Skull of Early Eocene Cantius abditus (Primates: Adapiformes) and its phylogenetic implications, with a reevaluation of 'Hesperolemur' actius

A substantially complete skull and mandible of the primitive adapiform Cantius is reported from the Early Eocene Willwood Formation of Wyoming. The mandible contains an almost complete lower dentition in which the lower incisors are strongly inclined and have spatulate crowns, I₂ is larger than I₁, and the canine is large and projecting. The cranium shares many features with those of Notharctus and Smilodectes but differs in having nasals that broaden proximally. Presence of a prominent canine and strong sagittal crest may indicate that it represents a male. The basicranium preserves auditory structures almost identical to those in extant noncheirogaleid lemurs, including a large bony tube for the stapedial artery and a small, open sulcus for the distal portion of the promontorial artery. The dentition is sufficiently primitive to be compatible with a relationship to either strepsirrhines or anthropoids, but the anatomy of the auditory region is more consistent with either specific relationship to lemurs or, more likely, a basal position that approximates the euprimate morphotype. Certain features of the basicranium of 'Hesperolemur' actius, described by Gunnell ([1995] Am. J. Phys. Anthropol. 98:447-470) as being unlike that of any other adapiform, were either misinterpreted or are apparently no longer present in the holotype. Reassessment of these and other features indicates that in fact 'H.' actius differs little from Cantius and should not be separated from the latter at the genus level, although on dental grounds the species appears to be distinct (as C. actius).