Phylogenetic relationships and palaeobiology of a new xenungulate (Mammalia: Eutheria) from the Palaeogene of Argentina

Abstract

Xenungulates are a peculiar group of bilophodont ungulates, widespread over South America during the Paleocene and early Eocene. Here we describe, illustrate and analyse a new Xenungulata, Rodcania kakan gen. et sp. nov., the first from the lower latitudes of Argentina. It was recovered from the Río Loro Formation (Tucumán Province), for which a Paleocene age was estimated. The material comprises a fragment of the left dentary with an incomplete m2, eroded over the enamel surface, and an almost complete m3. Both teeth share with Xenungulata strong enamel, vertically oriented Hunter–Schreger bands (HSB), and wear facets sloping distally. Rodcania kakan differs from other xenungulates in the m3 trigonid, simplified and mesiodistally short; absence of a paraconid; more oblique protolophid in relation to the mesiodistal axis and a straight cristid obliqua, pointing to the protoconid position; and has a wider and longer talonid. A phylogenetic analysis of 13 species and 36 craniodental characters recovered R. kakan as a basal member of the Carodniidae, supported by 10 synapomorphies in the four most parsimonious trees. The body mass estimation of the Río Loro Fauna and of the rest of the xenungulate taxa revealed that R. kakan is the largest mammal of the Río Loro assemblage, weighing around 165 kg and with a size comparable to Carodnia feruglioi. The increase in body mass showed no clear relation with evolutionary trends in xenungulates.