Smilodon

Smilodon is a genus of extinct felids. It is one of the best-known saber-toothed predators and prehistoric mammals. Although commonly known as the saber-toothed tiger, it was not closely related to the tiger or other modern cats, belonging to the extinct subfamily Machairodontinae, with an estimated date of divergence from the ancestor of living cats around 20 million years ago. Smilodon was one of the last surviving machairodonts alongside Homotherium. Smilodon lived in the Americas during the Pleistocene to early Holocene epoch (2.5 mya – at latest 8,200 years ago). The genus was named in 1842 based on fossils from Brazil; the generic name means 'scalpel' or 'two-edged knife' combined with 'tooth'. Three species are recognized today: S. gracilis, S. fatalis, and S. populator. The two latter species were probably descended from S. gracilis, which itself probably evolved from Megantereon. The hundreds of specimens obtained from the La Brea Tar Pits in Los Angeles constitute the largest collection of Smilodon fossils.

Overall, Smilodon was more robustly built than any extant cat, with particularly well-developed forelimbs and exceptionally long upper canine teeth. Its jaw had a bigger gape than that of modern cats, and its upper canines were slender and fragile, being adapted for precision killing. S. gracilis was the smallest species at 55 to 100 kg (121 to 220 lb) in weight. S. fatalis had a weight of 160 to 280 kg (350 to 620 lb) and height of 100 cm (39 in). Both of these species are mainly known from North America, but remains from South America have also been attributed to them (primarily from the northwest of the continent). S. populator from South America was the largest species, at 220 to 436 kg (485 to 961 lb) in weight and 120 cm (47 in) in height, and was among the largest known felids. The coat pattern of Smilodon is unknown, but it has been artistically restored with plain or spotted patterns.

In North America, Smilodon hunted large herbivores such as bison and camels, and it remained successful even when encountering new prey taxa in South America such as Macrauchenia and ground sloths. Smilodon is thought to have killed its prey by holding it still with its forelimbs and biting it, but in what manner the bite itself was delivered is unclear. Scientists debate whether Smilodon had a social or a solitary lifestyle; analysis of modern predator behavior, as well as of Smilodon's fossil remains, could be construed to lend support to either view. Smilodon probably lived in relatively closed habitats such as forests and bush, which would have provided cover for ambushing prey, although S. populator has been suggested to have hunted in open terrain. Smilodon died out as part of the end-Pleistocene extinction event, which occurred around 13-9,000 years ago, along with most other large animals across the Americas. Its reliance on large animals has been proposed as the cause of its extinction. Smilodon may have been impacted by habitat turnover and loss of prey on which it specialized, due to possible climatic impacts, the effects of recently arrived humans on prey populations, and other factors.

During the 1830s, Danish naturalist Peter Wilhelm Lund and his assistants collected fossils in the calcareous caves near the small town of Lagoa Santa, Minas Gerais, Brazil. Among the thousands of fossils found, he recognized a few isolated cheek teeth as belonging to a hyena, which he named Hyaena neogaea in 1839. After more material was found (including incisor teeth and foot bones), Lund concluded the fossils instead belonged to a distinct genus of felids, though transitional to the hyenas. He stated it would have matched the largest modern predators in size, and was more robust than any modern cat. Lund originally wanted to call the new genus Hyaenodon, but realizing this name had recently been applied to another prehistoric predator, he instead named it Smilodon populator in 1842. He explained the Ancient Greek meaning of Smilodon as σμίλη (smilē), 'scalpel' or 'two-edged knife', and οδόντος (odóntos), 'tooth'. This has also been translated as "tooth shaped like double-edged knife". He explained the species name populator as "the destroyer", which has also been translated as "he who brings devastation". Lund based the name on the shape of the incisors, and the large canine teeth were not known until 1846. By 1846, Lund had acquired nearly every part of the skeleton (from different individuals), and more specimens were found in neighboring countries by other collectors in the following years. Though some later authors used Lund's original species name neogaea instead of populator, it is now considered an invalid nomen nudum, as it was not accompanied with a proper description and no type specimens were designated. Some South American specimens have been referred to other genera, subgenera, species, and subspecies, such as Smilodontidion riggii, Smilodon (Prosmilodon) ensenadensis, and S. bonaeriensis, but these are now thought to be junior synonyms of S. populator.

Fossils of Smilodon were discovered in North America from the second half of the 19th century onwards. In 1869, American paleontologist Joseph Leidy described a maxilla fragment with a molar, which had been discovered in a petroleum bed in Hardin County, Texas. He referred the specimen to the genus Felis (which was then used for most cats, extant and extinct), but found it distinct enough to be part of its own subgenus, as F. (Trucifelis) fatalis. The species name means "deadly". In an 1880 article about extinct American cats, American paleontologist Edward Drinker Cope pointed out that the F. fatalis molar was identical to that of Smilodon, and he proposed the new combination S. fatalis. Most North American finds were scanty until excavations began in the La Brea Tar Pits in Los Angeles, where hundreds of individuals of S. fatalis have been found since 1875. S. fatalis has junior synonyms such as S. mercerii, S. floridanus, and S. californicus. American paleontologist Annalisa Berta considered the holotype of S. fatalis too incomplete to be an adequate type specimen, and the species has at times been proposed to be a junior synonym of S. populator. Nordic paleontologists Björn Kurtén and Lars Werdelin supported the distinctness of the two species in an article published in 1990. A 2018 article by the American paleontologist John P. Babiarz and colleagues concluded that S. californicus, represented by the specimens from the La Brea Tar Pits, was a distinct species from S. fatalis after all, and that more research is needed to clarify the taxonomy of the lineage.

In his 1880 article about extinct cats, Cope also named a third species of Smilodon, S. gracilis. The species was based on a partial canine, which had been obtained in the Port Kennedy Cave near the Schuylkill River in Pennsylvania. Cope found the canine to be distinct from that of the other Smilodon species due to its smaller size and more compressed base. Its specific name refers to the species' lighter build. This species is known from fewer and less complete remains than the other members of the genus. S. gracilis has at times been considered part of genera such as Megantereon and Ischyrosmilus. S. populator, S. fatalis and S. gracilis are currently considered the only valid species of Smilodon, and features used to define most of their junior synonyms have been dismissed as variation between individuals of the same species (intraspecific variation). One of the most famous of prehistoric mammals, Smilodon has often been featured in popular media and is the state fossil of California.

Long the most completely known saber-toothed cat, Smilodon is still one of the best-known members of the group, to the point where the two concepts have been confused. The term "saber-tooth" itself refers to an ecomorph consisting of various groups of extinct predatory synapsids (mammals and close relatives), which convergently evolved extremely long maxillary canines, as well as adaptations to the skull and skeleton related to their use. This includes members of Gorgonopsia, Thylacosmilidae, Machaeroidinae, Nimravidae, and Machairodontinae. Within the family Felidae (true cats), members of the subfamily Machairodontinae are referred to as saber-toothed cats, and this group is itself divided into three tribes: Metailurini (false saber-tooths); Homotherini (scimitar-toothed cats); and Smilodontini (dirk-toothed cats), to which Smilodon belongs.

Members of Smilodontini are defined by their long slender canines with fine to no serrations, whereas Homotherini are typified by shorter, broader, and more-flattened canines, with coarser serrations. Members of Metailurini were less specialized and had shorter, less-flattened canines, and are not recognized as members of the Machairodontinae by some researchers.