Thylacosmilus

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. The cladogram below shows the position of Thylacosmilus within Sparassodonta, according to Suarez and colleagues, 2015. 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 apparently even eliminated the need for functional incisors, while carnivorans like Smilodon and Barbourofelis still have a full set of incisors. 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 more anchored in the skull, with more than half of the tooth contained within the alveoli, which were extended over the braincase. They were hypsodont and may even have been self-sharpening through the wear and the occlusion with the lower canine tooth. 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.