The hindlimb of Amphicynodon leptorhynchus from the lower Oligocene of the Quercy Phosphorites (France): Highlight of new climbing adaptations of this early arctoid

The Itardies locality (MP23, 30-31 Ma), in the Quercy Phosphorites (France), has yielded an impressive amount of fossil remains attributed to Amphicynodon leptorhynchus, a genet-sized carnivoran. This species belongs to the family Amphicynodontidae, which was very diversified during the lower Oligocene in Eurasia and whose paleoecology is still poorly known. This study complements our previous analysis of the forelimb of Am. leptorhynchus, by describing and discussing several aspects of the functional anatomy of its hindlimb to infer its locomotor abilities and lifestyle. The locomotor adaptations of the hindlimb of Am. leptorhynchus portray a talented and agile climber; the great range of mobility of the hip articulation and the shape of the bony structures where the powerful hip abductors, adductors and hamstring muscles attach indicate its ability to firmly grip the substrate, and the knee articulation morphology and the osteological structures supporting the hindlimb extensors suggest powerful extension movements, linked to jumping and fast accelerations, as in half-bounding carnivorans. Its hindfoot morphology characterises a mobile articulation, able to perform a hindfoot reversal movement, perhaps even to 180 degrees, as in some extant climbing carnivorans with similar osteological features. This adaptation, highlighted for the first time in a fossil carnivoran, permitted Am. leptorhynchus to skillfully descend vertical supports headfirst and to hang by its hindlimbs to reach food and new supports and to capture preys with its forepaws. Although a slow climber model has been suggested based on the morphology of its forelimb, its hindlimb seems to portray a faster and more agile animal, able to climb with control headfirst upwards and downwards and also to perform jumps and bounds between branches or to capture prey, such as the extant ringtail or African palm civet.