The Colodon skulls described above document a tapir that, although only half the size of an extant tapir skull, had a surprisingly modern aspect to the facial skeleton. Many of the observed features are directly comparable with features in Tapirus, including several skeletal indicators of a prehensile proboscis similar to that of Tapirus. In particular, details of the anatomy surrounding the retracted nasoincisive incisure are similar to those of Tapirus, including the occurrence of apomorphic fossae for cartilaginous meatal diverticula on the dorsal frontals and nasal, and ascending maxillae. Other similarities with Tapirus include details of the telescoping of the skull, which is correlated with: the anteroposterior shortening of the frontals, and the development of frontal sinuses; the conformation of the rostrolateral processes of the frontals, the descending processes of the nasals, and the ascending process of the maxillae; and the apparent embracing of the cartilaginous nasal septum by the premaxillae.
By contrast, the skull of Protapirus, which is larger, shares less apomorphies with Tapirus. Among the more plesiomorphic conditions of Protapirus are: a less retracted nasoincisive incisure; a pronounced postorbital constriction; and a lack of frontal sinuses above the cranial cavity, to name a few. In these features, Protapirus more closely resembles the late Eocene Plesiocolopirus (which some consider to be Protapirus, see Hanson 1996), and also a new taxon of diminutive tapir from the Uintan of southern California (see Colbert, 1999; Figure 12). Indeed, it is possible that these three taxa form a monophyletic group, based on their similarities (Colbert, 1999).
Because the scanned Colodon skulls are subadults, much of their morphology potentially records juvenile conditions. The ontogenetic trajectory of these features in Colodon can be interpreted by comparison with Tapirus, however, as well as with other mammals. During the ontogeny of Tapirus, the facial skeleton shows the following transformations: a posterior retraction of the nasoincisive incisure relative to the anterior margin of the orbit; posterior migration of the palatal edge of the choanae relative to the molars; the formation and expansion of the frontal sinuses, and other sinuses; and the elongation of the face relative to the cranium as a whole. The last two of these transformations are common in many other mammals.
These observations suggest that, if there were any further ontogenetic transformations in the facial skeleton of Colodon from the condition described above, they would have led to a more deeply retracted nasoincisive incisure, larger frontal sinuses, and a relatively longer face. All of these hypothetical more mature conditions are also considered to represent more derived evolutionary states for these characters, such that further ontogenetic change would be expected to strengthen the special relationship of Colodon to Tapirus relative to Protapirus. In a broader evolutionary context, it might be hypothesized that the evolution of the facial skeleton in the Tapiroidea involves peramorphosis (e.g., see Alberch et al. 1979), with the ontogeny of Tapirus ‘recapitulating’ various evolutionary character transformations in the facial skeleton of ancestral forms.
The hypothesis that Colodon is more closely related to Tapirus than is Protapirus potentially extends the chronostratigraphic range of the Tapiridae to the middle Eocene. This is based on the reported occurrence of Colodon from the late Uintan Sage Creek Formation of North America (Radinsky 1963). Unfortunately, as discussed above, Colodon is a very poorly known taxon based primarily on dental remains and may not be a natural group.