Sparassodonta

Sparassodonta 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 thought to be a separate side branch that split before the last common ancestor of all modern marsupials.
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 shown that sparassodonts died out long before eutherian carnivores arrived in South America.
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 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 is usually diminished, though the zygomatic arch 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 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, to the point that early analysis could not even find evidence for them. 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, 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 oriented anterobuccally 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 parallel or oblique with respect to lower jaw axis.
a very tall protoconid that bulges to the side and is wider at its midpoint than its base
talonid of lower molar narrow in relation to trigonid.

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 is reduced and fused to the metacone, inflating the postmetacrista ; and they almost always lack the stylar shelf and associated stylar cusps. In the lower molars, the trigonids have an inflated paracristid and marginalized or absent metaconid; and the talonid is either reduced or gone.

Taxonomy

Classification

Sparassodonts can be divided into six major groups; basal sparassodonts, species that cannot be easily assigned to any of the other sparassodont groups and whose teeth often exhibit adaptations for omnivory; hathliacynids, which range from a marten to a thylacine in size, and have long, fox-like muzzles and teeth strongly suited for carnivory; basal borhyaenoids, borhyaenoids which are unable to be easily classified into the families Borhyaenidae, Thylacosmilidae, or Proborhyaenidae and range in form and size; borhyaenids, the sparassodont group most specialized for running, but not as much as living carnivorans or even thylacines; proborhyaenids, robust, wolverine-like forms with ever-growing upper and lower canines; and thylacosmilids, 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, 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, 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
* Genus Mayulestes
**Mayulestes ferox
* Genus Argyrolestes
* Genus Nemolestes
**Nemolestes spalacotherinus
* Genus Patene
**Patene campbelli
**Patene coloradensis
**Patene coluapiensis
**Patene simpsoni
* Genus Procladosictis
**Procladosictis anomala
* Family Hondadelphidae
** Genus Hondadelphys
***Hondadelphys fieldsi
** Genus Stylocynus
***Stylocynus paranensis
* Family Hathliacynidae
** Genus Acyon
***Acyon ?herrerae
***Acyon myctoderos
***Acyon tricuspidatus
** Genus Australogale
***Australogale leptognathus
** Genus Borhyaenidium
***Borhyaenidium altiplanicus
***Borhyaenidium riggsi
***Borhyaenidium musteloides
** Genus Chasicostylus
***Chasicostylus castroi
** Genus Cladosictis
***Cladosictis centralis
***Cladosictis patagonica
** Genus Notictis
***Notictis ortizi
** Genus Notocynus
***Notocynus hermosicus
** Genus Notogale
***Notogale mitis
** Genus Pseudonotictis
***Pseudonotictis chubutensis
***Pseudonotictis pusillus
** Genus Perathereutes
***Perathereutes pungens
** Genus Sallacyon
***Sallacyon hoffstetteri
** Genus Sipalocyon
***Sipalocyon externus
***Sipalocyon gracilis
***Sipalocyon "obusta"
* Superfamily Borhyaenoidea
** Genus Angelocabrerus
***Angelocabrerus daptes
** Genus Chlorocyon
***Chlorocyon phantasma
** Genus Dukecynus
***Dukecynus magnus
** Genus Eomakhaira
***Eomakhaira molossus
** Genus Fredszalaya
***Fredszalaya hunteri
** Genus Lycopsis
***Lycopsis longirostrus
***Lycopsis padillai
***Lycopsis torresi
***Lycopsis viverensis )
** Genus Pharsophorus
***Pharsophorus lacerans
***Pharsophorus tenax
** Genus Plesiofelis
***Plesiofelis schlosseri
** Genus Prothylacynus
***Prothylacynus patagonicus
** Genus Pseudolycopsis
***Pseudolycopsis cabrerai
** Family Borhyaenidae
*** Genus Acrocyon
****Acrocyon riggsi
****Acrocyon sectorius
*** Genus Arctodictis
****Arctodictis munizi
****Arctodictis sinclairi
*** Genus Australohyaena
****Australohyaena antiquua
*** Genus Borhyaena
****Borhyaena macrodonta
****Borhyaena tuberata
*** ?Genus Eutemnodus
** Family Proborhyaenidae
*** Genus Arminiheringia
*** Genus Callistoe
****Callistoe vincei
*** Genus Paraborhyaena
****Paraborhyaena boliviana
*** Genus Proborhyaena
****Proborhyaena gigantea
**Unranked clade Thylacosmiliformes
*** Genus Dimartinia
****Dimartinia pristina
*** Family Thylacosmilidae
**** Genus Anachlysictis
*****Anachlysictis gracilis
**** Genus Patagosmilus
*****Patagosmilus goini
**** Genus Thylacosmilus
*****Thylacosmilus atrox

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 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.
Cladogram of Borhyaenoidea after Suarez et al. 2025, who coined the clade Thylacosmiliformes to encompass Dimartinia and Thylacosmilidae: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.