Cranial osteology of Cynodictis (Amphicyonidae), the oldest European carnivoran

The amphicyonids, colloquially called ‘beardogs’, are one of the oldest known groups of caniformians, taking part in the initial radiation of this carnivoran clade. While the oldest American occurrences from the Middle Eocene have been investigated in detail, the European material remains understudied. The oldest European occurrences suggest an appearance of caniformians in the Priabonian of southern France and a diversification during the Oligocene, after a major faunal turnover following drastic climatic and environmental changes, the ‘Grande Coupure’. Their first representative is the amphicyonid Cynodictis lacustris, the cranial osteology of which is of much relevance for the systematics of Caniformia. A well-preserved cranium of Cynodictis lacustris was collected in the Phosphorites of Quercy (Lot, France) in the late 1960s. The exceptional preservation of the specimen allows us to describe the osteology, make substantial comparative observations and propose biological interpretations, leading to a partial reconstruction of the cranial vascularization, innervation, and musculature. We also reconstruct the ecological evolution of the European amphicyonids from the Paleogene based on their body masses and diets, leading to identify three different faunas: (1) the oldest one (Priabonian) is characterized by body mass around 10 kg, well-exemplified by Cynodictis lacustris; (2) the second (Rupelian) groups taxa from 30 to 50 kg; (3) the last one (Chattian) differs from the two others by the presence of large amphicyonids (ca. 140 kg) and the low number of hypercarnivorous amphicyonids. The in-depth investigation of this exceptional specimen provides new material for the systematic and paleoecological understanding of Paleogene amphicyonids.