A saber-tooth (alternatively spelled sabre-tooth) is any member of various extinct groups of predatory therapsids, 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" were Middle-Late Permian (~274-252 million years ago) non-mammalian therapsids including the gorgonopsians as well as some basal therocephalians (lycosuchids and scylacisaurids). Saber-toothed mammals have been found almost worldwide from the Eocene epoch to the early Holocene epoch (52.4 million years ago – 8,200 years ago). 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 mammalian saber-tooths were closely related to saber-toothed cats or felids in-general. For example, many members are classified into different family of Feliformia, the Nimravidae; or from a separate order such as Machaeroides and Apataelurus, which are members of the order Oxyaenodonta, and one extinct lineage of metatherian mammals (related to marsupials), represented by the thylacosmilids of the order Sparassodonta. In this regard, these saber-toothed mammals can be viewed as examples of convergent evolution. 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." Among the feliform sabertooth lineages, the family Nimravidae is the oldest, with the oldest fossils dated to 42 Ma of the Eocene. Some sabertooth predators have been known to have lived in the same habitats as seen with basal therocephalians and gorgonopsians in Middle-Late Permian, as well as machairodonts and nimravids (specifically Barbourofelinae) in the Late Miocene.
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.
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. 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. The second and third instances of sabertooth predators were presented by basal therocephalians (scylacosaurids and lycosuchids). The oldest known scylacosaurid, Eutheriodon, was found in the Eodicynodon Assemblage Zone, which dates to the Wordian stage. While earliest lycosuchid fossils were found in the Tapinocephalus Assemblage Zone of the Capitanian stage of the Middle Permian, phylogeny suggests lycosuchids were the most basal therocephalians. Gorgonopsians were subordinate to the dinocephalians and basal therocephalians, with the exceptions such as Phorcys and Jirahgorgon, who were probably the top predators of their locality. However, all dinocephalians and most basal therocephalians would later go extinct during the primary phase of the Capitanian mass extinction event, which may have been caused by volcanism that originated from the Emeishan Traps. While some basal therocephalians, such as Lycosuchus and Glanosuchus, survived the primary phase, they would later go extinct during the secondary wave of extinction. The extinction of dinocephalians and basal therocephalians, gorgonopsians occupied niches left vacant by increasing in size and becoming top predators. Gorgonopsians were able to replace the basal therocephalians as they were less restricted by prey sizes due to their more powerful jaws. During Wuchiapingian-Changhsingian stages of the Late Permian, many gorgonopsians began to go extinct in Africa and Eastern Europe. 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. In Africa, most of the large-bodied rubidgeines died out early in the Daptocephalus Assemblage Zone. Rubidgea, the last rubidgeine, went extinct during a turnover event that separates the lower and upper subzones, being replaced by Inostrancevia. The last gorgonopsians, Inostrancevia africana and Cyonosaurus, went extinct during the Permian–Triassic extinction event, which was likely due to volcanic activities that originated from 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. The extinction of the gorgonopsians marked the start of the sabertooth hiatus that lasted for 200 million years.
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