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Smilodon

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Smilodon
Temporal range: Early Pleistocene to Early Holocene, 2.5–0.01 Ma
Smilodon californicus mount.jpg
Smilodon fatalis skeleton at National Museum of Natural History, Washington, D.C.
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Carnivora
Family: Felidae
Subfamily: Machairodontinae
Tribe: Smilodontini
Genus: Smilodon
Lund, 1842
Species

Smilodon /ˈsmlədɒn/ is an extinct genus of machairodont felid. It is perhaps the best known saber-toothed cat and lived in the Americas during the Pleistocene epoch (2.5 mya–10,000 years ago). One of the largest collections of its fossils has been obtained from the La Brea Tar Pits. Three species of the genus are known; they vary in size and build.

Overall, Smilodon was more robustly built than any modern cat, with particularly well-developed forelimbs and exceptionally long upper canines. Its jaw had a bigger gape than modern cats and its upper canines were slender and fragile, being adapted for precision killing. These attributes likely made Smilodon a specialized hunter of large herbivores, such as bison and camels.

Smilodon likely lived in closed habitats, such as forests and bush, which would have provided cover for ambushing prey. Its reliance on large animals may have been the cause of its extinction; Smilodon died out at the same time that most North and South American megafauna disappeared, about 10,000 years ago. Scientists debate over 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.

Taxonomy[edit]

S. populator skull and syntype canine from Lund's collection, Zoological Museum, Copenhagen

During the 1830s, Danish naturalist and palaeontologist 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 recognised a few isolated pieces as belonging to a hyaena which he named Hyaena neogaea in 1839. After more material was found (including teeth and foot bones) Lund realised the fossils belonged to a felid larger than any extant species, and named the animal Smilodon populator in 1842. The genus name Smilodon means "tooth shaped like double-edged knife" in Greek, from the words σμίλη, (smilē) and ὀδoύς (odoús). The species name populator means "he who brings devastation". By 1846, Lund had acquired nearly every part of the skeleton (from different individuals), and more specimens were found in Brazil and Argentina by other collectors in the following years.[1][2] Some Argentinian specimens have been referred to species such as S. neogaeus and S. necator, but these are now thought to be junior synonyms of S. populator.[3]

Fossils were found in North America from the second half of the 19th century and onwards; in 1869, American paleontologist Joseph Leidy referred this animal to the extant genus Felis, in the subgenus F. (Trucifelis) fatalis. The species name means "fate" or "destiny", but it is thought Leidy intended it to mean "fatal". The species was later considered part of Smilodon, as S. fatalis, with synonyms such as S. mercerii, S. floridanus, and S. californicus. 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.[1][3][4]

Evolution[edit]

S. populator skeleton, Museo de La Plata

The nickname "saber-tooth" refers to the extreme length of their maxillary canines. Despite the colloquial name "saber-toothed tiger", Smilodon is not closely related to the tiger (or any other living felid); the latter belongs to the subfamily Pantherinae, whereas Smilodon belongs to the subfamily Machairodontinae. It is grouped with several other species of saber-toothed cats in the subfamily Machairodontinae within the family Felidae. An early ancient DNA analysis suggested that Smilodon should be grouped with modern cats (subfamily Felinae)[5] However, a 2005 study found that Smilodon belonged to a separate lineage.[6] Another study published a year later confirmed this, showing that the Machairodontinae diverged early from the ancestors of modern cats and were not closely related to any living feline species.[7]

The skull and mandible morphology of the earliest saber-toothed cats were similar to that of clouded leopards. 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.[8] Smilodon belongs to the tribe Smilodontini, which is known as "dirk-toothed cats". These cats were defined by their long slender canines with fine serrations.[9]

While a number of Smilodon species have been described, today usually only three are recognized.[10] S. gracilis existed 2.5 million–500,000 years ago and was the successor of Megantereon in North America, from which it probably evolved. This species reached the north of South America in the early Pleistocene, along with Homotherium, as part of the Great American Interchange.[11] The other Smilodon species are probably derived from this species. As its specific name suggests, this species is the most lightly built of the genus.[12][13] S. fatalis existed 1.6 million–10,000 years ago, replaced S. gracilis in North America and invaded western South America.[14] S. populator existed 1 million–10,000 years ago; occurred in the eastern parts of South America and was larger than the North American species.[15]

Description[edit]

S. fatalis shown to scale

Smilodon was around the size of a modern lion or tiger, but was more robustly built. It had long canines, a reduced lumbar region, high scapula, short tail, and broad limbs with relatively short feet.[10][16] The brain of Smilodon was relatively small compared to other cat species.[17] In reconstructing the facial appearance of Smilodon, Miller (1969) proposed that it looked very different from a typical cat: having a lower lip line (to allow its mouth to open so wide without tearing the facial tissues), a more retracted nose and lower placed ears.[18] However this is disputed, and Antón, et al. (1998) wrote that the facial features of Smilodon were overall not different from those of other cats.[19]

Smilodon is most famous for its relatively long canines, which are the longest found in the saber-toothed cats, at about 28 cm (11 in) long in the largest species Smilodon populator.[10] Those of S. fatalis reached their full size in 18 months at a growth rate of 7 mm/month.[20] These canine teeth were slender and had fine serrations.[9]

Restoration of S. populator by Charles R. Knight, 1903

There is some dispute over whether Smilodon was sexually dimorphic. Some studies of Smilodon fatalis fossils have found little difference between the sexes.[21][22] 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.[23]

S. populator was perhaps the largest known felid, with a body mass range of 220 to 400 kg (490 to 880 lb).[24][25] It stood at a shoulder height of 120 cm (47 in).[10] Compared to S. fatalis, S. populator had a more elongated and narrow skull, higher positioned nasals, more massive metapodials and slightly longer forelimbs relative to hindlimbs.[14] S. fatalis was intermediate in size between S. gracilis and S. populator.[10] It ranged from 160 to 280 kg (350 to 620 lb).[24] and reached a shoulder height of 100 cm (39 in) and body length of 175 cm (69 in).[26] S. gracilis was the smallest species, estimated at 55 to 100 kg (120 to 220 lb) in weight.[24]

Paleobiology[edit]

Predatory behavior[edit]

S. fatalis and dire wolves fight over a Columbian mammoth carcass in the La Brea Tar Pits, Robert Bruce Horsfall, 1913

Smilodon was an apex predator and primarily hunted large mammals like bison, camels, ground sloths, horses and Columbian mammoths. Isotopes preserved in the bones of S, fatalis in the La Brea Tar Pits reveal that ruminants like bison and camels were most commonly taken by the cats.[27] In addition, isotopes preserved in the enamel of S. gracilis specimens from Florida show that this species feed on the pig-like Platygonus and the llama-like Hemiauchenia.[28] Isotopic studies of dire wolf and American lion bones show an overlap with S. fatalis in prey, which suggests that they were competitors.[27] The availability of prey in the Rancho La Brea area was likely comparable to modern East Africa.[29] Smilodon probably avoided eating bone and would have left enough food for scavengers.[30] Smilodon itself may have scavenged dire wolf kills.[31]

Smilodon was likely an ambush predator that concealed itself in dense vegetation.[32] The heel bone of Smilodon was fairly long which suggests it was a good jumper.[10] Its well developed flexors and extensors in its forearms, likely enabled it to pull down and securely hold down large prey so it could deliver a killing bite without endangering the vulnerable elongate canines. Analysis of the cross-sections of S. fatalis humeri indicated that they were strengthened by cortical thickening to such an extent that they would have been able to sustain greater loading than those of extant big cats, or of the extinct American lion. However, the thickening of S. fatalis femurs was within the range of extant felids.[33] As its canine were fragile and could not have bitten into bone; these cats did not use their long teeth while taking down prey, due to the risk of breaking and had to subdue and restrain their prey so they could use the teeth.[33]

S. fatalis skull cast with jaws positioned to show maximum gape

Smilodon may have then used its long canines to deliver a deep stabbing bite or open-jawed stabbing thrust to the throat, which would generally cut through the jugular vein and/or the trachea and thus kill the prey very quickly.[33][34] Alternatively, it may have used its canines to puncture the thoracic wall of its prey (a larger, easier target) with a closed-mouth stab, creating a condition of pneumothorax (collapsed lungs).[35] By contrast, modern cats kill large prey with a suffocating bite.[33] Another hypothesis suggests that Smilodon targeted the belly of its prey. This is disputed, however, as the curvature of their prey's belly would likely have prevented the cat from getting a good bite.[36] Whether Smilodon generally used its canines to deliver a point-to-point bite, open-jawed stab or closed-jawed stab is unclear.[35]

Despite being more powerfully built than other large cats, Smilodon actually had a weaker bite. Modern big cats have more pronounced zygomatic arches, while Smilodon had smaller zygomatic arches which restricted the thickness and therefore power of the temporalis muscles, and thus reduced Smilodon's bite force. Analysis of its narrow jaws indicates that it could produce a bite only a third as strong as that of a lion.[37] There seems to a be a general rule that the saber-toothed cats with the largest canines had proportionally weaker bites. However, analyses of canine bending strength (the ability of the canine teeth to resist bending forces without breaking) and bite forces indicate that the saber-toothed cats' teeth were stronger relative to the bite force than those of modern "big cats".[38] In addition, Smilodon's gape could have reached almost 120 degrees,[39] while that of the modern lion reaches 65 degrees.[40] This makes the gape wide enough to allow Smilodon to grasp large prey despite the long canines.[19]

Social life[edit]

S. fatalis skeletal displays in the Los Angeles County Museum of Natural History (ca.1920)

Scientists debate whether Smilodon was social. One study of African predators found that social predators like lions and spotted hyenas respond more to the distress calls of prey than solitary species. Since Smilodon fatalis fossils are common at the La Brea Tar Pits, and were likely attracted by the distress calls of stuck prey, this could mean that La Brea S. fatalis were social as well.[41] Critics claim that the study neglects other factors, such as body mass (heavier animals are more likely to get stuck than lighter ones), intelligence (some social animals, like the American lion, may have avoided the tar because they were better able to recognize the hazard), lack of visual and olfactory lures, the type of audio lure, and the length of the distress calls (the actual distress calls of the trapped prey animals would have lasted longer than the calls used in the study). In addition, they note that solitary cats like tigers are known to aggregate around a single carcass.[42] The authors of the original study have responded to these criticisms.[43]

Another argument for sociality is based on the healed injuries in several Smilodon fossils, which would suggest that the animals needed others to provide it food.[44] This argument has been questioned, as cats can recover quickly from even severe bone damage and an injured Smilodon could survive if it had access to water.[45] Some researchers have argued that Smilodon's brain would have been too small for it to have been a social animal.[17] However, an analysis of brain size in living big cats found no correlation between brain size and sociality.[46] Another argument against Smilodon being social is based on it being an ambush hunter in closed habitat which would likely have negated sociality.[45]

Two S. fatalis approach a ground sloth (Paramylodon) mired in the La Brea Tar Pits, Charles R. Knight, 1921

Whether Smilodon was sexually dimorphic has implications for its reproductive behavior. Based on their conclusions that Smilodon fatalis had no sexual dimorphism, Van Valenburgh and Sacco (2002) suggest that, if the cats were social, they would likely have lived in monogamous pairs (along with offspring) with no intense competition among males for females.[21] Likewise, Meachen-Samuels and Binder (2010) conclude that aggression between males was less pronounced in S. fatalis than the American lion.[22] However, Christiansen and Harris (2012) find that as S. fatalis did exhibit some sexual dimorphism, there would have been evolutionary selection for competition between males.[23]

In recent years, Smilodon cubs have been found in tar pits close to the Rancho La Brea site, near the bones of larger adults. Unlike the mature specimens, juvenile Smilodon had smaller saber teeth, which were used more for cutting flesh away from bones than killing prey. This likely implies that they were coming in to feed after the adults had made a kill and fed on parts of the kill that were inaccessible to the adults. In attempting to feed, the cubs and subadults would also become mired and starve to death. This behavior was recently revealed in popular culture in the first episode of the 2013 BBC documentary miniseries "Ice Age Giants".[47]

Development[edit]

Undersides of La Brea skulls, showing tooth replacement

Smilodon started developing its adult saber-teeth when the animal turned one-and-a-half years of age, shortly after the completion of the eruption of the cat's baby teeth. Both baby and adult sabers would be present in the mouth for an 11-month period, and the muscles used in making the powerful bite were developed at about one-and-a-half years old as well, eight months earlier than in a modern lion. After Smilodon turned 20 months of age, the infant teeth were shed while the adult canines grew in, a process that continued until the cats reached 3 years of age, later than for modern species of big cat. Specimens of juvenile and adolescent Smilodon are extremely rare at Rancho La Brea, where the study was performed, indicating that they remained hidden or at denning sites during hunts.[48][49]

Paleopathology[edit]

Several Smilodon fossils show signs of ankylosing spondylitis, hyperostosis and trauma;[50] some also had arthritis which gave them fused vertebrae. One study of 1,000 Smilodon skulls found that 30% of them had eroded parietal bones, which is where the largest jaw muscles attach. They also showed signs of microfractures, and the weakening and thinning of bones possibly caused by mechanical stress from the constant need to make stabbing motions with the canines.[51]

Distribution and habitat[edit]

Smilodon was perhaps the most recent of the saber-toothed cats and lived during the Pleistocene epoch (2.5 mya–10,000 years ago).[10] Fossils of the genus have been found throughout the Americas.[52] In particular, numerous specimens have been discovered in the La Brea Tar Pits in Los Angeles, California and the Talara Tar Seeps in Peru.[26] Smilodon likely lived in "closed" habitat (forest and bush).[53]

Extinction[edit]

Smilodon went extinct 10,000 years ago, along with most of the Pleistocene megafauna, in the Quaternary extinction event. Its extinction may be linked to the decline and extinction of large herbivores, which were replaced by smaller and more agile ones like deer. Hence, Smilodon could have been too specialized at hunting large prey and may have been unable to adapt.[33] However, a 2012 study of Smilodon tooth wear found no evidence that they were limited by food resources. Other explanations include climate change and competition with humans.[54]

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Bibliography[edit]

  • Antón, M. (2013). Sabertooth (1 ed.). Bloomington: Indiana University Press. ISBN 978-0253010421.