Abstract

Hyaenodontidae is a diverse component of the carnivorous guild in early Eocene (Wasatchian) North American mammalian faunas. Although there have been suggestions that early members of the family had diverse locomotor repertoires, few Wasatchian postcrania are adequately described. Here we describe the first known postcranial remains of the rare Wasatchian hyaenodontid Galecyon, based principally on a well-preserved partial skeleton from the Willwood Formation, Wyoming. Galecyon is reconstructed as a 5.2–7.9 kg terrestrial carnivore. Small but significant differences throughout the skeleton distinguish Galecyon from Prolimnocyon. Galecyon is reconstructed as scansorial/generalized terrestrial because these postcranial differences consistently indicate a more terrestrial habit than in the scansorial Prolimnocyon. Among other hyaenodontids, Arfia shows evidence for parallel terrestrial adaptation, whereas Pyrocyon is morphologically intermediate between Galecyon and Prolimnocyon. Manipulation of the well-preserved bones forming the crurotarsal joint of Galecyon provides evidence that contradicts a previous proposal that Arfia, another Wasatchian hyaenodontid, was capable of hind foot reversal. In both genera, movements at the crurotarsal joint were largely restricted to a parasagittal plane. Phylogenetic analysis of a new postcranial character set supports a pattern of relationships for early Eocene hyaenodontids different from that supported by dental morphology. Combining the two data partitions favors most aspects of the dental topology, but the conflict between dental and postcranial data indicates that confident phylogenetic resolution can be elusive, even with relatively dense sampling.