Temporal range: Early Pleistocene to Early Holocene, 2.5–0.01 Ma
|S. fatalis skeleton at National Museum of Natural History, Washington, D.C.|
Smilodon //, is an extinct genus of machairodont felid. It is perhaps the best known saber-toothed cat, and is commonly known as the saber-toothed tiger though it was not closely related to the tiger and 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, and 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 largest collection of Smilodon fossils has been obtained from the La Brea Tar Pits.
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 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 (120 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. S. populator from South America is perhaps the largest known felid at 220 to 400 kg (490 to 880 lb) in weight and 120 cm (47 in) in height. The coat pattern of Smilodon is unknown, but it has been 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 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. 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.
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 pieces as belonging to a hyena which he named Hyaena neogaea in 1839. After more material was found (including teeth and foot bones), Lund realized 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 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 as well as extinct) but found it distinct enough to be part of its own subgenus, as F. (Trucifelis) fatalis. The species name means "fate" or "destiny", but it is thought Leidy intended it to mean "fatal". 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 adequate as a type specimen, and the species has at times been proposed to be a junior synonym of S. populator. Swedish paleontologists Björn Kurtén and Lars Werdelin supported the distinctness of the two species in 1990.
In his 1880 article about extinct cats, Cope also named a third species of Smilodon, S. gracilis. The type specimen is a partial canine, which had been obtained in a 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 its lighter build. 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 intraspecific variation.
Smilodon was long the most completely known saber-toothed cat, and 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 Felidae. 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. The skull and mandible morphology of the earliest saber-toothed cats was similar to that of the 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.
The earliest species of Smilodon is S. gracilis, which existed from 2.5 million to 500,000 years ago and was the successor of Megantereon in North America, from which it probably evolved. This species reached the northern regions of South America in the early Pleistocene, along with Homotherium, as part of the Great American Interchange. The younger Smilodon species are probably derived from S. gracilis. S. fatalis existed 1.6 million–10,000 years ago, and replaced S. gracilis in North America. S. populator existed 1 million–10,000 years ago; 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. An 1992 ancient DNA analysis suggested that Smilodon should be grouped with modern cats (subfamilies Felinae and Pantherinae). However, 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 et al., 2011:
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 28 cm (11 in) 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 P3 molar 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, and they do appear to show the same difference between the sexes in some traits.
S. gracilis was the smallest species, estimated at 55 to 100 kg (120 to 220 lb) 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 160 to 280 kg (350 to 620 lb). and reached a shoulder height of 100 cm (39 in) and body length of 175 cm (69 in). 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 perhaps the largest known felid, with a body mass range of 220 to 400 kg (490 to 880 lb). It stood at a shoulder height of 120 cm (47 in). 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.
Traditionally, saber-toothed cats have been 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, with a lower lip line (to allow its mouth to open wide without tearing the facial tissues), a more retracted nose and lower-placed ears. Paleoartist Mauricio Antón and coauthors disputed this in 1998 and maintained that the facial features of Smilodon were overall not very different from those of other cats. Antón noted that modern animals like the hippopotamus are able to achieve a wide gap without tearing tissue by the moderate folding of the orbicularis oris muscle, and such a muscle configuration exists in modern large felids. Antón stated extant phylogenetic bracketing (where the features of the closest extant relatives of a fossil taxon are used as reference) is the most reliable way of restoring life-appearance and the cat-like Smilodon restorations by Knight are therefore still accurate today.
Smilodon and other saber-toothed cats have been reconstructed with both plain-colored coats and with spotted patterns (which appears to be the ancestral condition for feliforms), both of which are considered possible. Studies of modern cat species have found that species that live in the open tend to have uniform coats while those that live in more vegetated habitats have more markings. However, exceptions to both cases exist. Some coat features, such as the manes of male lions or the stripes of the tiger, are too unusual to predict from fossils.
Smilodon was an apex predator and primarily hunted large mammals. Isotopes preserved in the bones of S. fatalis in the La Brea Tar Pits reveal that ruminants like bison (Bison antiquus, which was much larger than the modern American bison) and camels (Camelops) were most commonly taken by the cats there. In addition, isotopes preserved in the tooth enamel of S. gracilis specimens from Florida show that this species fed on the pig-like Platygonus and the llama-like Hemiauchenia. Isotopic studies of dire wolf (Canis dirus) and American lion (Panthera leo atrox) bones show an overlap with S. fatalis in prey, which suggests that they were competitors. The availability of prey in the Rancho La Brea area was likely comparable to modern East Africa. As Smilodon migrated to South America, its diet changed; bisons were absent, the horses and proboscideans were different, and native ungulates such as toxodonts and litopterns were completely unfamiliar, yet S. populator thrived as well there as its relatives in North America. The differences between the North and South American species may be due to the difference in prey between the two continents. Smilodon probably avoided eating bone and would have left enough food for scavengers. Smilodon itself may have scavenged dire wolf kills. It has been suggested that Smilodon was a scavenger that mainly used its canines for display to assert dominance over carcasses, but this theory is not supported today, and no modern terrestrial mammals are pure scavengers.
The brain of Smilodon had sulcat patterns similar to modern cats, which suggests an increased complexity of the regions that control the sense of hearing, sight, and coordination of the limbs. Felid saber-tooths in general had relatively small eyes which were not as forward-facing as those of modern cats, which have good binocular vision to help them move in trees. Smilodon was likely an ambush predator that concealed itself in dense vegetation. The heel bone of Smilodon was fairly long, which suggests it was a good jumper. Its well-developed flexor and extensor muscles in its forearms probably enabled it to pull down, and securely hold down, large prey. 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. As its canines 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. By contrast, modern cats can subdue and kill large prey with a slow suffocating bite.
There is on-going debate as to how Smilodon killed its prey. Traditionally, the most popular theory is that the cat delivered a deep stabbing bite or open-jawed stabbing thrust to the throat, generally cutting through the jugular vein and/or the trachea and thus kill the prey very quickly. 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). 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 or stab. Whether Smilodon generally used its canines to deliver a point-to-point bite, open-jawed stab or closed-jawed stab is unclear. In regard to how Smilodon delivered its bite, the "canine shear-bite" hypothesis has been favored, where flexion of the neck and rotation of the skull assisted in biting the prey, but this may be mechanically impossible. The protruding incisors were arranged in an arch, and were used to hold the prey still and stabilize it while the canine bite was delivered. The contact surface between the canine crown and the gum was enlarged, which helped stabilize the tooth, and helped the cat sense when the tooth had penetrated to its maximum extent. Since saber-toothed cats generally had a relatively large infraorbital foramen (opening) in the skull, which housed nerves associated with the whiskers, it has been suggested the improved senses would have helped the cats' precision when biting outside their field of vision, and thereby prevent breakage of the canines. The blade-like carnassial teeth were used to cut skin to access the meat, and the reduced molars suggest that they were less adapted for crushing bones than modern cats.
Despite being more powerfully built than other large cats, Smilodon had a weaker bite. Modern big cats have more pronounced zygomatic arches, while these were smaller in Smilodon, 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. 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. In addition, Smilodon's gape could have reached almost 120 degrees, while that of the modern lion reaches 65 degrees. This made the gape wide enough to allow Smilodon to grasp large prey despite the long canines.
Many Smilodon specimens have been excavated from asphalt seeps that once acted as natural carnivore traps, wherein animals were accidentally trapped and became bait for predators that came to scavenge, but were then trapped themselves. The best-known of such traps are the La Brea Tar Pits of Los Angeles, which have produced the largest sample of saber-toothed cat fossils in the world. The sediments of the pits there were accumulated 40,000 to 10,000 years ago, in the Late Pleistocene. Though the trapped animals were buried quickly, predators often managed to remove limb bones from them, but they were themselves often trapped and then scavenged by other predators; 90% of the excavated bones belonged to predators.
The Talara Tar Seeps in Peru represent a similar scenario, and have also produced fossils of Smilodon. Unlike in La Brea, many of the bones were broken or show signs of weathering. This may have been because the layers were shallower, so the thrashing of trapped animals damaged the bones of previously trapped animals. Many of the carnivores at Talara were juveniles, possibly indicating that inexperienced and less fit animals had a greater chance of being trapped. Though Lund thought accumulations of Smilodon and herbivore fossils in the Lagoa Santa Caves were due to the cats using the caves as dens, these are probably the result of animals dying on the surface, and water currents subsequently dragging their bones to the floor of the cave, but some individuals may also have died after becoming lost in the caves.
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 S. 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 this species was social as well. One critical study claims 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). The author of that study ponders what predators would have responded if the recordings were played in India, where the otherwise solitary tigers are known to aggregate around a single carcass. The authors of the original study responded that though effects of the calls in the tar pits and the playback experiments would not be identical, this would not be enough to overturn their conclusions. In addition, they state that weight and intelligence will not likely effect the results as lighter carnivores are far more numerous than heavy herbivores and the social (and seemingly intelligent) dire wolf is also found in the pits.
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. 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. The brain of Smilodon was relatively small compared to other cat species. Some researchers have argued that Smilodon's brain would have been too small for it to have been a social animal. However, an analysis of brain size in living big cats found no correlation between brain size and sociality. Another argument against Smilodon being social is that being an ambush hunter in closed habitat would likely have made group-living unnecessary, as in most modern cats. Yet it has also been proposed that being the largest predator in an environment comparable to the savanna of Africa, Smilodon may have had a social structure similar to modern lions, which possibly live in groups primarily to defend optimal territory from other lions.
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) suggested 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. Likewise, Meachen-Samuels and Binder (2010) concluded that aggression between males was less pronounced in S. fatalis than in the American lion. However, Christiansen and Harris (2012) found that, as S. fatalis did exhibit some sexual dimorphism, there would have been evolutionary selection for competition between males. The structure of the hyoid bones suggest that Smilodon communicated by roaring, like modern big cats.
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 canines would be present side by side 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. Juvenile and adolescent Smilodon specimens are extremely rare at Rancho La Brea, where the study was performed, indicating that they remained hidden or at denning sites during hunts. The teeth of S. fatalis reached their full size in 18 months at a growth rate of 7 mm/month.
Several Smilodon fossils show signs of ankylosing spondylitis, hyperostosis and trauma; 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. Bony growths where the deltoid muscle inserted in the humerus is a common pathology in La Brea specimens, which was probably due to repeated strain when Smilodon attempted to pull down prey with its forelimbs. Sternum injuries are also common, probably due to collision with prey. Some bones also show evidence of having been bitten by other Smilodon, with one skull showing an unhealed wound, which indicates the individual died as a result.
Distribution and habitat
Smilodon lived during the Pleistocene epoch (2.5 mya–10,000 years ago), and was perhaps the most recent of the saber-toothed cats. Smilodon probably lived in a "closed" habitat such as forest or bush. Fossils of the genus have been found throughout the Americas. In North America, the varied habitat supported other saber-toothed cats in addition to Smilodon, such as Homotherium and Xenosmilus; the habitat here varied from subtropical forests and savannah in the south, to treeless mammoth steppes in the north. North American Smilodon inhabited the temperate latitudes, where the mosaic vegetation of woods, shrubs, and grasses in the southwest supported large herbivores such as horses, bison, antelope, deer, camels, mammoths, mastodons, and ground sloths. Other large carnivores include dire wolves and the American lion..
S. gracilis entered South America during the early to middle Pleistocene, where it probably gave rise to S. populator, which lived in the eastern part of the continent. S. fatalis also entered western South America in the late Pleistocene. The American interchange resulted in a mix of native and invasive species sharing the prairies and woodlands in South America; North American herbivores included proboscideans, horses, camelids and deer, South American herbivores included toxodonts, litopterns, ground sloths, and glyptodonts. Native metatherian predators (including the saber-toothed thylacosmilids) had gone extinct by the Pliocene, and were replaced by North American carnivores such as canids, bears, and large cats. S. populator was very successful here, while Homotherium never became widespread. The extinction of the thylacosmilids has been attributed to competition with Smilodon, but this is probably incorrect, as they seem to have disappeared before the arrival of the large cats. The phorusrhacid "terror birds" may have dominated the large predator niche in South America until Smilodon arrived.
Along with most of the Pleistocene megafauna, Smilodon went extinct 10,000 years ago in the Quaternary extinction event. Its extinction has been 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. 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 (which entered the Americas around the time Smilodon disappered), or a combination of several factors, all of which apply to the general Pleistocene extinction event, rather than specifically to the extinction of the saber-toothed cats.
Some early writers theorized that the last saber-toothed cats, Smilodon and Homotherium, went extinct through competition with the faster and more generalized felids that replaced them. It was even proposed that the saber-toothed predators were inferior to modern cats, as the ever-growing canines were thought to inhibit their owners from feeding properly. Yet fast felids such as the American lion and the American cheetah also went extinct during the Late Pleistocene. The fact that saber-teeth evolved many times in unrelated lineages also attests to the success of this feature.
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