section_path: Taxonomy/Evolution/Morphological evidence
section_name: Morphological evidence
paragraph_text: Morphological evidence
  
  Morphological evidence based on fossil remains indicates an expansion of genus Canis from out of Eurasia led to the dire wolf.
  
  In 1974 Robert A. Martin proposed that the large North American wolf C. armbrusteri (Armbruster's wolf) was C. lupus. Nowak, Kurtén, and Annalisa Berta proposed that C. dirus was not derived from C. lupus. In 1987, a new hypothesis proposed that a mammal population could give rise to a larger form called a hypermorph during times when food was abundant, but when food later became scarce the hypermorph would either adapt to a smaller form or go extinct. This hypothesis might explain the large body sizes found in many Late Pleistocene mammals compared to their modern counterparts. Both extinction and speciation – a process by which a new species splits from an older one – could occur together during periods of climatic extremes. Gloria D. Goulet agreed with Martin, proposing further that this hypothesis might explain the sudden appearance of C. dirus in North America and, judging from the similarities in their skull shapes, that C. lupus had given rise to the C. dirus hypermorph due to an abundance of game, a stable environment, and large competitors.
  
  The three paleontologists Xiaoming Wang, Richard H. Tedford, and Ronald M. Nowak propose that C. dirus evolved from Canis armbrusteri, with Nowak stating that both species arose in the Americas and that specimens found in Cumberland Cave, Maryland, appear to be C. armbrusteri diverging into C. dirus. Nowak believed that Canis edwardii was the first appearance of the wolf in North America, and it appears to be close to the lineage which produced C. armbrusteri and C. dirus. Tedford believes that the early wolf from China, Canis chihliensis, may have been the ancestor of both C. armbrusteri and the gray wolf C. lupus. The sudden appearance of C. armbrusteri in mid-latitude North America during the Early Pleistocene 1.5 million years ago, along with the mammoth, suggests that it was an immigrant from Asia, with the gray wolf C. lupus evolving in Beringia later in the Pleistocene and entering mid-latitude North America during the Last Glacial Period along with its Beringian prey. In 2010 Francisco Prevosti proposed that C. dirus was a sister taxon to C. lupus.
  
  C. dirus lived in the Late Pleistocene to the early Holocene, 125,000–10,000 YBP (years before present), in North and South America. The majority of fossils from the eastern C. d. dirus have been dated 125,000–75,000 YBP, but the western C. d. guildayi fossils are not only smaller in size but more recent; thus it has been proposed that C. d. guildayi derived from C. d. dirus. However, there are disputed specimens of C. dirus that date to 250,000 YBP. 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 YBP 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 250,000 YBP 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 250,000 YBP 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 Canis gezi and Canis nehringi. Given their similarities and timeframes, it is proposed that C. gezi was the ancestor of Canis nehringi. One study found that C. dirus was more evolutionarily derived than 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 suggested that C. dirus and C. nehringi were the same species, though this possible synonymy is not officially formalized yet, 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 and should be classified under the subtribe Cerdocyonina; however, no genus was proposed.
  
  The 2020 discovery of a claimed dire wolf fossil in northeast China indicates that dire wolves may have crossed Beringia when it existed, though Ruiz-Ramoni et al. (2022) doubted that this specimen represents a dire wolf. They also proposed that the Armbruster's wolf should be given a new genus name, possibly included within the genus Aenocyon, as it was probably ancestral to the dire wolf and its assignment within the genus Canis is not well-founded. This was also considered plausible by other authors. In their revision of the Pleistocene assemblage from the Cumberland Bone Cave, Eshelman et al. (2025) proposed the new combination of the Armbruster's wolf within the genus Aenocyon (A. armbrusteri), which would expand the earliest known occurrence of this genus up to the Middle Pleistocene.