We here describe the first partial cranium of Hyaenodon leptorhynchus, the type species of the taxonomically diverse and widely distributed hypercarnivorous genus Hyaenodon (Hyaenodonta). The cranium is from the Séon Saint-André deposits (Marseille, France; Chattian, MP26). It is preserved in a dense red marl matrix that obscures key morphological features. CT-scans were used to reconstruct the specimen. The morphology of the cranium reinforces the homogeneity previously observed in Hyaenodon despite its specific diversity. The fossil represents a juvenile: it preserves its deciduous canines and the P3 is almost fully erupted. This pattern of delayed canine eruption is a trait shared among North American and European Hyaenodon. This discovery is the third occurrence of this species in the early Chattian: indeed, over a period of 5 My (from MP24 to MP27), only two occurrences (Rigal-Jouet and Saint-Martin de Casselvi, MP25) have been reported. Based on body mass and the general Hyaenodon body plan, we confidently identify H. leptorhynchus as a cursorial hypercarnivorous predator, hunting prey such as small artiodactyls. In order to understand the evolution of the European carnivorous faunas, we compared taxonomic diversity and the evolution of body mass in Hyaenodon and amphicyonids: this reveals stasis in Hyaenodon through the late Eocene and Oligocene, while amphicyonids show an extensive ecological diversification, especially during the Chattian.

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