INTRODUCTION

Perhaps the most extraordinary feature of the living tapirs is their prehensile proboscis. It is derived from modified muscles of the face and upper lip, and its presence is indicated by several osteological features (Witmer et al. 1999). These osteological correlates include the reduction of the bony wall of the nasal chamber (Witmer et al. 1999); the presence of processes and scars for attachment of proboscis musculature (Witmer et al. 1999); and a posterior displacement of the dorsal facial skeleton (i.e., telescoping; see Colbert 1999). Further conditions correlated with the telescoping of the skull are the development of frontal sinuses overlying the anterior cranial cavity, the loss of contact between the premaxillae and nasals, the apparent loss of a true maxillary sinus, and the unique condition of having the maxilloturbinals, premaxillae, and maxillae embrace the cartilaginous nasal septum (Witmer et al. 1999). The fossil record of the tapiroids includes crania that document less derived states for these character complexes, which provides insight into the evolution of their facial skeleton.

In this report, I present the first detailed description of the facial skeleton and some of the elements surrounding the facial skeleton of early Oligocene Colodon from North America. This description uses high-resolution X-ray computed tomography (HRXCT) to explore the internal anatomy of two Colodon skulls from South Dakota. Even though both Colodon specimens are subadult, they nonetheless preserve several indicators of prehensile proboscis development and have a derived telescoped condition. Although these remarkable skulls provide a glimpse of a facial morphology less derived than Tapirus, they are nevertheless more similar to Tapirus than to other Eocene and early Oligocene tapirs. This similarity is contrary to the observations presented in an earlier cranial description of Colodon (Radinsky 1963, figure 21) and supports a different phylogenetic placement for Colodon than previously hypothesized (e.g., Colbert and Schoch 1998; Dashzeveg and Hooker 1997; Holbrook 1999; Radinsky 1963; Schoch, 1989b).