derived from C.d.dirus. However, there are disputed specimens of C.dirus that date to 250,000YBP. Fossil specimens of C.dirus discovered at four sites in the Hay Springs area of Sheridan County, Nebraska, were named Aenocyon dirus nebrascensis (Frick 1930, undescribed), but Frick did not publish a description of them. Nowak later referred to this material as C.armbrusteri; then, in 2009, Tedford formally published a description of the specimens and noted that, although they exhibited some morphological characteristics of both C.armbrusteri and C.dirus, he referred to them only as C.dirus. A fossil discovered in the Horse Room of the Salamander Cave in the Black Hills of South Dakota may possibly be C.dirus; if so, this fossil is one of the earliest specimens on record. It was catalogued as Canis cf. C.dirus (where cf. in Latin means confer, uncertain). The fossil of a horse found in the Horse Room provided a uranium-series dating of 252,000 yearsYBP and the Canis cf. dirus specimen was assumed to be from the same period. C.armbrusteri and C.dirus share some characteristics (synapomorphies) that imply the latter's descent from the former. The fossil record suggests C.dirus originated around 250,000YBP in the open terrain of the mid-continent before expanding eastward and displacing its ancestor C.armbrusteri. The first appearance of C.dirus would therefore be 250,000YBP in California and Nebraska, and later in the rest of the United States, Canada, Mexico, Venezuela, Ecuador, Bolivia, and Peru, but the identity of these earliest fossils is not confirmed. In South America, C.dirus specimens dated to the Late Pleistocene were found along the north and west coasts, but none have been found in Argentina, an area that was inhabited by Canisgezi and Canis nehringi. Given their similarities and timeframes, it is proposed that Canis gezi was the ancestor of Canis nehringi. One study found that C.dirus was more evolutionarily derived compared with C.nehringi, and was larger in the size and construction of its lower molars for more efficient predation. For this reason, some researchers have proposed that C.dirus may have originated in South America. Tedford proposed that C.armbrusteri was the common ancestor for both the North and South American wolves. Later studies concluded that C.dirus and C.nehringi were the same species, and that C.dirus had migrated from North America into South America, making it a participant in the Great American Interchange. In 2018, a study found that Canis gezi did not fall under genus Canis