Thylacosmilidae is an extinct family of metatherian predators, related to the modern
marsupials
Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in a po ...
Sparassodonta
Sparassodonta (from Ancient Greek, Greek to tear, rend; and , gen.
, '
The comma is a punctuation mark that appears in several variants in different languages. It has the same shape as an apostrophe or single closing quotation mark () in many typefaces, but it differs from them in being placed on the baseline ...
tooth) is an extinct order (biology), order of carnivore, carnivorous metatherian mammals native to South America, related to modern marsupials. They were once con ...
, which occupied the ecological niche of many eutherian mammals of the order
Carnivora
Carnivora is a Clade, monophyletic order of Placentalia, placental mammals consisting of the most recent common ancestor of all felidae, cat-like and canidae, dog-like animals, and all descendants of that ancestor. Members of this group are f ...
from other continents. The family's most notable feature are the elongated, laterally flattened fangs, which is a remarkable evolutionary convergence with other saber-toothed mammals like ''
Barbourofelis
''Barbourofelis'' is an extinct genus of large, predatory, feliform carnivoran mammals of the family Barbourofelidae (false saber-tooth cats). The genus was endemic to North America and Eurasia during the Miocene until its extinction during t ...
The family Thylacosmilidae was originally erected by Riggs in 1933, to accommodate '' Thylacosmilus'', found in the Pliocene Brochero Formation of Argentina. Later, the family was demoted to a subfamily, as Thylacosmilinae, within Borhyaenidae, a group of superficially canid-like sparassodonts, under the assumption that ''Thylacosmilus'' was merely a late and specialized borhyaenid. Later, with the discovery of fragmentary specimens of new sparassodonts related to ''Thylacosmilus'' from Miocene and Pliocene strata, Thylacosmilidae was promoted back to familial status.Analía M. Forasiepi & Alfredo A. Carlini (2010). A new thylacosmilid (Mammalia, Metatheria, Sparassodonta) from the Miocene of Patagonia, Argentina. ''Zootaxa'' 2552: 55–68
In 1997, a second genus and species of thylacosmilid was described from the Laventan
Honda Group
is a Japanese public multinational conglomerate manufacturer of automobiles, motorcycles, and power equipment, headquartered in Minato, Tokyo, Japan.
Honda has been the world's largest motorcycle manufacturer since 1959, reaching a product ...
Colombia
Colombia (, ; ), officially the Republic of Colombia, is a country in South America with insular regions in North America—near Nicaragua's Caribbean coast—as well as in the Pacific Ocean. The Colombian mainland is bordered by the Car ...
, '' Anachlysictis gracilis''. This animal, less specialized than ''Thylacosmilus'', was the first indication that the family's origin dates back to before the end of the Miocene. In fact, the anatomy of ''Anachlysictis molar teeth suggests a closer relationship with basal sparassodonts like ''
Hondadelphys
''Hondadelphys'' is an extinct genus of carnivorous sparassodonts, known from the Middle Miocene of Colombia. The type species, ''H. fieldsi'', was described in 1976 from the fossil locality of La Venta, which hosts fossils from the Villavieja For ...
The family is perhaps the best known among sparassodonts because of their dental and cranial specializations, which are superficially similar to that of the
saber-toothed cats Sabretooth or sabertooth may refer to:
Animals
* Saber-toothed cat, several prehistoric felines
** ''Smilodon'', a prehistoric genus of felidae
* Sabertooth fish, a deep-sea fish found in the tropics
** Sabre-toothed blenny, ''Aspidontus taenia ...
, often cited as an example of convergent evolution between placental and metatherian mammals. However, there were several differences between thylacosmilids and the other saber-toothed mammals, and these unique traits diagnose the family: unique traits include canine teeth that grew continuously, less specialized carnassial molars and tremendous, flange-like outgrowths of the lower jaw that protected the saber-teeth.
In ''Thylacosmilus'', the last and most specialized known member, the incisors are very small and the lower teeth are poorly developed, and tack-shaped; in the other genera these elements are unknown. In ''Thylacosmilus'', the upper incisors are unknown other than by wear indentations on the lower incisors, as no premaxilla has ever been found complete in this genus. Another evolutionary trend in families is the progressive reduction of masseter and temporal muscles, resulting in relatively weak bites, but compensated by the increase in the size of the neck muscles to lower the head and the fangs into the necks of their prey. Fossils of ''Thylacosmilus'' forelimbs, the only reported for this group, indicate that animals were not fast runners, and were in turn adapted to exert force in order to subdue their prey, helping with their semiopposable thumb.
A recent study has since proposed a lack of analogy between thylacosmilids and saber toothed eutherians, speculating that rather than using the fangs to pierce prey they were instead used to open up corpses. Their lack of incisors and crushing molars suggest an entrail-based diet, and irrigated bone on the maxillae suggests some sort of extensive, soft tissue-based structure.