Sparassodonta (from Greek σπαράσσειν [sparassein], to tear, rend; and ὀδούς, gen. ὀδόντος [odous, odontos], tooth) is an extinct order of carnivorous metatherian mammals native to South America. They were once considered to be true marsupials, but are now thought to be either a sister taxon to them,[1][2][3][4] or far more distantly related, part of a separate clade of Gondwanan metatherians.[5] A number of these mammalian predators closely resemble placental predators that evolved separately on other continents, and are cited frequently as examples of convergent evolution. They were first described by Florentino Ameghino, from fossils found in the Santa Cruz beds of Patagonia. Sparassodonts were present throughout South America's long period of "splendid isolation" during the Cenozoic; during this time, they shared the niches for large warm-blooded predators with the flightless terror birds. Previously, it was thought that these mammals died out in the face of competition from "more competitive" placental carnivorans during the Pliocene Great American Interchange, but more recent research has showed that sparassodonts died out long before eutherian carnivores arrived in South America (aside from procyonids, which sparassodonts probably did not directly compete with).[6][7][8] Sparassodonts have been referred to as borhyaenoids by some authors,[9][10][11] but currently the term Borhyaenoidea refers to a restricted subgroup of sparassodonts comprising borhyaenids and their close relatives.[12][13]
Although members of the order Sparassodonta showed many similarities with placental carnivores, they were not closely related and are a very good example of convergent evolution. For example, sparassodonts' molars were very similar to the sharp teeth of placental carnivores. The canines were also lengthened, and in some cases resemble those of saber-toothed cats. Sparassodonts spanned a wide range of body sizes, from 2.2-pound (1 kg) weasel or civet-like forms to Thylacosmilus, which was the size of a leopard.[7][14] Along with the Australian thylacoleonids, sparassodonts include some of the largest metatherian carnivores.[7]
Sparassodonta is characterized by dental synapomorphies that distinguish the group from other closely related mammals. Unequivocal traits uniting the earliest Sparassodonts include:[12][15]
The dental formula of most sparassodonts is 4.1.3.43.1.3.4, with the exception of borhyaenids, which have three upper incisors, proborhyaenids (except for Callistoe), which have only two lower incisors, and thylacosmilids, which have only two lower incisors, at least two upper incisors, and only two upper and lower premolars on each side of the jaw.[12][16] Some specimens of Borhyaena and Arctodictis are also missing the last upper molar, showing that the presence of this tooth was variable in these species.[17] Sparassodonts had a typical metatherian mode of dental replacement, replacing only the upper and lower third premolar throughout their lifetime.[18] In thylacosmilids, the deciduous upper premolar was never replaced.[19]
Sparassodonts have highly reduced epipubic bones,[17] to the point that early analysis could not even find evidence for them.[20] This is a characteristic shared with the Australian thylacine, and historically argued as a synapomorphy,[17] though nowadays it is considered to have developed independently for poorly understood reasons. Like with thylacines, it is very likely that they possessed long cartilaginous elements instead.[10]