A new derorhynchid (Mammalia, Metatheria) from the Itaboraí Basin (early Eocene), Brazil, and the trophic diversity of derorhynchids during the onset of the Eocene

Abstract

Derorhynchidae is a Paleogene clade of South American and Antarctic metatherians. Among the localities with representatives of this clade, the Itaboraí Basin stands out by the identification of three genera and at least five species. Herein, we present a new derorhynchid from this locality, Streptorhynchus tropicalis gen. et sp. nov., and discuss its affinities and dietary inferences. Streptorhynchus is distinguished by the presence of a mesially placed ectoflexus, StC close to StD, pyramidal entoconid, and m4 with a labial contact of the cristid obliqua with the distal border of the trigonid. Phylogenetic analysis recovers Streptorhynchus, Pauladelphys, and Coona as a clade (Cooninae new subfamily), based on the presence of a pyramidal entoconid and m4 with a labial contact of the cristid obliqua with the distal wall of the trigonid. Streptorhynchus differs from Pauladelphys and Coona in having an unreduced hypoconulid (reduced in species of Pauladelphys and Coona). Moreover, two PCAs were performed based on morphometric and morphofunctional indexes of Itaboraí’s derorhynchids. The results indicate that lower molars of derorhynchids are morphometrically similar, except those of Diogenesia brevirostris, which are smaller than Itaboraí’s other derorhynchids. Nevertheless, their morphofunctional indexes are very different,especially those related to dentary shape, suggesting the exploitation of differential trophic niches. Streptorhynchus was probably an omnivore, Derorhynchus a “durophagous” insectivore, and Diogenesia an insectivore. Derorhynchus singularis has a longer and more robust jaw than Derorhynchus sp., and Diogenesia brevirostris has taller trigonids, smaller talonids, and a shallower dentary than Diogenesia sp.