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Sparassodonta〔From Greek (''sparassein''), to tear, rend; and , gen. (''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 a sister taxon to them.〔Guillermo W. Rougier, John R. Wible and Michael J. Novacek. First Implications of ''Deltatheridium'' specimens for early marsupial history, ''Nature'' 396, 459-463(3 December 1998)〕 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). ==Features== 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.〔 Along with the Australian thylacoleonids, sparassodonts include some of the largest metatherian carnivores.〔 Sparassodonta is characterized by dental synapomorphies that distinguish the group from other closely related mammals. Unequivocal traits uniting the earliest Sparassodonts include:〔 * a ridge on the upper molar (preparacrista) oriented anterobuccally (towards the cheek) with respect to the long axis of the tooth. * ridges on lower molars (postprotocristid-metacristid) parallel or oblique with respect to lower jaw axis. * talonid (crushing end) of lower molar narrow in relation to trigonid (shearing end). The dental formula of most sparassodonts is , with the exception of borhyaenids, which have lost an upper incisor, proborhyaenids (except for ''Callistoe''), which have lost a lower incisor, and thylacosmilids, which have lost one premolar, two lower incisors, and at least two upper incisors.〔 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. Sparassodonts had a typical metatherian mode of dental replacement, replacing only the upper and lower third premolar throughout their lifetime. In thylacosmilids, this tooth was never replaced. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Sparassodonta」の詳細全文を読む スポンサード リンク
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