T Odobenocetops peruvianus, the Walrus-Convergent Delphinoid (Mammalia: Cetacea) from the Early Pliocene ofPeru Christian de Muizon, Daryl P. Domning, andDarlene R. Ketten etops and the tusks ofOdobenus was as orientation guides in feed­ ABSTRACT ing. This reopens the question of whether the tusks of walruses playa role in feeding, as it seems that these also may be useful as Odobenocetops peruvianus Muizon, 1993 (early Pliocene, orientation guides for the mouth and vibrissal array. southern Peru), is a bizarre cetacean that is convergent in its skull, general aspect, and presumably feeding habits with the modem walrus Odobenus rosmarus (Linnaeus). Its cranial specializations are unique among cetaceans and include loss of the elongated ros­ Introduction trum, development of large premaxillary processes housing asym­ '.ctrical tusks, forward migration ofthe bony nares, reversal of the Abundant remains of fossil odontocete cetaceans have been typical cetacean telescoping of the skull, dorsal binocular vision, found in the rocks of the Pisco Formation near the southern large vaulted palate, and an inferred upper lip. The structure of the coast of Peru. Although the occurrence of cetacean bones in basicranium (possession of palatine expansions of the pterygoid this area has been known for more than one hundred years (Lis­ sinus and presence of a large cranial hiatus) and face (possession of a medial portion of the maxillae at the anterior border of the son, 1890), the first odontocete described from this formation nares) indicates that it belongs to the odontocete infraorder Del­ was Incacetl/s brogii Colbert, 1944. Subsequently, Hoffstetter phinida and to the superfamily Delphinoidea. Within this group (1968) was the first to show the importance of the faunal as­ Odobenocetops is related to the Monodontidae because of the lat­ semblage (fish, reptiles, birds, and mammals) of the locality of eral lamina of its palatine flooring the optic groove, the anteropos­ terior elongation of the temporal fossa, and the thickness of the Sacaco, in the southern outcrops of the Pisco Formation, 540 'isphenoid and squamosal in the region of the foramen ovale. We km south of Lima. Further studies by Muizon (1981, 1983a, hypothesize that Odobenocetops, like the walrus, fed upon shal­ 1983b, 1983c, 1984, 1986, 1988), Pilleri (1989, 1990), and low-water benthic invertebrates and probably used its tongue and Cheneval (1992) have described part of the vert,ebrate fauna of upper lip jointly in extracting the, soft parts of bivalves or other the Pisco Formation, but abundant material still remains un­ invertebrates by suction. The highly modified morphology of the rostrum indicates that there was no melon as in all other odonto­ studied (work in progress includes that ofC. de Muizon and G. cetes, and therefore that Odobenocetops was probably unable to McDonald on mammals, and 1. Cheneval on birds). As estab­ echolocate; binocular vision could have compensated for this lished by Marocco and Muizon (1988), the Sacaco vertebrate inability. The most probable function of the tusks themselves was fauna was deposited under shallow waters in a littoral and social, as ir the living walrus, but we suggest that the historically ',rimary function ofboth the premaxillary processes ofOdobenoc- beach environment. The preservation of the fossils is excep­ tional, and associated skeletons are not rare; both characteris­ tics indicate calm waters. Christian de Muizon, UMR 8569 CNRS, Laboratoire de Paleontolo­ In 1990, the skull of an unexpected walrus-like cetacean gie, Museum National d'Histoire Naturel/e, 75005 Paris, France. (Muizon, 1993a) was found in the locality called "Sud-Sacaco" Email: mui::[email protected]. Daryl P. Domning,Research Associ­ by Muizon (198 I). In the Sacaco area, Muizon (1981, 1984, ale, Department ofPaleobiology, National Museum ofNatural His­ 1988a), Muizon and DeVries (1985), and Muizon and Bellon lory, SWithsonial/ II/stitutiol/, Washingtol/, D.C. 20560-0121, and LaboratoJ)' o/Evolutionary Biology, Department ofAnatomy, College (1986) recognized five vertebrate horizons, which range from of}vledicine, Howard University, Washington, D.C. 20059, United the late Miocene to the late Pliocene (approximately 9 Ma to 3 Slates. En/ail: [email protected]. Darlene R. Ketten, Massa- Ma). The specimen came from the Sud~Sacaco (SAS) horizon, husetts Eye and Ear Infirmary, Harvard Medical School, Boston, earliest Pliocene, which has yielded an abundant vertebrate .>fassachusetts 02114, United States. Email: [email protected]. fauna (Muizon, 1981, 1984). Cetaceans are represented there 223 224 SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY by balaenopterids, cetotheriids, pontoporiids (Pliopontos lit­ Genus Odobenocetops Muizon (1993a) toratis), ziphiids (Ninoziphius platirostris), and phocoenids TYPE SPECIES.-Odobenocetops peruvianus Muizon (1993). (Piscolithax longirostris). The walrus-like cetacean, named DIAGNOSls.-As for the species. Odobenocetops peruvianus, was briefly described by Muizon (1993a, 1993b), who concluded that its feeding adaptations Odobenocetops peruvianus Muizon (1993a) were convergent with those of the walrus. Herein we describe that specimen more thoroughly and present a more detailed FIGURES 1-18, TABLE I study of its features as a foundation for observations on the pa­ TYPE SPECIMEN.-USNM 488252 (originally numbered leobiology of this unique cetacean. Its phylogenetic relation­ USNM 460306), an incomplete skull lacking much of the left ships with delphinoid odontocetes are confirmed. side and all ofthe auditory bones. INSTITUTIONAL ABBREVIATIONS.-The following abbrevia­ REFERRED SPECIMENS.-MNHN SAS 1613, a left periotic tions for institutions have been used: and partial tympanic; and MNHN SAS 1614, a left periotic. MNHN Museum National d'Histoire NatureIle, Paris One other, recently discovered specimen (MNHN SAO 202) NMNH National Museum of Natural History, Smithsonian Institution, is relevant to this study but is not described herein in detail; it Washington, D.C. will be described in a separate report (Muizon andDomning, SMNK Stnatliches Museum fUr Naturkunde, Karlsruhe, Gcnnany USNM Collections of the NMNH, which include those of the former 2002). It is a badly weathered partial skeleton with a partial United States National Museum skull, including the left tympanic, periotic and asymmetrical tusk; the distal half of a humerus; a complete radius; several ACKNOWLEDGMENTS.-This research was supported by dorsal and caudal vertebrae; and rib fragments. It was found in funds from the Centre National de la Recherche Scientifique the SAO horizon of the Pisco Formation (ca. 3.5 Ma), which is (CNRS), Paris, in 1991; by two short-term visitor awards from younger than the SAS horizon (ca. 5 Ma), and is provisionally the Smithsonian Institution, Washington D.C., in 1992 and referred herein to Odobenocetops sp. (see "Addendum," be­ 1993; and by funds from the Institut Franyais d'Etudes Andines low; Muizon et aI., 1999). (Lima, Peru) in 1994 (to C. de Muizon). CT scanning and peri­ ETYMOLOGY.-From odon, tooth, and baino, walk (Greek); otic analyseswere supported by Office ofNaval Research con­ and from cetus (Latin, from Greek ketos), whale, and ops tracts NOOOI4-2-J-4000 and NOOOI4-93-1-0940 (to D.R. (Greek, masculine), like: "the cetacean that seems to walk on \ Ketten) and by the Three-Dimensional Head and Neck Imaging its teeth," to refer to its feeding position (see below) and also to n Service, Department of Otolaryngology, Massachusetts Eye its similarity to the walrus (Odobenus). Species: peruvianus, s and Ear Infirmary. Beluga periotics were provided by S. Ridg­ from Peru. r way, Naval Research and Development (NRD, San Diego. We DIAGNOsls.-Delphinoid cetacean characterized by loss of s' thank C. Martin, who found the specimen, and P.L. Larson, elongate cetacean rostrum and concomitant probable absence of melon; development of large, ventrally directed premaxil­ who collected it and donated it to the Smithsonian Institution. lary alveolar processes housing straight tusks; right tusk longer P. Kroehler prepared the specimen. Illustrations, including the J than 55 cm, cylindrical, with oval section, with long, open pulp life reconstruction (Figure 20), are by Mary Parrish (NMNH); s cavity 23 cm deep; left tusk probably unerupted and probably s photographs are by Vic Krantz and Chip Clark (both NMNH), no longer than 20 cm, with short conical pulp cavity 1.5 cm a' and D. Serrette (MNHN). Irina Koretsky helped greatly with deep; important muscular insertion and numerous neurovascu­ d the illustrations. Valuable insights through discussion were pro­ lar foraminaon anterior side ofpremaxillae and on anteriorex­ r vided by L.G. Barnes, T.A. Demere, R.E. Fordyce, J.G. Mead, tremity of palate, suggesting presence of strong upper lip and c C.E. Ray, J.S. Reidenberg, and A. Werth. Ray and Werth also possible vibrissae; regression in telescoping of skull by for­ provided detailed formal reviews of the manuscript. ward migration of nares and anterior withdrawal of maxillae We are pleased to dedicate this paper to Clayton E. Ray, who and frontals; nasal on vertex of skull, contacting occipital and l invited us to study the specimen under his care andwho greatly right maxilla, lying upon frontals but separated from meseth­ I helped and encouraged us in our work. moid, which followed bony nares in their forward migration; t large temporal fossa open dorsally; large dorsal exposure ofpa­ Systematic Paleontology rietals and concomitant development