Postcrania and paleobiology of Patriofelis ulta (Mammalia, Oxyaenodonta) of the Bridgerian (lower–middle Eocene) of North America

Oxyaenodonta is one of several groups of early carnivorous placental mammals that are now extinct. Patriofelis is a genus of Bridgerian oxyaenodont, one of the last known from North America. We describe two partial skeletons of Patriofelis ulta to reevaluate hypotheses about the locomotion of Patriofelis. Patriofelis ulta had short limbs relative to a longer body, similar in proportion to other oxyaenodonts. The forelimb shows a wide capitulum of the humerus, indicative of the ability to pronate and supinate the hand. Likewise, the hindlimb shows features associated with flexibility such as a shallow acetabulum, and shallowly grooved astragulus. In contrast, the lumbar vertebrae are inflexible with interlocking zygapophyses. We performed a 2D geometric morphometric analysis of lumbar vertebrae from modern mammals and P. ulta to identify a close functional match. We found that while the lumbar shape in anterior view of P. ulta fell closest to carnivorans, the interlocking zygapophyses matched most closely to modern artiodactyls. We confirmed the occurrence of interlocking zygapophyses in other oxyaenodonts, as well as hyaenodonts and mesonychids, and infer that this is a plesiomorphic feature. Based on this information, we rule out the previously proposed semiaquatic hypothesis for locomotion, as well as the scansorial hypothesis based on the large size of P. ulta. We interpret P. ulta, one of the largest carnivores in its ecosystem, as an ambush predator capable of grappling prey with its flexible forearms.