Dire wolf
The dire wolf is an extinct species of canine which was native to the Americas during the Late Pleistocene and Early Holocene epochs. The species was named in 1858, four years after the first specimen had been found. Two subspecies are proposed, Aenocyon dirus guildayi and Aenocyon dirus dirus, but this assignment has been recently considered questionable. The largest collection of its fossils has been obtained from the Rancho La Brea Tar Pits in Los Angeles.
Dire wolf remains have been found across a broad range of habitats including plains, grasslands, and some forested mountain areas of North America and the arid savanna of South America. The sites range in elevation from sea level to. Dire wolf fossils have rarely been found north of 42°N latitude; there have been only five unconfirmed records above this latitude. This range restriction is thought to be due to temperature, prey, or habitat limitations imposed by proximity to the Laurentide and Cordilleran ice sheets that existed at the time.
The dire wolf was about the same size as the largest modern forms of gray wolf. A.d.guildayi weighed on average and A.d.dirus was on average. Its skull and dentition matched those of C.lupus, but its teeth were larger with greater shearing ability, and its bite force at the canine tooth was stronger than any known Canis species. These characteristics are thought to be adaptations for preying on Late Pleistocene megaherbivores; in North America, its prey is suggested to have included western horses, dwarf pronghorn, flat-headed peccary, ground sloths, ancient bison, and camels.
Dire wolves lived as recently as 10,000 years ago, according to dated remains. Its extinction occurred during the Quaternary extinction event, disappearing along with its main prey species; its reliance on megaherbivores has been proposed as the cause of its extinction, along with climatic change and competition with other species, or a combination of those factors.
Taxonomy
From the 1850s, the fossil remains of extinct large wolves were being found in the United States, and it was not immediately clear that these all belonged to one species. The first specimen of what would later become associated with Aenocyon dirus was found in mid-1854 in the bed of the Ohio River near Evansville, Indiana. The fossilized jawbone with cheek teeth was obtained by geologist Joseph Granville Norwood from an Evansville collector, Francis A. Linck. Paleontologist Joseph Leidy determined that the specimen represented an extinct species of wolf and reported it under the name of Canis primaevus. Norwood's letters to Leidy are preserved along with the type specimen at the Academy of Natural Sciences of Philadelphia. In 1857, while exploring the Niobrara River valley in Nebraska, Leidy found the vertebrae of an extinct Canis species that he reported the following year under the name C.dirus. The name C.primaevus was later renamed Canis indianensis when Leidy found out that the name C.primaevus had previously been used by the British naturalist Brian Houghton Hodgson for the dhole.In 1876, zoologist Joel Asaph Allen discovered the remains of Canis mississippiensis and associated these with C.dirus and Canis indianensis. As so little was found of these three specimens, Allen thought it best to leave each specimen listed under its provisional name until more material could be found to reveal their relationship. In 1908 paleontologist John Campbell Merriam began retrieving numerous fossilized bone fragments of a large wolf from the Rancho LaBrea tar pits. By 1912 he had found a skeleton sufficiently complete to be able to formally recognize these and the previously found specimens under the name C.dirus. Because the rules of nomenclature stipulated that the name of a species should be the oldest name ever applied to it, Merriam therefore selected the name of Leidy's 1858 specimen, C.dirus. In 1915 paleontologist Edward Troxell indicated his agreement with Merriam when he declared C.indianensis a synonym of C.dirus. In 1918, after studying these fossils, Merriam proposed consolidating their names under the separate genus Aenocyon to become Aenocyon dirus, but at that time not everyone agreed with this extinct wolf being placed in a new genus separate from the genus Canis. Canis ayersi and Aenocyon dirus were recognized as synonyms of C.dirus by the paleontologist Ernest Lundelius in 1972. All of the above taxa were declared synonyms of C.dirus in 1979, according to the paleontologist Ronald M. Nowak. However, Hill et al. examined the taxonomic history of C. mississippiensis and directly compared the bones attributed to C. mississippiensis with those of the Pleistocene gray wolf and the dire wolf, reaching a conclusion that C. mississippiensis is most likely synonymous with C. lupus.
In 1984, a study by Finnish paleontologist Björn Kurtén recognized a geographic variation within the dire wolf populations and proposed two subspecies: Canis dirus guildayi for specimens from California and Mexico that exhibited shorter limbs and longer teeth, and Canis dirus dirus for specimens east of the North American Continental Divide that exhibited longer limbs and shorter teeth. Kurtén designated a maxilla found in Hermit's Cave, New Mexico, as representing the nominate subspecies C. d. dirus. In 2019, this subspecific assignment was questioned by paleontologists Damián Ruiz-Ramoni and Marisol Montellano-Ballesteros at National Autonomous University of Mexico, who were unable to find a significant difference between the specimens assigned to each proposed subspecies.
In 2021, a DNA study found the dire wolf to be a highly divergent lineage when compared with the extant wolf-like canines, and this finding is consistent with the previously proposed taxonomic classification of the dire wolf as genus Aenocyon as proposed by Merriam in 1918.
Evolution
The canid family first appears in the North American fossil record around 40 million years ago, and the canine subfamily Caninae about 32 million years ago. From the Caninae, the ancestors of the fox-like Vulpini and the dog-like Canini branched off about 9 million years ago. The Canini are first represented by Eucyon, and mostly by coyote-like Eucyon davisi that was widely spread across North America. From the Canini the Cerdocyonina developed 6–5 million years ago, today represented by their canid descendants distinctly native to South America. Fossils of its sister clade, the wolf-like Canina, first appear 5 million years ago; however, they are believed to have likely originated as far back as 9 million years ago. Around 7 million years ago, the canines expanded into Eurasia and Africa, with Eucyon giving rise to the first members of the Canis genus in Europe. Around 4–3 million years ago C. chihliensis, the first wolf-sized member of Canis, arose in China and radiated into multiple other wolf-like canids across Eurasia and Africa. Members of the genus Canis later expand back into North America.The dire wolf evolved in North America. However, its ancestral lineage is debated, with two competing theories: The first theory is based on fossil morphology, which indicates that an expansion of the genus Canis out of Eurasia led to the dire wolf. The second theory is based on DNA evidence, which indicates that the dire wolf arose from an ancestral lineage that originated in the Americas and was separate from the genus Canis.
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 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 speciationa process by which a new species splits from an older onecould 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.
File:The American Museum journal .jpg|thumb|left|Life restoration by Erwin S. Christman, 1916
C. dirus lived in the Late Pleistocene to the early Holocene, 125,000–10,000 YBP, 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 fossils are not only smaller in size but more recent; thus it has been proposed that derived from 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, 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. 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 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. 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. proposed the new combination of the Armbruster's wolf within the genus Aenocyon, which would expand the earliest known occurrence of this genus up to the Middle Pleistocene.