The diversification of carnivorous trophic niches among Pucadelphyda (Mammalia, Metatheria) during the Palaeocene and early Eocene in South America

Pucadelphyda was a group of metatherians that were the primary mammalian predators during the Palaeogene and Neogene in South America. Among Palaeogene mammal-bearing localities, the faunas from Tiupampa and Itaboraí are relevant due to the occurrence of different members of Pucadelphyda. Among them, Xenocynus from the Itaboraí Basin was identified as a sparassodont supported by a phylogenetic analysis. A principal component analysis (PCA) based on morphometric and morphofunctional variables, including small-sized extinct and extant South American metatherians, positioned sparassodonts in a similar morphospace to Lutreolina crassicaudata, suggesting they were carnivorous-faunivorous taxa. Xenocynus and Patene display opposed jaw biomechanical trade-offs: a weaker but faster bite and a stronger but slower bite, respectively. This result suggests different prey preferences, with Xenocynus having a unique carnivorous trophic niche compared to other Palaeogene sparassodonts. The combined results of the phylogenetic, morphometric, and morphofunctional analyses suggest that Sparassodonta already occupied carnivorous trophic niches during the early Palaeocene. Sparassodonts evolved throughout the Palaeocene and Eocene, progressively acquiring lower molars characterised by smaller metaconids, bladed m1s, and carnassialized m4s, with more apical lineages displaying more carnassialized lower molars than earlier ones. This evolutionary pattern likely limited the disparity of carnivorous forms in apical clades.