[1] Traditionally, three different tribes of machairodontines were recognized, the Smilodontini with typical dirk-toothed forms, such as Megantereon and Smilodon, the Machairodontini or Homotherini with scimitar-toothed cats, such as Machairodus or Homotherium, and the Metailurini, containing genera such as Dinofelis and Metailurus.

[1] The phylogenetic relationships of Machairodontinae are shown in the following cladogram:[10][11][12][13][14] †Dinofelis paleoonca †Dinofelis petteri †Dinofelis aronoki †Dinofelis barlowi †Dinofelis cristata †Dinofelis darti †Dinofelis diastemata †Dinofelis piveteaui †Metailurus boodon †Metailurus major †Metailurus mongoliensis †Metailurus ultimus †Adelphailurus kansensis †Stenailurus teilhardi †Yoshi garevskii †Yoshi minor †Tchadailurus adei †Megantereon cultridens †Megantereon ekidoit †Megantereon whitei †Megantereon hesperus †Megantereon inexpectatus †Megantereon microta †Megantereon nihowanensis †Megantereon vakhshensis †Smilodon fatalis †Smilodon gracilis †Smilodon populator †Rhizosmilodon fiteae[16] †Paramachairodus maximiliani †Paramachairodus orientalis †Paramachairodus transasiaticus †Promegantereon ogygia[8] †Machairodus alberdiae †Machairodus aphanistus †Machairodus horribilis †Machairodus laskerevi †Machairodus robinsoni †Miomachairodus pseudaeluroides †Hemimachairodus zwierzyckii †Amphimachairodus kurteni †Amphimachairodus alvarezi †Amphimachairodus coloradensis †Amphimachairodus giganteus †Amphimachairodus kabir †Nimravides catacopsis †Nimravides galiani †Nimravides hibbardi †Nimravides pedionomus †Nimravides thinobates †Lokotunjailurus emageritus †Lokotunjailurus fanonei †Xenosmilus hodsonae †Homotherium ischyrus †Homotherium latidens †Homotherium serum †Homotherium venezuelensis Until the recent discovery of the Late Miocene fossil depository known as Batallones-1 in the 1990s, specimens of Smilodontini and Homotheriini ancestors were rare and fragmentary, so the evolutionary history of the saber-toothed phenotype, a phenotype affecting craniomandibular, cervical forelimb and forelimb anatomy, was largely unknown.

[17][18] Although the exact cause is uncertain, current findings have supported the hypothesis that a need for the rapid killing of prey was the principal pressure driving the development of the phenotype over evolutionary time.

These muscles have the capacity to be powerful and undergo a great degree of modification for ranging bite forces, but are not very elastic due to their thickness, placement, and strength.

The homotherines were overall more specialized and already the earliest taxa like Lokotunjailurus were remarkably long-legged and lean, though as large as a modern lion, a trend that further magnified in the Pliocene-Pleistocene genus Homotherium, which was once thought to be plantigrade, but was proven to be digitigrade.

[13] Homotherium serum, the most derived known species from the Pleistocene of North America bore a sloped back that might have made it excellent at running long distances, similar to the living spotted hyena.

It also had a well-developed visual cortex, a large nasal cavity that would have allowed for better oxygen intake and smaller, only partially retractable claws that might have functioned like spikes for a better grip on the ground, all of which seems to point to a highly active lifestyle and cursoriality.

[30] Xenosmilus however, a mid-Pleistocene homotherine from Florida and close relative of Homotherium, broke this trend in that it had both scimitar-like teeth and a bulky and strong build that is more typical for dirk-tooths.

[35] The main issue was the stresses suffered by the mandible: a strong force threatened to break the jaw as pressure was placed on its weakest points.

[31] Examinations published in 2022 of tooth wear patterns on Smilodon and bite marks on the bones of the peccary Platygonus by Xenosmilus suggest that machairodonts were capable of efficiently stripping and de-fleshing a carcass of meat when feeding.

[citation needed] According to Antón, García-Perea and Turner (1998), the nostrils of living felids always extend to a similar position, independently of the length of the nasal bones, which in Smilodon falls within the range observed in modern species.

Studies of Homotherium and Smilodon suggest that scimitar-toothed machairodonts like Homotherium itself possessed upper lips and gum tissue that could effectively hide and protect their upper canines; a trait they shared in common with modern cat species, while Smilodon had canines that remained partially exposed and protruded past the lips and chin even while the mouth was closed due to their great length.

It is considered the best Pleistocene fossil bed in North America for the number of animals caught and preserved in the tar, and may be similar to the situation created in the study.

The study concluded that this latter situation most closely fit the ratio of animals found at the La Brea tar pits, and therefore that Smilodon was most likely social.

[47] The modern lion is capable of, in large numbers, killing weakened adult and healthy subadult elephants, so similar sized Homotherium likely could have managed the same feat with juvenile mammoths.

But the idea that a cat, even one of very large size and possibly social, was able to cooperatively 'drag' a 180-kilogram (400 lb) mammoth calf any real distance into a cave without damaging its teeth has aroused great criticism.

Its sloped back and powerful lumbar section of its vertebrae suggested a bear-like build, so it might have been capable of pulling weights, but breaking canines, a fate suffered by Machairodus and Smilodon with some frequency, is not seen in Homotherium.

[49] Machairodus is another genus with few fossil records to suggest a social nature, but canines on these species are broken more often than others and show signs of extensive healing afterward.

A male Amphimachairodus giganteus from China housed by the Babiarz Institute of Paleontological Studies is an older individual with a broken canine, worn from usage after the break.

However, the individual died of a severe nasal infection, an injury that a social predator would have had a better chance of healing, so the skull can be interpreted in different ways.

[citation needed] In another example of paleopathology supporting the social hypothesis, a large number of Smilodon fossils from the La Brea tar pits feature hunting injuries.

Animals may have been crippled long after the injury healed, suffering swollen ankles, prominent limps, and limited mobility that persisted for years.

It is far more likely that such an animal would have been unable to move from a single spot on the ground for several months and might have only survived by being brought food or dragging itself towards kills made by relatives.

Larger, more sturdily built cat species, such as lions and leopards, have been observed to recover from severe injuries, such as broken jaws and torn muscles.

A strong sense of smell and good hearing could have helped find carcasses or steal the kills of other predators, such as dire wolves or short-faced bears, and sprinting would not have been needed, as is seen in the stocky conformation of most machairodonts.

If anything, the dangers to breaking teeth held in the throat of a panicked animal, even if well restrained, outweighs the possible benefits, so this method has often been viewed as improbable.

Another variation[13] suggests the advanced machairodonts were highly specialized, enough to obtain the specific geometry to puncture the four major blood vessels in the throat of a prey animal in one bite.

The bite would not need to be specific, and could be repeated to hasten the death of the animal, and it is already seen in the killing methods of several extant species, such as the spotted hyena.

Most ungulates are highly sensitive around the belly and hindquarters, and most predators find it much easier to capture and subdue an animal similar to the domestic cow, by manipulating the head and forequarters.

A third issue with the shearing bite is that the canines would need to tear a large hole in the belly of the animal to be successful, but might instead simply flay the skin and produce two long slits.