Sparassodonta (from Greek [], to tear, rend; and , gen. [, ], tooth) is an extinct order of carnivorous metatherian mammals related to modern marsupials. 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 Fossils of the order have been found exclusive to South America, although trackways suggests they were possibly present in Antarctica.
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. It has historically been suggested tha these mammals died out in the face of competition from "more competitive" placental carnivorans during the Great American Interchange, but more recent research has shown that sparassodonts had already substantially declined by the time placental carnivores arrived in South America, though it has been shown that the last known sparassodont, Pampaluctor probably lived alongside placental carnivores.
Sparassodonts have been referred to as borhyaenoids by some authors, but currently the term Borhyaenoidea refers to a restricted subgroup of sparassodonts comprising borhyaenids and their close relatives.
Anatomy
Almost all sparassodonts have an exceptionally shortened snout—most especially thylacosmylids. Hathliacynids usually have a longer snout than the other groups. The nasal bones extend past the eye sockets, often reaching the lacrimal bone. Except for thylacosmylids beyond Patagosmilus, sparassodonts feature an open eye socket, with more marginalized (though nonetheless prominent) postorbital processes which would otherwise form the postorbital bar connecting the forehead to the cheek, thus framing the eye. They exhibit marked postorbital constriction. The orbital process (between the cheek and the eye socket) is usually diminished, though the zygomatic arch (the cheekbone) is strong. They feature a prominent sagittal crest along the midline of the flattened skull, the crest strength is quite variable among borhyaenids. They have an expanded occipital bone with a well defined nuchal crest.
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. The largest known sparassodont was Proborhyaena which was about the size of a spectacled bear. Along with the Australian thylacoleonids, 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.
Teeth
The dental formula of sparassodonts varies considerably. In borhyaenids, it is , with three upper and lower incisors, one upper and lower canine, three upper and lower premolars, and four upper and lower molars in each half of either jaw. Proborhyaenids usually only have two lower incisors instead of three, except for Callistoe. Thylacosmylids have at least two upper and only two lower incisors (the uppers grew into elongated sabers), and two upper and lower premolars. 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.
Sparassodonta is characterized by dental synapomorphies that distinguish the group from other closely related mammals. Unequivocal traits uniting the earliest Sparassodonts include: a snout that forms a pronounced bulge around the canine teeth when viewed from above a ridge on the upper molar (preparacrista) oriented anterobuccally (towards the cheek) with respect to the long axis of the tooth. a pronounced keel near the base of the front of the paraconid ridges on lower molars (postprotocristid-metacristid) parallel or oblique with respect to lower jaw axis. a very tall protoconid (>90% tooth length, secondarily lost in Hondadelphys and Stylocynus) that bulges to the side and is wider at its midpoint than its base talonid (crushing end) of lower molar narrow in relation to trigonid (shearing end).
In borhyaenids, only the third premolar was ever replaced 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.
Taxonomy Classification Sparassodonts can be divided into six major groups; basal sparassodonts (?earliest Paleocene–late Miocene), species that cannot be easily assigned to any of the other sparassodont groups and whose teeth often exhibit adaptations for omnivory; hathliacynids (late Oligocene–early Pliocene/late Pliocene), which range from a marten to a thylacine in size, and have long, fox-like muzzles and teeth strongly suited for carnivory; basal borhyaenoids (middle Eocene–late Miocene), borhyaenoids which are unable to be easily classified into the families Borhyaenidae, Thylacosmilidae, or Proborhyaenidae and range in form and size; borhyaenids (early–late Miocene), the sparassodont group most specialized for running, but not as much as living carnivorans or even thylacines; proborhyaenids (middle Eocene–late Oligocene), robust, wolverine-like forms with ever-growing upper and lower canines; and thylacosmilids (early Miocene–early Pleistocene), another terrestrially specialized group with ever-growing saber-like upper canines.
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.
Order Sparassodonta Genus Allqokirus Allqokirus australis (earliest Paleocene, Tiupampan SALMA) Genus Mayulestes Mayulestes ferox (earliest Paleocene, Tiupampan SALMA) Genus Argyrolestes Genus Nemolestes Nemolestes spalacotherinus (late middle Eocene, Barrancan SALMA) Genus Patene Patene campbelli (late ?Eocene, Santa Rosa local fauna) Patene coloradensis (middle Eocene) Patene coluapiensis (middle Eocene, Barrancan SALMA) Patene simpsoni (late Paleocene/early Eocene, Itaboraian SALMA) Genus Procladosictis Procladosictis anomala (late Eocene, Mustersan SALMA) Family Hondadelphidae Genus Hondadelphys Hondadelphys fieldsi (late middle Miocene, Laventan SALMA) Genus Stylocynus Stylocynus paranensis (latest Miocene, Huayquerian SALMA) Family Hathliacynidae Genus Acyon Acyon ?herrerae (early Miocene, Colhuehuapian SALMA) Acyon myctoderos (late middle Miocene, Laventan SALMA) Acyon tricuspidatus (late early Miocene, Santacrucian SALMA) Genus Australogale Australogale leptognathus (late middle Miocene, Laventan SALMA) Genus Borhyaenidium Borhyaenidium altiplanicus (latest Miocene, Huayquerian SALMA) Borhyaenidium riggsi (unknown, either early or late Pliocene, Montehermosan or Chapadmalalan SALMA) Borhyaenidium musteloides (latest Miocene, Huayquerian SALMA) Genus Chasicostylus Chasicostylus castroi (early late Miocene, Chasicoan SALMA) Genus Cladosictis Cladosictis centralis (early Miocene, Colhuehuapian SALMA) Cladosictis patagonica (late early to earliest middle Miocene, Santacrucian-Friasian SALMA) Genus Notictis Notictis ortizi (latest Miocene, Huayquerian SALMA) Genus Notocynus Notocynus hermosicus (early Pliocene, Montehermosan SALMA) Genus Notogale Notogale mitis (late Oligocene, Deseadan SALMA) Genus Pseudonotictis Pseudonotictis chubutensis (early middle Miocene, Colloncuran SALMA) Pseudonotictis pusillus (late early Miocene, Santacrucian SALMA) Genus Perathereutes Perathereutes pungens (late early Miocene, Santacrucian SALMA) Genus Sallacyon Sallacyon hoffstetteri (late Oligocene, Deseadan SALMA) Genus Sipalocyon Sipalocyon externus (early Miocene, Colhuehuapian SALMA) Sipalocyon gracilis (late early Miocene, Santacrucian SALMA) Sipalocyon "obusta" (late early Miocene, Santacrucian SALMA) Superfamily Borhyaenoidea Genus Angelocabrerus Angelocabrerus daptes (middle Eocene) Genus Chlorocyon Chlorocyon phantasma (late Eocene, Mustersan SALMA) Genus Dukecynus Dukecynus magnus (late middle Miocene, Laventan SALMA) Genus Eomakhaira Eomakhaira molossus (early Oligocene, ?Tinguirirican SALMA) Genus Fredszalaya Fredszalaya hunteri (late Oligocene, Deseadan SALMA) Genus Lycopsis Lycopsis longirostrus (late middle Miocene, Laventan SALMA) Lycopsis padillai (middle Miocene, Colloncuran SALMA) Lycopsis torresi (early Miocene, Colhuehuapian SALMA) Lycopsis viverensis (early late Miocene, Chasicoan SALMA)) Genus Pharsophorus Pharsophorus lacerans (middle to late Oligocene, La Cantera Local Fauna to Deseadan SALMA) Pharsophorus tenax (late Oligocene, Deseadan SALMA) Genus Plesiofelis Plesiofelis schlosseri (late Eocene, Mustersan SALMA) Genus Prothylacynus Prothylacynus patagonicus (late early to earliest middle Miocene, Santacrucian-Friasian SALMA) Genus Pseudolycopsis Pseudolycopsis cabrerai (early late Miocene, Chasicoan SALMA Genus Pseudothylacynus Pseudothylacynus rectus (early Miocene, Colhuehuapian SALMA) Family Borhyaenidae Genus Acrocyon Acrocyon riggsi (early Miocene, Colhuehuapian SALMA) Acrocyon sectorius (late early Miocene, Santacrucian SALMA) Genus Arctodictis Arctodictis munizi (late early Miocene, Santacrucian SALMA) Arctodictis sinclairi (early Miocene, Colhuehuapian SALMA) Genus Australohyaena Australohyaena antiquua (late Oligocene, Deseadan SALMA) Genus Borhyaena Borhyaena macrodonta (early Miocene, Colhuehuapian SALMA)