A 2005 study found that the bite forces of Thylacosmilus and Smilodon were low, which indicates the killing-techniques of saber-toothed animals differed from those of extant species.

[1] In 1926, the Marshall Field Paleontological Expeditions collected mammal fossils from the Ituzaingó Formation of Corral Quemado, in Catamarca Province, northern Argentina.

In 1933, the American paleontologist Elmer S. Riggs named and preliminarily described the new genus Thylacosmilus based on these specimens, while noting that a full description was being prepared and would be published at a later date.

[3] Riggs found the genus distinct enough to warrant a new subfamily within Borhyaenidae, Thylacosmilinae, and stated it was "one of the most unique flesh-eating mammals of all times".

Specimen P 14344 was designated as the paratype of T. atrox, and consists of the skull, the mandible, seven cervical, two dorsal, two lumbar, and two sacral vertebrae, a femur, a tibia, a fibula, and various foot bones.

Recent methods, like Ercoli and Prevosti's (2011) linear regressions on postcranial elements that directly support the body's weight (such as tibiae, humeri and ulnae), comparing Thylacosmilus to both extinct and modern carnivorans and metatherians, suggest that it weighed between 80 and 120 kilograms (180 and 260 lb),[12][13] with one estimate suggesting up to 150 kg (330 lb),[14] about the same size as a modern jaguar.

[16] However, evidence in the form of wear facets on the internal sides of the lower canines of Thylacosmilus indicate that the animal did indeed have incisors, though they remain hitherto unknown due to poor fossilization and the fact that no specimen thus far has been preserved with its premaxilla intact.

[17] In Thylacosmilus there is also evidence of the reduction of postcanine teeth, which developed only a tearing cusp, as a continuation of the general trend observed in other sparassodonts, which lost many of the grinding surfaces in the premolars and molars.

They were protected by the large symphyseal flange and they were powered by the highly developed musculature of the neck, which allowed forceful downward and backward movements of the head.

The teeth had open roots and grew constantly, which eroded the abrasion marks that are present in the surface of the enamel of other sabertooths, such as Smilodon.

[19] Although the postcranial remains of Thylacosmilus are incomplete, the elements recovered so far allow the examination of characteristics that this animal acquired in convergence with the sabertooth felids.

[21] Recent comparative biomechanical analysis have estimated the bite force of T. atrox, starting from maximum gape, at 38 newtons (8.5 lbf), much weaker than that of a leopard, suggesting its jaw muscles had an insignificant role in the dispatch of prey.

Its skull was similar to that of Smilodon in that it was much better adapted to withstand loads applied by the neck musculature, which, along with evidence for powerful and flexible forelimb musculature and other skeleton adaptations for stability, support the hypothesis that its killing method consisted on immobilization of its prey followed by precisely directed, deep bites into the soft tissue driven by powerful neck muscles.

[22][23][24] It has been suggested that its specialized predatory lifestyle could be linked to more extensive parental care than in modern marsupial predators, due that the killing technique only could be used by adult individuals with a full development of its peculiar dental anatomy and grasping abilities; it could require some time for young individuals to learn the necessary skills, although there are no clear evidence in the fossils Thylacosmilus, and this kind of cooperative behavior is unknown in modern marsupials.

[25] The analysis published by Christine Argot in 2002 about the evolution of predatory borhyaenoids suggests that Thylacosmilus was a specialized form, which have a limited stereoscopic vision with small eyes, with an overlap of 50-60°, very low compared with modern predators, but the ossified and great auditory bulla and the muscular body would indicate that it could be an ambush predator in open and relatively dry environments, where the sound absorption is lesser than in more humid areas, and the acute hearing could compensate the limited vision.

This study also suggests that Thylacosmilus was largely unimpeded in predatory capability by the reduction in binocular vision created by its hypertrophied canines.

[28] A 2020 study found several functional disparities between Thylacosmilus's cranial anatomy and that of saber-toothed eutherians that cannot be explained by its metatherian status, such as the lack of a jaw symphysis, subtriangular canines instead of blade-like ones, lack of incisors (that would render feline-like feeding behaviours impossible), weak jaw musculature and unaligned teeth with no evidence of shearing activity, as well as a post-cranial skeleton more akin to that of a bear than a cursorial predator like a cat.

[16] The comparative studies of Argot 2004, indicates that the basicranium had rugose crests that served as attachments for the neck flexor muscles, which are associated to the increase of the bite strength.

This animal had an absent entepicondylar foramen in the humerus, which is correlated with the reduction of the abduction movement in that bone in cursorial ungulates and carnivores (Borhyaena also show this condition), although it contrasts with its probable powerful adductor muscles.

[21] Based on studies of its habitat, Thylacosmilus is believed to have hunted in savanna-like or sparsely forested areas, avoiding the more open plains where it would have faced competition with the more successful and aggressive Phorusrhacids and the giant vulture-like Teratornithid Argentavis.