Thylacosmilus is an extinct genus of saber-toothed metatherian mammals that inhabited South America from the Late Miocene to Late Pliocene epochs. Though Thylacosmilus looks similar to the "saber-toothed cats", it was not a felid, like the well-known American Smilodon, but a sparassodont, a group closely related to marsupials, and only superficially resembled other saber-toothed mammals due to convergent evolution, with the aforementioned Thylacosmilus being one of the last known sparassodonts. A 2005 study found that the bite forces of Thylacosmilus and Smilodon were low, which indicates that the killing techniques of saber-toothed animals differed from those of extant species. Remains of Thylacosmilus have been found primarily in Catamarca, Entre Ríos, and La Pampa Provinces in northern Argentina. Mass estimates of Thylacosmilus have varied depending on mass regressions, weighing , this makes Thylacosmilus one of the largest known metatherian predators.
Thylacosmilus was one of the last sparassodonts to go extinct, going extinct during the Late Pliocene. Traditionally, it was hypothesized that carnivorans such as Smilodon, outcompeted Thylacosmilus, however recent analysis found no evidence of temporal overlap between Smilodon and Thylacosmilus. Instead the extinction of Thylacosmilus was likely due to climatic changes in its environment.
Taxonomy
In 1926, the Marshall Field Paleontological Expeditions collected mammal fossils from the Ituzaingó Formation of Corral Quemado, in Catamarca Province, northern Argentina. Three specimens were recognized as representing a new type of marsupial, related to the borhyaenids, and were reported to the Paleontological Society of America in 1928, though without being named. 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. He named two new species in the genus, T. atrox and T. lentis. The generic name Thylacosmilus means "pouch knife", while the specific name atrox means "cruel". 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".
The holotype specimen of T. atrox, FMNH P 14531, was collected by Riggs and an assistant. It consists of a skull with the teeth of the right side entirely preserved as well as the left canine found separate in the matrix, fragments of the mandibles, and a partial skeleton consisting of a humerus, a broken radius and broken femora, and foot bones. Missing and scattered parts of the skull and mandible were reconstructed and fitted together. 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. It was one fourth smaller than the holotype, and may have been a young adult. It was collected by the American paleontologist Robert C. Thorne. The holotype of T. lentis, specimen P 14474, is a partial skull with the teeth of the right side preserved, and is about the same size as the T. atrox paratype. It was collected a few miles away from the site of the T. atrox holotype discovery, by the German biologist Rudolf Stahlecker. These specimens were housed at the Field Museum of Natural History in Chicago, while the T. lentis type later became part of the Museum of La Plata collection. In 1934, Riggs fully described the animal, after the fossils had been prepared and compared with other mammals from the same formation and better known borhyaenids from the Santa Cruz Formation.
More fragmentary Thylacosmilus specimens have since been discovered. Riggs and the American paleontologist Bryan Patterson reported in 1939 that a canine (MLP 31-XI-12-4) tentatively assigned to Achlysictis or Stylocynus by the Argentinian paleontologist Lucas Kraglievich in 1934 belonged to Thylacosmilus. A partial right ramus and front half of a skull (MLP 65_VI 1-29-41.) was collected in 1965. In a 1972 thesis, the Argentinian paleontologist Jorge Zetti suggested that T. atrox and T. lentis represented a single species, and the American paleontologist Larry G, Marshall agreed in 1976, stating the features distinguishing the two were of dubious taxonomic value, and probably due to differences in age and sex. He also found it hard to explain how two sympatric species (related species that lived in the same area at the same time) would be virtually identical in their specializations. Marshall also suggested Hyaenodonops could be cogeneric, though it was impossible to determine from the available specimens.Riggs, E.S. and Patterson, B., 1939, Stratigraphy of the late Miocene and Pliocene deposits of the Province of Catamarca (Argentina) with notes on the fauna. Physis., 14: 143–162.Marshall, L.G. (1976) Evolution of the Thylacosmilidae, extinct saber- tooth marsupials of South America. PaleoBios, 23, 1–30.
Evolution
Though Thylacosmilus is one of several predatory mammal genera typically called "saber-toothed cats", it was not a felid placentalian, but a sparassodont, a group closely related to marsupials, and only superficially resembled other saber-toothed mammals due to convergent evolution.
The term "saber-tooth" refers to an ecomorph consisting of various groups of extinct predatory synapsids (mammals and close relatives), which convergently evolved extremely long maxillary canines, as well as adaptations to the skull and skeleton related to their use. This includes members of Gorgonopsia, Thylacosmilidae, Machaeroidinae, Nimravidae, and Machairodontinae. The oldest known thylacosmilid was found in the Early Miocene roughly 20.2 Ma, although a Eocene-Oligocene boundary origin has been hypothesized.
The cladogram below shows the position of Thylacosmilus within Sparassodonta, according to Suarez and colleagues, 2015. Cladogram of Borhyaenoidea based on de del Campo et al. (2026):
Description
Size Thylacosmilus was a large metatherian predator, standing at the shoulders. Body mass for sparassodonts is difficult to estimate, since these animals have relatively large heads in proportion to their bodies, leading to overestimations, particularly when compared with skulls of modern members of Carnivora, which have different locomotive and functional adaptations, or with those of the recent predatory marsupials, which do not exceed of body mass. Sorkin (2008) suggested based on condylobasal skull length, Thylacosmilus could've weighed up to , about the same size as a modern jaguar. However, other experts suggested a smaller size. Ercoli and Prevosti's (2011) linear regressions based 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 . Mauricio Antón suggested Thylacosmilus was a leopard-sized predator. Suarez et al. (2023) estimated that Thylacosmilus may much smaller than the previous estimates based on upper molar regressions, weighing about . The mass estimates within these ranges would make Thylacosmilus one of the largest known carnivorous metatherians.
The differences in weight estimations may be due to the individual size variation of the specimens studied in each analysis, as well as the different samples and methods used. In any case, the weight estimations are consistent for terrestrial species that are generalists or have some degree of cursoriality.
Skull
In contrast to hathliacynids, T. atrox lacked large alisphenoid tympanic processes. T. atrox also differed from other sparassodonts in having large paratympanic spaces intruding into all the cranial elements bounding the middle ear.
Thylacosmilus had large, saber-like canines. The roots of these canines grew throughout the animal's life, growing in an arc up the maxilla and above the orbits. Thylacosmilus teeth are in many aspects even more specialized than the teeth of other sabertoothed predators. In these animals the predatory function of the "sabres" gave rise to a specialization of the general dentition, in which some teeth were reduced or lost. In Thylacosmilus the canines are relatively longer and more slender, relatively triangular in cross-section, in contrast with the oval shape of carnivorans' saber-like canines. The function of these large canines was once thought to have apparently even eliminated the need for functional incisors, while carnivorans like Smilodon and Barbourofelis still have a full set of incisors. 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.
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. The canines were hypsodont and more anchored in the skull, with more than half of the tooth contained within the alveoli, which were extended over the braincase. 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 canines had only a thin layer of enamel, just 0.25 mm in its maximum depth at the lateral facets, this depth being consistent down the length of the teeth. 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. The sharp serrations of the canines were maintained by the action of the wear with the lower canines, a process known as thegosis.
The convex upper portion of the maxilla is ornamented with extensive furrows and pits. This texturing has been correlated with an extensive network of blood vessels, which may suggest that the upper maxilla was covered by some form of soft tissue which tentatively has been hypothesized as a "horn covering" (keratinous structure).
The infraorbital foramen in Thylacosmilus was relatively small, though it did not differ substantially in size relative to the infraorbital foramina of other predatory sparassodonts. Postcranial skeleton 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. Its cervical vertebrae were very strong and to some extent resembled the vertebrae of Machairodontinae; also the cervical vertebrae have neural apophysis well developed, along with ventral apophysis in some cervicals, an element that is characteristic of other borhyaenoids. The lumbar vertebrae are short and more rigid than in Prothylacynus. The bones of the limbs, like the humerus and femur, are very robust, since they probably had to deal with larger forces than in the modern felids. In particular, the features of the humerus suggest a great development of the pectoral and deltoid muscles, not only required to capture its prey, also to absorb the energy of the impact of the collision with such prey.