A saber-tooth (alternatively spelled sabre-tooth) is any member of various extinct groups of predatorytherapsids, predominantly carnivoran mammals, that are characterized by long, curved saber-shaped canine teeth which protruded from the mouth when closed.
Among the earliest animals that can be described as "sabertooths" are the gorgonopsids, a group of non-mammalian therapsids that lived during the Middle-Late Permian, around 270-252 million years ago (mya).[1] Saber-toothed mammals have been found almost worldwide from the Eocene epoch to the end of the Pleistocene epoch (42 million years ago – 11,000 years ago).[2][3][4]
One of the best-known genera is the "saber-toothed cat" Smilodon, the species of which, especially S. fatalis, are popularly referred to as "saber-toothed tigers", although they are not closely related to tigers (Panthera), and are instead members of the extinct cat subfamily Machairodontinae (the true saber-toothed cats), most members of which had saber-teeth of varying lengths. Despite some similarities, not all saber-tooths are closely related to saber-toothed cats or felids in-general. For example, many members are classified into different family of Feliformia, the Nimravidae;[5] or from a separate order such as Machaeroides and Apataelurus, which are members of the order Oxyaenodonta, and two extinct lineages of metatherian mammals (related to marsupials), including the thylacosmilids of Sparassodonta. In this regard, these saber-toothed mammals can be viewed as examples of convergent evolution.[6] This convergence is remarkable due not only to the development of elongated canines, but also a suite of other characteristics, such as a wide gape and bulky forelimbs, which is so consistent that it has been termed the "saber-tooth suite."[7]
Of the feliform lineages, the family Nimravidae is the oldest, entering the landscape around 42 mya and becoming extinct by 7 mya.[8] Both machairodonts and nimravids (specifically Barbourofelinae) would have shared some habitats.[9][10][11][12]
Evolution
The different groups of saber-toothed predators evolved their saber-toothed characteristics entirely independently. They are most known for having maxillary canines which extended down from the mouth when the mouth was closed.[13][14]
The first saber-tooths to appear were non-mammalian synapsids, such as the gorgonopsians; they were one of the first groups of animals within Synapsida to experience the specialization of saber teeth, and many had long canines.[14] The oldest definitive worldwide gorgonopsian fossil was found in Port des Canonge Formation of Mallorca in the western Mediterranean dating to at least the earliest Wordian.[15] During the Middle Permian, gorgonopsians were subordinate to the dinocephalians, another clade of therapsids,[16] with the exception of Phorcys, who was probably the top predator in its assemblage.[17] Dinocephalians would later go extinct during the Capitanian mass extinction event.[16] Following their extinction, gorgonopsians occupied niches left vacant by increasing in size and becoming top predators.[18][1][19] Gorgonopsians disappeared in the Late Lopingian during the Permian–Triassic extinction event, mainly due to volcanic activities that originated in the Siberian Traps. The resulting eruption caused a significant climatic disruption unfavorable to their survival, leading to their extinction. Their ecological niches gave way to modern terrestrial ecosystems including sauropsids, mostly archosaurs, and among the few therapsids surviving the event, mammals.[20]
The second instance of saber-teeth is from machaeroidines of the extinct order Oxyaenodonta. The earliest known machaeroidine was Machaeroides simpsoni, which first appeared in the early Eocene. The last macheroidines died out in the middle Eocene, which was linked to the significant faunal overturn during the transition period between the middle and late Eocene.[21] Some experts hypothesized that their extinction was the result of competition with nimravids,[21] other experts found no evidence of competitive displacement,[22][23] as the excellent North American fossil record indicates oxyaenids were declining before the appearance of the replacement taxa such as nimravids.[22] In addition, the youngest records of machaeroidines predate the oldest records of nimravids.[21][24]
Instead, their extinction was likely due to the changing climate.[23][22] During the last 10 million years of the Eocene (43 to 33 Ma), the planet transitioned from a warm moist tropical world to a cooler, seasonally arid one.[22] Because of their low mobility, as well as preference for closed habitats, oxyaenids couldn’t adapt to the more open, temperate forests.[23][21]
The third saber-tooth appearance is the ancient feliform (carnivoran) family Nimravidae. This family also included the barbourofelines.[25][26][27] The oldest known nimravid was found in the Lushi and Dongjun formations of China, which dated to the middle Eocene. The next oldest nimravid was Maofelis middle of the Eocene epoch, about 41.03 Ma.[28] The earliest record of nimravids in North America was represented by Pangurban, which first appeared around 40 Ma.[29] The diversification of the early nimravids was likely the result of the decline and extinction of oxyaenids, which opened the niche of felid-like carnivore niche.[30] Nimravids made their first appearance in Europe during the early Oligocene.[31]
The family saw a decline in diversity in both Europe and North America, both corresponding with the increase in aridity, and in the case of North American nimravids, competition with amphicyonids.[32][33] The extinction of North American nimravids started the infamous cat gap, a 7 million year period when no cat-like predators were present in North America.[34] Barbourofelines probably evolved from Nimravinae dispersing into Africa during the early Miocene. The presence of large hyaenodonts prevented them from reaching a large size but were able to carve a niche due to their dental morphology. Eventually, they dispersed from Africa into Eurasia and later into North America. The last barbourofelines would go extinct around 7 Ma.[35] Some experts hypothesized the cause of their extinction was the result of competition with machairodonts.[36][34] Other experts suggested it was more likely they went extinct due to the expansion of grasslands and the decline of favorable prey such as antilocaprids, camelids, dromomerycids, and equids.[37][38][35]
The fourth appearance of saberteeth wasThylacosmilidae. They were part of an order of metatherian mammals known as sparassodonts. The earliest known thylacosmilid fossil was dated to around 21 Ma.[39] The last member of the family Thylacosmilus, went extinct during the Pliocene, around 3 Ma.[40] Originally, it was hypothesized that Thylacosmilus was outcompeted by carnivorans such as Smilodon.[41] Although more recent evidence found no evidence of temporal or niche overlap between the two predators,[40][42] instead extinction of Thylacosmilus and other sparassodonts was the result of the environmental changes.[40]
The fifth and last saber-toothed group to evolve were the machairodonts themselves. Mitochondrial DNA found that machairodonts split off from ancestors of living cats around 20 Ma, with Homotherium and Smilodon diverging from one another around 18 Ma.[43] Machairodonts began to decline during late Miocene, with the decline of North American machairodonts coinciding with the decline of horses.[44] This decline would persist into the Pleistocene,[44] with Homotherium and Smilodon being the last known members of the subfamily. These two genera became extinct around 13,000-10,000 years ago as part of the wave of extinctions of most large animals across the Americas.[43]
Some gorgonopsians had two pairs of upper canines with two jutting down from each side, but most had one pair of upper extreme canines. Because of their primitiveness, they are extremely easy to tell from machairodonts. Several defining characteristics are a lack of a coronoid process, many sharp "premolars" more akin to pegs than scissors, and very long skulls. Despite their large canines, however, most gorgonopsians probably lacked the other specializations found in true saber-toothed predator ecomorphs.[1] Two gorgonopsians, Smilesaurus and Inostrancevia, had exceptionally large canines and may have been closer functional analogues to later sabertooths.[1]
Some machairodonts, such as Smilodon, were more robust than modern day felids.[45]
It was initially hypothesized that sabertooth predators were more adapted for hunting relatively larger prey.[49][50][51][52] In the case of large sabertooth predators, such as Barbourofelis[50] and Smilodon,[51] being thought to have been specialized on predating on megaherbivores.
However, this hypothesis has been questioned by experts.[53][54][55] Andersson et al. (2011) found that for saber-tooth cats, the depth of the killing bite decreases dramatically with increasing prey size. The extended gape of saber-toothed cats results in a considerable increase in bite depth when biting into prey with a radius of less than 10cm. For the saber-tooth, this size-reversed functional advantage suggests predation on species within a similar size range to those attacked by present-day carnivorans, rather than megaherbivores as previously believed.[53]
Antón (2013) pointed out that the prey of the large gorgonopsians were smaller than them, not ten times larger. Additionally, given their large sizes, they weren’t the most abundant potential prey in most of their ecosystems. In comparison, medium-sized prey, such as ungulates (e.g. bovids and horses), were more plentiful and were the more ideal prey for sabertooth predators.[55]
Lautenschlager et al. (2020) found that the effective jaw gape in most sabertooths was restricted between 45° and 65°, furthermore proving not all sabertooths were large prey specialists. Their analysis found majority of gorgonopsians had a jaw gape below 80° and effective jaw gape below 60°, suggesting a specialization towards smaller prey instead of larger prey. On the other hand, nimravids generally showed high jaw gapes, with majority of taxa wide jaw gape of over 90°, combined with little bending strength values over time suggests they have an intermediate killing strategy towards large prey.[54]
A disputing view of the cat's hunting technique and ability is presented by C. K. Brain in The Hunters or the Hunted?, in which he attributes the cat's prey-killing abilities to its large neck muscles rather than its jaws.[56] Large cats use both the upper and lower jaw to bite down and bring down the prey. The strong bite of the jaw is accredited to the strong temporalis muscle that attach from the skull to the coronoid process of the jaw. The larger the coronoid process, the larger the muscle that attaches there, so the stronger the bite. As C.K. Brain points out, the saber-toothed cats had a greatly reduced coronoid process and therefore a disadvantageously weak bite. The cat did, however, have an enlarged mastoid process, a muscle attachment at the base of the skull, which attaches to neck muscles. According to C.K. Brain, the saber-tooth would use a "downward thrust of the head, powered by the neck muscles" to drive the large upper canines into the prey. This technique was "more efficient than those of true cats".[56]
The similarity in all these unrelated families involves the convergent evolution of the saber-like canines as a hunting adaptation. Meehan et al.[57] note that it took around 8 million years for a new type of saber-toothed cat to fill the niche of an extinct predecessor in a similar ecological role; this has happened at least four times with different families of animals developing this adaptation. Although the adaptation of the saber-like canines made these creatures successful, it seems that the shift to obligate carnivorism, along with co-evolution with large prey animals, led the saber-toothed cats of each time period to extinction. As per Van Valkenburgh, the adaptations that made saber-toothed cats successful also made the creatures vulnerable to extinction. In her example, trends toward an increase in size, along with greater specialization, acted as a "macro-evolutionary ratchet": when large prey became scarce or extinct, these creatures would be unable to adapt to smaller prey or consume other sources of food, and would be unable to reduce their size so as to need less food.[58]
More recently, it has been suggested that Thylacosmilus differed radically from its placental counterparts in possessing differently shaped canines and lacking incisors. This suggests that it was not ecologically analogous to other saber-teeth and possibly an entrail specialist.[59] Another study has found that other saber toothed species similarly had diverse lifestyles and that superficial anatomical similarities obscure them.[60]
The following cladogram shows the relationships between the feliform saber-tooths, including the Nimravidae, Barbourofelidae and Machairodontinae.[62][63] Saber-toothed groups are marked with background colors.
All saber-toothed mammals lived between 33.7 million and 9,000 years ago, but the evolutionary lines that led to the various saber-tooth genera started to diverge much earlier. It is thus a polyphyletic grouping.
↑Barrett, P. Z.; Hopkins, W. S. B.; Price, S. A. (2021). "How many sabertooths? Reevaluating the number of carnivoran sabertooth lineages with total-evidence Bayesian techniques and a novel origin of the Miocene Nimravidae". Journal of Vertebrate Paleontology. 41 (1) e1923523. Bibcode:2021JVPal..41E3523B. doi:10.1080/02724634.2021.1923523. S2CID236221655.
↑Antón, Mauricio (2013). Sabertooth (Life of the Past). Indiana University Press.
↑Meachen-Samuels, Julie A. (2012). "Morphological convergence of the prey-killing arsenal of sabertooth predators". Paleobiology. 38 (1): 1–14. doi:10.1666/10036.1. S2CID86749260.
123Prevosti, Francisco J.; Forasiepi, Analía; Zimicz, Natalia (5 November 2011). "The Evolution of the Cenozoic Terrestrial Mammalian Predator Guild in South America: Competition or Replacement?". Journal of Mammalian Evolution. 20 (1): 3–21. doi:10.1007/s10914-011-9175-9. hdl:11336/2663. S2CID15751319.
12Antón, Mauricio (2013). Sabertooth. Bloomington, Indiana: University of Indiana Press. p.74. ISBN978-0-253-01042-1.
12Brain, C. K. "Part 2: Fossil Assemblages from the Sterkfontein Valley Caves: Analysis and Interpretation." In: The Hunters or the Hunted?: An Introduction to African Cave Taphonomy. Chicago: University of Chicago, 1981. ISBN0226070891
↑Piras P, Maiorino L, Teresi L, Meloro C, Lucci F, Kotsakis T, Raia P (2013) Data from: Bite of the cats: relationships between functional integration and mechanical performance as revealed by mandible geometry. Dryad Digital Repository. https://dx.doi.org/10.5061/dryad.kp8t3
Further reading
Mol, D., W. v. Logchem, K. v. Hooijdonk, R. Bakker. The Saber-Toothed Cat. DrukWare, Norg 2008. ISBN978-90-78707-04-2.
Sardella, R. (10 March 2012). "Web of Knowledge (v5.6)". Soc Paleontologica Italiana. Archived from the original on 13 April 2020. Retrieved 27 October 2012.
Anton, Mauricio (2013). Sabertooth. Bloomington: Indiana University Press. ISBN978-0-253-01049-0.
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