Temporal range: Late Pleistocene to Early Holocene, 0.013–0.011Ma
|Composite male skeleton in the Page Museum at the La Brea Tar Pits, Los Angeles|
The Columbian mammoth (Mammuthus columbi) was a species of mammoth, the common name for the extinct elephant genus Mammuthus. The Columbian mammoth lived during the Pleistocene epoch, and inhabited North America from the United States, across Mexico, and as far south as Costa Rica. It was one of the last in a line of mammoth species, beginning with M. subplanifrons in the early Pliocene. The Columbian mammoth diverged from the steppe mammoth, M. trogontherii, which had entered North America about 1.5 million years ago. Its closest extant relative is the Asian elephant. The pygmy mammoths, M. exilis, of the Channel Islands were descended from Columbian mammoths.
The Columbian mammoth reached 4 m (13 ft) at the shoulders, and weighed up to about 10 tonnes. It had long, curved tusks and four molars, which were replaced six times during the lifetime of an individual. Its behaviour was probably similar to that of modern elephants, and it used its tusks and trunk for manipulating objects, fighting, and foraging. Hair, dung and stomach contents have been discovered, but no preserved carcasses are known. The Columbian mammoth preferred open areas, such as parkland landscapes, and fed on sedge, grass, and other plant. It did not live in the Arctic regions of Canada, which were instead inhabited by woolly mammoths. The range of the two species may have overlapped, and genetic evidence suggests that they interbred. Many Columbian mammoths skeletons are known from the same sites, some of these are the results of single incidents such as flash floods, or natural traps in which individuals were accumulated over time.
The Columbian mammoth coexisted with Palaeoamericans, who used its bones for making tools, and the species was also hunted for food and depicted in ancient art. Columbian mammoth remains have been found in association with Clovis culture artefacts, but it is unknown how many of these sites represent human hunting of mammoths. The Columbian mammoth disappeared at the end of the Pleistocene 12,850 years ago, most likely through climate change and consequent shrinkage of its habitat, hunting by humans, or a combination of the two.
The Columbian mammoth was first scientifically described by Scottish naturalist Hugh Falconer in 1857, who named the extinct proboscidean Elephas columbi, after Christopher Columbus, the discoverer of America. The animal had been brought to Falconer's attention by Charles Lyell in 1846, who had sent some fragmentary molars found during the 1838 excavation of the Brunswick Altamaha Canal in Georgia, in the south eastern United States. At the time, such fossils from all of North America were attributed to woolly mammoths (then Elephas primigenius), but Falconer found them to be distinct, and confirmed this after studying additional molars from Mexico, and after examining the internal structure of a Brunswick Canal molar. William Phipps Blake and Richard Owen claimed the name E. texianus was more appropriate for the species, but this was rejected by Falconer, who also suggested that E. imperator and E. jacksoni were based on too fragmentary molars to be properly classified. More material that may be from the same quarry as Facloner's holotype molar was reported in 2012.
The taxonomy of extinct elephants was complicated by the early 20th century, and in 1942, Henry Fairfield Osborn's posthumous monograph on the Proboscidea was published, wherein he used various genus and subgenus names that had previously been proposed for mammoth species, such as Archidiskodon, Metarchidiskodon, Parelephas, and Mammonteus. Osborn also retained names for many regional and intermediate subspecies or "varieties", and created recombinations such as Parelephas columbi felicis and Archidiskodon imperator maibeni. Mammoth taxonomy was simplified by various researchers from the 70s onwards, all species were retained in the genus Mammuthus, and many proposed differences between species were instead interpreted as intraspecific variation. In 2003, palaeontologist Larry Agenbroad summarised current views about North American mammoth taxonomy, and concluded that several species had been declared synonyms, and that M. columbi and M. exilis were the only species of mammoth endemic to the Americas. The idea that species such as the imperial mammoth (M. imperator) and Jefferson's mammoth (M. jeffersoni) were either more primitive or advanced stages in Columbian mammoth evolution was largely dismissed, and they were regarded as junior synonyms. In spite of these conclusions, Agenbroad cautioned that American mammoth taxonomy is not yet resolved.
The earliest known proboscideans, the clade that contains the elephants, existed about 55 million years ago around the Tethys Sea area. The closest living relatives of the Proboscidea are the sirenians and the hyraxes. The family elephantidae existed six million years ago in Africa, and includes the living elephants and the mammoths. Among many now extinct clades, the mastodon is only a distant relative of the mammoths, and part of the distinct family Mammutidae, which diverged 25 million years before the mammoths evolved. The following cladogram shows the placement of the Columbian mammoth among other proboscideans, based on hyoid characteristics:
Since many remains of each species of mammoth are known from several localities, it is possible to reconstruct the evolutionary history of the genus through morphological studies. Mammoth species can be identified from the number of enamel ridges (or lamellar plates) on their molars; primitive species had few ridges, and the number increased gradually as new species evolved and replaced the preceding ones. The crowns of the teeth lengthened and the skulls became taller to accommodate this. At the same time, the skulls became shorter from front to back to minimise the weight. The short and tall skulls of woolly and Columbian mammoths are the culmination of this process. These adaptations were acquired gradually as mammoths turned to more abrasive food items.
The first known members of the genus Mammuthus are the African species M. subplanifrons from the Pliocene, and M. africanavus from the Pleistocene. The former is thought to be the ancestor of later forms. Mammoths entered Europe around 3 million years ago. The earliest type known from there has been named M. rumanus, which spread across Europe and China. Only its molars are known, which show that it had 8–10 enamel ridges. A population evolved 12–14 ridges, splitting off from and replacing the earlier type, becoming M. meridionalis about 2–1.7 million years ago. In turn, this species was replaced by the steppe mammoth, M. trogontherii, with 18–20 ridges, which evolved in eastern Asia c. 2-1.5 million years ago. The Columbian mammoth evolved from a population of M. trogontherii (formerly thought to have been M. meridionalis) that had crossed the Bering Strait and entered North America about 1.5 million years ago, and retained a similar number of molar ridges. Mammoths derived from M. trogontherii evolved molars with 26 ridges 400,000 years ago in Siberia and became the woolly mammoth, M. primigenius. Woolly mammoths entered North America about 100,000 years ago.
A 2011 study of the complete mitochondrial genome of the Columbian mammoth showed that the two examined specimens, including the morphologically typical "Huntington mammoth", were grouped within a subclade of woolly mammoths. This suggests that the two populations interbred and produced fertile offspring, but it is unknown whether the results mean that the Columbian mammoth haplogroup had been introduced to the woolly mammoths through Introgression and dominated that species, or the other way around. The latter scenario is similar to behaviour seen in modern species of African elephant (Loxodonta), the African bush elephant (L. africana) and the African forest elephant (L. cyclotis). The males of the former species are larger, and therefore out-compete males of the smaller species. The same behaviour may have occurred between the larger Columbian and smaller woolly mammoths, and the authors of the study also suggested that the North American form sometimes referred to as M. jeffersonii may have been a hybrid between the two species, as it is morphologically intermediate. These findings were not expected by scientists, and more specimens will have to be analysed to clarify the situation.
A population of Columbian mammoths that lived between 50,000 and 13,000 years ago on the Channel Islands of California, 10 km (6 miles) away from the mainland, evolved very small size. They were 1.2-1.8 m (4-6 ft) tall at the shoulders and weighed 1 ton, which is less than half the size of the mainland Columbian mammoths, and they are therefore considered the distinct species M. exilis, the pygmy mammoth (or a subspecies, M. c. exilis). These mammoths presumably reached the islands by swimming there, and decreased in size due to scarcity of food on the small areas. Bones of larger mammoths have also been found on the islands, but it is unknown whether these were stages in the dwarfing process, or later arrivals to the islands.
The shoulder height of the Columbian mammoth reached 4 m (13 ft), and it weighed up to about 10 tonnes, which is equal to the weight of 130 adult humans. This is larger than modern African elephants and the woolly mammoth, which reach about 2.7 to 3.4 m (9–11 ft), and about the same size as the earlier mammoth species M. meridionalis and M. trogontherii. Males were generally larger and more robust, while females were smaller and more lightly built. The best indication of sex is the size of the pelvic girdle, as the birth canal is always wider in females than in males. Like other mammoths, Columbian mammoths had a high, single-domed head and a sloping back with a high shoulder hump; the shape resulting from the spinous processes of the back vertebrae decreasing in length from front to rear. These features were not present in juveniles, which had convex backs like Asian elephants. Other skeletal features include a rostrum that was short and deep, a rounded mandibular symphysis, and the coronoid process of the mandible extending above the molar surfaces.
Apart from their larger size and more "primitive" molars, Columbian mammoths also differed from woolly mammoths by the mandibular symphysis being more down turned, and the dental alveoli of the tusks being directed laterally, more away from the midline. Soft tissue of the Columbian mammoth has not been found, so much less is known about its appearance than that of the woolly mammoth. They lived in warmer habitats than the woolly mammoth, so they probably lacked many of the adaptations seen in that species. The tail of the Columbian mammoth was intermediate in length between that of modern elephants and the woolly mammoth. Hair believed to have belonged to the Columbian mammoth has been discovered in the Bechan Cave, Utah, where mammoth dung has also been found. Some of this hair is coarse, and identical to that known to belong to woolly mammoths. Since this location is so far south, it is unlikely that the hair belonged to woolly mammoths. The distribution and density of this fur on the living animal is unknown, but it was probably less dense than that of the woolly mammoth, due to the warmer habitat.
Columbian mammoths had very long tusks, which were more curved than those of modern elephants. The largest known mammoth tusk belonged to a Columbian mammoth and is 4.9 metres (16 ft) long. Others range between 3.5 metres (11 ft) and 4.121 metres (13.52 ft) long, and Columbian mammoth tusks were usually not much larger than those of woolly mammoths, which reached 4.2 metres (14 ft). The tusks of females were much smaller and thinner. About a quarter of the length was inside the sockets. The tusks grew spirally in opposite directions from the base and continued in a curve until the tips pointed towards each other, sometimes crossing. In this way, most of the weight would have been close to the skull, and there would be less torque than with straight tusks. The tusks were usually asymmetrical and showed considerable variation, with some tusks curving down instead of outwards and some being shorter due to breakage. Columbian mammoth tusks were generally less twisted than those of woolly mammoths. Calves developed small milk tusks a few centimetres long at six months old, which were replaced by permanent tusks a year later. Tusk growth continued throughout life but became slower as the animal reached adulthood. The tusks grew by 2.5–15 cm (0.98–5.91 in) each year.
Columbian mammoths had four functional molar teeth at a time, two in the upper jaw and two in the lower. About 23 cm (9.1 in) of the crown was within the jaw, and 2.5 cm (1 in) was above. The crown was continually pushed forwards and up as it wore down, comparable to a conveyor belt. The teeth had separated ridges of enamel, which were themselves covered in "prisms" that were directed towards the chewing surface. These were quite wear resistant and kept together by cementum and dentine. A mammoth had six sets of molars throughout a lifetime, which were replaced five times. The first molars were about the size of those of a human, 1.3 cm (0.51 in), the third were 15 cm (6 in) 15 cm (5.9 in) long, and the sixth were about 30 cm (1 ft) long and weighed 1.8 kg (4 lb). The molars grew larger and contained more ridges with each replacement. The amount of plates also varied between individuals. It took about 10.6 years to grow 180.9 mm of ridge.
As in modern elephants, the sensitive and muscular trunk worked as a limb-like organ with many functions. It was used for manipulating objects, and in social interactions. Adult mammoths could effectively defend themselves from predators with their tusks, trunks and size, but juveniles and weakened adults were vulnerable to pack hunters such as wolves and large felines. Bones of juvenile Columbian mammoths were accumulated by Homotherium, the scimitar-toothed-cat, in Friesenhahn Cave, Texas, but no animals could have hunted healthy adult mammoths. The tusks may have been used in intra-species fighting, such as territorial fights or fights over mates. Display of the large tusks of males could also have been used to attract females, and to intimidate rivals. Two Columbian mammoths from Nebraska that died with their tusks interlocked provide evidence of fighting behaviour. The mammoths used their tusks as weapons by thrusting them, swiping them, or crashing them down. They also used the tusks in pushing contests by interlocking them, which sometimes resulted in breakage. Because of their curvature, the tusks were not suitable for stabbing. On Goat Rock Beach in the Sonoma Coast State Park of California, blueschist and chert outcrops now nicknamed "Mammoth Rocks" show evidence of having been rubbed by Columbian mammoths or mastodons. The rocks have polished areas 3-4 ms (10-13 ft) above the ground, mainly near their edges, and are similar to the rubbing rocks in present day Africa, used by elephants and other herbivores to rid themselves of mud and parasites. Similar sites are known from Hueco Tanks in Texas, and Cornudas Mountain in New Mexico.
Accumulations of modern elephant remains have been termed "elephants' graveyards", as these sites were erroneously thought to be where old elephants went to die. Similar accumulations of mammoth bones have been found; it is thought these are the result of individuals dying near or in rivers over thousands of years, and their bones eventually being brought together by the streams (such as in the Acuilla River in Florida), or due to animals being mired in mud. Some accumulations are also thought to be the remains of herds that died together at the same time, perhaps due to flooding. Many specimens also accumulated in "natural traps" such as sink holes and tar pits. Columbian mammoths are also rarely preserved in volcanic deposits, such as in Tocuila, Mexico, where volcanic lahar mud flow covered at least seven individuals 12,500 years ago. It is unknown how many mammoths that lived at one location at a time, but it is likely that the amount varied by season and life-cycle events. Modern elephants form large herds, sometimes formed by multiple family groups, and these herds can include thousands of animals migrating together. Mammoths may have formed herds more often than modern elephants, as animals that live in open areas are more likely to form herds than those in forested areas. It is unclear to what extent Columbian mammoths migrated, but an isotope study of Columbian mammoths from Blackwater Draw in New Mexico indicated that they had spent part of the year in the Rocky Mountains, 200 km (124 miles) away. Study of tusk rings may also help in further study of mammoth migration.
Like modern elephants, Columbian mammoths were likely very social and lived in matriarchal (female led) family groups. It is therefore probable that most of their other social behaviour was similar to those of modern elephants. This is supported by fossil assemblages, such as in the Dent Site in Colorado and the Waco Mammoth Site in Waco, Texas, where groups consisting entirely of female and juvenile Columbian mammoths have been found, implying female-led family groups. The latter assemblage includes 22 skeletons, with 15 individuals representing a herd of females and juveniles that died in a single event, probably a flash flood; the arrangement of some of the skeletons may imply that the females had formed a defensive ring around the juveniles. At the same site, another group consisting of a bull and six females also appears to have been killed by a flash flood; both incidents occurred between 64,000 and 73,000 years ago, but it is unknown whether the two groups died in the same event. At the Murray Springs Site in Arizona, where several Columbian mammoth skeletons have been excavated, a trackway similar to those of modern elephants leads up to one of the skeletons. This has been interpreted as having either been placed by the dead individual before it died, or as belonging to another individual that approached the dead or dying animal, similar to how modern elephants guard dead relatives for days.
The Hot Springs Mammoth Site in South Dakota represents a 26,000 year old and ca. 40 metre long (130 ft) sink hole that operated for around 300 to 700 years, before it was filled with sediment. The site represents the opposite scenario of that in Waco; all but one of the at least 55 skeletons (additional skeletons are excavated each years) are male, accumulated over time rather than in a single event. It is assumed that like modern male elephants, male mammoths mainly lived alone and were more "adventurous" (especially the young adults), therefore more likely to end up in dangerous situations than the more cautious females. The mammoths may have lured near the hole by warm water or vegetation near the edges, then slipped in and died of drowning or starvation. Isotope studies of growth rings have shown that most of the mammoths there had died during spring and summer, which may have correlated with the amount of vegetation near the sink hole. One individual, nicknamed "Murray", lies sprawled on its side, and probably ended in this pose while struggling to get free. Deep footprints that resulted from mammoths attempting to free themselves from the mud of the sink hole can be seen in vertically excavated sections of the site.
Excavations at the La Brea Tar Pits in Los Angeles, California, have since the early 20th century yielded 100 tonnes of fossils representing 600 species of flora and fauna, including several Columbian mammoths. The fossils from the tar pits are the remains of animals that became stuck in asphalt puddles that seeped to the surface of the pits, mainly between 40,000 and 11,500 years ago. Mired animals would die from hunger or exhaustion, and their corpses would attract predators, which were subsequently stuck themselves; the fossil record of the tar pits is dominated by predator remains, such as large canids and felids. Fossils of various different animals are found stuck together when excavated from the pits. The tar pits do not preserve soft tissue, and a 2014 study concluded that asphalt may degrade DNA of animals deposited in it, after attempting to extract DNA from a Columbian mammoth from the tar pits.
An adult Columbian mammoth would have needed more than 180 kg (400 lb) of food, and may have foraged as long as twenty hours every day. Mammoths chewed their food by using their powerful jaw muscles to move the mandible forwards and close the mouth, then backwards while opening, which made the sharp enamel ridges cut across each other and grind the food. The ridges were wear-resistant to enable the animal to chew large quantities of food, which often contained grit. The trunk could be used for pulling off large grass tufts, picking buds and flowers, and tearing off leaves and branches where trees and shrubs were present. The tusks were also used for obtaining food, such as digging up plants and stripping off bark. This is indicated on many preserved tusks by flat, polished sections on the part of the surface that would have reached the ground. Isotope studies of Columbian mammoths from Mexico and the United States have shown that their diet consisted of a mix of C3 and C4 plants, which means they were not restricted to either grazing or browsing, and that their diet depended on the location.
Stomach contents from Columbian mammoths are rare, since no carcasses are known, but plant remains were found between the pelvis and ribs of the "Huntington mammoth" when it was excavated in Utah. These chewed remains were shown to consist of sedge, grass, fir twigs and needles, oak, and maple, through microscopic analysis. A large amount of Columbian mammoth dung has also been found in two caves in Utah. The dry conditions and stable temperature of the Bechan Cave ("Bechan" is Navajo for large faeces) has preserved 16,000 to 13,500 thousand year old elephant dung most likely belonging to Columbian mammoths. The dung consists of 95% grass and sedge, with 0-25% woody plants between boluses, including saltbush, sagebrush, water birch, and blue spruce. This is similar to the diet documented for the woolly mammoth, though browsing seems to have been more important for the Columbian mammoth in comparison. The cover of dung is 41 cm (16 in) thick, and has a volume of 227 m³ (8,000 cubic ft), with the largest boluses being 20 cm (8 in) in diameter. The Bechan dung could have been produced by a small group of mammoths over a relatively short time, since adult African elephants drop 11 kg (25 lb) of dung on average every second hour, and 90–135 kg (200-300 lb) each day.
It has been proposed that giant North American fruits of plants such as the Osage-orange, Kentucky coffee and Honey locust evolved in tandem with now extinct American megafauna, such as mammoths and other proboscideans, as there are no extant endemic herbivores able to ingest these fruits and disperse their seeds. Introduced cattle and horses have since taken over this ecological role.
Since modern elephants have gestation periods of 21–22 months, this was possibly true for mammoths as well. The lifespan of mammals is related to their size, and since modern elephants can reach the age of 60 years, the same is thought to be true for woolly mammoths which were of a similar size. Since Columbian mammoths were even larger, they may have lived up to 80 years. The age of a mammoth can be roughly determined by counting the growth rings of its tusks when viewed in cross section, but this does not account for their early years, as these are represented by the tips of the tusks, which are usually worn away. In the remaining part of the tusk, each major line represents a year, and weekly and daily lines can be found in between. Dark bands correspond to summers, and it is therefore possible to determine the season in which a mammoth died. The growth of the tusks slowed when it became harder to forage, for example during disease, or when a male was banished from the herd. Mammoths continued growing past adulthood, like other elephants. Unfused limb bones show that males grew until they reached the age of 40, and females grew until they were 25. At the age of 6–12 months, the second set of molars would be in the process of erupting, and the first set would be worn out at 18 months of age. The third set of molars lasted for ten years, and this process was repeated until the final, sixth set emerged when the animal was 30 years old. When the last set of molars was worn out, the animal would be unable to chew and feed, and it would die of starvation.
Almost all vertebrae of the "Huntington mammoth", a very old specimen, were deformed by arthritic disease, and four of its lumbar vertebrae were fused. Some bones also have evidence of bacterial infections, such as osteomyelitis. The condition of the bones suggests the specimen died of old age and malnutrition.
Distribution and habitat
Columbian mammoths inhabited the southern half of North America, ranging from the United States, across Mexico, and as far south as Costa Rica. The single Costa Rican specimen, a molar, was reported in 1963, but has since been lost. The environment here may have had more varied habitats than those areas inhabited by woolly mammoths in the north (mammoth steppe). Some areas were covered by grass, herbaceous plants, trees and shrub; the exact composition varied from region to region, including grassland, savannah-like, and parkland habitats. There were also various woodlands, and though mammoths would not have preferred forests, clearings within them could provide them with grass and herbs.
The Columbian mammoth shared its habitat with other now-extinct Pleistocene mammals such as Glyptodon, the sabre-toothed cat (Smilodon), ground sloths, Camelops, the distantly related American mastodon (Mammut americanum), as well as horses and buffalos. It did not live in the Arctic regions of Canada, which were instead inhabited by woolly mammoths. Fossils of woolly mammoths and Columbian mammoths have been found in the same localities in a few areas of North America where their regions overlapped, including the Hot Springs Site. It is unknown whether the two species were sympatric and lived there simultaneously, or if the woolly mammoths may have entered these southern areas during times when Columbian mammoth populations were absent there. The Columbian mammoth also co-occurred alongside the extinct proboscideans Stegomastodon mirificus and Cuvieronius tropicus at sites in Texas and New Mexico during the early Irvingtonian.
Relationship with humans
Humans entered the Americas through Beringia, and various lines of evidence document their interactions with Columbian mammoths. Columbian mammoth remains modified to make tools have been discovered in several sites in North America. At Tocuila, mammoth bones were quarried to produce lithic flakes and lithic cores, 13,000 years ago. An adult and juvenile mammoth were found alongside two cleaver choppers made from a mammoth shoulder blade at the Lange-Ferguson Site in South Dakota, and a microscope study confirmed it had been used to butcher the carcasses, 12.800 years ago. A flake knife made from mammoth long bone was also found wedged against some vertebrae. At Murray Springs, a 13,100 year old object made from a mammoth femur is thought to be shaft wrench, a tool for straightening wood and bone.
It is believed that Columbian mammoths were hunted by Palaeoamericans of the Clovis culture, named after the Clovis site in New Mexico. The Clovis hunters used characteristic spear points, known as Clovis points, which have been found in association with Columbian mammoth remains at several sites. There is some disagreement on whether these finds represent hunting or scavenging of dead mammoths, or accidental association of Clovis points and mammoth bones. A female mammoth at the Naco-Mammoth Kill Site in Arizona was found with eight Clovis points near its skull, shoulder blade, ribs and other bones, and is considered the most convincing evidence for hunting. It is unknown whether Clovis hunters threw or thrusted their spears, but modern experiments have shown that replicas were able to penetrate the ribcage of African elephants and that reuse led to little damage of the points.
Other sites show more circumstantial evidence, as the piled bones bear butcher marks, but are not closely associated with Clovis points. The Lehner Mammoth-Kill Site and the Dent Site, where multiple juvenile and adult mammoths have been found with butcher marks and in association with Clovis points, have been interpreted as the killing of entire herds by Clovis hunters. Isotope studies have instead shown that the accumulations represent individual deaths at different seasons of the year, and therefore not herds killed in single incidents. Many other such assemblages of bones with butcher marks may also represent accumulations over time, and are therefore ambiguous as evidence for large scale hunting. There is evidence of mammoth butchering dated to as much as 21,500 to 22,000 years ago, 7000 years before the Clovis culture, but Clovis may have been the first humans to hunt mammoths extensively.
Petroglyphs in the Colorado plateau may depict Columbian mammoths, if they are not mastodons instead. A 13,000 year old bone fragment from Vero Beach, Florida, with an engraving of either a mammoth or a mastodon was shown to be authentic in 2009, and thus the earliest example of art in the Americas. 11,000-13,000 year old petroglyphs thought to depict two Columbian mammoths and a bison are known from the San Juan River in Utah. A mastodon identity has been ruled out due to the animals being depicted with domed heads. They are also shown with two "fingers" on their trunks, a feature known from European depictions of mammoths. The tusks are short, which may indicate they are meant to be females. Though the bison is less worn than the mammoths, and possibly a later addition, it may depict the extinct Bison antiquus. Other possible depictions of American proboscideans have been shown to be either misinterpretations or fraudulent.
In 1998, the Washington State Legislature approved the Columbian mammoth as the state fossil. Additionally, a specimen found in Nebraska in 1922 and named "Archie" is that state's official fossil. "Archie" is currently on display at Elephant Hall in Lincoln, Nebraska, and is the largest mounted mammoth specimen in the United States.
The Columbian and woolly mammoths both disappeared during the late Pleistocene and early Holocene, alongside most of the Pleistocene megafauna, during the Quaternary extinction event, which began 40,000 years ago, and peaked between 14,000 and 11,500 years ago. 40 mammal species disappeared from North America, which accounts for 70% of the total, almost all weighing over 40 kg (90 lb), so the extinction of the mammoths cannot be explained in isolation. Scientists are divided over whether hunting or climate change was the main factor that contributed to the extinction of the Columbian mammoths, or whether it was due to a combination of the two. The exact sequence of events and whether extinctions happened abruptly or were drawn out is not clear either.
The "overkill hypothesis" was first proposed by Paul S. Martin in 1967. A 2002 study concluded that only 14 Clovis sites (12 with mammoth remains, 2 with mastodons) out of 76 examined provided strong evidence of hunting, and concluded that the archaelogical record did not support the hypothesis that humans caused Pleistocene extinctions in America. A 2007 study instead pointed out that the Clovis record indicated the highest frequency of prehistoric exploitation of proboscideans for subsistence in the world. According to the climate change hypothesis, warmer weather led to the shrinkage of suitable habitat for Columbian mammoths, which turned from parkland to forest, grassland and semi-desert with less diverse vegetation. Whatever the actual cause of extinction was, large mammals are generally more vulnerable than smaller ones due to their smaller population size and low reproduction rates. The last Columbian mammoths have been dated to 12,850 years ago.
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