|Phylogeny and Evolution||Morphology|
Perhaps one of the most iconic animals of the Pleistocene, aside from mammoths, are the Smilodons. These so-called “saber-toothed tigers”, or saber-toothed cats, have appeared in all forms of popular media: movies, television, books, comics, and toys. Though there are some mysteries about these cats, they were nevertheless fascinating creatures.This is the cover of a paperback edition of The Eternal Savage by Edgar Rice Burroughs, featuring a Smilodon fighting a Neanderthal. This image is from commons.wikimedia.org
The first member of the Smilodon genus to be described was Smilodon populator, which was discovered in Brazil by Lund in the 1842. Since that time, other species have been documented. Smilodon fatalis, was discovered in 1869 by Leidy and is found in North America, and Smilodon gracilis was discovered in 1880 by Cope (De Castro and Langer, 2008) and ranged from Eurasia into North and South America (Martin, 1980). S. fatalis is probably the best- known of these species in North America, as it is found abundantly in the Rancho La Brea tar pits of California.
Though it is called the “saber-toothed tiger” by some, Smilodon was actually neither closely related to tigers nor a single species. Smilodon is actually a genus with several distinct species, as mentioned in the previous section. Exactly how many species are in the genus is a matter of slight contention, as some argue that all members of the genus should be considered subspecies of S. populator (De Castro and Langer, 2008), others suggest that S. fatalis should be divided into two species, S. fatalis, which can be found in western North America, and Smilodon floridanus, which, as its name implies, can be found primarily in Florida (Martin, 1980). Others suggest that they should just be considered subspecies.
Phylogenetic analysis has enabled scientists to place the Smilodon genus in relation to modern day fauna. Like all cats, Smilodons were part of the family Felidae, in the order Carnivora (Barnett et al., 2005). However, while early DNA analysis placed Smilodon within the subfamily Felinae with modern cats (Janczewski et al., 1992), more recent studies have revealed that they are not closely related to the Felinae. Instead, they are placed into a subfamily that evolved extant of the Felinae, called the Machairodontinae (Barnett et al., 2005).
The evolutionary history of Smilodon is much easier to trace, compared to its phylogeny. The first known species to appear in the fossil record was S. gracilis, which evolved during the Pleistocene Epoch, approximately 2.5 million years ago. S. fatalis appeared next, evolving about 1.6 million years ago, and lastly, S. populator, approximately 1.0 million years ago (Turner, 1997).
The most distinctive features of Smilodons are, of course, their oversized upper canine teeth, which were 28 cm long in S. populator. This is what gives them their colloquial name of saber-toothed cats. There are, in fact, two types of saber-toothed cat. These are the scimitar-toothed, which had short, broad canines with coarse serrations, and the dirk-toothed, which had longer, more slender canines with fine serrations . It was into this latter group that Smilodon fits. In order for these canines to clear the lower jaw when open, smilodons were able to open their jaws quite widely, to an approximate 100° angle with the rest of their skull (Martin, 1980). In spite of this, reconstructions of the musculature of the jaw implies that Smilodon actually had a rather weak bite (McHenry et al., 2007).Image of a Smilodon skull on display in the University of Kansas Natural History Museum. Note the elongated dirk-teeth.(photo by the author)
The skeletons of Smilodons were robust. The front limbs in particular had thick bones and large muscle attachments (Martin, 1980), implying dense musculature. Of the three species, S. populator was the largest and stoutest (De Santa and Langer, 2008). Meanwhile, S. gracilis was, as its name implies, the smallest and slenderest (Turner, 1997).Image of a S. fatalis skeleton found in the Rancho La Brea Tar Pits on display in the University of Kansas Natural History Museum (photo by the author)
Like all cats, Smilodon was a carnivore. Because of their heavy musculature in their forelimbs, it is often thought that they were ambush predators, sneaking up on prey and attacking suddenly to get a quick kill (Martin, 1980). Others argue that they may, in fact, have been able to chase prey over long distances, at least some of the time, due to their physical similarity to hyenas (Turner, 1997). They tended to prefer large herbivores, such as bison, horses, or even mammoths, for prey (McHenry et al., 2007). In order to take down such large prey, their population density in the Rancho La Brea tar pits suggests that Smilodon was likely a pack hunter (Carbone et al., 2009).
What is less well-known is exactly how Smilodon used their oversized canine teeth. Some think they used them as stabbing implements to pierce the flesh of prey (Martin, 1980). Others suggest that they may have been used primarily to slice meat off of carcasses that were already dead, implying that Smilodons were primarily scavengers (Turner, 1997). However, the most plausible hypothesis seems to be that the upper canines were used in concert with the lower ones to pierce and shear flesh, especially from the throat of their prey (McHenry et al., 2007). However, because these teeth were somewhat fragile and so essential, Smilodon was unable to eat all of a carcass in order to avoid damaging them (DeSantis et al., 2012).
S. gracilis went extinct approximately 50,000 years ago, but S. fatalis and S. populator both survived until the end of the Pleistocene Epoch approximately 10,000 years ago (Turner, 2007). The exact cause of Smilodon’s extinction is not certain, but there are several hypotheses. One hypothesis that was popular at one time, but has lost favor in recent years, is that Paleo-Indians either hunted them to extinction or outcompeted them for prey. This “overkill hypothesis” is supported by the fact that they disappear from the fossil record soon after evidence for fluted-point spear hunters appears (Martin, 1966). However, studies looking at paleoclimate suggest that climate change caused by receding ice caps caused their prey to dwindle, and thus, they could not survive (Martin and Nuener, 1978).
The genus Smilodon was discovered during the 19th century, and consisted of three species of dirk-toothed cats. They were heavily built and featured long upper canine teeth. They are thought to have been pack predators who ambushed their prey, killing it quickly by using their canines to slice through its throat. It is unclear how they died out, but it may have been due to loss of prey when the megafauna of the Pleistocene went extinct.