Extinct Dire Wolves

The Dire Wolf is an extinct famous prehistoric wolf like animal that lived mainly in North America, along with its extinct competitor Smiladon. The dire wolf lived in the Americas and eastern Asia during the Late Pleistocene and Early Holocene epochs (125,000–9,500 years ago).

The species was named in 1858, four years after the first specimen had been found. Two subspecies are recognized: Aenocyon dirus guildayi and Aenocyon dirus dirus. The largest collection of its fossils has been obtained from the Rancho La Brea Tar Pits in Los Angeles.

Dire wolf bones have been found across the plains, grasslands, and some forested mountain areas of North America, the arid savanna of South America, and the steppes of eastern Asia. These kinds of places range in elevation from sea level to 2,255 meters (7,400 ft).

The 2020 discovery of dire wolf fossils in northeast China indicates that dire wolves had crossed Beringia when it existed.

The dire wolf weighed on average 68 kg (150 lb).

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.

Dire wolves lived as recently as 9,500 years ago, according to dated remains.

In 1876 the zoologist Joel Asaph Allen discovered the remains of Canis mississippiensis (Allen 1876) and associated these with C. dirus (Leidy 1858) and Canis indianensis (Leidy 1869).

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 the paleontologist John Campbell Merriam began retrieving numerous fossilized bone fragments of a large wolf from the Rancho La Brea 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 (Leidy 1858).

Because the rules of nomenclature stipulated that the name of a species should be the oldest name ever applied to it,[12] Merriam therefore selected the name of Leidy's 1858 specimen, C. dirus.[13]

In 1915 the paleontologist Edward Troxell indicated his agreement with Merriam when he declared C. indianensis a synonym of C. dirus.[14]

In 1918, after studying these fossils, Merriam proposed consolidating their names under the separate genus Aenocyon (from ainos, 'terrible' and cyon, 'dog') to become Aenocyon dirus,[2]

but at that time not everyone agreed with this extinct wolf being placed in a new genus separate from the genus Canis.[15]

Canis ayersi (Sellards 1916) and Aenocyon dirus (Merriam 1918) were recognized as synonyms of C. dirus by the paleontologist Ernest Lundelius in 1972.[16]

All of the above taxa were declared synonyms of C. dirus in 1979, according to the paleontologist Ronald M. Nowak.[17]

In 1984 a study by Björn Kurtén recognized a geographic variation within the dire wolf populations and proposed two subspecies: Canis dirus guildayi (named by Kurtén in honor of the paleontologist John E. Guilday) 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.[3][18][19][20]

Kurtén designated a maxilla found in Hermit's Cave, New Mexico as representing the nominate subspecies C. d. dirus.[3]

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 (Ancient Greek: "terrible wolf") as proposed by Merriam in 1918.[21]

Evolution

In North America, the canid family came into existence 40 million years ago,[22][23] with the canine subfamily Caninae coming into existence about 32 million years ago.[24]

From the Caninae, the ancestors of the fox-like Vulpini and the dog-like Canini came into existence 9 million years ago.

This group was first represented by Eucyon, and mostly by coyote-like Eucyon davisi that was spread widely across North America.[25] From the Canini the Cerdocyonina, today represented by the South American canids, came into existence 6–5 million years ago.[26]

Its sister the wolf-like Canina came into existence 5 million years ago, however, they are likely to have originated as far back as 9 million years ago.[25]

Around 7 million years ago, the canines expanded into Eurasia and Africa, with Eucyon giving rise to the first of genus Canis in Europe.[27]

Around 4-3 million years ago C. chihliensis, the first wolf-sized member of Canis, arose in China and expanded to give rise to other wolf-like members across Eurasia and Africa. Members of genus Canis would later expand into North America.[26]

The dire wolf evolved in North America.[26][21] However, the ancestral lineage that gave rise to it is debated with two competing theories.

The first theory is based on fossil morphology, which indicates an expansion of genus Canis from out of Eurasia led to the dire wolf.[26]

The second theory is based on DNA evidence, which indicates the dire wolf arose from an ancestral lineage that originated in the Americas and was separate to genus Canis.[21]

In 1974 Robert A. Martin proposed that the large North American wolf C. armbrusteri (Armbruster's wolf) was C. lupus.[29]

Nowak, Kurtén, and Annalisa Berta proposed that C. dirus was not derived from C. lupus.[17][30][31]

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.[32][33]

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.[34]

The three paleontologists Xiaoming Wang, Richard H. Tedford, and Ronald M. Nowak propose that C. dirus evolved from Canis armbrusteri,[26][28] with Nowak stating that both species arose in the Americas[35] and that specimens found in Cumberland Cave, Maryland, appear to be C. armbrusteri diverging into C. dirus.[36][37]

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.[38]

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.[39]

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,[28] 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.[26][28][37]

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 years before present or YBP) in North and South America.[3] 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. armbrusteria 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 years 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 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 and should be classified under the subtribe Cerdocyonina, however no genus was proposed.

DNA evidence indicates the dire wolf arose from an ancestral lineage that originated in the Americas and was separate to genus Canis.

In 1992 an attempt was made to extract a mitochondrial DNA sequence from the skeletal remains of A. d. guildayi to compare its relationship to other Canis species. The attempt was unsuccessful because these remains had been removed from the La Brea pits and tar could not be removed from the bone material.[50] In 2014 an attempt to extract DNA from a Columbian mammoth from the tar pits also failed, with the study concluding that organic compounds from the asphalt permeate the bones of all ancient samples from the La Brea pits, hindering the extraction of DNA samples.

In 2021, researchers sequenced the nuclear DNA (from the cell nucleus) taken from five dire wolf fossils dating from 13,000 to 50,000 years ago. 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. The study also measured numerous dire wolf and gray wolf skeletal samples that showed their morphologies to be highly similar, which had led to the theory that the dire wolf and the gray wolf had a close evolutionary relationship. The morphological similarity between dire wolves and gray wolves was concluded to be due to convergent evolution. Members of the wolf-like canines are known to hybridize with each other but the study could find no indication of genetic admixture from the five dire wolf samples with extant North American gray wolves and coyotes nor their common ancestor. This finding indicates that the wolf and coyote lineages evolved in isolation from the dire wolf lineage.

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. The study's findings are consistent with the previously proposed taxonomic classification of the dire wolf as genus Aenocyon.

Radiocarbon dating

The age of most dire wolf localities is determined solely by biostratigraphy, but biostratigraphy is an unreliable indicator within asphalt deposits.[52][53] Some sites have been radiocarbon dated, with dire wolf specimens from the La Brea pits dated in calendar years as follows: 82 specimens dated 13,000–14,000 YBP; 40 specimens dated 14,000–16,000 YBP; 77 specimens dated 14,000–18,000 YBP; 37 specimens dated 17,000–18,000 YBP; 26 specimens dated 21,000–30,000 YBP; 40 specimens dated 25,000–28,000 YBP; and 6 specimens dated 32,000–37,000 YBP.[44]: T1  A specimen from Powder Mill Creek Cave, Missouri, was dated at 13,170 YBP.

The average dire wolf proportions were similar to those of two modern North American wolves:

the Yukon wolf (Canis lupus pambasileus) and the

Northwestern wolf (Canis lupus occidentalis).

The largest northern wolves today have an average shoulder height of 38 in (97 cm) and a body length of 69 in (180 cm).[55]: 1  Some dire wolf specimens from Rancho La Brea are smaller than this, and some are larger.

The dire wolf had smaller feet and a larger head when compared with a northern wolf of the same body size. The skull length could reach up to 310 mm (12 in) or longer, with a broader palate, frontal region, and zygomatic arches compared with the Yukon wolf. These dimensions make the skull very massive. Its sagittal crest was higher, with the inion showing a significant backward projection, and with the rear ends of the nasal bones extending relatively far back into the skull.

A connected skeleton of a dire wolf from Rancho La Brea is difficult to find because the tar allows the bones to disassemble in many directions.

Parts of a vertebral column have been assembled, and it was found to be similar to that of the modern wolf, with the same number of vertebrae.

Geographic differences in dire wolves were not detected until 1984, when a study of skeletal remains showed differences in a few cranio-dental features and limb proportions between specimens from California and Mexico (A. d. guildayi) and those found from the east of the Continental Divide (A. d. dirus).

A comparison of limb size shows that the rear limbs of A. d. guildayi were 8% shorter than the Yukon wolf due to a significantly shorter tibia and metatarsus, and that the front limbs were also shorter due to their slightly shorter lower bones.[56][57] With its comparatively lighter and smaller limbs and massive head, A. d. guildayi was not as well adapted for running as timber wolves and coyotes.

A. d. dirus possessed significantly longer limbs than A. d. guildayi. The forelimbs were 14% longer than A. d. guildayi due to 10% longer humeri, 15% longer radii, and 15% longer metacarpals.

The rear limbs were 10% longer than A. d. guildayi due to 10% longer femora and tibiae, and 15% longer metatarsals. A. d. dirus is comparable to the Yukon wolf in limb length.

The largest A. d. dirus femur was found in Carroll Cave, Missouri, and measured 278 mm (10.9 in).

Gray wolf skeleton (left) compared with a dire wolf skeleton

Aenocyon dirus guildayi compared with the Yukon wolf by the mean length of limb bones in millimeters (inches)

Limb variable

A. d. guildayi

Yukon wolf

A. d. dirus

Humerus (upper front leg)

218 mm (8.6 in)

237 mm (9.3 in)

240 mm (9.4 in)

Radius (lower front leg)

209 mm (8.2 in)

232 mm (9.1 in)

240 mm (9.4 in)

Metacarpal (front foot)

88 mm (3.4 in)

101 mm (4.0 in)

101 mm (4.0 in)

Femur (upper back leg)

242 mm (9.5 in)

251 mm (9.9 in)

266 mm (10.5 in)

Tibia (lower back leg)

232 mm (9.1 in)

258 mm (10.2 in)

255 mm (10.0 in)

Metatarsal (back foot)

93 mm (3.7 in)

109 mm (4.3 in)

107 mm (4.2 in)

A. d. guildayi is estimated to have weighed on average 60 kg (130 lb), and

A. d. dirus weighed on average 68 kg (150 lb) with some specimens being larger,

but these could not have exceeded 110 kg (240 lb) due to skeletal limits.

In comparison, the average weight of the Yukon wolf is 43 kg (95 lb) for males and 37 kg (82 lb) for females.

Individual weights for Yukon wolves can vary from 21 kg (46 lb) to 55 kg (121 lb), with one Yukon wolf weighing 79.4 kg (175 lb):

These figures show the average dire wolf to be similar in size to the largest modern gray wolf.

The remains of a complete male A. dirus are sometimes easy to identify compared to other Canis specimens because the baculum (penis bone) of the dire wolf is very different from that of all other living canids.

Adaptation

Ecological factors such as habitat type, climate, prey specialization, and predatory competition have been shown to greatly influence gray wolf craniodental plasticity, which is an adaptation of the cranium and teeth due to the influences of the environment. Similarly, the dire wolf was a hypercarnivore, with a skull and dentition adapted for hunting large and struggling prey; the shape of its skull and snout changed across time, and changes in the size of its body have been correlated to climate fluctuations.

Extinction

During the Quaternary extinction event around 12,700 YBP, 90 genera of mammals weighing over 44 kilograms (97 lb) became extinct.

The extinction of the large carnivores and scavengers is thought to have been caused by the extinction of the megaherbivore prey upon which they depended.

The cause of the extinction of the megafauna is debated[92] but has been attributed to the impact of climatic change, competition with other species including overexploitation by newly arrived human hunters, or a combination of both.

One study proposes that several extinction models should be investigated because so little is known about the biogeography of the dire wolf and its potential competitors and prey, nor how all these species interacted and responded to the environmental changes that occurred at the time of extinction.

Ancient DNA and radiocarbon data indicate that local genetic populations were replaced by others within the same species or by others within the same genus.

Both the dire wolf and the Beringian wolf went extinct in North America, leaving only the less carnivorous and more gracile form of the wolf to thrive, which may have outcompeted the dire wolf.

One study proposes an early origin of the dire wolf lineage in the Americas which led to its reproductive isolation, such that when coyotes, dholes, gray wolves, and Xenocyon expanded into North America from Eurasia in the Late Pleistocene there could be no admixture with the dire wolf.

Gray wolves and coyotes may have survived due to their ability to hybridize with other canids - such as the domestic dog - to acquire traits that resist diseases brought by taxa arriving from Eurasia. Reproductive isolation may have prevented the dire wolf from acquiring these traits.