American lion

The American lion (Panthera atrox, with the species name meaning "savage" or "cruel", also called the North American lion) is an extinct pantherine cat native to North America during the Late Pleistocene from around 130,000 to 12,800 years ago. Genetic evidence suggests that its closest living relative is the lion (Panthera leo), with the American lion representing an offshoot from the lineage of the largely Eurasian cave lion (Panthera spelaea), from which it is suggested to have split around 165,000 years ago. Its fossils have been found across North America, from Canada to Mexico. It was about 25% larger than the modern lion, making it one of the largest known felids to ever exist, and an important apex predator. The American lion is suggested to have lived in prides like living lions.

The American lion became extinct as part of the end-Pleistocene extinctions of most large mammals across the Americas. The extinctions followed human arrival in the Americas. Proposed factors in its extinction include climatic change reducing viable habitat, as well as hunting by Paleoindians of herbivore prey causing a trophic cascade.

Initial discovery and North American fossils
The first specimen now assigned to Panthera atrox was collected in the 1830s by William Henry Huntington, Esq., who announced his discovery to the American Philosophical Society on April 1, 1836 and placed it with other fossils from Huntington's collection in the Academy of Natural Sciences in Philadelphia. The specimen had been collected in ravines in Natchez, Mississippi that were dated to the Pleistocene; the specimen consisted only of a partial left mandible with 3 molars and a partial canine. The fossils did not get a proper description until 1853 when Joseph Leidy named the fragmentary specimen (ANSP 12546) Felis atrox ("savage cat"). Leidy named another species in 1873, Felis imperialis, based on a mandible fragment from Pleistocene gravels in Livermore Valley, California. F. imperialis however is considered a junior synonym of Panthera atrox. A replica of the jaw of the first American lion specimen to be discovered can be seen in the hand of a statue of famous paleontologist Joseph Leidy, currently standing outside the Academy of Natural Sciences in Philadelphia.

Few additional discoveries came until 1907, when the American Museum of Natural History and College, Alaska collected several Panthera atrox skulls in a locality originally found in 1803 by gold miners in Kotzebue, Alaska. The skulls were referred to a new subspecies of Felis (Panthera) atrox in 1930, Felis atrox "alaskensis". Despite this, the species did not get a proper description and is now seen as a nomen nudum synonymous with Panthera atrox. Further south in Rancho La Brea, California, a large felid skull was excavated and later described in 1909 by John C. Merriam, who referred it to a new subspecies of Felis atrox, Felis atrox bebbi. The subspecies is synonymous with Panthera atrox.

Throughout the early to mid 1900s, dozens of fossils of Panthera atrox were excavated at La Brea, including many postcranial elements and associated skeletons. The fossils were described by Merriam & Stock in detail in 1932, who synonymized many previously named taxa with Felis atrox. At least 80 individuals are known from La Brea Tar Pits and the fossils define the subspecies, giving a comprehensive view of the taxon. It was not until 1941 that George Simpson moved Felis atrox to Panthera, believing that it was a subspecies of jaguar. Simpson also referred several fossils from central Mexico, even as far south as Chiapas, as well as Nebraska and other regions of the western US, to P. atrox. 1971 witnessed the description of fragmentary remains from Alberta, Canada that extended P. atrox's range north. In 2009, an entrapment site at Natural Trap Cave, Wyoming was briefly described and is the second most productive Panthera atrox-bearing fossil site. It most importantly contains well-preserved mitochondrial DNA of many partial skeletons.

Panthera onca mesembrina and possible South American material
In the 1890s in the "Cueva del Milodon" in southern Chile, fossil collector Rodolfo Hauthal collected a fragmentary postcranial skeleton of a large felid that he sent to Santiago Roth who described them as a new genus and species of felid, "Iemish listai", in 1899, though the name is considered a nomen nudum. 5 years later in 1904, Roth reassessed the phylogenetic affinities of "Iemish" and named it Felis listai and referred several cranial and fragmentary postcranial elements to the taxon. Notably, several mandibles, a partial skull, and pieces of skin were some of the specimens referred. 30 years later in 1934, Felis onca mesembrina was named by Angel Cabrera based on that partial skull from "Cueva del Milodon" and the other material from the site was referred to it. Unfortunately, the skull (MLP 10-90) was lost, and was only illustrated by Cabrera. Further material, including feces and mandibles, was referred to as F. onca mesembrina from Tierra del Fuego, Argentina and other southern sites in Chile.

In 2016, the subspecies was referred to Panthera onca in a genetic study, which supported its identity as a subspecies of jaguar. Later in 2017, the authors of a study synoymised P. onca mesembrina with Panthera atrox based on morphological similarities, though this does not have broad acceptance.

Evolution
The American lion was initially considered a distinct species of Pantherinae, and designated as Panthera atrox, which means "cruel" or "fearsome panther" in Latin. Some paleontologists accepted this view, but others considered it to be a type of lion closely related to the modern lion (Panthera leo) and its extinct relative, the Eurasian cave lion (Panthera leo spelaea or P. spelaea). It was later assigned as a subspecies of P. leo (P. leo atrox) rather than as a separate species. Most recently, both spelaea and atrox have been treated as full species.

Cladistic studies using morphological characteristics have been unable to resolve the phylogenetic position of the American lion. One study considered the American lion, along with the cave lion, to be most closely related to the tiger (Panthera tigris), citing a comparison of the skull; the braincase, in particular, appears to be especially similar to the braincase of a tiger. Another study suggested that the American lion and the Eurasian cave lion were successive offshoots of a lineage leading to a clade which includes modern leopards and lions. A more recent study comparing the skull and jaw of the American lion with other pantherines concluded that it was not a lion but a distinct species. It was proposed that it arose from pantherines that migrated to North America during the mid-Pleistocene and gave rise to American lions and jaguars (Panthera onca). Another study grouped the American lion with P. leo and P. tigris, and ascribed morphological similarities to P. onca to convergent evolution, rather than phylogenetic affinity.

Mitochondrial DNA sequence data from fossil remains suggests that the American lion (P. atrox) represents a sister lineage to Late Pleistocene populations of the Eurasian cave lion (P. spelaea), and likely arose when an early cave lion population became isolated south of the North American continental ice sheet. While initial studies suggested that the divergence between American and Eurasian cave lions took place around 340,000 years ago, later studies suggested that the split took place considerably later, around 165,000 years ago, consistent with the earliest appearance of cave lions in eastern Beringia (now Alaska) during the Illinoian (190-130,000 years ago).

Genetic studies indicate that the living lion is the closest living relative of P. atrox and P. spelaea. Genome-wide sequencing of modern lions and Eurasian cave lions suggests that the lineage of the cave lion and American lion diverged from that of the modern lion around 500,000 years ago.

Description
The American lion is estimated to have measured 1.6 to 2.5 m from the tip of the nose to the base of the tail and stood 1.2 m at the shoulder. Panthera atrox was at least as sexually dimorphic as African lions, with an approximate range of between 235kg to 523 kg (518lbs-1153lbs) in males and 175kg to 365 kg (385lbs-805lbs) for females. In 2008, the American lion was estimated to weigh up to 420 kg. A study in 2009 showed an average weight of 256 kg for males and 351 kg for the largest specimen analyzed.

Panthera atrox had limb bones more robust than those of an African lion, and comparable in robustness to the bones of a brown bear. About 80 American lion individuals have been recovered from the La Brea Tar Pits in Los Angeles, so their morphology is well known. Their features strongly resemble those of modern lions, but they were considerably larger, similar to P. spelaea and the Pleistocene Natodomeri lion of eastern Africa.

Preserved skin remains found with skeletal material considered by some to belong to the American lion found in caves in Patagonia is reddish in colour, though the attribution of Patagonian Panthera remains to P. atrox is highly controversial and not accepted by many authors. Preserved fur of the closely related P. spelaea found in Siberia is yellowish in colour, with cave art of European P. spelaea indicating that males lacked substantial manes unlike modern lions. These characteristics may also apply to P. atrox.

Distribution
The earliest lions known in the Americas south of Alaska are from the Sangamonian Stage (equivalent to the global Last Interglacial ~130-115,000 years ago) during which American lions rapidly dispersed across North America, with their distribution ultimately ranging from Canada to southern Mexico and from California to the Atlantic coast. It was generally not found in the same areas as the jaguar, which favored forests over open habitats. It was absent from eastern Canada and the northeastern United States, perhaps due to the presence of dense boreal forests in the region. Farther south, fossilised remains of the American lion have been discovered in Extinction Cave, Belize. The American lion was formerly believed to have colonized northwestern South America as part of the Great American Interchange. However, the fossil remains found in the tar pits of Talara, Peru actually belong to an unusually large jaguar. On the other hand, fossils of a large felid from late Pleistocene localities in southern Chile and Argentina traditionally identified as an extinct subspecies of jaguar, Panthera onca mesembrina, have been considered by some authors actually represent remains of the American lion, though this interpretation is highly controversial, with many authors favouring a jaguar attribution for these remains.

Habitat
The American lion inhabited savannas and grasslands like the modern lion. In some areas of their range, American lions lived under cold climatic conditions. They probably used caves for shelter from the cold weather in those areas, and might have lined their dens with grass or leaves, as the modern Siberian tiger does.

Paleoecology
American lions likely preyed on deer, horses, camels, tapirs, American bison, mammoths, and other large ungulates (hoofed mammals). Evidence for predation of bison by American lions is particularly strong as a mummified carcass nicknamed "Blue Babe" was discovered in Alaska with clear bite and claw marks from lions. Based on the largely intact nature of the carcass, it probably froze before the lions could devour it. Paired nitrogen and carbon isotopic evidence from Natural Trap Cave in Wyoming reveals that the extant pronghorn was an important food source for American lions, which probably hunted them regularly, although probably also could be due to kleptoparasitism from the kills of Miracinonyx (sometimes called the "American cheetah"). It has been suggested that the American lion probably lived in prides like modern lions.

La Brea Tar Pits
The remains of American lions are not as abundant as those of other predators like Smilodon fatalis or dire wolves (Aenocyon dirus) at the La Brea Tar Pits. This suggests that they were better at evading entrapment, possibly due to greater intelligence. While the ratio of recovered juveniles to adults suggests that Panthera atrox was social, its rarity suggests that it was at least more solitary than Smilodon and Aenocyon, or was social but lived in low densities.

Analyses of dental microwear suggest that the American lion actively avoided bone just like the modern cheetah (more so than Smilodon). Panthera atrox has the highest proportion of canine breakage in La Brea, suggesting a consistent preference for larger prey than contemporary carnivores. Dental microwear additionally suggests that carcass utilization slightly declined over time (~30,000 BP to 11,000 radiocarbon BP) in Panthera atrox. The fragment of a femur from a gray wolf from the La Brea Tar Pits shows evidence of a violent bite which possibly amputated the leg. Researchers believe that Panthera atrox is a prime candidate for the injury, due to its bite force and bone shearing ability.

Extinction
The American lion went extinct as part of the end-Pleistocene extinctions around 13-12,000 years ago, approximately simultaneously with most large (megafaunal) mammals across the Americas. The most recent fossil, from Edmonton, Canada dates to ~12,877 calibrated years Before Present, and is 400 years younger than the youngest cave lion in Alaska. These extinctions post-date human arrival to the Americas. The causes of the extinctions have been long the subject of controversy, with most authors positing climate change, humans or some combination of the two as the causes of the extinctions. A 2017 study suggested that the viable habitat for the American lion in North America had been greatly reduced over the course of the Last Glacial Period, which would have made it more vulnerable to extinction. Other authors have suggested that the extinction of the American lion and other competing carnivores like dire wolves, and the sabertooth cats Smilodon and Homotherium may have been due to trophic cascade effects caused by Paleoindian hunting of herbivores. These authors suggested that the herbivores already probably existed at low population numbers prior to Paleoindian arrival due to their abundance being limited by predators, rather than being at the carrying capacity of the ecosystem based on food resources. Due to humans having a more flexible omnivorous diet they may have been less subject to competition with other apex predators, allowing their population numbers to increase even as the number of herbivores declined.