New mandibular remains of Callistoe (Metatheria, Sparassodonta) reveal unexpected anatomical, functional, and evolutionary aspects of this carnivorous genus

Abstract

We present a detailed description of the anatomy of the dentary and lower teeth of a new specimen of Callistoe vincei, a large carnivorous metatherian from the Eocene (?Ypresian) of northwestern Argentina. The recently collected specimen is a young adult represented by a partial right dentary with the canine, p1, roots of p3, and very well-preserved m1 to m4. The description includes a comparison with the holotype specimen, a much older individual, and other closely related large sparassodonts (e.g., Arminiheringia). The analysis of this new material allowed identifying plesiomorphic molar features in Callistoe, such as the presence of a reduced metaconid on the m3 and a tricuspated, basined talonid on m1–m3. We also described the mesowear facets in the lower dentition, showing that the self-sharpening facet typically present in extinct and extant placental and some marsupial carnivorous forms, was absent in Callistoe. The presence of a short-term cutting edge in the trigonid related to the thinness of the enamel layer, and the associated tooth wear susceptibility, were likely compensated by a dental mechanism (overeruption) to maintain occlusal contact among antagonist teeth. This process could explain the marked extrusion of the tooth roots observed in Callistoe as well as in other large closely related members of the group.

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)

“…This structure is less developed in A. gracilis than in T. atrox and probably P. goini (inferred from a portion present in the specimen from Quebrada Honda; see Material and methods, and Appendix 1, Material examined) and poorly developed in the specimen IGM 251108. The more expanded symphyseal flange, is associated in thylacosmilids with a lower canine more vertical and laterally compressed, and premolars reduced in size and number, compared to other sparassodonts (e.g., Sinclair 1906; Petter & Hoffstetter 1983; Marshall 1976; Babot et al 2002, 2022; Forasiepi 2009). In the specimen IGM 251108 the first premolar (P/p1) is almost vestigial, while this tooth is absent in thylacosmilids (e.g., A. gracilis , P. goini, and T. atrox ).…”

“…However, this would differ from the two conditions seen in other borhyaeonoids: 1) the metaconid present only in the m2-4; or 2) absent (or vestigial) in all molars. However, a recently described specimen of Callistoe vincei (Babot et al 2022) shows another unusual condition, with a strongly reduced metaconid in the m3, probably also present in the m1 (inferred from a wear facet located on the mesiolingual corner of the trigonid); but “given the absence of a metaconid in the m2, that option is possible but unlikely” (Babot et al 2022: 475). The antero-lingual vertical crest of the paraconid (preparacristid interlocking mechanism) forms a keel.…”

“…Eomakhaira molossus Engelman, Flynn, Wyss & Croft, 2020Photographs of the holotype (SGOPV 3490) shared by Engelman.Additional information: Engelman et al (2020). Proborhyaenidae Callistoe vincei Babot, Powel & Muizon, 2002PVL 4187 (holotype), complete skull and dentary and almost complete postcranium; IBIGEO P-110, fragment of right mandible with partial intra alveolar and complete extra alveolar portion of the canine, p1 with complete crown and distal root, roots of the p3, and complete m1 to m4.Additional information: Babot et al (2002, 2022). Paraborhyaena boliviana Hoffstetter & Petter, 1983Hoffstetter & Petter (1983); Petter & Hoffstetter (1983).…”

“…As such, Thylacosmilidae was redefined using a stem-based definition. However, considering this proposed definition, not only Eomakhaira molossus (see Engelman et al 2020) but other sparassodonts (e.g., Callistoe vincei Babot, Powell & Muizon, 2022 and Paraborhyaena boliviana Hoffstetter & Petter, 1983, traditionally considered Proborhyaenidae Ameghino, 1897) should be included in the group because, according to some phylogenetic hypotheses (e.g., Babot et al 2002; Forasiepi et al 2015: fig. 7; Suarez et al 2016: fig.…”