Temporal range: Rancholabrean, 0.25–0.010Ma
The dire wolf (Canis dirus, "fearsome dog") is an extinct carnivorous mammal of the genus Canis, roughly the size of the extant gray wolf, but with a heavier build. It evolved in North America and later moved into South America. Canis dirus is assigned to the Rancholabrean land mammal age of North America (240,000–10,000 years BP) and was among the many large carnivores and megaherbivores that became extinct in North and South America near the end of the Pleistocene epoch. Salamander Cave in the Black Hills of South Dakota has produced the oldest known fossil of what is thought to be a dire wolf. The site preserves a fauna approximately 252,000 years old, based on uranium-series dating of a horse fossil found there. Its ending is associated with the Quaternary extinction event.
The dire wolf averaged about 1.5 m (5 ft) in length and weighed between 50 and 79 kg (110 and 174 lb), which made it the largest species in the genus Canis. Limb elements are rarely found outside the La Brea Tar Pits, which makes it hard to compare the size of average individuals between populations. The dire wolf is estimated to have been 8% shorter at the shoulder than the modern Northwestern wolf, and of equal height to the typical gray wolf, but more heavily built. With the exception of the canine teeth in some populations, male and female body and teeth sizes evidence no major sexual dimorphism, similar to most canines. In some populations, males’ canine teeth were considerably larger, suggesting male competition for breeding access. In other populations, lack of dimorphism in the canine teeth suggests little competition.
Despite superficial similarities to the gray wolf, the two species differed significantly. Today’s largest gray wolves would have been of similar size to an average dire wolf; the largest dire wolves would have been considerably larger than any modern gray wolf. The dire wolf is calculated to have weighed 25% more than living gray wolves.
Many of these characteristics were needed to fight off and prey on larger megafauna. The legs of the dire wolf were proportionally shorter and sturdier than those of the gray wolf, and its brain case was smaller than that of a similarly sized gray wolf.
Skull and dentition
The dire wolf's teeth were similar to the gray wolf's, only slightly larger, pointing to a hypercarnivorous to mesocarnivorous activity. The dietary characteristics were primarily carnivorous, as well as partially omnivorous. Other than size, there is no significant special adaptations in the mandibular corpus of the dire wolf to set it apart from the gray wolf.
Between C. lupus and C. dirus, C. lupus had a significantly greater mechanical advantage of the temporalis muscle over C. dirus at the lower carnassial and P4 (MATM1, MATP4).
The slicing teeth (P4, the carnassial) on the upper jaw of C. dirus are larger than those of the gray wolf, but those on the lower jaw are similar. The temporalis of the dire wolf could generate more force than seen in modern gray wolves, suggesting stronger killing bites.
Many paleontologists have proposed that the dire wolf may have used its relatively large teeth to crush bone, an idea supported by the frequency of large amounts of wear on the crowns of their fossilized teeth. The upper carnassial had much larger blade than that of the gray wolf, indicating greater slicing ability. It had a longer temporal fossa and broader zygomatic arches, indicating the presence of a large temporalis muscle capable of generating slightly more force than a gray wolf's. However, other scientists have noted the dorsoventral and labiolingual force profiles are indistinguishable from those of other canids, such as coyotes and African wild dogs, indicating a similar diet.
Dire wolf teeth lacked the craniodental adaptations of habitual bonecrushers such as hyenas and borophagines. The dire wolf's robust skull and susceptibility to tooth breakage suggests that it hunted for large prey or scavenged the carcasses of large prey. The dorsoventrally weak symphyseal region indicates it killed in a manner similar to its modern relatives, by delivering a series of shallow bites, strongly indicating pack hunting behaviour. However, the incidence of broken postcarnassial molars is much higher than in fossil gray wolves, indicating the species was probably less adapted to bone crushing than the gray wolf.
Behavior and ecology
Dire wolves are part of the same carnivorous guild as the smaller gray wolves and coyotes. Dire wolves' overpowering bite, 129% of the force of the modern gray wolf, could hold and subdue their prey. As inferred from their large bodies and carnivorous teeth, they often took on large prey or megafauna, made possible by traveling in packs. Dire wolves were not specialized hunters—they fed on whatever megafauna was abundant.
Compared to modern species, a remarkable number of dire wolf specimens from the La Brea pits showed evidence of having broken their teeth in life. Specimens in the older part of the pit exhibited more tooth wear than those in the younger pit, which could be a result of either the older part of the pit containing more senior dire wolf specimens or a diet that included harder foods such as carcasses and bones. Another explanation for increased tooth wear and breakage could be intense competition among carnivores forcing C. dirus to eat as much of the carcass as quickly as possible.
The dire wolf has an extremely extensive fossil range. It is known to be located as far north as Canada and south into southern Bolivia. Ten localities in Mexico are known to contain Canis dirus: Cedazo, Aguascalientes; Comondu, Baja California; El Cedral, San Luis Potosí; El Tajo Quarry, Tequixquiac, and Valsequillo, all in Distrito Federal, Mexico; Lago de Chapala, Jalisco; Loltun Cave, Yucatán; Potrecito, Sinaloa; and San Josecito Cave, Nuevo León. The majority of these localities occur in the central and southeast-central portions of Mexico with few occurrences in the north or west. Of the central localities, San Josecito Cave and Cedazo have the greatest number of individuals of Canis dirus collected from a single locality; other localities in Mexico are known by only a few specimens.
Due to the large amount of remains at the Rancho La Brea tar pits it is possible dire wolves used animals trapped here as a food source and some end up being trapped and preserved themselves. It can also be concluded that they were social predators as they are most successful in defending carcasses.
Habitat and distribution
The habitat of C. dirus varied considerably. In North America, it ranged from plains and grasslands to forested mountain areas. In South America, it occupied areas of arid savannah. The dire wolf lived in several habitats, tropical marsh with thorn-scrub to deciduous forest including some component of nearby grassland, and from sea level up to 2255 m (7400 feet). It was widespread, and its remains have been found in 136 places, from Alberta, Canada, to Tarija, Bolivia. Although the dire wolf was recorded as far north as Alberta, its remains have never been found at higher latitudes. The dire wolf may have migrated to northern Canada during interglacial periods, but its remains would likely have been obliterated by later glacial activity. However, the last named species of dire wolf was said to be found in North America stemming from Florida. The largest known dire wolves specimens have also said to have been recovered from this area of North America. So even though the dire wolf has migrated it may have found to be seasonal.
The dire wolf is well known for its unusually high representation in La Brea Tar Pits in California. Over 200,000 fragments representing more than 4,000 individual dire wolves have been recovered from the tar pits, more than any other mammal species. This large number suggests the dire wolf, like modern wolves and dogs, hunted in packs. The abundance of remains of the gray wolf (C. lupus, also known as C. furlongi) in the tar pits is about 1% that of the dire wolf.
Canis dirus was named by Joseph Leidy in 1858 and recombined as Aenocyon dirus by Merriam (1918), Hibbard (1949) and Hibbard and Taylor in 1960. In 1916, Canis ayersi was named by Sellards. It was recombined as Aenocyon ayersi by Merriam in 1918 and was synonymized subjectively with C. dirus by Lundelius in 1972, Martin (1974), Nowak (1979), Kurten and Anderson (1980) and Kurten in 1984. Leidy also named the dire wolf as Canis indianensis in 1869 which was synonymized subjectively with C. dirus by Troxell in 1915. Canis mississippiensis was named by Allen in 1876 and synonymized subjectively with Canis dirus by Nowak (1979), Kurten and Anderson (1980) and again by Kurten in 1984.
The type specimen of the dire wolf was found in Evansville, Indiana, in the summer of 1854, when the Ohio River was quite low. The specimen, a fossilized jawbone, was obtained by Joseph Granville Norwood from an Evansville collector named Francis A. Linck. Norwood, who at that time was the first state geologist of Illinois, sent the specimen to Joseph Leidy at the Academy of Natural Sciences in Philadelphia. Leidy determined the specimen represented an extinct species of wolf and published a note to that effect in November 1854. In a publication dated 1858, Leidy assigned the name Canis dirus.
Norwood's letters to Leidy are preserved along with the type specimen at the Academy of Natural Sciences, although one of the letters indicates the specimen was to be returned to Linck's family, as Linck died in August 1854.
Whether the dire wolf originated in North America versus South America is the subject of controversy. Most paleontologists lean toward a North American origin for three reasons: first, more potential progenitors are present in the middle Pleistocene of North America; second, distribution of C. dirus is much better represented in North America, with 136 sites versus only three localities in South America; and last, C. dirus appears earlier in the fossil record in North America than South America. A North American origin implies that C. dirus migrated into South America from North and Central America. The dire wolves most likely migrated to South America via the Andean corridor, a proposed pathway for temperate mammals to migrate from Central to South America because of the favorable cool, dry and open habitats that characterized the region at times. This most likely happened during a glacial period, however, as the pathway then consisted of open, arid regions and savanna whereas during inter-glacial periods, it would have been characterized by tropical rain forest habitat.
The fossil record suggests that the genus Canis diverged from the small, foxlike Leptocyon in North America sometime in the Late Miocene epoch 9 to 10 million years (Ma) ago, along with two other genera, Urocyon, and Vulpes. Canids soon spread to Asia and Europe (8 Ma BP) and became the ancestors of modern wolves, jackals, foxes, and the raccoon dog.
By 3–5 Ma BP, canids had spread to Africa (Early Pliocene) and South America (Late Pliocene). Their invasion of South America as part of the Great American Interchange was enabled by the formation of the Isthmus of Panama 3 Ma ago.
Over the next nine million years, extensive development and diversification of the North American wolves took place by the Middle Pleistocene. Canis armbrusteri appeared, possibly from C. chihliensis in Asia. There is good evidence that the dire wolf evolved from C. armbrusteri, with the two taxa sharing in the open plains and grasslands of what is now the central United States.
C. dirus eventually displaced C. armbrusteri, with the latter's final range shrinking to what is now the southeastern U.S., more specifically Florida. While this occurred, C. dirus expanded its range to include that of C. armbrusteri and moved into Central America and South America, appearing in the Late Pleistocene fossil record in northwestern South America.
Two subspecies of the dire wolf are known to have inhabited what is now the United States. C. dirus guildayi was smaller and ranged west of the Rocky Mountains. C. dirus dirus was larger and ranged east of the Rockies.
In 2010, a genetic study of extinct South American canids indicated that the South American Canis nehringi and Canis dirus were the same species, with the Gray wolf Canis lupus being in a sister clade. 
Although it was closely related to the gray wolf and other sister species, C. dirus is not the direct ancestor of any modern species. Unlike the gray wolf, which is of Eurasian origin, the dire wolf evolved on the North American continent, along with the coyote. The dire wolf co-existed with the gray wolf in North America for about 100,000 years.
The dire wolf was one of the abundant Pleistocene megafauna—a wide variety of large mammals that lived during the Pleistocene. Approximately 10,000 years ago the dire wolf became extinct along with most other North American megafauna.
During the Late Pleistocene (300,000 years ago) the gray wolf (C. lupus) crossed into North America on the Bering Strait land bridge and competed with the dire wolf. Overlapping fossil findings of the extinct saber tooth cat, the Smilodon, shows that the dire wolf had these as competition in North America. Both species were social animals that hunt in packs and preyed on the same animals. Starting about 16,000 years ago, coinciding with the end of the last glacial period and the arrival of humans in North America, most of the large mammals upon which the dire wolf depended for prey began to die out, possibly as a result of climate and/or human-induced changes.
Slower than the other wolf species on the continent at the time, like the gray wolf and red wolf, the dire wolf could not hunt the remaining swifter prey, and was forced to subsist on scavenging. By approximately 10,000 years ago, the large mammals and the dire wolf were extinct. Their demise, along with that of other large Pleistocene carnivores, was found to be related to the extinction of megafaunal prey.
More information is required in order to understand the extinction of C. dirus, including information on its biogeographical range, population size, competition, interactions with predators and prey, and its physical environment. Further discoveries about the dire wolf's interactions with its competition and prey around the time of the extinction events would also be integral in understanding why C. dirus went extinct.
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