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MEMBERS OF the subfamily Ursinae dispersed into North America from Africa and Asia during the Miocene, with the appearance of Ursavus (Schlosser, 1899), Indarctos (Pilgrim, 1913), and Agriotherium (Wagner, 1837) (Dalquest, 1986; Miller and Carranza-Castañeda, 1996; Hunt, 1998). However, none of these genera were thought to have survived past the Hemphillian Land Mammal Age in North America. It is thought that these genera were replaced, and possibly outcompeted, by members of the extant genus Ursus (Linnaeus, 1758), or Plionarctos (Frick, 1926), as suggested by several sources (Bjork, 1970; Dalquest, 1986; Bell et al, 2004). It has also been suggested that the Ursavini (Agriotherium and Indarctos) may have given rise to the extant ursids and the Tremarctinae (Harrison, 1983; Miller and Carranza-Castañeda, 1996). Of the Ursavini, Agriotherium is consistently found in the Hemphillian Land Mammal Age, and so is used as an index fossil in that its absence is assumed to indicate that a site is Blancan rather than Hemphillian (Lundelius et al., 1987; Bell et al., 2004; Hunt, 2004).
Agriotherium is the most widely distributed member of the Ursidae, and its fossils are known from approximately 20 localities in North America, including fossil sites in Arizona, California, Florida, Kansas, Oklahoma, Nevada, Nebraska, Texas, and Mexico. These sites range from approximately 6.0 to 4.6 Ma in age and all are considered mid-late Hemphillian (Tedford et al., 2004). The latest recorded occurrence of Agriotherium from North America was -4.8 to 4.6 million years ago from the Yepomera Fauna, Chihuahua, Mexico (Lindsay et al, 1984; Bell et al., 2004). However, middle to late Pliocene and Pleistocene specimens are known from Poland, China, Ukraine, and South Africa. The dentary of Agriotherium described in this paper from the Hagerman Fossil Beds National Monument in Idaho is the first specimen of Agriotherium from Idaho, and the most northern occurrence of the genus to date in North America. Its occurrence in the Blancan is also the latest known record of the genus in North America.
METHODS
Measurements were made with Mitutoyo Absolute digital calipers to the nearest 0.01 mm. All measurements follow Van Valkenburgh and Koepfli (1993). Comparisons were made with known specimens of Agriotherium from the National Museum of Natural History (holotype specimen of A. schneidert), Field Museum of Natural History,...