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Dire wolf
Temporal range: Late Pleistocene – early Holocene 0.125–0.009 Ma
↓
Mounted skeleton
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Carnivora
Family: Canidae
Genus: Canis
Species:
†C. dirus
Binomial name
†Canis dirus
Leidy, 1858.[1]
Subspecies
Synonyms

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, the "sabre-toothed cat". The dire wolf lived in the Americas during the Late Pleistocene epoch (125,000–10,000 years ago). The species was named in 1858 but the first specimen had been found in 1854. 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 m (7,400 feet). 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; however, 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 adaptions for preying on Late Pleistocene megaherbivores, and in North America its prey is known to have included horse, sloth, mastodon, 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.

Taxonomy

Display at the Page Museum of 404 dire wolf skulls found in the La Brea Tar Pits[9]

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 the one species. The first specimen of what would later become associated with Canis dirus[10] was found in the summer of 1854 at Evansville, Indiana. A fossilized jawbone was obtained by Joseph Granville Norwood from Francis A. Linck, an Evansville collector. Joseph Leidy determined the specimen represented an extinct species of wolf and reported it under the name of Canis primaevus.[3] 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 which he reported the following year under the name C. dirus.[1] The name C. primaevus (Leidy 1854) was later renamed Canis indianensis (Leidy 1869) when Leidy discovered that the name C. primaevus had previously been used overseas.[4]

Canis indianensis (Leidy 1869) was first associated with C. dirus (Leidy 1858) by Allen in 1876 along with his discovery of Canis mississippiensis. As there were so few pieces of these 3 specimens, it was thought best to leave each listed under these 3 provisional names until more material could be provided to show their relationship.[5] In 1912, 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.[11] In agreement with Merriam, C. indianensis (Leidy 1869) was declared a synonym with C. dirus by Troxell in 1915.[12] 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,[7] however not everyone agreed with this wolf departing from genus Canis and the opinion of paleontologists remained divided.[13] Canis ayersi (Sellards 1916) and Aenocyon dirus (Merriam 1918) were recognized as synonyms with C. dirus by Lundelius in 1972.[14][15] All of the above taxa were declared as synonyms with C. dirus in 1979, according to R. M. Novak.[16]

There are two subspecies of C. dirus. In 1984, a study by 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.[2][17][18][19]

Evolution

See also: Evolution of the wolf
Dire wolf divergence
†Canis chihliensis
Canis lupus

†Canis armbrusteri
†Canis gezi

†Canis nehringi

†Canis dirus
†Canis dirus guildayi

†Canis dirus dirus

Phylogeny of Canis dirus based on morphology.[20]: p148 [21]: p181 

The three noted paleontologists Xiaoming Wang, Richard H. Tedford and R. M. Nowak proposed that C. dirus evolved from the large wolf Canis armbrusteri[21]: 181 [20]: p52  (Armbruster's wolf),[22] with Nowak stating that there were specimens from Cumberland Cave, Maryland that indicate C. armbrusteri diverging into C. dirus.[23][24]: p243  The early wolf from China, Canis chihliensis, may have been the ancestor of both C. armbrusteri and the gray wolf C. lupus.[21]: 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.[21]: 144  Martin disagreed and believed that C. ambrusteri was C. lupus,[25] with Goulet agreeing with Martin and proposing that the C. dirus was a variant of C. lupus.[26] Nowak, Kurten and Berta disagreed with Goulet and proposed that C. dirus was not derived from C. lupus.[16][27][28] Prevosti later proposed that C. dirus was a sister of C. lupus.[29]: 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.[2] The majority of fossils from the eastern C. d. dirus have been dated to 125,000–75,000 YBP but the western C. d. guildayi fossils are more recent and smaller in size, therefore it has been proposed that C. d. guildayi derived from C. d. dirus.[2][19] However, there are disputed specimens of C. dirus that date back 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 record a description of them. Nowak later referred to this material as C. armbrusteri,[16]: 93  however more recently Tedford formally recorded 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 as C. dirus.[21]: 146  These may represent the earliest record of C. dirus.[8][21]: 146 

A fossil discovered in the Horse Room of the Salamander Cave in the Black Hills of South Dakota may possibly be C. dirus, and if so then it is also one of the earliest on record.[30][18] The specimen was catalogued as Canis cf. C. dirus[31] (where cf. in Latin means confer, uncertain). The fossil of a horse found in the 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.[18][31] 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 in the midcontinent, and then later expanding eastward and displacing its ancestor C. ambrusteri.[21]: 181  The timing of C. dirus would therefore be the 250,000 YBP in California and Nebraska, and the later in Canada, the rest of the United States, Mexico, Venezuela, Ecuador, Bolivia and Peru,[21]: 146  however the identity of these earliest fossils are not confirmed.[32]

An artistic rendition of two possible appearances of the dire wolf: one based on a North American origin (left) and the other on a South American origin (right)[33]

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 that was inhabited by Canis gezi and Canis nehringi.[21]: 148  One study found that C. dirus was the most evolutionary derived genus Canis species in the New World, and compared to C. nehringi was larger in size and construction of its lower molars for more efficient predation.[28]: 113  Therefore, some researchers have proposed that C. dirus may have originated in South America.[18][27][16]: 116  In 2009, a proposal was made that C. ambrusteri was the common ancestor for both the North and South American wolves.[21]: 148  In the following year, a study indicated that C. dirus and C. nehringi were the same species, which supports C. dirus having migrated from North America into South America.[29]: 472 

DNA analysis

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.[34] 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 and hinders the extraction of DNA samples.[35]

Radiocarbon dating

The age of most dire wolf localities is determined solely by biostratigraphy. Canis dirus and the sabre-tooth cat (Smilodon fatalis) are the two most common carnivorans from the La Brea pits but biostratigraphy is an unreliable indicator within asphalt deposits.[36][37] 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.[32]: T1  A specimen from Powder Mill Creek Cave, Missouri was dated at 13,170 YBP.[18]

Description

Dire wolf skeleton compared with that of the gray wolf (left)

Canis dirus was the largest of all Canis species.[20]: 52  A study of the length and circumference of femur bones have 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).[19] 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).[38] 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)).[19][39][40][41][42] The figures indicate that on average, dire wolves were the same size as the extant Yukon wolf (Canis lupus pambasileus) and the extant Northwestern wolf (Canis lupus occidentalis),[21] with the largest individuals possibly exceeding this size.[19]

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. It was also slightly shorter in the lower bones of the front legs. Canis dirus dirus had significantly longer legs than C. d. guildayi, with 14.3% longer front legs and a 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.[43][44]

The remains of a complete C. dirus are sometimes easy to identify compared to other Canis specimens because the baculum of the dire wolf is very different from that of all other living canids.[43][18]

Adaption

Ecological factors including habitat type, climate, prey specialization and predatory competition have been shown to greatly influence gray wolf craniodental plasticity, which is an adaption of its cranium and teeth due to the influences of its environment.[45][46][47] Similarly, C. dirus was a hypercarnivore that possessed a skull and dentition that were adapted for hunting large and struggling prey,[48][49][50] its skull shape and snout shape changed across time, and changes in its body size correlated with climate fluctuations.[51]

Paleoecology

The last glacial period, commonly referred to as the 'Ice Age', spanned 125,000 YBP[52] to 14,500 YBP[53] and was the most recent glacial period within the current ice age, which occurred during the last years of the Pleistocene era.[52] 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 this was the primary source for an abrupt rise in the sea level 14,500 YBP.[53] 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.[54]

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 down to 5 degrees C and the 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 between 28,000 YBP – 26,000 YBP it was dominated by coastal sage scrub but there were cypress and pines at the more 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 indicate a winter rainfall similar to that of modern coastal southern California, but the presence of coast redwood that is now found 600 kilometers 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.[55]

Prey

Two dire wolves and Smilodon on the carcass of a Columbian mammoth at the La Brea tar pits by R. Bruce Horsfall[56]

A range of animal and plant specimens have been preserved in tar pits and later removed and studied so that researchers can learn about the past. The Rancho La Brea tar pits located near Los Angeles, southern California are a collection of sticky asphalt deposits made up of pits which differ in deposition time from 40,000–12,000 YBP. Commencing 40,000 YBP, trapped asphalt has been moved to the surface through fissures by methane pressure that formed seeps, which can cover several square meters in area and 9–11 meters deep.[36] Over 200,000 specimens (fragments) have been recovered from the tar pits,[19] where the remains range from saber-toothed cats to squirrels, invertebrates and plants.[36] The time period includes the Last Glacial Maximum when global temperatures were eight degrees C. 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 became extinct.[37][51]

Isotope analysis can be used to identify some chemical elements that allows researchers to make inferences about diet. An isotope analysis of bone collagen extracted from La Brea specimens indicates that C. dirus, the sabre-tooth cat (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, however at the Last Glacial Maximum and its associated shift in vegetation the camels and bison were forced to rely more heavily on conifers.[55]

A study of isotope data of La Brea C. dirus fossils dated 10,000 YBP indicates 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.[48][57] 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.[57]

Skull and dentition

Wolf mandible diagram showing the names and positions of the teeth

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."[28]: 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 feliform, the spotted hyena (117), challenged the common assumption that high bite force in the canines and the carnassials was necessary in order to consume bone.[50]

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 which indicates enhanced slicing ability at the carnassial. The jaw indicated that C. dirus had a relatively broader and more massive temporalis muscle, indicating that it could 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 and lower P4 but the functional significance of this is not known. The lower premolars were relatively slightly larger than C. lupus,[49] and the C. dirus M1 was much larger and had more shearing ability.[27][49][11] The dire wolf canines had greater bending strength than those of extant canids of equivalent size.[58] These differences indicate that C. dirus was able to deliver stronger bites, and together with flexible and more rounded canines indicates that it was adapted for struggling with its prey.[48][49] 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 that had originated in Eurasia.[24]: p242 [26]

The skull of the Dire wolf[59]
Canis lupus and Canis dirus compared by mean mandible tooth measurements (millimeters)
Tooth variable lupus extant[60] lupus La Brea[60] lupus Beringia[60] dirus dirus Sangamon era[2][48]

(125,000–75,000 YBP)

dirus dirus Late Wisconsin[2][48]

(50,000 YBP)

dirus guildayi[2][48]

(40,000–13,000 YBP)

m1 length 28.2 28.9 29.6 36.1 35.2 33.3
m1 width 10.7 11.3 11.1 14.1 13.4 13.3
m1 trigonid length 19.6 21.9 20.9 24.5 24.0 24.4
p4 length 15.4 16.6 16.5 16.7 16.0 19.9
p4 width - - - 10.1 9.6 10.3
p2 length - - - 15.7 14.8 15.7
p2 width - - - 7.1 6.7 7.4

Behavior

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.[36][61] Herbivore entrapment was estimated to have occurred once every fifty years,[61] and for every herbivore remains that was found in the pits there were an estimated ten carnivores.[36] C. d. guildayi and the sabre-tooth cat are the two most common carnivorans from La Brea,[37] with C. d. guildayi the most common.[51] Remains of C. dirus outnumber remains of lupus in the tar pits by a ratio of five to one.[32] During the Last Glacial Maximum, coastal California is thought to have been a refuge with the climate slightly cooler and wetter than today,[57] and a comparison of the frequency of C. dirus and other predator remains at La Brea to other parts of California and North America indicated significantly greater abundances, therefore the higher C. dirus numbers in the La Brea region did not reflect the wider area.[62] 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.[63]

Skeleton from the La Brea Tar Pits mounted in running pose

Sexual dimorphism is the difference between the male and female of a species apart from their sex organs and there is little variance among the canids regardless of their group size. A study of the skull length, canine tooth size, and lower molar length of C. dirus remains, dated between 15,360–14,310 YBP and taken from one pit, showed little dimorphism and therefore indicated that C. dirus lived in monogamous pairs. The level of dimorphism was similar to that of the gray wolf.[63] Their large size and highly carnivorous dentition indicates that C. dirus was a predator that feeds on large prey.[63][64][65] 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 consist 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.[63] Large and social carnivores would have been successful at defending carcasses of trapped prey from smaller solitary predators, and the most likely to become trapped themselves, which indicates that both C. d. guildayi and Smilodon fatalis were social predators.[62][66]

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.[38][67] The large size of C. dirus provides an estimated prey size in the 300 kg (660 lb) to 600 kg (1,300 lb) range.[19][64][65] Stable isotope analysis of C. dirus bones indicate 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.[19][49][68] A pack of timber wolves can bring down a 500 kg (1,100 lb) moose that is their preferred prey,[19][69] and a pack of dire wolves bringing down a bison is conceivable.[19] Although some studies have suggested that because of tooth breakage that C. dirus 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, however due to their larger size it could crack larger bones to give access to bone marrow.[49]

Tooth breakage

A study has proposed that tooth breakage is related to behavior. The study of nine modern carnivores found that one in four adults had suffered tooth breakage, half of these breakages were of the canine teeth, with the most breakage occurring in the spotted hyena that is known to consume all of its prey including the bone.[70][71] A later study of the fossil remains of large carnivores from La Brea pits dated 36,000–10,000 YBP indicated tooth breakage rates of 5–17% for the C. dirus, coyote, American lion and the saber-tooth cat, compared to 0.5–2.7% for 10 modern predators. These higher fracture rates were across all teeth and not more often the canine teeth as when compared to the modern carnivores. The dire wolf broke its incisors more often when compared to the extant gray wolf, therefore it was 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. This study proposed that the higher frequency of tooth breakage compared with extant carnivores was not the result of hunting larger game because the Pleistocene carnivores were larger than their modern counterparts. Carnivores feed more rapidly when food is scarce and competition is high, often consuming bone. The study proposed that the breakage was due to increased carcass consumption, including bone, due to low or seasonal prey availability or greater competition, or both.[51][71][72] 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).[71][72]

A later La Brea pits study compared tooth breakage of C. dirus between 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 that competition was higher for C. dirus, and by 13,000 YBP as the prey species moved towards extinction that predator competition had declined and therefore the frequency of tooth breakage in C. dirus had also declined.[72][73]

In carnivores, the solitary hunters depend on a powerful bite at the canine teeth to subdue their prey and these have a strong mandibular symphysis, while the pack hunters that deliver many shallow bites have a relatively weaker one. 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 packhunter. This suggests that C. dirus may have processed bone but was not as well adapted at this as C. lupus.[74] The incidence of fracture for C. dirus reduced in frequency in the late Pleistocene to that of its extant relatives[71][73] indicates that reduced competition allowed C. dirus to return to a feeding behaviour with a lower amount of bone consumption for which it was best suited for.[72][74] A later study found that just before extinction, C. dirus carcass utilization was less than among large carnivores today.[75]

Climate impact

Past studies have indicated that changes in C. dirus body size correlates with climate fluctuations.[37][51][76][77][78] A later study compared C. dirus craniodental morphology from four La Brea pits, each representing four different time periods. The results indicated 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. Food stress is indicated by a smaller body size, skulls with the larger cranial base and shorter snouts (shape neoteny and size neoteny), and more tooth breakage and wear. Canis dirus dated 17,900 YBP showed all of these features, which indicated 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 to this date were even bigger in size.[51] Nutrient stress is likely to lead to stronger bite forces to more fully consume carcasses and to crack bones,[51][79] 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.[51] 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.[16][51][26]

Canis dirus information based on skull measurements[51]
Variable 28,000 YBP 26,100 YBP 17,900 YBP 13,800 YBP
Body size largest large smallest medium/small
Tooth breakage high low high low
Tooth wear high low high low
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

Competitors

Smilodon and dire wolf skeletons near ground sloth bones

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.[20]: p60  Stable isotope analysis indicates that C. dirus, the sabre-toothed cat, and the American lion competed for the same prey.[55][75] 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.[75]

Specimens that have been identified by morphology as Beringian wolves (C. lupus) and radiocarbon dated between 25,800–14,300 YBP have been recently found in the Natural Trap Cave at the base of the Bighorn Mountains in Wyoming, 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, that 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.[32]

Range

Restoration of a pack in Rancho La Brea, by Charles R. Knight, 1922[80]

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 m (7400 feet).[18] The location of these fossil remains suggest that C. dirus lived predominantly in the open lowlands along with its prey the large herbivores.[16] Canis dirus does not occur at high latitudes, unlike its close relative, C. lupus.[18]

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,[10] and Nevada.[81] However, the identity of fossils reported further north than California are not confirmed.[10][32] 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).[10][32] This indicates a range restriction on C. dirus due to temperature, prey, or habitat.[32] The major fossil-producing sites for C.d. dirus are located east of the Rocky Mountains and include Friesenhahn Cave, Texas, Carroll Cave, Missouri, and Reddick, Florida.[19]

Ten localities in the central and southeast-central areas of 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. Of the central localities, San Josecito Cave and 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.[48]

In South America, C. dirus has been dated younger than 17,000 YBP and reported from only three localities: Muaco, Venezuela; Talara, Peru; and Tarija, Bolivia.[18] If C. dirus originated in North America, the species likely dispersed into South America via the Andean corridor,[18][28] 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.[18][82]

Extinction

The American megafaunal extinction event occurred 12,700 YBP when 90 genera of mammals weighing over 44 kilograms became extinct.[37][51] The extinction of the large carnivores and scavengers is thought to be caused by the extinction of their megaherbivore prey,[18][57][71][73][83][84][85][86][87] when the dire wolf became extinct in both North and South America.[18] However, 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 evidence suggests that these carnivores were not food-stressed just before their extinction and that carcass utilization was less than among large carnivores today.[75] 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[75] but the cause remains controversial.[88]

The latest dire wolf remains were dated 9,440 YBP and found in Brynjulfson Cave, Boone County, Missouri.[27]

See also

References

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  88. ^ Brannick, Alexandria L.; Meachen, Julie A.; O'Keefe, F. Robin (September 15, 2015). John M. Harris (ed.). "Microevolution of Jaw Shape in the Dire Wolf, Canis dirus, at Rancho La Brea" (PDF). Science Series 42. Contributions in Science (A special volume entitled La Brea and Beyond: the Paleontology of Asphalt-Preserved Biotas in commemoration of the 100th anniversary of the Natural History Museum of Los Angeles County's excavations at Rancho La Brea). Natural History Museum of Los Angeles County: 23–32. {{cite journal}}: Cite journal requires |journal= (help)
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