family
Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...
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 ...
, which lived in
South America
South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the sou ...
between the
Miocene
The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...
and
Pliocene
The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Great American Biotic Interchange, these animals belonged to the order
Sparassodonta
Sparassodonta (from Greek to tear, rend; and , gen. , ' tooth) is an extinct order of carnivorous metatherian mammals native to South America, related to modern marsupials. They were once considered to be true marsupials, but are now t ...
, 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 ...
'' and ''
Smilodon
''Smilodon'' is a genus of the extinct machairodont subfamily of the felids. It is one of the most famous prehistoric mammals and the best known saber-toothed cat. Although commonly known as the saber-toothed tiger, it was not closely rela ...
''.
Taxonomic history
The family Thylacosmilidae was originally erected by Riggs in 1933, to accommodate ''
Thylacosmilus
''Thylacosmilus'' is an extinct genus of saber-toothed metatherian mammals that inhabited South America from the Late Miocene to Pliocene epochs. Though ''Thylacosmilus'' looks similar to the " saber-toothed cats", it was not a felid, like the ...
'', 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
The Laventan ( es, Laventense) age is a period of geologic time (13.8 to 11.8 Ma) within the Middle Miocene epoch of the Neogene, used more specifically within the SALMA classification in South America. It follows the Colloncuran and precedes th ...
Lagerstätte
A Lagerstätte (, from '' Lager'' 'storage, lair' '' Stätte'' 'place'; plural ''Lagerstätten'') is a sedimentary deposit that exhibits extraordinary fossils with exceptional preservation—sometimes including preserved soft tissues. These f ...
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 ...
, '' 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 F ...
'' than with advanced sparassodonts like '' Borhyaena''.Goin, F.J. 2003. Early marsupial radiations in South America. En: M. Jones, C. Dickman y M. Archer (eds.), ''Predators with Pouches, The Biology of Carnivorous Marsupials'', CSIRO Publishing, Australia, pp. 30-42. Also, additional materials of a small predatory sparassodont of Colombia have been found, which has certain features diagnostic of thylacosmilids, but much less specialized, as well as indeterminate remains in
Uruguay
Uruguay (; ), officially the Oriental Republic of Uruguay ( es, República Oriental del Uruguay), is a country in South America. It shares borders with Argentina to its west and southwest and Brazil to its north and northeast; while bordering ...
and the Argentinean
Patagonia
Patagonia () refers to a geographical region that encompasses the southern end of South America, governed by Argentina and Chile. The region comprises the southern section of the Andes Mountains with lakes, fjords, temperate rainforests, and g ...
, from the early Pliocene, has been tentatively assigned to family. Forasiepi and Carlini in 2010 unveiled a third genus and species, '' Patagosmilus goini'', from the Collón Cura Formation of Argentina from the mid-Miocene, with characteristics intermediate between ''Anachlysictis'' and ''Thylacosmilus''.
Description
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, often cited as an example of
convergent evolution
Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...
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
Carnassials are paired upper and lower teeth modified in such a way as to allow enlarged and often self-sharpening edges to pass by each other in a shearing manner. This adaptation is found in carnivorans, where the carnassials are the modified f ...
molars and tremendous,
flange
A flange is a protruded ridge, lip or rim (wheel), rim, either external or internal, that serves to increase shear strength, strength (as the flange of an iron beam (structure), beam such as an I-beam or a T-beam); for easy attachment/transfer of ...
-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
In human anatomy, the masseter is one of the muscles of mastication. Found only in mammals, it is particularly powerful in herbivores to facilitate chewing of plant matter. The most obvious muscle of mastication is the masseter muscle, since it ...
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.