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 : 424-426

ODOBENOCETOPS MUIZON, 1993

Type species: Odobenocetops peruvianus Muizon, 1993 . Emended diagnosis: delphinoid cetacean which does not possess the elongated rostrum present in all other cetaceans. The rostrum is short, rounded, and blunt; it is formed by the premaxillae which are greatly enlarged and have large, asymmetrical ventral alveolar sheaths holding sexually dimorphic tusks. The right tusk of the male is large and can reach one metre or more in length. It is cylindrical and its section is mediolaterally compressed (the long axis measures c. 4–5 cm). The left tusk of the male was only between 20 and 25 cm long, of which a few cm were erupted. Its section is also mediolaterally compressed. Both tusks are straight. In the female, the right tusk is only slightly larger than the left and both protruded only a few cm from the gum. Premaxillary sheaths and tusks form an angle of approximately 60–70° with the dorsal plane of the skull. The bony nares have been displaced anteriorly and are anteroposteriorly elongated. The palate is very deep and arched, and its anterior border (which is formed by the anterior edges of the alveolar sheaths) is U- to V-shaped. On the palate the vomer is very large and the premaxillae and the pterygoids are in contact with it. The maxillae have been excluded from the palate and relegated to a small triangular area on the lateral side of the rostrum and an elongated strip on the dorsal face of the skull. The postorbital processes are large and lateral to the posterior end of the nares. The orbits face anterolaterally and dorsally. The portions of the frontals and maxillae which cover the temporal fossae in the other odontocetes have been reduced and narrowed in such a way that the temporal fossae are opened dorsally. Ventrally, the crest of the hamular processe of the pterygoid is strong and rounded and the choanae are very wide. The basioccipital is extremely wide and long. The glenoid cavity of the squamosal is an anteroposteriorly elongated groove. The occipital condyles are very salient and more convex than in any other Pliocene or living cetacean.

ODOBENOCETOPS LEPTODON MUIZON, DOMNING & PARRISH, 1999

( FIGS 1–12 , TABLES 1 AND 2)

Holotype: SMNK PAL 2492 , an incomplete skull (lacking most of the dorsal and right sides of the braincase, right orbit, and right side of the right alveolar sheath), with the right ear bones and the atlas.

Referred specimen: MNHN SAO 202, partial skeleton with partial skull lacking the right tusk and with the alveolar sheaths very damaged. Most of the braincase has been weathered away but the left periotic and a partial tympanic are preserved in situ. The postcra-

Skull

Basicondylar length: 415e

Bizygomatic width: 150 ¥ 2 = 300e

Minimum width of the skull posterior to the postorbital processes: 70 ¥ 2 = 140e

Maximum width between the postorbital processes:

130 ¥ 2 = 260

Maximum width of both nares: 59e

Maximum length of the left naris: 53.5e

Width of the choanae: 104

Maximum width of the skull at the level of the exoccipitals: 123.5 ¥ 2 = 257e

Maximum width between the pterygoid crests: 142 Width between the apices of the hamular processes: 112

Tympanic

Length of the lateral lobe of the left tympanic: 54.2 Atlas

Maximum width of the atlas: 174

Maximum height of the atlas at sagittal plane: 106 Maximum vertical diameter of neural canal: 64.5 Maximum horizontal diameter of neural canal: 56.5 Limb

Maximum length of the radius: 117

Anteroposterior length of distal end of humerus: 60.1 Mediolateral width of distal end of humerus: 43

nial skeleton is very poorly preserved, comprising only a few hyoid fragments, 26 partial vertebrae, several rib fragments, the distal half of the left humerus, most of the left radius, a proximal portion of the ulna lacking the olecranon, and fragments of carpals and metacarpals. MNHN PPI 249, a right periotic is referred to O. cf. leptodon .

Etymology of the species name: from Greek: leptos (thin) and odon (tooth), in reference to the needle-like morphology of the large right tusk.

Diagnosis: Odobenocetops leptodon differs from O. peruvianus in the morphology of the snout, which is more rounded and wider in dorsal view; in the lack of large premaxillary foramina; in the presence of small depressions for premaxillary sacs in the premaxillae, anterior to the bony nares; in the presence of supplementary rostral bones at the anterodorsal apex of the snout; in the broader, deeper, and longer palate; in the anterior border of the palate, which is more U-shaped, while it is more V-shaped in O. peruvianus ; and in the straight (or slightly concave) anterodorsal border of the orbit, which is deeply concave in O. peruvianus .

Locality, Horizon and Age: the holotype and MNHN SAO 202 are from the SAO level of the Pisco Formation in Peru (at approximately km 540 of the Panamerican Highway south of Lima). The SAO horizon has been defined by de Muizon & Bellon (1980) but see also de Muizon (1981), de Muizon & DeVries (1985), and de Muizon & Bellon (1986). Its age spans approximately from 4 to 3 Ma; therefore, the specimens are early Pliocene in age. The SAO level of the Pisco Formation

Periotic

Anteroposterior length: 77.5e

Maximum width (measured ventrally, approximately perpendicular to anteroposterior length: 42e

Length of pars cochlearis from contact with anterior process anteriorly to medial edge of aqueductus cochleae (measured dorsally): 29.5

Maximum width of the pars cochlearis in medial view: 30.8

Length of internal auditory meatus: 14.3

Tympanic

Length of the involucrum (measured medially): 58.8

Malleus

Maximum length in posteromedial view (cf. Fig. 10A ): 11.4

Maximum width in posteromedial view (cf. Fig. 10A ): 6.05

is slightly younger than the SAS level (5–4 Ma), which yielded the holotype of O. peruvianus and the third specimen referred to here as a female of O. peruvianus . MNHN PPI 249, an isolated periotic, is from beds of the Pisco Formation located at c. 3 km east of Yauca (these beds may be slightly younger than those of the SAO horizon and may belong to the late Pliocene).

Figures

Figure 1. Odobenocetops leptodon, holotype (SMNK PAL 2492). Skull: left ventrolateral view (A), anterior view (approximately parallel to the feeding plane) (B).

Figure 2. Odobenocetops leptodon, holotype (SMNK PAL 2492). Skull: dorsal (A), ventral (B), and lateral (left side) (C) views.

Figure 3. Odobenocetops leptodon, referred specimen (MNHN SAO 202). Skull in dorsal (A), ventral (B), and anteroventral (C) views.

Figure 4. Odobenocetops leptodon, referred specimen (MNHN SAO 202). Skull in lateral left (A) and lateral right (B) views.

Figure 5. Reconstruction of the dorsal view of the rostrum of Odobenocetops leptodon. Abbreviations: Fps, fossa for the premaxillary sac; Fr, frontal; Mx, maxilla; NaF, narial fossa; Pmx, premaxilla; Sb, supplementary bone.

Figure 6. Auditory region of Odobenocetops leptodon, holotype (SMNK PAL 2492). Lateral view (A) and posterolateral view showing the tympanic aperture (B). On B, the outline of the tympanic aperture has been enhanced with a white line.Abbreviations:Ap, anterior process of the periotic; Bo, basioccipital; Cho, choanae; Eam, external auditory meatus; Fo, foramen ovale; Gf, glenoid fossa; Llpt, lateral lamina of the pterygoid; Oc, occipital condyle; Pa, palatine; Ptc, pterygoid crest; Ptp, post-tympanic process of the squamosal; po, paroccipital process of the exoccipital; Pts, pterygoid sinus; Sp, sigmoid process of the tympanic; Tym, tympanic.

Figure 7. Odobenocetops leptodon, referred specimen (MNHN SAO 202). Periotic in dorsal (A), lateral (B), ventral (C), and medial (D) views. Abbreviations: ac, aqueductus cochleae; ap, anterior process; av, aqueductus vestibuli; dfc, dorsal opening of the facial canal; ef, fovea epitubaria; fc, fenestra cochleae; fi, fossa incudis; fm, fossa mallei (= epitympanic recess); iam, internal auditory meatus; pbf, posterior bullar facet; pc, pars cochlearis (= promontorium); pp, posterior process; slf, sulcus facialis; so, supernumerary ossicles; st, stapes; stf, fossa for stapedial muscle; ts, tractus spiralis; vfc, ventral opening of the facial canal; vr, ventral rim of the anterior process.

Figure 8. Odobenocetops cf. leptodon, referred specimen (MNHN PPI 249). Periotic in dorsal (A), lateral (B), ventral (C), and medial (D) views.

Figure 9. Odobenocetops leptodon, referred specimen (MNHN SAO 202). Partial tympanic in ventral (A) and medial (B) views.

Figure 10 Odobenocetops leptodon, referred specimen (MNHN SAO 202). Malleus in posteromedial view (A); incus in ventral view (B). Abbreviations: ast, articular facet for the stapes; cb, crus breve; cl, crus longum; icf, articular facets for the incus; mn, manubrium; pm, processus muscularis; tu, tuberculum.

Figure 11. Odobenocetops leptodon, holotype (SMNK PAL 2492). Atlas in anterior (A), posterior (B), dorsal (C), ventral (D), lateral right (E), and lateral left (F) views.

Figure 12. Odobenocetops leptodon, referred specimen (MNHN SAO 202). Left forelimb in lateral (A) and medial (B) views.