Cranial Morphology of a Pantolestid Eutherian Mammal from the Eocene Bridger Formation,Wyoming, USA: Implications for Relationships and Habitat

Pantolestinae is a eutherian subfamily of mammals whose members are known from the middle early Paleocene through at least the beginning of the Oligocene of North America. They are also known from Europe, and possibly Africa. A lack of information on pantolestine skulls has prevented the use of cranial anatomy in evaluation of this group’s enigmatic higher-level phylogenetic relationships. Conversely, postcranial skeletons are well known and locomotor interpretations based on them are robust. The most complete known skull of a pantolestine, Pantolestes longicaudus (YPM 13525), is described here and compared to potential close fossil relatives and extant mammals. Semicircular canal morphology is used to test locomotor hypotheses. YPM 13525 lacks an ossified bulla. It has a mediolaterally broad basioccipital, a large entoglenoid process, and a deeply incised glaserian fissure of the squamosal, caudal and rostral tympanic processes on the petrosal, a foramen for an internal carotid artery (ICA) that entered the tympanic cavity from a posteromedial position, bony tubes enclosing the main stem and transpromontorial branch of the ICA, a large anterior carotid foramen formed within the basisphenoid, evidence of a stapedial artery ramus superior, a groove on the dorsal aspect of the basisphenoid leading to the piriform fenestra possibly for drainage of the cavernous sinus to an extracranial inferior petrosal sinus, a dorsum sellae with well-developed posterior clinoid processes, a foramen rotundum within the alisphenoid, and a sphenorbital fissure between the alisphenoid and orbitosphenoid. Overall, the morphology is not strikingly similar to any potential close relative and the phylogenetic position of Pantolestinae cannot be estimated without cladistic analysis of a character matrix that includes this new morphology and broadly samples extant and extinct eutherian taxa. Semicircular canal morphology differs from that of two likely terrestrial Paleocene mammals, Aphronorus (another pantolestid) and Eoryctes (a palaeoryctid), suggesting a different, possibly semi-aquatic, lifestyle for Pantolestes.