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On the Skull of Radinskya (Mammalia) and Its Phylogenetic Position

ABSTRACT The skull of the Paleocene mammal Radinskya is redescribed here, based on direct study of the type and only specimen. A number of inaccuracies from previous studies are corrected. Radinskya possesses two potential synapomorphies with perissodactyls, namely, posteriorly broad nasals and a small, laterally facing exposure of the mastoid process of the periotic. The phylogenetic position of Radinskya is examined using matrices from three studies, two that originally included Radinskya and for which the scores for Radinskya are corrected, and another recent study of archaic ungulates that did not originally include Radinskya. In addition, a new analysis is performed combining data from all three previous studies, including modifications based on the present study. Reanalyses of the three previous studies give conflicting results regarding the issue of perissodactyl ancestry, placing Radinskya as sister taxon to perissodactyls, as sister taxon to a perissodactyl-phenacodontid clade, or as part of a polytomy with perissodactyls and paenugulates. The results of the new analysis (1) place perissodactyls closer to Radinskya than to phenacodontids; (2) place Hyracoidea and Proboscidea (i.e., Paenungulata) as sister taxa to Perissodactyla; (3) do not support inclusion of Radinskya in Phenacolophidae; and (4) are equivocal in placing Lophocion as the phenacodontid closest to perissodactyls. These results thus support neither an Asian nor a North American origin for perissodactyls. Future studies should examine the origins of perissodactyls and other ‘ungulates’ in a broader taxonomic context, in order to more directly address discrepancies between results supported by morphological data and those derived from molecular data.