Armbruster's wolf

Canis armbrusteri was named by James W. Gidley in 1913. The first fossils were uncovered at Cumberland Bone Cave, Maryland, in an Irvingtonian terrestrial horizon. Fossil distribution is widespread throughout the United States.[3]

Middle Pleistocene in North America.[4] The North American wolves became larger, with tooth specimens indicating that C. priscolatrans diverged into the large wolf C. armbrusteri.[5]^: p242[6] R. A. Martin disagreed, and believed that C. armbrusteri[7] was C. lupus.[8] Ronald M. Nowak disagreed with Martin and proposed that C. armbrusteri was not related to C. lupus but C. priscolatrans, which then gave rise to C. dirus. Richard H. Tedford proposed that the South American C. gezi and C. nehringi share dental and cranial similarities developed for hypercarnivory, suggesting C. armbrusteri was the common ancestor of C. gezi, C. nehringi and C. dirus.[1]^: 148 Based on morphology from China, the Pliocene wolf C. chihliensis may have been the ancestor for both C. armbrusteri and C. lupus before their migration into North America.[9]^: p148[1]^: p181 C. armbrusteri appeared in North America in the Middle Pleistocene and is a wolf-like form larger than any Canis at that time.[4]

The three noted paleontologists X. Wang, R. H. Tedford and R. M. Nowak have all proposed that C. dirus had evolved from C. armbrusteri,[1]^: 181[9]^: p52 with Nowak stating that there were specimens from Cumberland Cave, Maryland that indicated C. armbrusteri diverging into C. dirus.[10][5]^: p243 The two taxa share a number of characteristics (synapomorphy), which suggests an origin of dirus in the late Irvingtonian in the open terrain in the midcontinent, and then later expanding eastward and displacing armbrusteri.[1]^: 181

In 2021, researchers sequenced the nuclear DNA (from the cell nucleus) of the dire wolf. The sequences indicate the dire wolf to be a highly divergent lineage which last shared a most recent common ancestor with the wolf-like canines 5.7 million years ago, with morphological similarities with the grey wolf being convergent evolution. The study's findings are consistent with the previously proposed taxonomic classification of the dire wolf as genus Aenocyon. The study proposes an early origin of the dire wolf lineage in the Americas, and that this geographic isolation allowed them to develop a degree of reproductive isolation since their divergence 5.7 million years ago. Coyotes, dholes, gray wolves, and the extinct Xenocyon evolved in Eurasia and expanded into North America relatively recently during the Late Pleistocene, therefore there was no admixture with the dire wolf. The long-term isolation of the dire wolf lineage implies that other American fossil taxa, including C. armbrusteri and C. edwardii, may also belong to the dire wolf's lineage.[11]