Columbian mammoth

One of the largest mounted mammoth skeletons in the world, nicknamed "Archie", at Nebraska State Museum of Natural History, is the type specimen of the junior synonym Archidiskodon imperator maibeni

The earliest known members of Proboscidea, 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 (dugongs and manatees) and the hyraxes (an order of small, herbivorous mammals). 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 (Mammut) is only a distant relative, and part of the distinct family Mammutidae, which diverged 25 million years before the mammoths evolved.[6] The Asian elephant (Elephas maximus) is the closest extant relative of the mammoths. The following cladogram shows the placement of the Columbian mammoth among other proboscideans, based on characteristics of the hyoid bone in the neck:[7]

Excavation and skeleton of a pygmy mammoth, which evolved from Columbian mammoths on the Channel Islands

Since many remains of each species of mammoth are known from several localities, reconstructing the evolutionary history of the genus is possible 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 to feed on more abrasive food items. The crowns of the teeth became taller in height and the skulls became taller to accommodate this. At the same time, the skulls became shorter from front to back to reduce the weight of the head.[8][9] The short, tall skulls of woolly and Columbian mammoths are the culmination of this process.[6]

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 European mammoth has been named M. rumanus; it 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 2.0–1.7 million years ago. In turn, this species was replaced by the steppe mammoth (M. trogontherii) with 18–20 ridges, which evolved in eastern Asia around 2.0–1.5 million years ago.[8] The Columbian mammoth evolved from a population of M. trogontherii that had crossed the Bering Strait and entered North America about 1.5 million years ago; it 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.[6]

A population of Columbian mammoths that lived between 80,000 and 13,000 years ago on the Channel Islands of California, 10 km (6.2 mi) away from the mainland, evolved to be less than half the size of the mainland Columbian mammoths.[10] They are, therefore, considered to be the distinct species M. exilis, the pygmy mammoth (or a subspecies, M. c. exilis). These mammoths presumably reached the islands by swimming there when sea levels were lower, and decreased in size due to the limited food provided by the islands' small areas. Bones of larger specimens have also been found on the islands, but whether these were stages in the dwarfing process, or later arrivals of Columbian mammoths is unknown.[7][11][12]

Specimen formerly assigned to the junior synonym M. jeffersonii, now suggested to be a hybrid between Columbian and woolly mammoths, at the American Museum of Natural History (left), and 1909 life restoration by Charles R. Knight based on the same specimen

A 2011 ancient DNA study of the complete mitochondrial genome (inherited through the female line) showed that two examined Columbian mammoths, 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. One possible explanation is introgression of a haplogroup from woolly to Columbian mammoths, or vice versa. A similar situation has been documented in modern species of African elephant (Loxodonta), the African bush elephant (L. africana) and the African forest elephant (L. cyclotis). The authors of the study also suggest that the North American type formerly referred to as M. jeffersonii may have been a hybrid between the two species, as it is apparently morphologically intermediate.[13] These findings were not expected by scientists; nuclear DNA and more specimens will have to be examined to clarify the situation.[14]

A 2015 study of mammoth molars confirmed that M. columbi evolved from Eurasian M. trogontherii, not M. meridionalis as had been suggested earlier, and noted that M. columbi and M. trogontherii were so similar in morphology that their classification as separate species may be questionable. The study also suggested that the animals in the range where M. columbi and M. primigenius overlapped formed a metapopulation of hybrids with varying morphology.[15] In 2016, a genetic study of North American mammoth specimens confirmed that M. columbi and M. primigenius interbred extensively, were both descended from M. trogontherii, and concluded that morphological differences between fossils may, therefore, not be reliable for determining taxonomy. The authors also questioned whether M. columbi and M. primigenius should be considered "good species", considering that they were able to interbreed after supposedly being separated for a million years, but cautioned that more specimens need to be sampled.[16]

In 2021, DNA older than a million years was sequenced for the first time, from two mammoth teeth of Early Pleistocene age found in eastern Siberia. One tooth from Adyocha (1-1.3 million years old) belonged to a lineage that was ancestral to later woolly mammoths, whereas the other from Krestovka (1.1–1.65 million years old) belonged to new lineage, possibly a distinct species, perhaps descended from steppe mammoths that had become isolated. The study found that half of the ancestry of Columbian mammoths came from the Krestovka lineage, and the other half from woolly mammoths, with the hybridization happening more than 420,000 years ago, during the Middle Pleistocene. Later woolly and Columbian mammoths also interbred occasionally, and mammoth species perhaps hybridized routinely when brought together by glacial expansion. These findings were the first evidence of hybrid speciation from ancient DNA. The study also found that genetic adaptations to cold environments, such as hair growth and fat deposits, were already present in the steppe mammoth lineage, and was not unique to woolly mammoths.[17][18]

Tusk during excavation in Nevada, with human for scale (left), and complete molars (numbers m3-m6), Waco Mammoth National Monument (right)

Columbian mammoths had very long tusks (modified incisor teeth), which were more curved than those of modern elephants. The largest known mammoth tusk, 4.9 m (16 ft) long, belonged to a Columbian mammoth, and others range from 3.5 to 4.12 m (11.5 to 13.5 ft) long. Columbian mammoth tusks were usually not much larger than those of woolly mammoths, which reached 4.2 m (14 ft). The tusks of females were much smaller and thinner. About a quarter of the tusks' length was inside the sockets; they grew spirally in opposite directions from the base, curving until the tips pointed towards each other, and sometimes crossed. Most of their weight would have been close to the skull, with less torque than straight tusks would have generated. The tusks were usually asymmetrical, with considerable variation; some tusks curved down, instead of outwards, or were shorter due to breakage. Columbian mammoth tusks were generally less twisted than those of woolly mammoths. At six months of age, calves developed milk tusks a few centimeters long, which were replaced by permanent tusks a year later. Annual tusk growth of 2.5–15 cm (0.98–5.91 in) continued throughout life, slowing as the animal reached adulthood.[19]

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 crowns of the lower jaw were pushed forward and up as they wore down, comparable to a conveyor belt. The teeth had separated ridges of enamel, which were covered in "prisms" directed towards the chewing surface. Wear-resistant, they were held together with cementum and dentin. A mammoth's molars were replaced five times over the animal's lifetime. The first molars were about the size of those of a human, 1.3 cm (0.51 in); the third ones were 15 cm (5.9 in) long, and the sixth ones were about 30 cm (1 ft) long and weighed 1.8 kg (4 lb). With each replacement, the molars grew larger and gained more ridges; the number of plates varied between individuals.[19][22] Growing 18 cm (7.1 in) of ridge took about 10.6 years.[23]

Female "mammoth W" (left) and male "Q", with juveniles "R" and "T" (right), at the Waco Mammoth National Monument

Like modern elephants, Columbian mammoths were probably social and lived in matriarchal (female-led) family groups; most of their other social behavior was also similar to that of modern elephants. This is supported by fossil assemblages such as the Dent site in Colorado and the Waco Mammoth National Monument 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. The herd was originally proposed to have been killed by a flash flood, and the arrangement of some of the skeletons suggests that the females may have formed a defensive ring around the juveniles. In 2016, the herd was suggested to have died by drought near a diminishing watering hole; scavenging traces on the bones contradict rapid burial, and the absence of calves and the large diversity of other animal species found gathered at the site support this scenario. Another group, consisting of a bull and six females, was found at the same site; although both groups died between 64,000 and 73,000 years ago, whether they died in the same event is unknown. At the Murray Springs Clovis Site in Arizona, where several Columbian mammoth skeletons have been excavated, a trackway similar to that left by modern elephants leads to one of the skeletons. The mammoth may have made the trackway before it died, or another individual may have approached the dead or dying animal—similar to the way modern elephants guard dead relatives for several days.[19][27][28]

Skulls (left) and mandible (right) in a lahar deposit at the Paleontological Museum in Tocuila

Accumulations of modern elephant remains have been called "elephants' graveyards", because these sites were erroneously thought to be where old elephants went to die. Similar accumulations of mammoth bones have been found; these are thought to be the result of individuals dying near or in rivers over thousands of years and their bones being accumulated by the water (such as in the Aucilla River in Florida), or animals dying after becoming mired in mud. Some accumulations are thought to be the remains of herds that died at the same time, perhaps due to flooding. Columbian mammoths are occasionally preserved in volcanic deposits such as those in Tocuila, Texcoco, Mexico, where a volcanic lahar mudflow covered at least seven individuals 12,500 years ago. How many mammoths lived at one location at a time is unknown, but the number likely varied by season and lifecycle. Modern elephants can form large herds, sometimes consisting of multiple family groups, and these herds can include thousands of animals migrating together. Mammoths may have formed large herds more often than modern elephants, since animals living in open areas are more likely to do this than those in forested areas. [19][27][29]

Male "Murray" specimen (left) and footprints of a struggling mammoth (arrows), at the Hot Springs Mammoth Site

Many specimens also accumulated in natural traps, such as sinkholes and tar pits. The Mammoth Site in Hot Springs, South Dakota, is a 26,000-year-old, roughly 40 m (130 ft)-long sinkhole that functioned for 300 to 700 years before filling with sediment. The site is the opposite scenario of that in Waco; all but one of the at least 55 skeletons—additional skeletons are excavated each year—are male, and accumulated over time rather than in a single event. Like modern male elephants, male mammoths primarily are assumed to have lived alone, to be more adventurous (especially young males), and to be more likely to encounter dangerous situations than the females. The mammoths may have been lured to the hole by warm water or vegetation near the edges, slipping in and drowning or starving. Isotope studies of growth rings have shown that most of the mammoths died during spring and summer, which may have correlated with vegetation near the sinkhole. One individual, nicknamed "Murray", lies on its side, and probably died in this pose while struggling to get free. Deep footprints of mammoths attempting to free themselves from the sinkhole's mud can be seen in vertically excavated sections of the site.[19][27]

Smilodon and dire wolves fighting over a Columbian mammoth carcass in the La Brea Tar Pits, by Robert Bruce Horsfall, 1911

Since the early 20th century, excavations at the La Brea Tar Pits in Los Angeles have yielded 100 t (220,000 lb) of fossils from 600 species of flora and fauna, including several Columbian mammoths. Many of the fossils are the remains of animals that became stuck in asphalt puddles that seeped to the surface of the pits, 40,000 to 11,500 years ago. Dust and leaves likely concealed the liquid asphalt, which then trapped unwary animals. Mired animals died from hunger or exhaustion; their corpses attracted predators, which sometimes became stuck, themselves. The fossil record of the tar pits is dominated by the remains of predators, such as large canids and felids. Fossils of different animals are found stuck together when they are excavated from the pits.[27] The tar pits do not preserve soft tissue, and a 2014 study concluded that asphalt may degrade the DNA of animals mired in it after an attempt was made to extract DNA from a Columbian mammoth.[30]

A site in at airport construction area in Mexico nicknamed "mammoth central" is believed to have been the boggy shores of an ancient lake bed where animals were trapped 10,0000 to 20,000 years ago. Human tools have been found at the site. It remains unclear whether the 200 Columbian mammoths found there died of natural causes and were then carved by humans. Some have hypothesized that humans drove the Columbian mammoths into the area to kill them. The site is only 12 miles (19 km) from artificial pits which were once used by humans to trap and kill large mammals.[31][32]

Underside of the skull of a male La Brea specimen nicknamed "Zed", showing upper molars (left), and skull and teeth of the male "Rawlins mammoth", University of Wyoming Geological Museum (right)

An adult Columbian mammoth would have needed more than 180 kg (400 lb) of food per day, and may have foraged for 20 hours a day. Mammoths chewed their food using their powerful jaw muscles to move the mandible forward and close the mouth, then backward while opening; the sharp enamel ridges thereby cut across each other, grinding the food. The ridges were wear-resistant, enabling the animal to chew large quantities of food that contained grit. The trunk could be used for pulling up large tufts of grass, picking buds and flowers, or tearing leaves and branches from trees and shrubs, and the tusks were used to dig up plants and strip bark from trees. Digging is indicated on preserved tusks by flat, polished sections of the surface that would have reached the ground.[19] Isotope studies of Columbian mammoths from Mexico and the United States have shown that their diet varied by location, consisting of a mix of C3 (most plants) and C4 plants (such as grasses), and they were not restricted to grazing or browsing.[33][34]

Stomach contents from Columbian mammoths are rare, since no carcasses have been found, but plant remains were discovered between the pelvis and ribs of the "Huntington mammoth" when it was excavated in Utah. Microscopy showed that these chewed remains consisted of sedges, grasses, fir twigs and needles, oak, and maple.[35] A large amount of mammoth dung has been found in two caves in Utah. The dry conditions and stable temperature of Bechan Cave (bechan is Navajo for "large faeces") has preserved 16,000- to 13,500-year-old elephant dung, most likely from Columbian mammoths.[36] The dung consists of 95% grasses and sedges, and varies from 0 to 25% woody plants between dung boluses, including saltbush, sagebrush, water birch, and blue spruce. This is similar to the diet documented for the woolly mammoth, although browsing seems to have been more important for the Columbian mammoth. 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 20 cm (7.9 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 an average of 11 kg (24 lb) of dung every two hours and 90–135 kg (198–298 lb) each day.[19][37]

Giant North American fruits of plants such as the Osage-orange, Kentucky coffeetree, pawpaw and honey locust have been proposed to have evolved in tandem with now-extinct American megafauna such as mammoths and other proboscideans, since no extant endemic herbivores are able to ingest these fruits and disperse their seeds. Introduced cattle and horses have since taken over this ecological role.[38][39][40]

Skeleton of a juvenile (left) and cast of the male "Huntington mammoth" (right), which was old and sick when it died, both at the Natural History Museum of Utah

The lifespan of the Columbian mammoth is thought to have been about 80 years. The lifespan of a mammal is related to its size; Columbian mammoths are larger than modern elephants, which have a lifespan of about 60 years. The age of a mammoth can be roughly determined by counting the growth rings of its tusks when viewed in cross section. However, ring-counting does not account for a mammoth's early years; early growth is represented in tusk tips, which are usually worn away. In the remainder of the tusk, each major line represents a year, with weekly and daily lines found in between. Dark bands correspond to summer, making determining the season in which a mammoth died possible. Tusk growth slowed when foraging became more difficult, such as during illness or when a male mammoth was banished from the herd (male elephants live with their herds until about the age of 10).[19]

Mammoths continued growing during adulthood, as do other elephants. Males grew until age 40, and females until age 25. Mammoths may have had gestation periods of 21–22 months, like those of modern elephants. Columbian mammoths had six sets of molars in the course of a lifetime. At 6–12 months, the second set of molars would erupt, with the first set worn out at 18 months of age. The third set of molars lasted for 10 years, and this process was repeated until the sixth set emerged at 30 years of age. When the last set of molars wore out, the animal would be unable to chew, and would die of starvation.[19]

Almost all vertebrae of the "Huntington mammoth", a very aged specimen, were deformed by arthritic disease, and four of its lumbar vertebrae were fused; some bones also indicate bacterial infection, such as osteomyelitis.[19] The condition of the bones suggests the specimen died of old age and malnutrition.[35]