Smilodon

''Smilodon'' is a genus of the extinct machairodont subfamily of the felids. It is one of the most famous prehistoric mammals and the best known saber-toothed cat. Although commonly known as the saber-toothed tiger, it was not closely related to the tiger or other modern cats. ''Smilodon'' lived in the Americas during the Pleistocene epoch (2.5 mya – 10,000 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 individuals 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 in weight. ''S. fatalis'' had a weight of and height of . Both of these species are mainly known from North America, but remains from South America have also been attributed to them. ''S. populator'' from South America was the largest species, at in weight and 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 species in South America. ''Smilodon'' is thought to have killed its prey by holding it still with its forelimbs and biting it, but it is unclear in what manner the bite itself was delivered. 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 closed habitats such as forests and bush, which would have provided cover for ambushing prey. ''Smilodon'' died out at the same time that most North and South American megafauna disappeared, about 10,000 years ago. Its reliance on large animals has been proposed as the cause of its extinction, along with climate change and competition with other species, but the exact cause is unknown.

Taxonomy

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 canine teeth and foot bones), Lund concluded the fossils instead belonged to a distinct genus of felid, 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 name the new genus '' Hyaenodon'', but realizing this had recently become preoccupied by another prehistoric predator, he instead named it ''Smilodon populator'' in 1842. He explained the Ancient Greek meaning of ''Smilodon'' as (''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". 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'' ("naked name"), 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 as well as 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.

Evolution

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" 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, Barbourofelidae, 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, broad, 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 Machairodontinae by some researchers.

The earliest felids are known from the Oligocene of Europe, such as '' Proailurus'', and the earliest one with saber-tooth features is the Miocene genus '' Pseudaelurus''. The skull and mandible morphology of the earliest saber-toothed cats was similar to that of the modern clouded leopards (''Neofelis''). The lineage further adapted to the precision killing of large animals by developing elongated canine teeth and wider gapes, in the process sacrificing high bite force. As their canines became longer, the bodies of the cats became more robust for immobilizing prey. In derived smilodontins and homotherins, the lumbar region of the spine and the tail became shortened, as did the hind limbs. Based on mitochondrial DNA sequences extracted from fossils, the lineages of ''Homotherium'' and ''Smilodon'' are estimated to have diverged about 18 Ma ago. The earliest species of ''Smilodon'' is ''S. gracilis'', which existed from 2.5 million to 500,000 years ago (early Blancan ages) and was the successor in North America of ''Megantereon'', from which it probably evolved. ''Megantereon'' itself had entered North America from Eurasia during the Pliocene as part of the Great American Interchange. ''S. fatalis'' existed 1.6 million–10,000 years ago (late Irvingtonian to Rancholabrean ages), and replaced ''S. gracilis'' in North America. ''S. populator'' existed 1 million–10,000 years ago (Ensenadan to Lujanian ages); it occurred in the eastern parts of South America.

Despite the colloquial name "saber-toothed tiger", ''Smilodon'' is not closely related to the modern tiger (which belongs in the subfamily Pantherinae), or any other extant felid. A 1992 ancient DNA analysis suggested that ''Smilodon'' should be grouped with modern cats (subfamilies Felinae and Pantherinae). A 2005 study found that ''Smilodon'' belonged to a separate lineage. A study published in 2006 confirmed this, showing that the Machairodontinae diverged early from the ancestors of modern cats and were not closely related to any living species. The following cladogram based on fossils and DNA analysis shows the placement of ''Smilodon'' among extinct and extant felids, after Rincón and colleagues, 2011:

Description

''Smilodon'' was around the size of modern big cats, but was more robustly built. It had a reduced lumbar region, high scapula, short tail, and broad limbs with relatively short feet. ''Smilodon'' is most famous for its relatively long canine teeth, which are the longest found in the saber-toothed cats, at about long in the largest species, ''S.populator''. The canines were slender and had fine serrations on the front and back side. The skull was robustly proportioned and the muzzle was short and broad. The cheek bones (zygomata) were deep and widely arched, the sagittal crest was prominent, and the frontal region was slightly convex. The mandible had a flange on each side of the front. The upper incisors were large, sharp, and slanted forwards. There was a diastema (gap) between the incisors and molars of the mandible. The lower incisors were broad, recurved, and placed in a straight line across. The p3 premolar tooth of the mandible was present in most early specimens, but lost in later specimens; it was only present in 6% of the La Brea sample. There is some dispute over whether ''Smilodon'' was sexually dimorphic. Some studies of ''S. fatalis'' fossils have found little difference between the sexes. Conversely, a 2012 study found that, while fossils of ''S. fatalis'' show less variation in size among individuals than modern ''Panthera'', they do appear to show the same difference between the sexes in some traits.

''S. gracilis'' was the smallest species, estimated at in weight, about the size of a jaguar. It was similar to its predecessor ''Megantereon'' of the same size, but its dentition and skull were more advanced, approaching ''S. fatalis''. ''S. fatalis'' was intermediate in size between ''S. gracilis'' and ''S. populator''. It ranged from . and reached a shoulder height of and body length of . It was similar to a lion in dimensions, but was more robust and muscular, and therefore had a larger body mass. Its skull was also similar to that of ''Megantereon'', though more massive and with larger canines. ''S. populator'' was among the largest known felids, with a body mass range of , and one estimate suggesting up to . A particularly large ''S. populator'' skull from Uruguay measuring in length indicates this individual may have weighed as much as . It stood at a shoulder height of . Compared to ''S. fatalis'', ''S. populator'' was more robust and had a more elongated and narrow skull with a straighter upper profile, higher positioned nasal bones, a more vertical occiput, more massive metapodials and slightly longer forelimbs relative to hindlimbs. Large tracks from Argentina (for which the ichnotaxon name ''Smilodonichium'' has been proposed) have been attributed to ''S. populator'', and measure by . This is larger than tracks of the Bengal tiger, to which the footprints have been compared.

Traditionally, saber-toothed cats have been artistically restored with external features similar to those of extant felids, by artists such as Charles R. Knight in collaboration with various paleontologists in the early 20th century. In 1969, paleontologist G. J. Miller instead proposed that ''Smilodon'' would have looked very different from a typical cat and similar to a bulldog

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