RESULTS OF THE PHYLOGENETIC ANALYSIS

Phylogenetic analysis yields 189 most-parsimonious trees (treelength 311, CI excluding uninformative characters = 0.54, RI = 0.62). See Figure 12 and Figure 13 for majority-rule consensus tree and adam’s consensus tree, respectively.

This analysis supports the unorthodox placement of Colodon occidentalis closer to Tapirus than is Protapirus. Most previous analyses considered Colodon to be a divergent lineage tracing its ancestry to the Bridgerian Helaletes (e.g., Radinsky 1963), because it was considered ‘too specialized’ to be an ancestor of tapirids (e.g., Radinsky 1963). This was based partly on the supposition that “the molariform premolars of Colodon exclude all known species of that genus from the ancestry of Protapirus” (Radinsky 1963, p. 96). Protapirus was deemed closer to Tapirus than Colodon on merit of its presumed greater degree of narial incision retraction, and shortening of its nasals, which cannot be confidently coded in Protapirus (Colbert 1999). As noted above, the only figured Colodon skull in the literature (Radinsky 1963, figure 21) does not accurately depict the actual morphology of the narial region (see description) and has misled earlier interpretations of Colodon’s evolutionary affinities (e.g., Colbert and Schoch 1998).

Several cranial characters support Colodon being closer to Tapirus than is Protapirus, including: the nasoincisive incisure extending to the level of M2 (character 56), the supraorbital process being reduced to a small rugosity (character 76), and the moderate inflation of the frontals (character 78). The following dental characters also support this relationship: having a P1 with a hypocone (character 28), having P3 and P4 with a divided protocone and hypocone (characters 34 and 37), and having the postprotocrista join the protocone rather than the hypocone (character 38). Most of these dental characters relate to the degree of ‘molarization’ of the premolars. The chronostratigraphic occurrence the two skulls described here documents an earlier origin for these derived cranial morphologies than previously hypothesized.