Armbruster's wolf



Armbruster's wolf (Canis armbrusteri) is an extinct species that was endemic to North America and lived during the Irvingtonian stage of the Pleistocene epoch, spanning from 1.9 Mya—250,000 years BP. It is notable because it is proposed as the ancestor of one of the most famous prehistoric carnivores in North America, the dire wolf, which replaced it.

Taxonomy
Canis armbrusteri was named by James W. Gidley in 1913. It appeared in North America in the Middle Pleistocene, and is a wolf-like form larger than any Canis at that time. The first fossils were uncovered at Cumberland Bone Cave, Maryland, in an Irvingtonian terrestrial horizon. Fossil distribution is widespread throughout the United States.

In the Middle Pleistocene of North America, wolf species became larger, with some studies concluding that C. priscolatrans diverged into the large wolf C. armbrusteri based on tooth specimens. R. A. Martin disagreed, and believed that C. armbrusteri was C. lupus. However, Ronald M. Nowak proposed instead that C. armbrusteri was not related to C. lupus, but C. priscolatrans, positing that this then gave rise to C. dirus. Richard H. Tedford, on the other hand, suggested 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. 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.

The paleontologists X. Wang, R. H. Tedford and R. M. Nowak have all proposed that C. dirus had evolved from C. armbrusteri,  with Nowak stating that there were specimens from Cumberland Cave, Maryland that indicated C. armbrusteri diverged into C. dirus. The two taxa share a number of characteristics (synapomorphy), suggesting an origin of C. dirus in the late Irvingtonian around central America in open terrain habitats, with later eastward expansion and displacement of C. armbrusteri.

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 similarity to the grey wolf being a result of 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 also resulted in reproductive isolation, driving their divergence 5.7 million years ago. Concurrently, Coyotes, dholes, gray wolves, and the extinct Xenocyon ostensibly evolved in Eurasia and expanded into North America more recently, during the Late Pleistocene, and did not experience admixture with the dire wolf. This long-term isolation of the dire wolf lineage implies that other American fossil canines, including C. armbrusteri and C. edwardii, may belong to the same lineage as Aenocyon.

In a 2024 study, Armbruster's wolf was considered as a species of Aenocyon, while Canis edwardii was considered more closely related to the modern coyote.