Micro-computed tomography unveils anatomy of the oldest known plesiadapiform cranium

Palaechthonids are a likely paraphyletic or polyphyletic assemblage of dentally plesiomorphic plesiadapiforms known from the Paleocene of North America. This family is known primarily from isolated dental fossils, but one partial cranium of the palaechthonid Plesiolestes nacimienti (Division of Vertebrate Paleontology, Biodiversity Institute, University of Kansas [KUVP] 9557) exists and was studied a half-century ago to infer aspects of the paleobiology of basal or stem primates. Since then, additional plesiadapiform crania representing several families have been described, but KUVP 9557 remains the best preserved for a palaechthonid and is the geologically oldest known cranial fossil of any plesiadapiform or euarchontan mammal (primates + colugos + treeshrews). Here, for the first time, we scanned the partial cranium of P. nacimienti using micro-computed tomography (microCT) to assess previously described morphology, document novel morphology, and make comparisons with crania of other plesiadapiforms and euarchontans. While several previous cranial descriptions are reaffirmed here (e.g., caudal expansion of the nasals, an intraorbital lacrimal foramen), microCT scan data have clarified that certain previously identified structures (e.g., postorbital process) are not present and have unveiled previously unknown structures (e.g., foramen ovale, optic foramen). Comparisons indicate that the cranial anatomy of P. nacimienti is most like that of non-microsyopid plesiadapiforms and that unambiguous synapomorphies with an extant euarchontan clade are absent. Paleobiological inferences presented here suggest that P. nacimienti was broadly similar to the extant treeshrew Ptilocercus, albeit less insectivorous, which is in line with evolutionary scenarios proposed for the ancestral primatomorphan or the ancestral primate (sensu lato) that emphasize the importance of arboreality and angiosperm products.