Saber-toothed predator

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, 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 earliest Wordian of the Middle Permian, although the stratigraphic position of the gorgonopsian, as well as surrounding fossil track record may suggest it is significantly older, potentially Roadian stage of the Middle Permian or even of the Kungurian or Artinskian stages of the Early Permian.^[15] During the Middle Permian, gorgonopsians were subordinate to the dinocephalians and basal therocephalians,^[16][17][18] with the exceptions such as Phorcys and Jirahgorgon, who were probably the top predators of their locality.^[17][19] Dinocephalians and basal therocephalians would later go extinct during the Capitanian mass extinction event,^[18][20] which may have been caused y the eruption of Emeishan Traps.^[21] Following their extinction, gorgonopsians occupied niches left vacant by increasing in size and becoming top predators.^{[22][1][23][16]}

During the second half of the Late Permian, many gorgonopsians began to go extinct in Africa and Eastern Europe.^[24][25][26] The cause of the gorgonopsians extinction in Eastern Europe was the inability to adapt to the changing temperature and humidity and were replaced by archosauriforms and large therocephalians.^[24][27] In Africa, majority of the large-bodied rubidgeines died out early Daptocephalus Assemblage Zone. Rubidgea, the last rubidgeine, went extinct during a turnover event that separates the lower and upper subzones being replaced by Inostrancevia.^[25] The last gorgonopsians, Inostrancevia africana and Cyonosaurus, disappeared in the Late Lopingian during the Permian–Triassic extinction event,^[26][27] 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.^[28]

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.^[29] Some experts hypothesized that their extinction was the result of competition with nimravids,^[29] other experts found no evidence of competitive displacement,^[30][31] as the excellent North American fossil record indicates oxyaenids were declining before the appearance of the replacement taxa such as nimravids.^[30] In addition, the youngest records of machaeroidines predate the oldest records of nimravids.^[29][32]

Instead, their extinction was likely due to the changing climate.^[31][30] 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.^[30] Because of their low mobility, as well as preference for closed habitats, oxyaenids couldn't adapt to the more open, temperate forests.^[31][29]

The third saber-tooth appearance is the basal feliform (carnivoran) family Nimravidae, which also includes barbourofelines.^[33][34][35] 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.^[36] The earliest record of nimravids in North America was represented by Pangurban, which first appeared around 40 Ma.^[32] The diversification of the early nimravids was likely the result of the decline and extinction of oxyaenids, which opened the niche of felid-like carnivores.^[37][38] Nimravids made their first appearance in Europe during the early Oligocene following the Grand Coupre.^[39] Nimravids also saw their greatest diversity during the early Oligocene with 13 contemporary species.^[38]

The family then 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.^[40][41][42] 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.^[43] 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 around 18 Ma and later North America around 12 Ma.^[44][45][46]

The arrival of barbourofelines in North America was thought to have placed a role in the decline and extinction of hesperocyonines.^[47] Over course of the Late Miocene, barbourofelines declined in diversity, going extinct in Europe and East Asia around 9.1 and 8.5 Ma respectively.^[11][48][38] Their extinction in Europe was correlated with the Vallesian-Turolian turnover event, also known as the Vallesian Crisis, which saw high extinction rates along with high levels of appearances of mammalian fauna. The Vallesian Crisis saw an increase of precipitation of seasonality, which gave way for drier, more open environments. Albanosmilus jourdani, the last European barbourofeline, went extinct due to the inability to adapt to the open environments.^[11] The last barbourofelines, represented by Albanosmilus whitfordi and Barbourofelis fricki, went go extinct around 7 Ma,^[45] some experts hypothesized the cause of their extinction was the result of competition with machairodonts.^[43] 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.^[49][50][45]

The fourth appearance of saberteeth was Thylacosmilidae. They were part of an order of metatherian mammals known as sparassodonts. The earliest known thylacosmilid fossil was dated to around 21 Ma,^[51] although a suspected origin within the Eocene-Oligocene boundary has been hypothesized for the family.^[52] The last member of the family, Thylacosmilus, went extinct during the Pliocene, around 3 Ma.^[53] Originally, it was hypothesized that Thylacosmilus was outcompeted by carnivorans such as Smilodon.^[54] Although more recent evidence found no evidence of temporal or niche overlap between the two predators,^[53][55] instead extinction of Thylacosmilus and other sparassodonts was the result of the environmental changes.^[53]

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.^[56] Machairodonts probably evolved from the basal felid Pseudaelurus quadridentatus showed a trend towards elongated upper canines, and is believed to be at the base of the machairodontine evolution.^[57] The earliest known machairodont was Miomachairodus from the late Middle Miocene.^[58] The earliest known homothere, Machairodus, first appeared in the fossil record around 12 Ma in Africa, before appearing in Europe around 10.4 Ma.^[59] The genus would later evolve into Amphimachairodus, which evolved around 9.8 Ma in East Asia.^[60] It would later make an appearance in Europe during the early Turolian replacing Machairodus.^[61] It would also disperse into North America during the Hemphillian stage, with the oldest fossils being found in Withlacoochee River 4A of Florida.^[62] Amphimachairodus was initially rarer than Nimravides, another machairodont, however it would later replace it as grasslands began to expand in North America.^[63] The genus is also noted to have a pan-African distribution,^[64] and may have been the ancestor of Homotherium.^[65]

Smilodontin first appeared in the Late Miocene, Promegantereon being the oldest known genus of the tribe.^[66] Megantereon was another member of the tribe, with the oldest possible fossils associated to the genus being found in the Late Miocene Africa around 7 Ma.^[67] It has been hypothesized by many experts that Megantereon was possibly the ancestor of Smilodon.^[68] Smilodon first appeared in the fossil record during the Blancan stage of the Pleistocene.^[69][70] Smilodon first appears in South America during the Ensenadan stage.^[71]

Machairodonts began to decline during Late Miocene, with the decline of North American machairodonts coinciding with the decline of horses.^[72] This decline would persist into the Pleistocene,^[72] the cause of their decline in the Plio-Plestocene has been debated by experts. The extinction of African machairodonts has been hypothesized to be the result of competition with hominins.^[73][74][75] However, this has been contested by some experts, which argued that the expansion of grasslands and loss of forest covers were more detrimental to the large carnivorans as well the decline of megaherbivores. Additionally, evidence for anthropogenic extinctions by pre-erectus hominins to have been inconclusive, as it would suggest australopiths and early Homo were stealing prey from large carnivorans long before visible archaeological or anatomical evidence of carnivory among hominins.^[76]

In Europe, the extinction of Megantereon coincided with the expansion of grasslands,^[77] while Homotherium went extinct because of competition with early humans and Panthera fossilis.^[78][79][80] Homotherium and Smilodon were the last known members of the subfamily by the Late Pleistocene. 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.^[56][81]

• 
  1st saber-tooth instance: Gorgonopsia (Theriodontia, Therapsida, Synapsida) – Rubidgea atrox skull
• 
  2nd saber-tooth instance: Machaeroidinae (Oxyaenidae, Oxyaenodonta) – Machaeroides skull
• 
  3rd saber-tooth instance: Nimravidae (Feliformia, Carnivora) – Barbourofelis fricki skull
• 
  4th saber-tooth instance: Thylacosmilidae (Borhyaenoidea, Sparassodonta) – Thylacosmilus atrox skull
• 
  5th saber-tooth instance: Machairodontinae (Felidae, Feliformia, Carnivora) – Smilodon skull and upper cervical vertebrae

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 Megantereon and Smilodon, were more robust than modern day felids.^[82][83]

Large gorgonopsians, such as Inostrancevia, were thought to have hunted large contemporary tetrapods,^[84][85] such as dicynodonts and pareiasaurs.^[85][86] Isotopic analysis of the Venta Micena locality in southeast Spain dating to the Early Pleistocene, found evidence of niche partitioning between Homotherium and Megantereon, with the former hunting in more open habitats and hunting prey such as the equine Equus altidens, bison, and juvenile Mammuthus meridionalis. On the other hand, Megantereon hunted in more forested habitats and focused on prey such as Equus altidens, the muskox-relative Soergelia, and the giant deer Praemegaceros.^[87][88] In Swartkrans cave of South Africa, isotopic analysis suggests in this locality, Megantereon hunted hominins, including Paranthropus robustus, baboons, as well early Homo.^[89]

Within the Turkana Basin of Kenya, Megantereon focused on prey weighing ; which included the antelopes Aepyceros, Antidorcas recki, Megalotragus, Kobus sigmoidalis and Tragelaphus, the swines Kolpochoerus limnetes and Metridiochoerus andrewsi, and the three-toed equine Eurygnathohippus ethiopicum.^[90]

Phylogeny of feliform saber-tooths