Palaeontologia Electronica http://palaeo-electronica.org THE FACIAL SKELETON OF THE EARLY OLIGOCENE COLODON (PERISSODACTYLA, TAPIROIDEA) Matthew W. Colbert ABSTRACT Two skulls of the early Oligocene Colodon from the White River Group in South Dakota are much more derived than previously reported. In particular, morphologies of the facial skeleton and narial region are surprisingly modern, including a deeply retracted nasoincisive incisure, and other indicators of prehensile proboscis develop- ment. High-resolution X-ray computed tomography was used to explore the internal anatomy of these tapiroids, and revealed frontal sinuses, and an internal facial skele- ton approaching that of modern tapirs. This not only indicates an earlier origin for these anatomical conditions than previously recorded, but in a phylogenetic context indicates that Colodon is more closely related to Tapirus than is Protapirus. Matthew W. Colbert. The Jackson School of Geosciences, The University of Texas at Austin, Geological Sciences Department, 1 University Station C1100, Austin, Texas 78712-0254 USA col- [email protected] KEY WORDS: Tapiroidea; Colodon; anatomy, cranial; computed tomography; phylogeny PE Article Number: 8.1.12 Copyright: Society of Vertebrate Paleontology May 2005 Submission: 20 December 2004. Acceptance: 24 March 2005 INTRODUCTION attachment of proboscis musculature (Witmer et al. 1999); and a posterior displacement of the dorsal Perhaps the most extraordinary feature of the facial skeleton (i.e., telescoping; see Colbert 1999). living tapirs is their prehensile proboscis. It is Further conditions correlated with the telescoping derived from modified muscles of the face and of the skull are the development of frontal sinuses upper lip, and its presence is indicated by several overlying the anterior cranial cavity, the loss of con- osteological features (Witmer et al. 1999). These tact between the premaxillae and nasals, the osteological correlates include the reduction of the apparent loss of a true maxillary sinus, and the bony wall of the nasal chamber (Witmer et al. unique condition of having the maxilloturbinals, 1999); the presence of processes and scars for premaxillae, and maxillae embrace the cartilagi- Colbert, Matthew, W. 2005. The Facial Skeleton of the Early Oligocene Colodon (Perissodactyla, Tapiroidea) , Palaeontologia Electronica Vol. 8, Issue 1; 12A:27p, 600KB; http://palaeo-electronica.org/paleo/2005_1/colbert12/issue1_05.htm COLBERT: FACIAL SKELETON OF AN OLIGOCENE TAPIR nous nasal septum (Witmer et al. 1999). The fossil Preservation and Relative Maturity. Although record of the tapiroids includes crania that docu- both are partially crushed and distorted, the two ment less derived states for these character com- skulls described here are generally well preserved plexes, which provides insight into the evolution of and relatively complete. The AMNH skull is hori- their facial skeleton. zontally flattened (Figure 1A), and the SDSM skull In this report, I present the first detailed is compressed sagittally and sheared (Figure 1B), description of the facial skeleton and some of the with the right side elevated relative to the left. The elements surrounding the facial skeleton of early different compaction planes of these two speci- Oligocene Colodon from North America. This mens present somewhat complementary informa- description uses high-resolution X-ray computed tion for interpreting the degree of anatomical tomography (HRXCT) to explore the internal anat- deformation, but precise determination of the true omy of two Colodon skulls from South Dakota. skull shape is not possible. Thus, for example, Even though both Colodon specimens are sub- while the AMNH skull suggests a wide rostrum for adult, they nonetheless preserve several indicators Colodon, the SDSM rostrum is narrow, and the true of prehensile proboscis development and have a shape is some intermediate condition. derived telescoped condition. Although these The braincase of the SDSM skull is more remarkable skulls provide a glimpse of a facial completely preserved than the AMNH skull, which morphology less derived than Tapirus, they are lacks posterior parietals and most of its occipital nevertheless more similar to Tapirus than to other shield, including the basioccipital and supraoccipi- Eocene and early Oligocene tapirs. This similarity tals. The critical narial region, which is the focus of is contrary to the observations presented in an ear- this description, is better preserved on the AMNH lier cranial description of Colodon (Radinsky 1963, skull. The SDNH skull only preserves fragments of figure 21) and supports a different phylogenetic its nasals, and most of the narial opening margins placement for Colodon than previously hypothe- are damaged. sized (e.g., Colbert and Schoch 1998; Dashzeveg Based on dental eruption and suture closures, and Hooker 1997; Holbrook 1999; Radinsky 1963; the AMNH skull is less mature than the SDSM Schoch, 1989b). specimen, and neither represents an adult condi- tion. The immaturity of the AMNH skull is indicated MATERIALS AND METHODS by the incomplete eruption of its permanent premo- lars and molars; M3 is still within its crypt, and it The Specimens retains deciduous premolars. The dental formula Locality Data. F.AM 42891 (specimen from the differs on the left and right sides, most likely a con- Frick American mammals collection at the AMNH, sequence of dP3-4 having been prepared away on hereafter referred to as the AMNH skull) is an iso- the left side. The dental stage is as follows: right lated skull from the Whitneyan of Shannon County, side, P1 missing (adult roots intact), P2, P3-4 in South Dakota. Data from the original specimen crypts, M1-2, M3 in crypt; left side, P1-2, dP3-4 label cites Skinner and Mefferd as the collectors (P3-4 in crypts), M1-2, M3 in crypt (Figure 2). This from northeast of Indian Stronghold on the divide arrangement is consistent with an eruption between west Big Corral Draw and Cottonwood sequence pattern observed in recent tapirs (Col- Creek from a lower Protoceras channel. The label bert 1999). The presence of incisors is inferred mentions that this would be in the upper Oreodon from alveoli, but canines are lacking. The AMNH beds or lower Poleslide Member of the Brule For- specimen’s immaturity is also indicated by the mation of Jim Bump. largely open cranial sutures. For example, the SDSM 59566 (hereafter referred to as the basioccipital is missing, having separated along SDSM skull) is an isolated skull from the Orellan of the open spheno-occipital synchondrosis. South Dakota, collected by Japeth Boyce, who All the adult premolars and M1-2 are in place donated it to the South Dakota School of Mines, on the SDSM skull, but M3 lies within its crypt. I1-2 where it has been on display in the Museum of are preserved on the left premaxilla, and I1 and I3 Geology. Japeth Boyce provided the following on the right premaxilla (Figure 3). As on the AMNH provenance information (Boyce, personal com- skull, canines are lacking. Additional indicators of mun., 2005): specimen collected from the the immaturity of this specimen are the lack of ‘Metamynodon sandstone,’ a channel sand in the fusion of the exoccipitals to the supraoccipitals, lowest Brule Formation located just below the and of the basioccipital to the basisphenoid. lower nodular layer; site approximately midway The Referral of these Specimens to Colodon. between Cottonwood Pass and Stronghold Table. The monophyly of Colodon is suspect, as indicated by a series of phylogenetic analyses performed by 2 COLBERT: FACIAL SKELETON OF AN OLIGOCENE TAPIR Figure 1. Illustration of the horizontal compaction of the AMNH skull, and sagittal compression of the SDSM skull. A. AMNH 42891 in lateral view. B. SDSM 59566 in ventral view. Scale bar equals 10 mm. Colbert (1999). This condition is largely a conse- men might record an evolutionary increase in size quence of the limited material, particularly nonden- within the C. occidentalis lineage over time. tal material that has been referred to Colodon. The slightly smaller dentition of the Orellan Traditionally, Colodon has been identified by its SDSM skull falls within the size range for C. occi- distinctive teeth (e.g., Colbert and Schoch 1998; dentalis, to which it is referred (compare measure- Radinsky 1963). Radinsky (1963) recognized three ments presented in Radinsky 1963 and dental North American species of Colodon (C. kayi, C. measurements for the two skulls provided in Table woodi, and C. occidentalis). 1). Although it cannot be assumed that the two Col- Dental characters are the basis for the referral odon skulls described here belong to a single spe- of the skulls described here to Colodon. These cies, it is clear that they were closely related based similarities include both the degree of molarization on overall morphological similarities. of their premolars, and their lingually displaced and HRXCT Scanning and Image Processing reduced molar metacones (Figures 1, 2; Radinsky 1963). The AMNH skull cannot be unquestionably Both Colodon skulls were scanned at the Uni- referred to any particular species of Colodon, but versity of Texas High-Resolution X-ray CT Facility the SDSM skull falls comfortably within the range (UTCT) using the high-energy subsystem as of C. occidentalis. Species of Colodon were dis- described by Ketcham and Carlson (2001). The criminated by Radinsky (1963) based on size dif- original data sets for both as are saved as1024 by ferences in their teeth. The dentition of the AMNH 1024 pixel TIFF images with a 16-bit gray scale skull was compared favorably to Colodon occiden- depth. The slice thickness for the AMNH skull was talis by Radinsky (1963) who noted, however, that 0.50 mm, and the inter-slice spacing 0.40 mm. The it was generally larger than other C. occidentalis, field of reconstruction was 125 mm, yielding an in- and would also represent a chronostratigraphic plane resolution of 0.122 mm/pixel and 556 slices range extension from the Chadronian and Orellan in the original coronal plane.