<i>Odobenocetops peruvianus</i>, the Walrus-Convergent Delphinoid (Mammalia: Cetacea) from the Early Pliocene of Peru

Abstract

<i>Odobenocetops peruvianus</i> Muizon, 1993 (early Pliocene, southern Peru), is a bizarre cetacean that is convergent in its skull, general aspect, and presumably feeding habits with the modem walrus <i>Odobenus rosmarus</i> (Linnaeus). Its cranial specializations are unique among cetaceans and include loss of the elongated rostrum, development of large premaxillary processes housing asymmetrical tusks, forward migration of the bony nares, reversal of the typical cetacean telescoping of the skull, dorsal binocular vision, large vaulted palate, and an inferred upper lip. The structure of the basicranium (possession of palatine expansions of the pterygoid sinus and presence of a large cranial hiatus) and face (possession of a medial portion of the maxillae at the anterior border of the nares) indicates that it belongs to the odontocete infraorder Delphinida and to the superfamily Delphinoidea. Within this group <i>Odobenocetops</i> is related to the Monodontidae because of the lateral lamina of its palatine flooring the optic groove, the anteroposterior elongation of the temporal fossa, and the thickness of the alisphenoid and squamosal in the region of the foramen ovale. We hypothesize that <i>Odobenocetops</i>, like the walrus, fed upon shallow-water benthic invertebrates and probably used its tongue and upper lip jointly in extracting the soft parts of bivalves or other invertebrates by suction. The highly modified morphology of the rostrum indicates that there was no melon as in all other odontocetes, and therefore that <i>Odobenocetops</i> was probably unable to echolocate; binocular vision could have compensated for this inability. The most probable function of the tusks themselves was social, as in the living walrus, but we suggest that the historically primary function of both the premaxillary processes of <i>Odobenocetops</i> and the tusks of <i>Odobenus</i> was as orientation guides in feeding. This reopens the question of whether the tusks of walruses play a role in feeding, as it seems that these also may be useful as orientation guides for the mouth and vibrissal array.