Xenosmilus

''Xenosmilus'' is an extinct genus of homotherin machairodontine (saber-toothed cat) that roamed the North America from the Early Pleistocene. The type species of the genus, ''X. hodsonae,'' is known from Early Pleistocene deposits in Florida.

Over the recent years, scientists have proposed ''X. venezuelensis'' (formerly ''Homotherium venezuelensis'') could be a valid species within the genus, in addition to a possible third unnamed species, cf. ''Xenosmilus'' sp., from Uruguay. If valid, both species would extend the genus’ range into the Middle Pleistocene and South America.

Taxonomy

Discovery and Naming

Two fairly intact specimens were found by amateur fossil hunters in 1983 (1981 by some sources) in the Haile limestone mines in Alachua County, Florida. The genus and type species, ''Xenosmilus hodsonae'', was described in 2001 based on a nearly complete skeleton (BIOPSI 101) from the Florida site Haile 21A, with a second partial skeleton (UF 60,000) as the paratype. Both skeletons came from Early Pleistocene-aged rocks in Florida. A radius similar to ''X. hodsonae'' was found in Blacan rocks of Arizona, this represents the earliest record of the genus outside of Florida.

The genus name ''Xenosmilus'' was derived from the Greek / meaning "strange", and / meaning "knife". The species name ''hodsonae'' honors Debra Hodson, the wife of a researcher.

Classification

''Xenosmilus'' is in the tribe Homotherini in the subfamily Machairodontinae of the cat family. A paper published in 2022 proposed that ''Xenosmilus'' is a more derived member of the tribe Machairodontini (another name for Homotherini), and that ''Homotherium venezuelensis'' should be reassigned to ''Xenosmilus.'' The 2022 study found that ''Xenosmilus'' was nested within ''Homotherium'' as traditionally defined, making ''Homotherium'' without including the species in ''Xenosmilus'' paraphyletic. A 2024 paper argued that remains of a homotherin from Uruguay should also be assigned to the genus.

Description

The skull of ''Xenosmilus'' was in length. Compared to other machairodonts, ''Xenosmilus'' skull was relatively small, however, the occipital condyles was unusually large for the skull size. Overall, it had a more bearlike than catlike appearance.


Physically, the cat reached around tall at the shoulder, and is estimated about the same size or larger than ''Smilodon fatalis'', though the body mass estimates of the holotype is around . A 2019 book suggested a body mass range of . In 2024, Manzuetti and colleagues estimated cf. ''Xenosmilus'' sp. could have weighed .

Paleobiology

Before the discovery of ''Xenosmilus'', all known saber-toothed cats fell into two general categories. Dirk toothed cats had long upper canines and stout legs. Scimitar toothed cats had only mildly elongated canines, and long legs. ''Xenosmilus'' broke these groupings by possessing both stout muscular legs and body, and short broad upper canines. Unlike most other saber-toothed cats, all of ''Xenosmilus''s teeth were serrated, not just its fangs and incisors. ''Xenosmilus'' differs from ''Homotherium'' and most other cats in the lack of a gap separating the last incisor tooth and the canine, as well as the loss of the p3 tooth. Notably, only the later species of ''Smilodon'' have also lost the p3 tooth. The way its top teeth were lined up also allowed ''Xenosmilus'' to concentrate its bite force on two teeth at a time.

''Xenosmilus'' has also been theorized by some to have hunted via a "bite and retreat" strategy using its teeth to inflict deep wounds because of the way its canines and incisors could operate as a unit during a bite, leading to ''Xenosmilus'' bearing the occasional moniker of "cookie-cutter cat". It seems likely, with their muscular builds, that ''X. hodsonae'' preyed upon peccaries, due to the large numbers of peccaries found within the same site.

A study published in 2022 suggests that ''Xenosmilus'' and other machairodonts such as ''Smilodon'' were also capable of efficiently removing meat from a kill without damaging their teeth, as evidenced by bite marks on the bones of ''Platygonus''. The same study also suggests that machairodonts could consume at least smaller bones when feeding, similar to lions.

Because the skeletons were found beside each other, some suspect ''Xenosmilus'' was a social mammal. According to Martin and colleagues, the cave deposit the specimens were found in may be evidence of denning behavior.

Paleoenvironment

The holotype and paratype fossils of ''X. hodsonae'' were of Irvingtonian age (1.8 to 0.3 Ma). However, it has also been found in quarries dating to the late Blancan'', ''Paramylodon'', '' Hemiauchenia'', and ''Cuvieronius''. It also coexisted with '' Titanis'', one of the last phorusrhacids, and carnivorans such as the fellow machairodont '' Smilodon gracilis'', ''Canis edwardii'', and ''Arctodus pristinus''.

Potential species ''X. venezuelensis'' was found in El Breal de Orocual of the Mesa Formation in Venezuela, which may have been a similar environment to modern day Llanos. The rarity and scarcity of homotherins in South America may suggest that they lived in low population densities.

References

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{{Taxonbar, from1=Q141533, from2=Q20719562

Homotherini
Pleistocene genus extinctions
Monotypic prehistoric carnivoran genera
Pleistocene mammals of North America
Fossil taxa described in 2000