Members of the early Cenozoic family Plagiomenidae possess highly derived cheekteeth that phenetically resemble those of extant colugos (Galeopithecidae, Dermoptera), although the two groups notably differ in details of cusp morphology and in the form of the anterior teeth. Whether these dental resemblances should be considered as evidence of close, shared ancestry or merely as a set of interesting convergences is difficult to evaluate, in part because heretofore no other portions of the plagiomenid skeleton have been available for testing phylogenetic hypotheses. In this report we analyze the basicranial morphology of a skull referable to the nominotypical genus Plagiomene from the early Eocene Willwood Formation of the Bighorn Basin, Wyoming. Plagiomene exhibits one of the most highly derived basicranial regions found in Eutheria. No known mammal shares the combination of derived traits exhibited by Plagiomene, although there are points of resemblance to Talpidae, Macroscelidea, and (to a lesser extent) Primates. Surprisingly, there are no recognizable nonprimitive resemblances to the extant colugo Cynocephalus. Unless the traits in question are counted as synapomorphies of a highly implausible plagiomenid-talpid-macroscelidean-primate clade, some of these special resemblances must be due to convergence. Perhaps all of them are; for except in isolated features that also appear to be convergent resemblances, the teeth of moles, elephant shrews, and primates are quite unlike those of plagiomenids. By contrast, galeopithecids are dentally more similar to Plagiomene but lack all of its basicranial apomorphies. Tendentious selection of dental traits over basicranial ones is in our view indefensible, and we prefer to situate Plagiomenidae incertae sedis within Eutheria.
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