Temporal range: Late Pleistocene – early Holocene 0.125–0.009 Ma
|Mounted skeleton, Sternberg Museum of Natural History|
The dire wolf (Canis dirus, "fearsome dog") is an extinct species of the genus Canis. It is perhaps one of the most famous prehistoric carnivores in North America along with its extinct competitor Smilodon fatalis, a member of the "saber-toothed cat" group. The dire wolf lived in the Americas during the Late Pleistocene epoch (125,000–10,000 years ago). The species was named in 1858, four years after the first specimen had been found. Two subspecies are recognized, these being Canis dirus guildayi and Canis dirus dirus. The species probably descended from Canis ambrusteri and evolved from it in North America. The largest collection of dire wolf fossils has been obtained from the Rancho La Brea Tar Pits in Los Angeles, California.
Dire wolf remains have been found across a broad range of habitats including the plains, grasslands, and some forested mountain areas of North America, and in the arid savannah of South America. The sites range in elevation from sea level to 2,255 metres (7,400 ft). Dire wolf fossils have rarely been found north of 42°N latitude, with five unconfirmed reports above this latitude. This range restriction is thought to be due to temperature, prey, or habitat imposed by proximity to the Laurentide Ice Sheet and the Cordilleran Ice Sheet that existed at that time.
The dire wolf was about the same size as the largest extant gray wolves (Canis lupus), which are the Yukon wolf and the Northwestern wolf. C. d. guildayi weighed on average 60 kg (130 lb) and C. d. dirus 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 biteforce at the canine tooth was the strongest of any known Canis species. These characteristics are thought to be adaptations for preying on Late Pleistocene megaherbivores, and in North America its prey are known to have included horses, sloths, mastodons, bison, and camels. As with other large Canis hypercarnivores today, the dire wolf was thought to have been a pack hunter. Its extinction occurred during the Quaternary extinction event along with its main prey species. Its reliance on megaherbivores has been proposed as the cause of its extinction, along with climate change and competition with other species, but the cause remains controversial. The latest dire wolf remains were dated to 9,440 years ago.
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 Canis dirus was found in the summer of 1854 at Evansville, Indiana. A fossilized jawbone was obtained by the geologist Joseph Granville Norwood from an Evansville collector, Francis A. Linck. The 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 (the first of a species that has a written description) at the Academy of Natural Sciences. 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 (Leidy 1854) was later renamed Canis indianensis (Leidy 1869) when Leidy found out that the name C. primaevus had previously been used by the British naturalist Brian Hodgson for the Dhole.
Canis indianensis (Leidy 1869) was first associated with C. dirus (Leidy 1858) by the zoologist Joel Asaph Allen in 1876 along with his discovery of Canis mississippiensis. As there were so few pieces of these three specimens, it was thought best to leave each listed under these three provisional names until more material could be provided to show their relationship. In 1912 the paleontologist John Campbell Merriam formally recognized them all under the name of C. dirus (Leidy 1858) because Leidy's use of this name for the species in 1858 preceded his use of C. indianensis in 1859. In agreement with Merriam, C. indianensis (Leidy 1869) was declared a synonym of C. dirus by the paleontologist Edward Troxell in 1915. In 1918 Merriam studied these fossils and proposed consolidating their names under the separate genus Aenocyon (from Aenos:terrible and cyon:wolf) to become Aenocyon dirus; however, not everyone agreed with this wolf departing from genus Canis, and the opinion of paleontologists remained divided. Canis ayersi (Sellards 1916) and Aenocyon dirus (Merriam 1918) were recognized as synonyms of C. dirus by the paleontologist Ernest Lundelius in 1972. All of the above taxa were declared as synonyms of C. dirus in 1979, according to Ronald M. Nowak.
There are two subspecies of C. dirus. In 1984 a study by Björn Kurten 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.
|Dire wolf divergence|
|Phylogeny of Canis dirus based on morphology:p148:p181|
In 1974, R. A. Martin proposed that the large wolf C. ambrusteri (Armbruster's wolf) was C. lupus. Nowak, Kurten, and Annalisa Berta proposed that C. dirus was not derived from C. lupus. In 1993 G. D. Goulet agreed with Martin and proposed that C. dirus was a variant of C. lupus. The three paleontologists Xiaoming Wang, Richard H. Tedford, and Ronald M. Nowak have proposed that C. dirus evolved from Canis armbrusteri,:181:p52 with Nowak stating that specimens found in Cumberland Cave, Maryland, appear to be C. armbrusteri diverging into C. dirus.:p243 The early wolf from China, Canis chihliensis, may have been the ancestor of both C. armbrusteri and the gray wolf C. lupus.:146 The sudden appearance of C. armbrusteri in North America during the Early Pleistocene suggests that this was an immigrant from Asia, as was C. lupus later in the Pleistocene.:144 In 2010, Franciso Prevosti proposed that C. dirus was a sister taxon to C. lupus.:472
Canis dirus lived in the late Pleistocene to the early Holocene (125,000–10,000 years before present or 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 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 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. armbrusteri;:93 then in 2009 Tedford formally published a description of the specimens and noted that, although they exhibited some morphological characteristics of both C. ambrusteri and C. dirus, he referred to them only as C. dirus.:146 These Nebraskan fossil specimens may represent the earliest record of C. dirus.:146
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, then it is also 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. Both C. armbrusteri and C. dirus share a number of characteristics (synapomorphy), which suggests an origin of C. dirus around 250,000 YBP in the open terrain of the mid-continent, and then later expanding eastward and displacing its ancestor C. ambrusteri.:181 The timing of C. dirus would therefore be 250,000 YBP in California and Nebraska, and later in Canada, the rest of the United States, Mexico, Venezuela, Ecuador, Bolivia, and Peru;:146 however, the identity of these earliest fossils is not confirmed.
A South American origin for C. dirus has been proposed. In South America, C. dirus specimens that have been 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.:148 One study found that C. dirus was the most evolutionary derived Canis species in the New World, and compared to C. nehringi was larger in the size and construction of its lower molars for more efficient predation.:113 For this reason, some researchers have proposed that C. dirus may have originated in South America.:116 In 2009 a proposal was made that C. ambrusteri was the common ancestor for both the North and South American wolves.:148 In the following year, a study yielded evidence that led to the conclusion that C. dirus and C. nehringi were the same species and thus that C. dirus had migrated from North America into South America.:472
Attempts to extract DNA from tarpit specimens have been unsuccessful. In 1992 an attempt was made to extract a mitochondrial DNA sequence from the skeletal remains of C. d. guildayi in order to compare its relationship to other Canis species. However, these remains had been removed from the La Brea pits and the attempt was unsuccessful because tar could not be removed from the bone material. 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.
The age of most dire wolf localities is determined solely by biostratigraphy. Canis dirus and Smilodon fatalis are the two most common carnivorans from the La Brea pits, but biostratigraphy is an unreliable indicator within asphalt deposits. However, some sites have been radiocarbon dated, with C. dirus specimens from 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.:T1 A specimen from Powder Mill Creek Cave, Missouri, was dated at 13,170 YBP.
Canis dirus was the largest of all Canis species.:52 A study of the length and circumference of femur bones has been used for estimating the mean body mass of Canis dirus guildayi to be 60 kg (130 lb) and Canis dirus dirus to be 68 kg (150 lb). Another study proposed that the increased stress caused in the humerus when running at maximum speed may have imposed a biomechanical upper limit on the body mass for C. d. dirus to be 110 kg (240 lb). In comparison, the mean body mass of the extant gray wolf is 40 kg (88 lb) (with the smallest specimen recorded at 12 kg (26 lb) and the largest at 80 kg (176 lb)). The figures show that, on average, dire wolves were approximately the same size as the extant Yukon wolf (Canis lupus pambasileus) and the extant Northwestern wolf (Canis lupus occidentalis), with the largest individuals possibly exceeding this size.
A comparison of leg size shows that the rear legs of C. d. guildayi were 8% shorter than the Northwestern wolf due to significantly shorter tibia and metatarsus, and was also slightly shorter in the lower bones of the front legs compared to the Northwestern wolf. Canis dirus dirus had significantly longer legs than C. d. guildayi, with 14.3% longer front legs and 9.7% longer back legs. This was due to 15% longer radii and metapodials in the forelimbs, and 10% longer humeri, femora, and tibiae in the rear limbs. Canis lupus is comparable to C. d. dirus in limb length.
The remains of a complete male C. 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 is the evolutionary process whereby an organism becomes better able to live in its habitat or habitats. Ecological factors including habitat type, climate, prey specialization, and predatory competition have been shown to greatly influence gray wolf craniodental plasticity, which is an adaptation of its cranium and teeth due to the influences of its environment. Similarly, C. dirus was a hypercarnivore with a skull and dentition that were adapted for hunting large and struggling prey; its skull shape and snout shape changed across time, and changes in its body size correlated with climate fluctuations.
The last glacial period, commonly referred to as the "Ice Age", spanned 125,000 – 14,500 YBP and was the most recent glacial period within the current ice age, which occurred during the last years of the Pleistocene era. The Ice Age reached its peak during the Last Glacial Maximum, when ice sheets commenced advancing from 33,000 YBP and reached their maximum positions 26,500 YBP. Deglaciation commenced in the Northern Hemisphere approximately 19,000 YBP and in Antarctica approximately 14,500 years YBP, which is consistent with evidence that glacial meltwater was the primary source for an abrupt rise in sea level 14,500 YBP. Access into northern North America was blocked by the Wisconsin glaciation. The fossil evidence from many continents points to the extinction mainly of large animals, termed Pleistocene megafauna, near the end of the last glaciation.
Coastal southern California from 60,000 YBP to the end of the Last Glacial Maximum was cooler and with a more balanced supply of moisture than today. During the Last Glacial Maximum, the mean annual temperature decreased from 11 °C (52 °F) down to 5 °C (41 °F) degrees, and annual precipitation had decreased from 100 cm (39 in) down to 45 cm (18 in). By 24,000 YBP, the abundance of oak and chaparral decreased but pines increased, which created open parklands similar to today's coastal montane/juniper woodlands. After 14,000 YBP, the abundance of conifers decreased and those of the modern coastal plant communities, including oak woodland, chaparral, and coastal sage scrub, increased. The Santa Monica Plain lies north of the city of Santa Monica and extends along the southern base of the Santa Monica Mountains, and 28,000 YBP – 26,000 YBP it was dominated by coastal sage scrub, but there were cypress and pines at higher elevations. The Santa Monica Mountains supported a chaparral community on its slopes and isolated coast redwood and dogwood in its protected canyons, along with river communities that included willow, red cedar, and sycamore. These plant communities suggest a winter rainfall similar to that of modern coastal southern California, but the presence of coast redwood that is now found 600 kilometres (370 mi) to the north indicates a cooler, moister, and less seasonal climate than today. This environment supported large herbivores that were prey for C. dirus and its competitors.
A range of animal and plant specimens that became entrapped and were then preserved in tar pits have been removed and studied so that researchers can learn about the past. The Rancho La Brea tar pits located near Los Angeles in southern California are a collection of pits of sticky asphalt deposits that differ in deposition time from 40,000 – 12,000 YBP. Commencing 40,000 YBP, trapped asphalt has been moved through fissures to the surface by methane pressure, forming seeps that can cover several square meters and be 9–11 metres (30–36 ft) deep. Over 200,000 specimens (mostly fragments) have been recovered from the tar pits, with the remains ranging from Smilodon to squirrels, invertebrates, and plants. The time period represented in the pits includes the Last Glacial Maximum when global temperatures were 8 °C (46 °F) lower than today, the Pleistocene–Holocene transition (Bølling-Allerød interval), the Oldest Dryas cooling, the Younger Dryas cooling from 12,800 – 11,500 YBP, and the American megafaunal extinction event 12,700 YBP when 90 genera of mammals weighing over 44 kilograms (97 lb) became extinct.
Isotope analysis can be used to identify some chemical elements, allowing researchers to make inferences about the diet of the species found in the pits. An isotope analysis of bone collagen extracted from La Brea specimens provides evidence that C. dirus, Smilodon fatalis), and the American lion (Panthera leo atrox) competed for the same prey. Their prey included "Yesterday's camel" (Camelops hesternus), the Pleistocene bison (Bison antiquus), the "dwarf" pronghorn (Capromeryx minor), the western horse (Equus occidentalis), and the "grazing" ground sloth (Paramylodon harlani) native to North American grasslands. The Columbian mammoth (Mammuthus columbi) and the American mastodon (Mammut americanum) were rare at La Brea. The horses remained mixed feeders and the pronghorns mixed browsers, but at the Last Glacial Maximum and its associated shift in vegetation the camels and bison were forced to rely more heavily on conifers.
A study of isotope data of La Brea C. dirus fossils dated 10,000 YBP provides evidence that the horse was at that time an important prey species, and that sloth, mastodon, bison, and camel were less common in the C. dirus diet. This indicates that C. dirus was not a prey specialist, and at the close of the late Pleistocene before its extinction it was hunting or scavenging off the most available herbivores.
Skull and dentition
Dentition relates to the arrangement of teeth in the mouth, with the dental notation for the upper-jaw teeth using the upper-case letters I to denote incisors, C for canines, P for premolars, and M for molars, and the lower-case letters i, c, p and m to denote the mandible teeth. Teeth are numbered using one side of the mouth and from the front of the mouth to the back. In carnivores, the upper premolar P4 and the lower molar m1 form the carnassials that are used together in a scissor-like action to shear the muscle and tendon of prey.:74 A study of dentition found that C. dirus was the most advanced, or evolutionary derived, Canis in the Americas: "Canis dirus is regarded here as the most derived species of the genus Canis in the New World. The following combination of derived characters separates C. dirus from all other species of Canis: P2 with a posterior cusplet; P3 with two posterior cusplets; M1 with a mestascylid, entocristed, entoconulid, and a transverse crest extending from the metaconid to the hyperconular shelf; M2 with entocristed and entoconulid.":50
A study of the estimated bite force at the canine teeth of a large sample of living and fossil mammalian predators, when adjusted for the body mass, found that for placental mammals the bite force at the canines (in Newtons/kilogram of body weight) was greatest in the dire wolf (163), then followed among the extant canids by the four hypercarnivores that often prey on animals larger than themselves: the African hunting dog (142), the gray wolf (136), the dhole (112), and the dingo (108). The bite force at the carnassials showed a similar trend to the canines. A predator's largest prey size is strongly influenced by its biomechanical limits. The morphology of C. dirus was similar to that of its living relatives, and assuming that C. dirus was a social hunter, then its high bite force relative to extant canids suggests that it preyed on relatively large animals. However, the bite force rating of the bone-consuming spotted hyena (117), challenged the common assumption that high bite force in the canines and the carnassials was necessary in order to consume bone.
A study of the cranial measurements and jaw muscles of C. dirus found no significant differences with extant C. lupus in all but 4 of 15 measures. Upper dentition was the same except that C. dirus had larger dimensions, and the P4 had a relatively larger, more massive blade that enhances slicing ability at the carnassial. The jaw of C. dirus had a relatively broader and more massive temporalis muscle, able to generate slightly more bite-force than lupus. However, due to the jaw arrangement C. dirus had lower temporalis leverage than C. lupus at the lower carnassial (m1) and lower p4, but the functional significance of this is not known. The lower premolars were relatively slightly larger than C. lupus, and the C. dirus m1 was much larger and had more shearing ability. The dire wolf canines had greater bending strength than those of extant canids of equivalent size. These differences indicate that C. dirus was able to deliver stronger bites, and together with flexible and more rounded canines shows that it was adapted for struggling with its prey. One study that compared the craniodental morphology of C. dirus to C. lupus noted the similarity in skull shape between the two, and the variations found in the skull shape of each, and proposed that C. dirus evolved in North America from C. lupus, which had originated in Eurasia.:p242
|Tooth variable||lupus extant
|lupus La Brea||lupus Beringia||dirus dirus Sangamon era
|dirus dirus Late Wisconsin
|m1 trigonid length||19.6||21.9||20.9||24.5||24.0||24.4|
At La Brea, predatory birds and mammals were attracted to dead or dying herbivores that had become mired, and then these predators became trapped themselves. Herbivore entrapment was estimated to have occurred once every fifty years, and for every instance of herbivore remains found in the pits there were an estimated ten carnivores. C. d. guildayi and Smilodon are the two most common carnivorans from La Brea, with C. d. guildayi the most common. Remains of C. dirus outnumber remains of lupus in the tar pits by a ratio of five to one. During the Last Glacial Maximum, coastal California, with a climate slightly cooler and wetter than today, is thought to have been a refuge, and a comparison of the frequency of C. dirus and other predator remains at La Brea to other parts of California and North America indicates significantly greater abundances; therefore, the higher C. dirus numbers in the La Brea region did not reflect the wider area. Assuming that only a small number of the carnivores that were feeding became trapped, it is likely that fairly sizeable groups of C. dirus fed together on these occasions.
The difference between the male and female of a species apart from their sex organs is called sexual dimorphism, and in this regard little variance exists among the canids. A study of C. dirus remains dated 15,360–14,310 YBP and taken from one pit that focused on skull length, canine tooth size, and lower molar length showed little dimorphism, similar to that of the gray wolf, indicating that C. dirus lived in monogamous pairs. Their large size and highly carnivorous dentition supports the proposal that C. dirus was a predator that fed on large prey. In order to kill ungulates larger than themselves the African wild dog, the dhole, and the gray wolf depend on their jaws because they cannot use their forelimbs to grapple with prey, and they work together as a pack that consists of an alpha pair and their offspring from the current and previous years. It can be assumed that C. dirus lived in packs of relatives that were led by an alpha pair. Large and social carnivores would have been successful at defending carcasses of trapped prey from smaller solitary predators, and thus the most likely to become trapped themselves. Smilodon and C. d. guildayi are the two most common carnivorans found at La Brea, which suggests that both were social predators.
All social terrestrial mammalian predators feed mostly on terrestrial herbivorous mammals with a body mass similar to the combined mass of the social group members attacking the prey animal. The large size of C. dirus provides an estimated prey size in the 300 to 600 kg (660 to 1,320 lb) range. Stable isotope analysis of C. dirus bones provides evidence that they had a preference for consuming ruminants such as bison rather than other herbivores but moved to other prey when food became scarce, and occasionally scavenged on beached whales along the Pacific coast when available. A pack of timber wolves can bring down a 500 kg (1,100 lb) moose that is their preferred prey, and a pack of dire wolves bringing down a bison is conceivable. Although some studies have suggested that because of tooth breakage, C. dirus must have gnawed bones and may have been a scavenger, its widespread occurrence and the more gracile limbs of C. d. dirus indicate a predator. The dire wolf probably used its molars to crack bones similar to the gray wolf, but due to their larger size it could crack larger bones to give access to bone marrow.
Tooth breakage is related to a carnivore's behavior. A study of nine modern carnivores found that one in four adults had suffered tooth breakage and that half of these breakages were of the canine teeth. The most breakage occurred in the spotted hyena that consumes all of its prey including the bone; the least breakage occurred in the African wild dog, and the gray wolf ranked in between these two. The eating of bone increases the risk of accidental fracture due to the relatively high, unpredictable stresses that it creates. The most commonly broken teeth are the canines, followed by the premolars, carnassial molars, and incisors. Canines are the teeth most likely to break because of their shape and function, which subjects them to bending stresses that are unpredictable in both direction and magnitude. The risk of tooth fracture is also higher when killing large prey.
A study of the fossil remains of large carnivores from La Brea pits dated 36,000–10,000 YBP shows tooth breakage rates of 5–17% for the C. dirus, coyote, American lion, and Smilodon, compared to 0.5–2.7% for ten modern predators. These higher fracture rates were across all teeth but not more often the canines when compared to the modern carnivores. The dire wolf broke its incisors more often when compared to the extant gray wolf; thus it has been proposed that C. dirus used its incisors more closely to the bone when feeding. Canis dirus fossils from Mexico and Peru show a similar pattern of breakage. A 1993 study proposed that the higher frequency of tooth breakage among Pleistocene carnivores compared with extant carnivores was not the result of hunting larger game because the former were larger than their modern counterparts. It is known that, when food is scarce, competition among carnivores is high, causing them feed more rapidly and thus consume bone more frequently. The study proposed that the tooth breakage was due to increased carcass consumption, including bone, due to low or seasonal prey availability, greater competition, or both. As their prey became extinct around 10,000 years ago, so too did these competing carnivores, except for the coyote (although coyotes are technically an omnivore).
A later La Brea pits study compared tooth breakage of C. dirus in two time periods. One pit contained fossil C. dirus dated 15,000 YBP and another dated 13,000 YBP. The results showed that the 15,000 YBP C. dirus had three times more tooth breakage than the 13,000 YBP C. dirus, whose breakage matched those of nine modern carnivores. The study concluded that between 15,000–14,000 YBP prey availability was less or competition was higher for C. dirus, and that by 13,000 YBP, as the prey species moved towards extinction, predator competition had declined and therefore the frequency of tooth breakage in C. dirus had also declined.
Carnivores include both pack hunters and solitary hunters. The solitary hunter depends on a powerful bite at the canine teeth to subdue their prey, and thus exhibits a strong mandibular symphysis. In contrast, a pack hunter, which delivers many shallower bites, has a comparably weaker mandibular symphysis. Thus, researchers can use the strength of the mandibular symphysis in fossil carnivore specimens to determine what kind of hunter it was – a pack hunter or a solitary hunter – and even how it consumed its prey. The mandibles of canids are buttressed behind the carnassial teeth in order to crack bones with their post-carnassial teeth (molars M2 and M3). A study found that the mandible buttress profile of C. dirus was lower than that of the gray wolf and the red wolf, but very similar to the coyote and the African hunting dog. The dorsoventrally weak symphyseal region (in comparison to premolars P3 and P4) of C. dirus indicates that it delivered shallow bites similar to its modern relatives and therefore was a pack hunter. This suggests that C. dirus may have processed bone but was not as well adapted for it as was C. lupus. The fact that the incidence of fracture for C. dirus reduced in frequency in the late Pleistocene to that of its extant relatives suggests that reduced competition had allowed C. dirus to return to a feeding behaviour involving a lower amount of bone consumption, a behaviour for which it was best suited. A later study found that just before extinction, C. dirus carcass utilization was less than among large carnivores today.
Past studies propose that changes in C. dirus body size correlates with climate fluctuations. A later study compared C. dirus craniodental morphology from four La Brea pits, each representing four different time periods. The results are evidence of a change in C. dirus size, dental wear and breakage, skull shape, and snout shape across time. Dire wolf body size had decreased between the start of the Last Glacial Maximum and near its ending at the warm Allerød oscillation. Evidence of food stress is seen in a smaller body size, skulls with a larger cranial base and shorter snout (shape neoteny and size neoteny), and more tooth breakage and wear. Canis dirus dated 17,900 YBP showed all of these features, which indicates food stress. Canis dirus dated 28,000 YBP also showed to a degree many of these features but were the largest wolves studied, and it was proposed that these wolves were also suffering from food stress and that wolves earlier than this date were even bigger in size. Nutrient stress is likely to lead to stronger bite forces to more fully consume carcasses and to crack bones, and with changes to skull shape to improve mechanical advantage. North American climate records revealed cyclic fluctuations during the glacial period that included rapid warming followed by gradual cooling, called Dansgaard–Oeschger events. These cycles would have caused increased temperature and aridity, and at La Brea would have caused ecological stress and therefore food stress. A similar trend was found with C. lupus, which in the Santa Barbara basin was originally massive, robust, and possibly convergent with Canis dirus, but was replaced by more gracile forms by the start of the Holocene.
|Variable||28,000 YBP||26,100 YBP||17,900 YBP||13,800 YBP|
|Snout shape||shortening, largest cranial base||average||shortest, largest cranial base||average|
|Tooth row shape||robust||-||-||gracile|
|DO event||number 3 or 4||none||imprecise data||imprecise data|
Just before the appearance of C. dirus, North America was invaded by the genus Xenocyon (ancestor of the Asian dhole and the African hunting dog) that was as large as C. dirus and more hypercarnivorous. The fossil record shows them as rare, and it is assumed that they could not compete with the newly derived C. dirus.:p60 Stable isotope analysis provides evidence that C. dirus, Smilodon, and the American lion competed for the same prey. Other large carnivores included the extinct North American giant short-faced bear (Arctodus simus), the extant cougar (Puma concolor), the Pleistocene coyote (Canis latrans), and the Pleistocene gray wolf that was more massive and robust than today. These predators may have competed with humans who hunted for similar prey.
Specimens that have been identified by morphology as Beringian wolves (C. lupus) and radiocarbon dated 25,800–14,300 YBP have been found in the Natural Trap Cave at the base of the Bighorn Mountains in Wyoming, in the western United States. The location is directly south of what would at that time have been a division between the Laurentide Ice Sheet and the Cordilleran Ice Sheet. A temporary channel between the glaciers may have existed that allowed these large, Alaskan direct competitors of the dire wolf, which were also adapted for predating on megafauna, to come south of the ice sheets. Dire wolves were absent north of 42°N latitude in the Late Pleistocene; therefore, this region would have been available for Beringian wolves to expand south along the glacier line. How widely they were then distributed is not known. These also became extinct at the end of the Late Pleistocene, as did the dire wolf.
Canis dirus remains have been found across a broad range of habitats including the plains, grasslands, and some forested mountain areas of North America, and in the arid savannah of South America. The sites range in elevation from sea level to 2,255 metres (7,400 ft). The location of these fossil remains suggests that C. dirus lived predominantly in the open lowlands along with its prey the large herbivores. Canis dirus does not occur at high latitudes, unlike its close relative, C. lupus.
In the United States, its fossils have been reported in Arizona, California, Florida, Idaho, Indiana, Kansas, Kentucky, Missouri, Nebraska, New Mexico, Oregon, Pennsylvania, South Carolina, South Dakota, Texas, Utah, Virginia, West Virginia, Wyoming, and Nevada. The identity of fossils reported farther north than California is not confirmed. There have been five reports of unconfirmed C. dirus fossils north of 42°N latitude at Fossil Lake, Oregon (125,000–10,000 YBP), American Falls Reservoir, Idaho (125,000–75,000 YBP), Salamander Cave, South Dakota (250,000 YBP), and four closely grouped sites in northern Nebraska (250,000 YBP). This suggests a range restriction on C. dirus due to temperature, prey, or habitat. The major fossil-producing sites for C. d. dirus are located east of the Rocky Mountains and include Friesenhahn Cave, near San Antonio, Texas; Carroll Cave, near Richland, Missouri; and Reddick, Florida.
Ten localities in the central and southeast-central areas of Mexico are known to contain Canis dirus: El Cedazo in Aguascalientes; Comondú Municipality in Baja California; El Cedral in San Luis Potosí; El Tajo Quarry near Tequixquiac, and Valsequillo near Puebla City in the Distrito Federal; Lago de Chapala in Jalisco; Loltun Cave in Yucatán; Potrecito in Sinaloa; and San Josecito Cave near Aramberri in Nuevo León. Of the central localities, San Josecito Cave and El Cedazo have the greatest number of individuals of C. dirus collected from a single locality. Other localities in Mexico have only a few specimens. A study of specimens from Sonora were confirmed as Canis dirus guildayi.
In South America, C. dirus has been dated younger than 17,000 YBP and reported from only three localities: Muaco in Falcón state, Venezuela; Talara Province in Peru; and Tarija Department in Bolivia. If C. dirus originated in North America, the species likely dispersed into 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 because the pathway then consisted of open, arid regions and savanna, whereas during inter-glacial periods it would have consisted of tropical rain forest.
Extinction is the result of the elimination of the geographic range of a species with a reduction of its population size down to zero. The factors that affect biogeographic range and population size include competition, predator/prey interactions, variables of the physical environment, and chance events. The American megafaunal extinction event occurred 12,700 YBP when 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, and it is assumed that this also explains the extinction of the dire wolf in both North and South America.
One study has proposed that a number of extinction models should be investigated because we know little about the biogeography of C. dirus 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. A study of the dental microwear on tooth enamel was undertaken for specimens of all of the carnivore species from La Brea pits, including C. dirus. The resulting evidence suggests that these carnivores were not food-stressed just before their extinction and that carcass utilization (i.e., consuming as much of a carcass as possible, including breaking up and consuming bones) was less than among large carnivores today. The extinction of C. dirus has been proposed to be based on its reliance on megaherbivores, the impact of climate change, and competition with other species, including humans, but the cause remains controversial.
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