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Sabre-tooth cats, commonly termed “sabre-tooth tigers”, are a group of large, hyper-carnivorous predators of the Felidae family, widespread during the Late Tertiary. These cats were often very different from modern cats in cranial and mandibular morphology, as they had long and sharp maxillary canines, as well as unique features that accommodated for the increase in canine length. The elongated canines often had serrations on the anterior and posterior edges, and were better suited for piercing tough materials such as thick animal skin. These cats have some of the most extreme craniodental adaptations for predation within all of Mammalia. In general, they had shortened skulls to facilitate wide gape, reduced post-carnassials, and robust forelimbs for grasping and manipulating prey. They had moderately powerful bites, however with less adductor power for their body size, in comparison to extant felids.

The sabretooth morphological trait has evolved repeatedly in the past in more than one incidence. It has appeared in nimravids, barbourofelids, and machairodonts (Carnivora, Feliformia), as well as some other carnivorous placentals (such as Machaeroides: Creodonta), marsupials (Thylacosmilus: Borhyaenidae) and non-mammalian therapsids (such as gorgonopsids).

Ecomorphs
Sabretooths can be distinguished and separated into three different ecomorphs. The general morphology of each of these ecomorphs is also distinct.

Dirk-Tooth
The dirk-tooth ecomorph is characterised by very long, and laterally compressed canines with fine or no serrations, relatively shorter incisors, and a long sagittal crest. Dirk-tooths have a robust body, their skeletons resembling that of a badger or a wolverine rather than that of a lion or a tiger, with massive limb bones and large muscle attachments. They are capable of a powerful bite with incisors while restraining prey with their forelimbs. These characteristics are indicative of a predator that uses ambush-style techniques to capture prey. The carnassials of dirk-tooths are kept sharp by wearing away enamel to produce a dentine-enamel interface, promoting rapid wear and could thus limit the life span of the mammal. Dirk-tooths include Barbourofelis, Eusmilus, Hoplophoneus, Megantereon, Paramachairodus, and Smilodon.

Scimitar-Tooth
Scimitar-tooth ecomorph is characterised by shorter, thicker canines with serrations that are large and broadly spaced, longer incisors, a shorter sagittal crest/temporalis musculature associated with relatively weaker bite force, and a more felid-like body shape and limbs adapted for running. Scimitar-tooths includes Dinictis, Homotherium, Ischyrosmilus, Machairodus, Nimravides, and Nimravus.

A third ecomorph consists only of the species Xenosmilus hodsonae. It had short sabres of the scimitar-tooth ecomorph, as well as the robust body and increased sagittal of the dirk-tooth ecomorph. . This ecomorph may not have been as widespread as dirk- and scimitar-toothed forms.

Morphology
The maximum possible gape for a sabre-toothed cat ranges from 100 to more than 115 degrees, as compared to the African Lion’s gape of only 65 degrees. The large gapes cause stretching of the temporal muscles, and cause a decrease in the coronoid process of the mandible. The temporal muscles were not weak, as had been previously thought.

The large upper canines can reach lengths of up to 210mm, and are bladelike, unlike the conical teeth of modern cats. The canines are lateromedially compressed, and are also anteroposteriorly wider than the canines of conical-tooth felids. They lay exposed over the lower lip against a flange, served to protect both the tooth from damage, as well as protect from self-harm.

There are several unique skull modifications in evolution of sabrecats to accommodate for its unique features. The snout of the sabretooth cat was elevated to accommodate for extreme gape when the mouth opened, as the flange would contact the animal’s chest. There was also a ventral displacement of the jaw joint, a rotation of the occipital plane so it would be more vertical, a reduction of the coronoid process of the mandible, and an upward rotation of the palate to accommodate for a wide gape and elongate canines. These evolutionary changes are highly correlated with canine length, and are vital in increasing bite force.

All sabretooth felids are predicted to have had a higher body mass than that of extant felids.

Evolution
It is suggested that the canine morphology in Feliformia are driven more strongly by sexual selection rather than natural selection. However, natural selection was also probably an important driver for sabretooth evolution, as the sabretooth morphology has appeared more than once in sabretooth evolutionary history. The upper canines became differentiated in size in order to optimize capture of different prey species, and to reduce interspecific competition.

On the contrary, other researchers suggest that sexual selection is unlikely to be responsible for canine length because there is only a moderate amount of dimorphism in canine proportions.

Christiansen suggested that sabretooth evolution was strongly directed towards precision killing using the sharp upper canines. In supplementary to this hypothesis, some researchers suggested that the development of the sabretooth condition represented a shift in function and killing behaviour. The condition started as an extension of a jaw-powered killing bite, similar to that of today’s pantherine cats, evolving towards a neck-powered bite, and again towards neck-powered shear biting. Specialization in killing most likely resulted from extremely intense competition, which meant that rapidly killing prey could have been advantageous, and was thus a driving force of evolution of large canines.

Some researchers have also suggested that the sabretooth characteristics may have risen through mosaic evolution, a process that occurs when individual differ in their rates of evolution, resulting in taxa that display a mix of primitive and derived features. The evidence supporting this hypothesis lies in the fact that there is a combination of long canines and a lack of sabretooth cranial features in primitive machairodonts and the modern species N. nebulosa, also known as the clouded leopard.

Predatory Behaviour
There are several hypotheses in regards to the method of ambush or attack. One hypothesis suggests that the cats leaped on the prey, and used the impact force to insert the canines. Another hypothesis suggests that the cats used neck musculature to depress the head and thus produce a stabbing effect. The latter hypothesis is currently widely accepted. Sabrecats probably used their large canines in a shearing bite to the throat or stomach of prey to sever nerves and blood vessels to cause rapid or instant collapse.

Derived sabre-tooth cats had substantially lower bite forces than extant felids of similar size; they had proportionally smaller jaw adductor muscles than extant felids and primitive sabrecats. To augment the weak bite forces from the mandibular adductors, these cats probably bit down on their prey using a combination of mandibular adduction and head depression. Powerful downward forces from hypertrophied cranial depressor musculature are employed to force the upper elongated canines into the flesh during a biting sequence. Sabrecats close the mouth by moving the cranium towards the mandible, and not vice versa. However, it appears that they are less capable of powerful, sustained biting as often employed by extant felids.

The stabbing force of the long canines is not to be underestimated. Canine force can be illustrated by the presence of stab wounds found in the forehead of skulls made by dirk-toothed cats. The forehead includes some of the thickest bones in the head. However, it is unlikely that the sabre-toothed cats would attack an area with bone, due to the increased risk of breaking the elongated canine, a consequence that is detrimental to survival. Thus, the two feasible areas of attack are the throat and the stomach. Dirk-tooths probably did not pursue prey, as they had short legs, and may have primarily aimed for the nape using a powerful sabre bite. This method would result in severe damage to the vital organs of the prey, and would thus quickly subdue it. On the other hand, scimitar-tooth cats may have first attacked the stomach using a slashing sabre bite, and then chased the injured prey, as they had long legs adapted for running.

Sabre-cats, much like modern cats, must have grasped prey during attack. It is hypothesized that one paw is thrown over the shoulder of the prey, and the other one controlling the head, prior to and during a biting sequence.

Sabre-toothed cats were probably unable to tear flesh directly off the bone, because the enlarged canines would interfere. It is also unlikely that the canines were used for cutting out pieces of meat. Thus, foods must have been taken through the side of the mouth; the outward orientation of the upper and lower incisors, as well as the lower canines). This orientation would allow pulling the flesh away from the bone so that it could be taken into the side of the mouth.

Specific mechanisms by which each ecomorph hunted prey may vary, and is the subject of ongoing investigations.

Eutherian Sabre-Tooths
Eutherian sabretooths include families Machaeroidines, Nimravidae, Barbourofelidae, and Machairodontinae, Juvenile eutherian sabretooths had extended parental care and became independent around the same age as modern felids. Under parental care, the juvenile sabretooth learned and honed skills in hunting and killing using their sabres. This stage of life is crucial in maintaining a healthy canine; once it breaks, it does not regrow.

1. Machairoidines, Carnivora
These sabrecats existed during the lower to middle Eocene Epoch. In hunting its prey, it is able to deliver a powerful canine bite to kill in a similar fashion to felids. They primarily delivered these bites on prey that were less resistant, using the forelimbs to restrain prey before delivering the bite. They were probably often less efficient in restraining prey than more derived sabretooths.

2. Nimravidae, Carnivora
Nimravids roamed the Earth from the late Eocene to early Miocene Epoch. Generally, they are able to generate bite forces parallel to, or even greater than felids of similar jaw length, and have enlarged paws that aids in climbing and prey-killing (Meachen-Samuels, 2012). There are two distinct monophyletic lineages of Nimravidae. The Paleogene Nimravinae lineage includes both scimitar- and dirk-tooth ecomorphs. The dirk-tooth cats in this lineage may have emphasized the canine bite as a means to kill prey swiftly, while the scimitar-tooths may not have relied heavily on a single deadly bite to kill prey. Rather, scimitar-tooths use slashing movements to weaken prey and pursue it. The Miocene Barbourofelinae lineage is actually more closely related to felids than they are to other nimravids.

3. Barbourofelidae, Carnivora
The Barbourofelidae family is characterised by the dirk-tooth ecomorph only, and existed in early through late Miocene in North America, Europe, Asia, and Africa. Species belonging to the Barbourofelidae family had evolved some of the most extreme sabretooth features. They have extremely robust jawbones, and subsequently possess powerful bites with forces larger than those exerted by Nimravids and extant felids of similar jaw length. Its extremely robust limbs may have been greater than that of S. fatalis, and could thus efficiently immobilize their prey.

4. Machairodontinae, Carnivora
Members of the Machairodontinae family are the best known “true” sabrecats who existed from late Miocene to late Pleistocene in Africa, Eurasia, and the Americas. These sabrecats were hyper-carnivorous and hunted large prey. Machairodontinae was divided into two main tribes, Homotherini and Smilodontini. The Homotherini tribe consisted of the scimitar-tooth ecomorph, whereas Smilodontini consisted of dirk-tooths. A third tribe, Metailurini, had canines intermediate of sabre and conical-tooths.

Smilodon
This sabrecat is the largest and most powerful of all the sabretooth felids. It weighed approximately 322kg, and was a top predator because of its large body mass. It presumably hunted large prey such as Hippidion, Macrauchenia, and Mylodon, and possibly smaller prey as well because carnivores are often opportunistic. It is not likely these cats had a prey preference. Smilodon has much higher bite forces than other sabrecats, in exception to Machairodus. Generally, it avoided biting into bone; however, there is direct evidence of it being an aggressive predator. Its bite force is strong enough to pierce through bone.

5. Thylacosmilinae, Sprassodonta
The Thylacosmilinae family is represented by a single species, Thylacosmilus atrox, a carnivorous, dirk-toothed marsupial that lived in South America from late Miocene to late Pliocene. Despite being a marsupial, it was still capable of delivering powerful canine bites, as its jaw was adapted to sustain local high loads. It was also capable of ambushing and pulling down large prey using robust forelimbs, however less efficient than its eutherian counterparts. Thylacosmilus is unique in their ability to constantly grow their canines, possibly to supplement the lack of retractable claws. Breaking the canine would not be as detrimental to their survival as compared to eutherian sabre-tooths.