Cranial anatomy of Ignacius graybullianus and the affinities of the Plesiadapiformes

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Abstract
A nearly complete cranium of Ignacius graybullianus provides increased understanding of the cranial anatomy of Plesiadapiformes. In nearly all details of cranial anatomy, Ignacius differs markedly from primates. USNM 421608 exhibits a long tapering snout, small widely spaced orbits, and a complete lack of postorbital process or bar. Large olfactory bulbs are inferred from the wide interorbital space. The marked flare of the zygomatic arches suggests that Ignacius possessed large and powerful temporal muscles. The basicranial region is particularly well preserved and reveals a distinct suture between the petrosal bone and an entotympanic bulla. This suture is visible on both the left and right sides of the skull and dispels the hypothesis that Ignacius and, by inference, other Plesiadapiformes share the primate synapomorphy of a petrosal bulla. To test the phylogenetic position of Ignacius, cranial characters were identified and scored for Ignacius, Plesiadapis, Cynocephalus, and a number of primates, bats, and scandentians. Two erinaceomorph insectivores were also included to allow the assessment of archontan monophyly. These characters were incorporated into a maximum‐parsimony analysis to determine the phylogenetic position of Plesiadapiformes. There are several important phylogenetic conclusions that can be inferred from this analysis: 1) Ignacius and Plesiadapis make up a monophyletic clade; 2) Plesiadapiformes may be the sister group of Dermoptera; 3) Scandentia, not Plesiadapiformes, is the sister group of Primates; and 4) Primates, plesiadapiforms, bats, colugos, and scandentians may not form a monophyletic clade Archonta. Consequently, the taxon Archonta is in need of review.