Thylacosmilus atrox, Riggs, 1933

Suarez, Catalina, Forasiepi, Analia M., Babot, María Judith, Shinmura, Tatsuya, Luque, Javier, Vanegas, Rubén D., Cadena, Edwin A. & Goin, Francisco J., 2023, A sabre-tooth predator from the Neotropics: Cranial morphology of Anachlysictis gracilis Goin, 1997 (Metatheria, Thylacosmilidae), based on new specimens from La Venta (Middle Miocene, Colombia), Geodiversitas 45 (18), pp. 497-572 : 546-547

Thylacosmilus atrox

Character 19: changed from 2 to inapplicable (–) because despite a fronto-maxillary contact is present, this condition is secondary, not homologous to that present in other taxa codified with state 2 (e.g., marsupials). In this species, this contact is caused by the posterior expansion of the maxilla on the skull roof, interrupting the naso-lacrimal contact present in Sparassodonts, including more plesiomorphic thylacosmilids (e.g., A. gracilis and P. goini ).

Character 24: changed from polymorphic to 1 (fossa for levator labii muscle mainly on maxilla (following Forasiepi et al. 2019).

Characters 61, 134 and 135 were changed followingForasiepi et al. (2019) and observations examined material. Character 61 was changed from 0 to inapplicable (–), because Thylacosmilus atrox lacks a suprameatal foramen (Riggs 1934; Forasiepi et al. 2019). However, it has two couple of openings on the squamosal (two on each side), which correspond to the foramina for temporal rami (Forasiepi et al. 2019). Consequently, character 134 was changed from 1 to 0 (Foramina for temporal ramus with well-developed internal branch of stapedial artery present), and character 135 from inapplicable (–) to 1 (foramina for temporal rami located on squamosal). The functional role of the suprameatal foramen would presumably have been taken up by one of the canals for rami temporales (Forasiepi et al. 2019).

Character 52: changed from 1 to 0 (shape of fronto-parietal suture formed by a posterior wedge of frontals), according to observations on the material observed (see Material examined) and figures in Forasiepi et al. (2019), where the suture is not completely straight but with a small posterior wedge of frontals entering between parietals, though more reduced than in Anachlysictis .

Character 83: coded 2 (primary foramen ovale on alisphenoid), following Forasiepi et al. (2019).

Character 84: coded 1 (foramen ovale located on the ventral surface of the skull), following Forasiepi et al. (2019) and observations on the material examined.

Character 145: coded 1 (three or more mental foramina).

APPENDIX 1. — Continuation.

Character 146: coded inapplicable (–). The anterior-most mental foramen in Thylacosmilus does not correspond topographically to the same enlarged anterior foramen as in non-thylacosmilid sparassodonts because the symphyseal flange has additional foramina anterior to it. For this reason, we coded it as inapplicable, and homologies of these foramina should be tested in the future.

Character 151: changed from 1 to 2, following the changes in this character, including the addition of state 2.

Character 159: changed from 3 to “?”, following Churcher (1985) and Goin & Pascual (1987). The presence of at least two upper incisors is an inference based on the position of the lower incisors preserved and the wear facets and space between upper canines. However, no evidence has ever been collected with a clearly complete incisor series. Evidence of “at least two”, not exactly two, does not allow scoring the taxon with state 3 (i.e., two or fewer incisors) because the possibility of a third incisor has not been discarded.

Character 177: coded 3 (prominent median sulci present on labial and lingual faces of the lower canine only), according to the modification in states.

Character 193: changed from “?” to 0 (p2 smaller than p3).

Character 194: changed from “?” to “–”. With an incomplete premolar series, this character (change in the height of lower premolars) cannot be evaluated for Thylacosmilus .

Character 89 changed from 0 to 1 following Forasiepi et al. 2019.

Character 224: changed from “?” to 1 (carnassial notch in postmetacrista present) following Goin et al. (2007) and observations on examined material.

Character 238: coded 1 (talonid of m4 reduced and narrower than m3).

Characters 248, 262, 263, 268, and 269 were changed to “?” due to uncertainties and changes in the interpretation of homologies of the talonid cusps. Goin & Pascual (1987) describe the talonid of Thylacosmilus as composed of one main cusp. This cusp is well differentiable in the m1-3 and vestigial in the m4. Goin & Pascual (1987) identify the vestigial cusp in the m4 as an extremely reduced talonid, apparently without cusps. However, they do not mention a homology hypothesis for the main structure in the m1-3, which is evidently a cusp. Forasiepi (2009) codifies the absence of a hypoconid, presence of a hypoconulid and a vestigial entoconid. These scorings suggest that the main cusp in the talonid would be the hypoconulid.

In our revision, it was observed that the main cusp has, in fact, a posterior position and has a crest running to the base of the trigonid, which probably corresponds to the cristid obliqua. Additionally, there is an extremely reduced, almost vestigial cusp in lingual position. Due to this topographical location, this last cusp could be the entoconid, as codified by Forasiepi (2009). This condition is different in A. gracilis and P. goini , with the entoconid being the dominant cusp of the talonid, almost completely fused to the hypoconulid (the tips are still visible). On the other hand, the main cusp in Thylacosmilus could be either a hypoconulid (as coded by Forasiepi 2009), a hypoconid displaced posteriorly, or the fusion of these two.Goin et al. (2007) analyzed the homologies of the talonid in some borhyaenoids, grouped at that moment in two subfamilies within the family Borhyaenidae : “Borhyaeninae” and “Prothylacyninae” (currently invalid classification). These sparassodonts have a strong reduction in the talonid as part of a typical morphology related to carnivory, which, in more specialized groups, preserves basically one posterior cusp.

After observing and analyzing plesiomorphic and derived morphologies,Goin et al. (2007) concluded that the main cusp in “prothylacynids” is homologous to the hypoconid, while in the “borhyaenids” it would be the entoconid, or the entoconid fused to the metaconid. We do not pretend to establish a relationship of Thylacosmilus with any of these groups but to show the variability spectrum of these structures within other carnivore morphologies. Assuming that the vestigial cusp in the lingual margin of the talonid of Thylacosmilus is a reduced entoconid and, considering that the cristid obliqua connects with the main cusp, the more probable hypothesis is that this main cusp is the hypoconid displaced posteriorly and that the hypoconulid disappeared or fused with one of these two cusps. In fact, a fusion of the hypoconulid with the hypoconid would explain that posterior position, as it occurs with the block entoconid + hypoconulid in the other thylacosmilids, which is displaced to a more posterior position (posterolingual).

Character 264: changed from 0 to 1, because there is a vestigial entoconid, but it is still observable and clearly identifiable as an entoconid, as explained before (see explanation of the change in the scoring of characters 248, 262, 263, 268, and 269).