|Two reconstructed skeletons mounted in fighting pose, Museum of Paleontology Egidio Feruglio|
Thylacosmilus is an extinct genus of saber-toothed metatherian that inhabited South America from the Late Miocene to Pliocene epochs. Though Thylacosmilus is one of several predatory mammal genera typically called "saber-toothed cats", it was not a felid placentalian, like the well-known North American Smilodon, but a sparassodont, a group closely related to marsupials, and only superficially resembled other saber-toothed mammals due to convergent evolution. 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. Remains of Thylacosmilus have been found primarily in Catamarca, Entre Ríos, and La Pampa Provinces in northern Argentina.
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 a new genus based on these specimens, while noting that a full description was being prepared and would be published at a later date. He named the new genus Thylacosmilus, which means "pouch knife". He found the genus distinct enough to warrant a new subfamily within Borhyaenidae; Thylacosmilinae.
The type species of the genus is T. atrox, based on the holotype specimen P 14531, which consists of the skull and a partial skeleton. Specimen P 14344 was designated as the paratype of T. atrox, and consists of the cranium, mandible, vertebrae, a femur, a tibia, a fibula, and tarsal bones. He also named a second species, T. lentis, based on specimen P 14474, a partial skull with teeth, form the same location. These specimens are housed at the Field Museum of Natural History in Chicago. 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.
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, Barbourofelidae, and Machairodontinae.
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 they seems more specialized than the teeth of other sabertoothed predators. In these animals the predatory function of the "sabres" ended generating a specialization of the general dentition, reducing or losing teeth. In Thylacosmilus the canines are relatively longer and slender, with a cross section with a relatively triangular shape, in contrast with the oval shape of other saber-like canines. The function of these large canines even apparently eliminated the need of functional incisors, while animals like Smilodon and Barbourofelis they still have a full set of incisors.
In Thylacosmilus also is evident the reduction of postcanine teeth, which develop only tearing cusp, as a continuation of the general trend observed in other sparassodonts, which lost many of the grinding surfaces in premolars and molars. The canines were more anchored in the skull, with more than half of the tooth contained within the alveolus, which were extended over the braincase; they were hypsodont and maybe they could sharpen themselves through the wear and the occlusion with the lower canine tooth; they were more protected by the large symphyseal flange and they were powered by the developed musculature of the neck which allowed more forceful downward and backward movements with the head. The canines only had a thin layer of enamel, with just 0.25 mm in its maximum depth at the lateral facets, which was present through the length of the teeth. The teeth had open roots and grew constantly, which eroded the abrasion marks otherwise present in the surface of the enamel, as in Smilodon. The sharp serrations of the canines were maintained by the action of the wear with the lower canines, a process known as thegosis.
Although the postcranial remains of Thylacosmilus are incomplete, the elements recovered so far allow to examine the 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 have 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 preys, also to absorb the energy of the impact of the collision with such preys. The features of the hindlimb, with a robust femur equipped with a great trochanter in the lower part, the short tibia and plantigrade feet shows that this animal was not a runner, and probably stalked its prey items. The hindlimbs also allowed certain mobility of the hip, and possibily the ability to stand up only with its hindlimbs, like Prothylacynus and Borhyaena. Contrary to felids, barbourofelids and nimravids, the claws of Thylacosmilus were not retractable.
Body mass for sparassodonts are difficult to estimate, since that these animals have relatively large heads in proportion with their bodies which generates overestimations, particularly when is compared only with skulls of modern members of Carnivora, which have different locomotory and functional adaptations, or the recent predatory marsupials, which doesn't exceed the 30 kg of body mass. The methods used or are based of Thylacosmilus based, like the study by Ercoli and Prevosti (2011) using linear regression equations with postcranial elements like tibiae, humeri and ulnae that supports directly the body weight, from both extinct and modern members of carnivorans and metatherians suggests that this animal weighed between 80 to 120 kilograms (180 to 260 lb), and one estimate suggesting up to 150 kg (330 lb), about the same size as a modern jaguar. The differences in weight estimations possibly are due 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 generalist terrestrial species or with certain degree of cursoriality. This mass would make it one of the largest known carnivorous metatherians.
Recent comparative biomechanical analysis have estimated the bite force of T. atrox starting at 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. Its has been suggested that its specialized predatory lifestyle could be linked to a parental care more developed than in modern marsupial predators.
In 1988 Juan C. Quiroga published a study on the cerebral cortex of two proterotherids and Thylacosmilus. The endocranial casts of two Thylacosmilus specimens, MLP 35-X-41-1 from the Montehermosan age of the Catamarca Province which represents a natural cast of the left half of the cranial cavity lacking the anterior part of the olfatory bulbs and the brain hemispheres, and MMP 1443, from the Chapadmalalan age of the Buenos Aires Province which is a complete, artificial cast that shows some ventral displacement but with the anterior right part of the brain hemisphere and olfatory bulb. This analysis showed that the somatic nervous system of Thylacosmilus represented 27% of the entire cortex, with the visual area representing 18% and the auditive area 7%. The paleocortex was more than 8%. The sulci of the cortex are relatively complex and similar in pattern and number to the modern diprotodont marsupials. Compared with Macropus and Trichosurus, Thylacosmilus had a lesser development of the maxilar area with respect to the mandibular area and the rhinal fissure is taller than in Macropus and Thylacinus; this disproportion between the maxilar and mandibular areas, which are roughly similar in marsupials, seem to be a consequence of the extreme development of the neck and mandibular musculature, used in the functionality of the osteodentary anatomy of this animal. However, the area dedicated to the oral-mandible region comprises the 42% of the somatic area. The comparison between the endocranial casts of Thylacosmilus and a proterotherid specimen (possibly a species coevolving with Thylacosmilus and a potential prey item) indicates that the sparassodont only have a half of the encephalization and a quarter of the cortical area, while some have more somatized areas, similar visual areas and less auditive area, which suggests different sensomotoric qualities between the animals.
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.
A 2010 study found that the cranial morphology of Thylacosmilus and other metatherians was more similar to caniforms, despite them often being compared to felids.
Distribution and habitat
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 it shared its environment with. Fossils of Thylacosmilus have been found in the Ituzaingó, Epecuén, and Cerro Azul Formations in Argentina.
Although older references have often stated that Thylacosmilus became extinct due to competition with the "more competitive" saber-toothed cat Smilodon during the Great American Interchange, newer studies have shown this is not the case. Thylacosmilus died out during the Pliocene (3.6–2.58 mya) whereas saber-toothed cats are not known from South America until the Middle Pleistocene (781-126,000 years ago). As a result, the last appearance of Thylacosmilus is separated from the first appearance of Smilodon by over one and a half million years.
Thylacosmilus appears in the game Jurassic Park: Builder as a silver creature in the Glacier Park, and was available in the later version Jurassic World: The Game on January 12, 2018 as a VIP snow creature.
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