Mammals". Acta Palaeontologica Polonica. 56 (1): 11–32. doi:10.4202/app.2010.0021. ISSN 0567-7920.
- 1 2 3 4 5 6 Skinner, Shirley M.; Gooris, Raymond J. (1966). "A Note on Toxotherium (Mammalia, Rhinocerotoidea) from Natrona County, Wyoming". American Museum Novitates (2261).
- 1 2 3 Radinsky, Leonard B. (1969). "The Early Evolution of the Perissodactyla". Evolution. 23 (2): 308. doi:10.2307/2406794.
- 1 2 3 4 5 6 Prothero, Donald R.; Manning, Earl; Hanson, C. Bruce (1986). "The phylogeny of the Rhinocerotoidea (Mammalia, Perissodactyla)". Zoological Journal of the Linnean Society. 87 (4): 341–366. doi:10.1111/j.1096-3642.1986.tb01340.x.
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