Saber-toothed predator

Morphology

Size

Sabertooth predators vary widely in size, with some of the smallest known sabertooth predators, such as Eofelis edwardsii, Eusmilus cerebralis, and Lycaenops omatus, only being about the size of a bobcat or medium sized dog.

But many sabertooth predators have been noted to have reached larger sizes. Thylacosmilids were some of the largest metatherian predators with the smallest genus, Anachlysictis, weighing between 18–22 kilograms (40–49 lb). Thylacosmilus was one of the largest known thylacosmilid, standing 60 cm (2 ft 0 in) at the shoulders and weighing 41–150 kg (90–331 lb) based on various regressions. The closely related Pampaluctor calibar was noted to have been even larger than Thylacosmilus weighing 100 kg (220 lb). Among machairodonts, Smilodon fatalis was estimated to have stood 100 cm (3.3 ft) at the shoulders and weighed 160–280 kg (350–620 lb), while Homotherium stood 90–110 cm (3.0–3.6 ft) at the shoulders and weighed 200–250 kg (440–550 lb). Barbourofelis fricki, the largest known nimravid, was hypothesized to have been a lion-sized predator, standing 90 cm (2 ft 11 in) at the shoulders and weighing 256 kg (564 lb). ^{Including supplementary materials}

The largest gorgonopsianns, Inostrancevia latifrons, was estimated to have measured 3.5 m (11 ft) in length and could've weighed 300 kg (660 lb). Among gorgonopsians, only Rubidgea rivaled it in size. The largest machairodonts were of some of the largest known felids. Smilodon populator was estimated to have stood 120 cm (47 in) at the shoulders and typically weighed 220–360 kg (490–790 lb), with the large males weighing 436 kg (961 lb). Within the Amphimachairodus genus, Amphimachairodus horribilis is estimated to have weighed 405 kg (893 lb). In addition, the largest species within the genus, Amphimachairodus kabir, was estimated to have weighed 350–490 kg (770–1,080 lb).

Anatomy

A notable feature among the various clades of sabertoothed therapsids is their noticeably robust forelimbs. Postcranial analysis on Gorgonops found its forelimbs to have been rather robust, while its hindlimbs were slender. In general, the postcranial anatomy of gorgonopsians was rather conservative. Some machairodonts, such as Megantereon and Smilodon, were more robust than modern day felids. Humeral length regressions by Sorkin (2008) found that Thylacosmilus fell below the 95% prediction interval from Felinae than Smilodon, Panthera spelaea, and Panthera atrox, suggesting it had a more robust humeri than any felid. Therrien (2005) found the leopard-sized Barbourofelis morrisi had a crural index to the much larger Smilodon fatalis. However, its lower brachi index was much lower than the machairodont being closer to that of that of ambulators. This suggests Barbourofelis morrisi had similar leverage to the machairodont. The larger species of Barbourofelis were found to have a lower brachi and crural index than Smilodon, suggesting a greater power output of the limbs.

On the other hand, Homotherium forelimbs were less powerfully built, being between the forelimbs of cheetahs and lions.

Paleobiology

Diet

All sabertooth predators are hypercarnivores with their dentition specialized for only consuming flesh and lost the ability to consume other items such as plants and bone. Large gorgonopsians, such as Inostrancevia, were thought to have hunted large contemporary tetrapods, such as dicynodonts and pareiasaurs. Within Gratkorn, Albanosmilus jourdani may have hunted equids, suids, and palaeomerycids based on coprolites; additional fossil evidence suggests it may have occasionally scavenged on Deinotherium. Due to the robustness of its forelimbs, it was hypothesized that Barbourofelis loverum could've hunted prey larger than itself, such as Teleoceras and Gomphotherium. Nimravides catocopis was thought to have preferred hunting prey weighing 413–1,386.3 kg (911–3,056 lb), with the largest prey weighing 1.6 t (3,500 lb), although it may not have been a large prey specialist. Based on regressions, it has been hypothesized that a 436 kg (961 lb) Smilodon populator was capable of taking on prey up to 3 t (3.0 long tons; 3.3 short tons), with a pack potentially being capable of taking down fully grown megaherbivores such as Megatherium and Notiomastodon.

Isotopic analysis has been done on multiple sabertooth predators, which have recovered their preferred prey. Within the Quehué and Salinas Grandes de Hidalgo localities, Thylacosmilus preyed upon grazers, such notoungulates in C3 open areas, as well as litopterns, and large rodents.

Within the Venta Micena locality of southeast Spain 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. The Turkana Basin of Kenya also found evidence of machairodonts hunting in different habitats. Megantereon hunted in woody habitats and focused on prey weighing 72 kg (159 lb); 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. On the other, Homotherium hunted in open habitats focusing on prey weighing 294 kg (648 lb); which included antelopes such as Megalotragus, Kobus sigmoidalis, and Tragelaphus, swines Kolpochoerus limnetes, Nyanzachoerus kanamensis, and Metridiochoerus andrewsi, the three-toed equine Eurygnathohippus ethiopicum, and the extinct giraffe Giraffa pygmaea.

In Swartkrans cave of South Africa, isotopic analysis suggests in this locality, Megantereon hunted hominins, including Paranthropus robustus, baboons, as well early Homo.

Predatory behavior

Kammerer (2016) noted despite their large canines, most gorgonopsians probably lacked the other specializations found in true saber-toothed predator ecomorphs due to cranial morphology convergences with head-focused predators such as theropods. He noted only two gorgonopsians, Smilesaurus and Inostrancevia, had exceptionally large canines and may have been closer functional analogues to later sabertooths. However, postcranial analysis on Gorgonops suggests gorgonopsians were using their powerful forelimbs to capture prey before using their large canines, suggesting this hunting method was more widespread than Kammerer (2016) hypothesized.

It was initially hypothesized that sabertoothed predators were more adapted for hunting relatively larger prey, including megaherbivores. However, this hypothesis has been questioned by experts. 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 10 cm. 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.

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. 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 similar sized or 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 larger prey.

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. 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".

The similarity in all these unrelated families involves the convergent evolution of the saber-like canines as a hunting adaptation. Meehan et al. 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.

Janis et al. (2020) 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. Another study has found that other saber toothed species similarly had diverse lifestyles and that superficial anatomical similarities obscure them.

Social behavior

A gorgonopsian, comparable to Eriphostoma microdon in size, showed signs of a healed bite mark with an embedded tooth in its snout. Based on mammalian healing capabilities, the specimen was bitten 2–9 weeks before its death. The absence of a drainage channel for pus or any other trace of infection suggests the bite wasn't the cause of death. Experts hypothesized it was an example of social biting, meant to signal dominance and power within the hierarchy. Machairodonts, such as Megantereon and Promegantereon, were thought to have been solitary predators. In the case of Promegantereon, it was due to the lack of cubs found in the predator traps. However, given its level of sexual dimorphism, adults had a higher degree of tolerance for one another and young adults would've inhabited their mothers' territory after maturing.

However, some machairdonts have been speculated to have been social predators. Carbone et al. (2009) hypothesized that based on the ratio of juveniles to adults and high abundance of specimens found in the La Brea Tar Pits, Smilodon was a social carnivore, much like modern lions. However, some experts have questioned the evidence of Smilodon being gregarious. Kiffner (2009) questioned Carbone et al. (2009) as their analysis 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). Additionally, solitary felids, such as tigers and cougars, been known to aggregate around carcasses. The authors of the original study argued while the playback experiments and calls in the tar pits would've been different, it wouldn't have overturned their conclusions. Furthermore, they argued weight and intelligence would not likely affect 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. However, they do not rule out the possibility that Smilodon may have been solitary in part of its distribution.

Pathological analysis has also been used as evidence to support gregariousness in Smilodon, although experts have questioned this as cats can recover quickly from even severe bone damage and an injured Smilodon could survive if it had access to water. However, dental injuries suggests that injured individuals consumed softer flesh than non-injured individuals. The consideration of individuals surviving a good amount of time, it may suggest that Smilodon may have formed social groups. Additionally, a Smilodon specimen was suffering from hip dysplasia at a young age but managed survived to adulthood. This suggests that it could not have survived to adulthood without aid from a social group, as this individual was unable to hunt or defend its territory due to the severity of its congenital issue. Two Smilodon populator skulls from Argentina show seemingly fatal, unhealed wounds which appear to have been caused by the canines of another Smilodon (though it cannot be ruled out they were caused by kicking prey). If caused by intraspecific fighting, it may also indicate that they had social behavior which could lead to death, as seen in some modern felines (as well as indicating that the canines could penetrate bone).

Amphimachairodus hezhengensis was believed to have been gregarious due to its adaptation of living in open environments such as stereo vision, which would've aided in prey identification and cooperative hunting. Additionally, evidence of healed pathological forepaw suggests evidence of partner care, and the abundant of large carnivorans present in Linxia Basin, which likely prove the rapid morphological evolution and gregariousness. Pathological analysis also found that Machairodus aphanistus was likely gregarious due to specimens surviving severe injuries such as a broken jaw. Solitary felids with high levels of sexual dimorphism such as leopards typically die before severe injuries managed to heal, being unable to hunt for themselves or sustain themselves on carrion long-term. Lions, however, have a greater chance of survival, due to living in prides. Considering the high levels of sexual dimorphism in this species of Machairodus, this would likely indicate higher levels of intolerance to other individuals within their territory. This would suggest males likely formed coalitions of two or three individuals, while females were solitary predators. Homotherium was thought to have been a gregarious animal based on its anatomy, as well as fossil and genetic evidence.

Taxonomy

Gorgonopsians were believed to have split off from eutheriodonts either Kungurian stage of the Early Permian or the Roadian stage of the Middle Permian. Following their split, basal therocephalians diverged from eutherocephalians sometime in the Roadian or Wordian stage. The relationship between therocephalians and cynodonts, the lineage that includes mammals, has been debated by experts. If recovered as a clade, therocephalians were thought to have split off from cynodonts during the Roadian or Wordian stage of the Middle Permian.

Among mammals, metatherians split off from placentals during the Mesozoic between the earliest Cretaceous and latest Triassic. Thylacosmilids were believed to diverged from proborhyenids during the Eocene epoch. By the early Paleocene, oxyaenids already diverged from carnivoraformes. Following the caniform-feliform split, nimravids diverged from other feliforms during the Eocene epoch. The machairodonts would split off from felids during the Early Miocene.

• Clade Therapsida
  • Clade: †Gorgonopsia
    • Genus: †Nochnitsa
    • Genus: †Viatkogorgon
      • Clade: †Russian clade
        • Genus: †Sauroctonus
        • Genus: †Suchogorgon
        • Subfamily †Inostranceviinae
      • Clade: †African clade
        • †Smilesaurus
        • Family: †Phorcyidae
          • Genus: †Jirahgorgon
          • Genus: †Phorcys
        • Subfamily †Rubidgeinae
  • Clade: †Therocephalia
    • Family: †Lycosuchidae
    • Genus: †Gorynychus
    • Clade: †Scylacosauria
      • Family: †Scylacosauridae
  • Class: Mammalia
    • Clade: Metatheria
      • Order: †Sparassodonta
        • Family: †Thylacosmilidae
    • Subclass: Placentalia
      • Clade: Pan-Carnivora
        • Order: †Oxyaenodonta
          • Family: †Oxyaenidae
            • Subfamily †Machaeroidinae
              • Genus: †Apataelurus
              • Genus: †Diegoaelurus
              • Genus: †Machaeroides
        • Order Carnivora
          • Suborder Feliformia ('cat-like' carnivores)
            • Family: †Nimravidae (false-saber toothed cats)
              • Subfamily †Barbourofelinae
              • Subfamily †Hoplophoninae
              • Subfamily †Nimravinae
            • Family Felidae (true cats)
              • Subfamily †Machairodontinae (true saber-toothed cats)
                • Tribe †Homotherini
                • Tribe †Metailurini
                • Tribe †Smilodontini