Testing the occlusal relations between isolated upper and lower molars of an Eocene metatherian (Mammalia): The case of Xenocynus crypticus

Xenocynus crypticus is a metatherian from the Itaboraí Basin (early Eocene) in the state of Rio de Janeiro, Brazil. While this taxon was initially described based on a single upper molar and three isolated lower molars, the occlusal relations between these teeth have never been previously analyzed. In this study, the upper and lower molars assigned to this taxon, along with specimen AMNH 49805, were included in a scatterplot and two principal component analyses (PCA), assessing several morphometric parameters that were diagnostic of this taxon. We inferred the occlusal relationships between the assigned molars to assess their morpho-occlusal compatibility. Our analyses have demonstrated that the Xenocynus crypticus molars are morphometrically distinct from other sympatric taxa of similar size, with m1s having proportionally shorter talonids. Furthermore, the upper and lower molars of X. crypticus are occlusally compatible, exhibiting similar inferred proportions in the length and width of the talonid. These findings support the taxonomic validity of X. crypticus and an association of the lower molars with this species. Additionally, our results indicate that PCAs can reliably assess morphometric similarity between isolated upper and lower molars in metatherians, offering valuable insights for taxonomic studies on fragmentary dental-based taxa.