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Sparassodonta (from Ancient Greek σπαράσσειν sparassein, to tear, rend; and ὀδούς, gen. ὀδόντος odous,, tooth) is an extinct order of carnivore native to South America, related to modern . They were once considered to be true marsupials, but are now thought to be a separate side branch that split before the last common ancestor of all modern marsupials.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)Guillermo Rougier, New specimen of Deltatheroides cretacicus (Metatheria, Deltatheroida) from the Late Cretaceous of Mongolia, BULLETIN OF CARNEGIE MUSEUM OF NATURAL HISTORY 36(DEC 2004):245-266 · SEPTEMBER 2009
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 Phorusrhacidae. 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 shown that sparassodonts died out long before eutherian carnivores arrived in South America (aside from Procyonidae, which sparassodonts probably did not directly compete with).
Sparassodonts have been referred to as borhyaenoids by some authors, but currently the term Borhyaenoidea refers to a restricted subgroup of sparassodonts comprising Borhyaenidae and their close relatives.
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 Thylacoleonidae, sparassodonts include some of the largest metatherian carnivores.
Sparassodonts have highly reduced epipubic bones (pelvic bones which support the pouch), to the point that early analysis could not even find evidence for them.Wm. J. Sinclair, The Marsupial Fauna of the Santa Cruz Beds, Proceedings of the American Philosophical Society . Vol. 44, No. 179 (Jan. – Apr. 1905), pp. 73–81 . Published by: American Philosophical Society This is a characteristic shared with the Australian thylacine, and historically argued as a synapomorphy, though nowadays it is considered to have developed independently for poorly understood reasons. As with thylacines, it is very likely that they possessed long cartilaginous elements instead.
Sparassodonta is characterized by dental synapomorphies that distinguish the group from other closely related mammals. Unequivocal traits uniting the earliest Sparassodonts include:
In borhyaenids, only the third premolar was ever deciduous teeth in the animal's lifetime, similar to other metatherians. In thylacosmilids, only the lower third premolar was replaced.
The cusps of the sparassodont molar correlate to a cutting function rather than a crushing one. In the upper molars, the paracone (on the lingual, or tongueward, side) is reduced and fused to the metacone (distal, towards the back of the mouth), inflating the postmetacrista (the lingual border of the metacone); and they almost always lack the stylar shelf (on the buccal, or cheekward, side) and associated stylar cusps. In the lower molars, the trigonids (the buccal shearing side) have an inflated paracristid and marginalized or absent metaconid; and the talonid (the distal, or backendwards, crushing side) is either reduced or gone.
The taxonomic classification below follows the latest review of the group, that of Prevosti and Forasiepi (2018), with additions from more recent studies. Although Mayulestes was originally described as a sparassodont, later phylogenetic analyses have shown that it most likely does not belong to this group; however more recent studies show it to be closely related to sparassodonts. Similarly, while basal borhyaenoids such as Lycopsis and Prothylacynus were once thought to belong to a distinct family (Prothylacynidae), phylogenetic analyses have found that these animals do not represent a monophyletic group. The exact age of most Eocene species of sparassodonts is uncertain, given the lack of precise stratigraphic information associated with most specimens and the recent division of the Casamayoran SALMA into the Vacan and Barrancan SALMAs.
Several other metatherian taxa have been suggested to be sparassodonts or closely related to sparassodonts. The australian Murgon taxa Archaeonothos has been noted as being similar to sparassodonts, but currently its relationships are not fully concluded. Carneiro (2018) recovered the genus Varalphadon from the Late Cretaceous of North America as a basal member of Sparassodonta. However, this interpretation of Varalphadon as a sparassodont has not been supported by later phylogenetic analyses, and most of the purported synapomorphies between Varalphadon and sparassodonts are not actually present in Varalphadon or have been suggested to be due to convergent evolution. Sparassodonts have sometimes been considered closely related to the "Gurlin Tsav skull" an unnamed metatherian known from a partial skull found in the Late Cretaceous Nemegt Formation of Mongolia.
The following cladogram of sparassodont interrelationships is after Engelman et al., 2020. Not all studies agree on the sister group relationship between Thylacosmilidae and Borhyaenidae recovered here, with other studies finding thylacosmilids to be within Proborhyaenidae. The relationships among hathliacynids are also relatively unstable.
Within Metatheria, a 2016 phylogenetic analysis group found that borhyaenids form a clade with the Asian "Gurlin Tsav skull" as well as other South American taxa. The same phylogeny found that marsupials group among various North American Cretaceous species. The phylogenetic tree is reproduced below.
Sparassodonts have been suggested to be related to a variety of other groups of metatherians. Florentino Ameghino, who first described fossils of the group, thought that sparassodonts were closely related to creodonts and were a transitional group between metatherians and carnivorous placentals (including modern carnivorans). Contemporary authors in the late 19th and early 20th century rejected this hypothesis and considered sparassodonts to be closely related to Australian thylacines and Dasyuridae. The most popular hypothesis for much of the 20th century was that sparassodonts were closely related to opossums. In 1990, Marshall et al. (1990) considered the Cretaceous Stagodontidae to be members of Sparassodonta, but this was criticized by later authors. Marshall and Kielan-Jaworowska (1992) considered sparassodonts to be closely related to , but this was also criticized. Most of these hypotheses were based on similar adaptations for carnivorous diets in sparassodonts, opossums, dasyuromorphians, stagodonts, and deltatheroidans, which are highly prone to convergent evolution within mammals. Szalay (1994) considered sparassodonts to be closely related to based on features of the ankle. (1994). 9780521025928, Cambridge University Press. ISBN 9780521025928 In recent years there has been a growing consensus that sparassodonts are positioned just outside of crown-group Marsupialia, in a broader clade (Pucadelphyida) including pucadelphyids as well as sparassodonts.
Sparassodonts are presently regarded as an endemic South American group, and have not even been found in nearby continents like Antarctica (where other groups native to South America such as Litopterna, Astrapotheria, Microbiotheria, and Polydolopidae) are present.
Bite marks likely pertaining to hathliacynid sparassodonts have been found on the remains of penguins and flightless marine ducks in ancient seabird nesting colonies, suggesting that sparassodonts raided seabird colonies for eggs, carrion, and other prey like many predatory mammals do today.
Borhyaenid and proborhyaenid sparassodonts have been interpreted as being capable of crushing bones similar to modern hyenas, wolverines, or the Tasmanian devil ( Sarcophilus harrisii) based on their deep jaws, bulbous premolars with deep roots and pronounced wear at their tips, extensive fused or interlocking mandibular symphyses, large , microfractures in their tooth enamel, and high estimated bite forces. Australohyaena antiquua shows particularly pronounced adaptations for bone-cracking, with a very deep jaw and strongly arched nasal bone similar to what is seen in modern hyaenids.
Based on studies of the postcranial skeleton, it appears as though most sparassodonts were scansorial (adapted for climbing), although terrestrial adaptations evolved in Lycopsis longirostrus, borhyaenids, proborhyaenids, and thylacosmilids. Most sparassodonts were plantigrade, Borhyaena has been suggested to have been digitigrade but this has been questioned. The one exception was Thylacosmilus, which has been interpreted as having a digitigrade forefoot and a semiplantigrade hindfoot, this has been supported by fossil tracks.
One unusual aspect of sparassodont paleoecology is that at most fossil localities their remains are nearly ten times rarer than would be expected based on comparisons with carnivorous mammals at fossil sites in other parts of the world. The exact reasons for this are not clear, though this appears to be a broader pattern applicable to other groups of Cenozoic South American terrestrial carnivores (i.e., terror birds).
Wounds have been documented on the face of specimens of Borhyaena tuberata and Sipalocyon gracilis, potentially suggesting aggressive habits similar to the modern Tasmanian devil ( Sarcophilus harrisii).
The thylacosmilids, on the other hand, were more successful and abundant, being some of the only large mammalian carnivores in South America during the Pliocene, before dying out during a faunal turnover in the middle of the epoch (the youngest specimens of thylacosmilids are ~3.3 Ma). It is still not certain why Sparassodonta declined in diversity and became extinct during the late Cenozoic, but it appears as though competition from eutherian carnivorans was not a factor, as the placental analogues of sparassodonts (Canidae, weasels, and saber-toothed cats) did not enter South America until the middle Pleistocene, several million years after their sparassodont counterparts became extinct. Sparassodonts did coexist with Cyonasua-group procyonids during the late Miocene and Pliocene, but Cyonasua-group procyonids appear to have been primarily omnivorous and filled ecological niches that sparassodonts never occupied, which may be one reason that these animals were able to colonize South America despite the diverse predator guild in the late Miocene. The overall decline in sparassodont diversity from the Late Miocene to the end of the Pliocene may be linked to the climatic cooling that characterised the Late Neogene and the onset of the Quaternary glaciation. Additionally, the increased aridity of South America caused by the uplift of the Andes was likely responsible as well.
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