Odobenocetops, Muizon, 1993

Muizon, Christian de & Domning, Daryl P., 2002, The anatomy of Odobenocetops (Delphinoidea, Mammalia), the walrus-like dolphin from the Pliocene of Peru and its palaeobiological implications, Zoological Journal of the Linnean Society 134 (4), pp. 423-452 : 444-445

ODOBENOCETOPS

O. peruvianus and O. leptodon have well-defined stratigraphic relationships, since the former is from the SAS horizon of the Pisco Formation, which is earliest Pliocene in age, and the latter is from the SAO horizon which is stratigraphically younger in the early Pliocene. All four specimens were found in an area no larger than 5 km 2. Since Odobenocetops is, apparently, endemic to the Pacific coast of South America, one could expect an ancestor–descendant relationship between the two species.

Several features of O. leptodon are indeed more derived than in O. peruvianus . O. leptodon is more specialized than O. peruvianus in its larger, longer and wider palate, in the elevation and breadth of the snout, in the loss of the premaxillary foramina, and in the presence of rostral supplementary bones. Furthermore, a comparison of the sizes of the tusks in the holotypes of the two species seems to corroborate this hypothesis. The holotype of O. peruvianus is 46 cm long in the sagittal plane from the anterior edge of the palate to the occipital condyle, and that of O. leptodon is 41.5 cm. In the holotype of O. peruvianus , the diameters of the large tusk at the alveolar border are 38 mm and 31 mm. In the holotype of O. leptodon , which is slightly smaller, these diameters are 50 mm and 36 mm. If the length of the tusk relative to the diameters is the same in the two species, and considering that the total length of the long tusk of the holotype of O. leptodon is 135 cm, then the long tusk of the holotype of O. peruvianus should have been 102 cm long (based on the larger diameter). If a correction is made because of the smaller size of the holotype of O. leptodon , then the long tusk of O. peruvianus for a skull of the size of that of the holotype of O. leptodon would be 92 cm long. In other words, the long tusk of O. peruvianus was proportionally c. 30% shorter than in O. leptodon . A similar comparison can be made on the diameter of the small tusk. In the holotype of O. peruvianus the diameters of the small tusk at base are 24 mm and 30 mm, while in O. leptodon they are 18 mm and 22 mm. The estimated total length of the small tusk (partly reconstructed) of the holotype of O. leptodon is 25 cm. Therefore an estimate (based on the larger diameters) of the small tusk of the holotype of O. peruvianus would approach 34 cm. If a correction is made relative to the length of the skull, then the length of the small tusk of O. peruvianus should approach 31 cm (for a skull of the size of the holotype of O. leptodon ). The small tusk of O. peruvianus was therefore approximately 24% longer than that of O. leptodon . Furthermore, these comparisons indicate that the small tusk was approximately 18% the size of the long tusk in O. leptodon and approximately 33% in O. peruvianus . Therefore, assuming that the plesiomorphic condition was represented by smaller tusks subequal in size, then O. peruvianus is clearly more plesiomorphic than O. leptodon for these character states. If the approximations made above are correct, then an ancestor–descendant relationship between the two species is consistent with these characters.

In other respects, however, the earlier species appears to be more derived than the younger. de Muizon et al. (2002) have regarded the absence of fossae for the premaxillary sacs on the premaxillae anterior to the nares and the inferred lack (or extreme reduction) of the melon as derived conditions in O. peruvianus since such structures are present in all the other delphinoids. In O. leptodon , anterior to the nares, the premaxillae present distinct shallow oval fossae which are very probably homologous to the fossae for premaxillary sacs in the other odontocetes. Since the presence of premaxillary sacs is closely related to the presence of a melon ( Mead, 1975), it is probable that O. leptodon had a melon. This hypothesis is reinforced by the fact that the dorsal surface of the snout (between the nares and the apex) is much wider and larger in O. leptodon than in O. peruvianus in which there is just no space for a melon. Therefore, O. leptodon , which probably still retained a melon and premaxillary sacs, is more primitive than O. peruvianus , which had no melon or premaxillary sacs or only vestigial ones.

Furthermore, the deeply notched anterodorsal border of the orbit is a very specialized feature allowing good dorsal binocular vision, which probably compensated for the lack of echolocation ability (a consequence of the probable absence of a melon). In O. leptodon , the anterodorsal border of the orbit is slightly concave and is regarded as less derived than in O. peruvianus . Therefore, because of the probable presence of a melon and premaxillary sacs and because of the morphology of the anterodorsal edge of the orbit, O. leptodon is more primitive than O. peruvianus for this character. An ancestor–descendant relationship seems ruled out and the two species must be viewed as sister taxa.