New Cranial and Postcranial Remains of the Once Enigmatic Early Eocene Mammal Wyolestes (Mammalia, Ferae, Hyaenodonta) from North America, and Phylogenetic Evidence for its Interordinal Relationships

Wyolestes is an extinct placental mammal from early Eocene (Wasatchian) rocks of North America whose phylogenetic position has been enigmatic for decades. Known from three species with distinct, high-cusped molars, the genus has been difficult to assign to a higher clade within Placentalia in part because it has, until now, been known primarily from dentitions. Here we describe new postcranial material of Wyolestes iglesius; new cranial, dental, and postcranial material, including both axial and appendicular elements, of W. apheles; and new postcranial and dental material of W. dioctes. Examining the phylogenetic position of Wyolestes in a character-dense, total evidence matrix that sampled extant and extinct taxa across Mammalia, reveals that Wyolestes is the sister taxon of the hyaenodont Sinopa, a clade we refer to here as Hyaenodonta. This finding revives a phylogenetic attribution first proposed by Gazin over 60 years ago. Wyolestes is phylogenetically distant from both mesonychids and didymoconids, a result with strong jackknife support. The hyaenodont clade is sister to Carnivoraformes and nested within Ferae. Inclusion of Wyolestes in a more focused phylogenetic analysis with dense sampling of early Ferae supports placement of Wyolestes within Hyaenodonta. A third analysis of early hyaenodonts indicates that the two Wyoming species, W. apheles and W. dioctes are sister taxa with respect to the Baja California species, W. iglesius. Skull material of W. apheles indicates the first known presence of an ossified tentorium cerebelli in Wyolestes, a fragile feature whose preservation is significant because it has been cited as a synapomorphy of Ferae, and is known in Carnivora, Pholidota, and their fossil relatives as well as in Hyaenodonta. The postcranial skeleton reveals that Wyolestes was a relatively generalized Paleogene mammal. Body mass estimates using extant Carnivora as an analog indicate that all species of Wyolestes weighed less than 10 kg and two species less than 5 kg. Wyolestes lacked derived climbing abilities. Nor does the postcranial skeleton indicate that it was semifossorial, but it suggests it may have had more of a tendency toward scratch digging. The postcranial skeletons exhibit many generalized features such as a deltopectoral crest that extends at least halfway down the shaft of the humerus, and the presence of an entepicondylar foramen, as well as the likely presence of a centrale in the manus. For skeletal elements that can be directly compared, such as the humerus, W. apheles is more robust than W. iglesius. Dental morphology indicates a sister taxon relationship between W. apheles and W. dioctes to the exclusion of W. iglesius. FRONTISPIECE. Life reconstruction of the head of Wyolestes apheles based on the skull of USNM 493828. The bottom image shows the skull with muscles and other subcutaneous soft tissue added. The middle image shows skin and fur added over the soft tissue, and the top image shows the finished reconstruction. See text for further details. Illustration by U. Kikutani.