Sparassodonta

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Sparassodonta
Temporal range: Paleocene-Pliocene Possible Late Cretaceous record
Lycopsis
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Clade: Metatheria
Clade: Marsupialiformes
Order: †Sparassodonta
Ameghino, 1894
Families

†Hathliacynidae
†

Hondadelphidae

†Borhyaenidae
†Proborhyaenidae
†Thylacosmilidae

Sparassodonta (from

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]

Anatomy

Lycopsis longirostrus
, from the late middle Miocene of Colombia

Almost all sparassodonts have an exceptionally shortened snout—most especially thylacosmylids. Hathliacynids usually have a longer snout than the other groups. 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.[14]

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][15] Along with the Australian thylacoleonids, sparassodonts include some of the largest metatherian carnivores.[7]

Sparassodonts have highly reduced

synapomorphy,[16] 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.[10]

Teeth

Thylacosmilus had long sabers.

The

dental formula of sparassodonts varies considerably. In borhyaenids, it is 3.1.3.43.1.3.4, 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.[12][18] 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.[16]

Sparassodonta is characterized by dental

synapomorphies that distinguish the group from other closely related mammals. Unequivocal traits uniting the earliest Sparassodonts include:[12][19]

  • 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.[20] In thylacosmilids, only the lower third premolar was replaced.[21]

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.[22]

Taxonomy

Classification

Sparassodonts can be divided into six major groups; basal sparassodonts (?earliest

thylacines; proborhyaenids (middle Eocene-late Oligocene), robust, wolverine-like forms with ever-growing upper and lower canines; and thylacosmilids (early Miocene-late Pliocene), another terrestrially specialized group with ever-growing saber-like upper canines.[12]

The taxonomic classification below follows the latest review of the group, that of Prevosti and Forasiepi (2018),

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.[36] Carneiro (2018) recovered the genus Varalphadon from the Late Cretaceous of North America as a basal member of Sparassodonta.[37] 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[19] or have been suggested to be due to convergent evolution.[29] 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.[5]

The following cladogram of sparassodont interrelationships is after Engelman et al., 2020.[34] Not all studies agree on the sister group relationship between Thylacosmilidae and Borhyaenidae recovered here, with other studies finding thylacosmilids to be within Proborhyaenidae.[26] The relationships among hathliacynids are also relatively unstable.[32]

Sparassodonta

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.[38]

Marsupialiformes

Evolution

Skull of Borhyaena (Borhyaenidae)

The early history of the Sparassodonta is poorly known, as most

metatherian Mayulestes was considered to be the earliest known member of the Sparassodonta, but phylogenetic analyses suggest that this species represents an independent radiation of carnivorous metatherians more closely related to Pucadelphys;[12] however, recent studies show that these taxa were closely related to borhyaenids.[25] As of this writing[specify], the earliest known true sparassodonts are either Allqokirus australis, a species from the same site as Mayulestes that may turn out to not be a sparassodont, and an isolated astragalus from the earliest Paleocene site of Punta Peligro, Argentina.[39]

Sparassodonts have been suggested to be related to a variety of other groups of metatherians.

Marsupialia, in a broader clade (Pucadelphyida) including pucadelphyids as well as sparassodonts.[12][19][29]

Sparassodonts are currently considered to be

polydolopids).[49][29]

Paleobiology

Diet

Skeletal reconstruction of Thylacosmilus, a large specialised sabre-toothed sparassodont

Sparassodonts were carnivorous, and with the exception of some basal members of all members of this group were

C3 grazers in open habitats, likely notoungulates.[54]

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.[55][56]

Borhyaenid and proborhyaenid sparassodonts have been interpreted as being capable of crushing bones similar to modern

hyaenids.[33]

Based on studies of the

scansorial (adapted for climbing), although terrestrial adaptations evolved in Lycopsis longirostrus, borhyaenids, proborhyaenids, and thylacosmilids.[60][61][62] Most sparassodonts were plantigrade, Borhyaena has been suggested to have been digitigrade[63] but this has been questioned.[12] The one exception was Thylacosmilus, which has been interpreted as having a digitigrade forefoot and a semiplantigrade hindfoot,[63][64] this has been supported by fossil tracks.[65]

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.

terror birds).[69]

Sociality

Little is known of the behavior and biology of sparassodonts outside of general locomotor and dietary habits. Argot (2004) proposed that Thylacosmilus atrox may have exhibited protracted parental care after weaning of the offspring, given that

saber teeth in general have been suggested to require long juvenile periods for the young to gain the skill necessary to use them effectively.[11] However, this has not been tested further. Sparassodonts have relatively large and complex brains for metatherians, comparable to those of some Australian marsupials like Australian possums,[70] though the body masses used to produce these estimates of relative brain size are low compared later studies suggesting these values could be overestimated.[71]

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).[42]

Senses

Sparassodonts appear to have had very little binocular vision, with borhyaenids having the greatest degree of depth perception (but still lower than modern carnivorans) and the eyes of Thylacosmilus facing almost completely to the sides.[72] However, later studies have found that Thylacosmilus likely held its head in a downward-facing position, which would have allowed for more binocular vision than previously thought.[73]

Pathology

Several specimens of hathliacynids (Sipalocyon and Cladosictis) show a pathological disorder characterized by the presence of growths on the surface of the mandible, which in the most extreme cases can result in the loss of several teeth due to bony pathological growths.[16] The exact cause of this condition (i.e., infection, virus, parasite) and why it seems to only occur in small sparassodonts is unknown, though this condition has also been documented in microbiotherians.[74]

Extinction

After the middle Miocene, sparassodonts began to slowly decline in diversity. Basal borhyaenoids are last known from the early late Miocene (Pseudolycopsis cabrerai and Lycopsis viverensis), and after this time were at least partially replaced by large-bodied basal sparassodonts such as Stylocynus. It has been suggested that this shift in dominance was due to the more omnivorous habits of basal sparassodonts, which may have been better able to exploit the more seasonal climates of South America during the late

didelphids such as Lutreolina that appeared around this time.[76]
Hathliacynids are last definitively known from the early Pliocene, though their remains are rare.

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).

ice ages.[78]

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Further reading

External links

Wikimedia Commons has media related to Sparassodonta.
Wikispecies has information related to Sparassodonta.
Wikisource has the text of the 1911 Encyclopædia Britannica article "Sparassodonta".
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