Odobenocetops LEPTODON MUIZON, DOMNING & PARRISH 1999
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. Referr...
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2002
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| Summary: | 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 postcranial 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 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). 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 Periotic Maximum width of the skull at the level of the exoccipitals: 123.5 ¥ 2 = 257e Anteroposterior length: 77.5e Maximum width (measured ventrally, approximately perpendicular to anteroposterior length: 42e Maximum width between the pterygoid crests: 142 Width between the apices of the hamular processes: 112 Length of pars cochlearis from contact with anterior process anteriorly to medial edge of aqueductus cochleae (measured dorsally): 29.5 Tympanic Length of the lateral lobe of the left tympanic: 54.2 Atlas Maximum width of the pars cochlearis in medial view: 30.8 Maximum width of the atlas: 174 Length of internal auditory meatus: 14.3 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 Tympanic Length of the involucrum (measured medially): 58.8 Malleus Maximum length in posteromedial view (cf. Fig. 10A): 11.4 Maximum length of the radius: 117 Anteroposterior length of distal end of humerus: 60.1 Mediolateral width of distal end of humerus: 43 Maximum width in posteromedial view (cf. Fig. 10A): 6.05 DESCRIPTION The following description focuses on the differences between the two species of Odobenocetops . We refer to de Muizon et al . (2002) for the aspects of the cranial anatomy common to the two species. GENERAL FEATURES The general morphology of the skull of Odobenocetops leptodon is similar to that of O. peruvianus in its basic pattern (Figs 1–4). However, in dorsal view, the former differs from the latter in the morphology of the snout, which is much wider and more blunt. In anterior view the snout was more massive. Furthermore, if a melon was actually present, as hypothesized below, it is likely that the head of O. leptodon was more rounded than that of O. peruvianus . The skull of O. leptodon has the same bilobate outline as is observed in O. peruvianus, which is unique in a Neogene odontocete (de Muizon et al ., 2002). This condition, where the frontals and maxillae do not overhang the temporal fossa, is found in primitive odontocetes such as Xenorophus , Agorophius and Archaeodelphis and in archaeocetes because of the lesser telescoping of the skull (Kellogg, 1936; Fordyce, 1981). In Odobenocetops it is regarded as a reversal of this typically cetacean telescoping. In general, the skull of O. leptodon is very heavily built and the bone is much denser, thicker, and more solid than in other delphinoids, being almost osteosclerotic in its posteroventrolateral angle. PREMAXILLA The premaxillae of O. leptodon are built on the same pattern as those of O. peruvianus (de Muizon et al ., 2002). They both present a large alveolar process housing one tusk, the right tusk (of the presumed male, which is the only sex represented) being much larger than the left. Since the left alveolar process is not preserved on the holotype of O. peruvianus (the only specimen known when this species was first described) and because there are no other mammals which exhibit pronounced cranial asymmetry, it was hypothesized that the processes sheathing the tusks were both of the same size and that, consequently, the left tusk (which was judged to be shorter than the right sheath) was not erupted. The holotype of O. leptodon, however, indicates that the left sheath was clearly smaller than the right and that the left tusk was actually erupted. It is therefore probable that the condition of O. leptodon was also present in O. peruvianus . In the holotype of O. leptodon , the left sheath is transversely flattened and lies almost in a parasagittal plane (it diverges only slightly laterally). The right sheath is thicker because of the larger size of the tusk and diverges more markedly laterally. The region of the sheaths anterior to the tusks is strongly flattened and forms a posteriorly bent U-shaped lamina which borders the palate anteriorly. The anteroventral edge of this lamina is 0.5–1 cm thick, and presents longitudinal irregular ridges of bone to which was firmly attached the strong upper lip that was very probably present in O. leptodon as in O. peruvianus . The anterior and anterolateral side of each premaxillary sheath is a broad convex surface bearing several large grooves and foramina. These structures indicate an extensive vascularization of the anterior region of the skull, which is regarded here as related to the presence of a thick, muscular upper lip. In anterior view, in the sagittal plane, the snout is more elevated than in O. peruvianus and the surface of attachment of the upper lip is larger. Furthermore, the snout is wider in anterior view, and the anterior edge of the palate is grossly U-shaped rather than more Vshaped as in O. peruvianus . Therefore it is clear that the upper lip of O. leptodon was much larger than in O. peruvianus, indicating a larger vibrissal array and/or better suction ability in the former than in the latter. Dorsally, the morphology of the premaxillae of O. leptodon presents the major characters which differentiate this species from O. peruvianus . These bones are much wider than in O. peruvianus, and the right premaxilla is more expanded laterally because of the greater size of the tusk (Figs 2A and 3A). There are no enlarged premaxillary foramina as are observed in the other odontocetes, including O. peruvianus . This absence is regarded as a loss in O. leptodon which, in this respect, is more derived than O. peruvianus . The premaxillary foramina transmit branches of the internal maxillary artery and maxillary nerve from the infraorbital foramen (internal to the orbit) to parts of the nasofacial muscles anteriorly and posteriorly on the rostrum (see e.g. Schenkkan, 1973: fig. 5, on Mesoplodon bidens ). In Odobenocetops these vessels and nerves are probably related to the irrigation and innervation of the strong upper lips and associated musculature as well as the possible vibrissae. In O. leptodon the canals conducting the nerves and vessels do not open dorsally but are clearly present. This is observed on the referred skull MNHN SAO 202 (Fig. 3A), in which the apex of the snout is eroded above the pulp cavity of the tusks. The canals passed above the proximal extremities of the tusks and apparently (clearly observable on the right side) divided into several smaller canals perforating the premaxillae to reach the upper lip anteriorly. Ramifications of the premaxillary canals are also observable on the lateral side of the premaxilla at the level of the antorbital notch, where several large foramina are present. It is likely that irrigation and innervation for the pulp cavities of the tusks were also transported by the premaxillary canals. The premaxillae of O. leptodon bear a small concave area, anterior to the bony nares, where lay the premaxillary sacs of the air sac system (Mead, 1975). These fossae for the premaxillary sacs are limited medially by the medial border of the premaxilla which overhangs the vomerian gutter; they are limited anteromedially by the suture with the rostral supplementary bone (see below); and anterolaterally they are medial to the posterior edge of each tusk alveolus (Figs 2A, 3A and 5). Fossae for premaxillary sacs are absent in O. peruvianus, where the space between the large premaxillary foramina and the anterior edge of the nares is so reduced that it was supposed that the premaxillary sacs were either very reduced or absent in this species (de Muizon et al ., 2002). On the dorsal face of the skull the premaxillae of O. leptodon extend further posteriorly than in O. peruvianus . The posterior extremity of the left premaxilla is damaged on MNHN SAO 202, but it is clear that it lay slightly posterior to the posterior border of the nares. The posterior extremity of the right premaxilla lies about 5 cm posterior to the nares. On the holotype of O. peruvianus, the posterior extremity of the left premaxilla is located half-way between the anterior and posterior limits of the naris and on the right side (as well as on both sides of SMNK PAL 2491) it reaches the posterior edge of the naris. The Pmx-Mx suture is external to the nares, the border of which is formed by the maxillae only, in contrast to the condition of O. peruvianus in which the Pmx-Mx suture is internal to the nares in the anterior two-thirds of their length. In their dorsal portions the premaxillae of O. leptodon are relatively flat and not as salient as is observed in O. peruvianus . This feature is clearly observed on the lateral edges of the nares and at the level of the antorbital notch, where the premaxillae markedly overhang the maxillae in O. peruvianus , whereas the relief of these regions is much smoother in O. leptodon . The anterior sides of the premaxillae of O. leptodon (SMNK PAL 2492) bear strong foramina and grooves, which indicates an abundant vascularization related to the development of the upper lip. The premaxillae are much broader and much more elevated than in O. peruvianus , again indicating a larger and stronger upper lip (Figs 1B and 3C). The premaxillary sheaths, which house the tusks, are partly preserved on SMNK PAL 2492. The right is complete and the left is broken, apparently no more than 2 or 3 cm from the apex. Contrary to what was hypothesized in the case of O. peruvianus, the sheaths are not identical in size; the left one is clearly smaller and shorter than the right and reflects the size difference between the tusks. In lateral view, the right alveolar process forms an angle of c . 60° with the dorsal plane of the skull (from the vertex to the anterodorsal tip of the rostrum). The anteroventral edges of the sheaths (which include the anterior edge of the palate) are thickened and corrugated, as is observed in O. peruvianus , which indicates attachments for strong, possibly horny, connective tissue for the attachment of the upper lip. This narrow edge also bears (in both species) a series of foramina, which could have been related to a fringe of vibrissae. In the style of feeding we have hypothesized, these could have allowed the animal to detect when the front edges of the sheaths were in contact with the sea floor. The anteroventral crest of the premaxillae (Figs 1B and 3C) has a very wide U-shape whereas it is more V-shaped in O. peruvianus . This morphology is the result of the widening of the snout and is possibly related to the presence of the supplementary bones described below. The U-shaped anterior border of the palate is not perpendicular to the sagittal plane. In other words, in anterior view, if the sagittal plane is positioned vertically (based on pterygoids, basioccipital and vomerian groove), the plane of the anterior crest of the palate dips towards the right side of the skull. This is because the anteroventral edge of the smaller and less laterally divergent left sheath is dorsal to (more elevated than) that of the right sheath. This condition implies that when the animal was feeding (i.e. with the anterior border of the palate parallel to the sea floor; see de Muizon, 1993b and de Muizon et al ., 2002), the sagittal plane was inclined toward the left side of the animal (i.e. dipping toward the right side). Therefore, because of the asymmetry of the premaxillae, O. leptodon was feeding slightly inclined on left side. The premaxillae form the anterior region of the palate as is observed in O. peruvianus . Although the palate of O. leptodon is much broader than in O. peruvianus , the area occupied by the premaxillae in this part of the skull is only slightly larger than in O. peruvianus (see below). ROSTRAL BONES One of the most striking differentiating characters of O. leptodon is the presence of two supplementary bones (one on each side) anterior to the premaxillae at the anterodorsal apex of the snout. These bones are apparently appressed against the medial edges of the premaxillae and lie partly on the dorsal part of the skull and partly on the anterior slope of the snout. The dorsal portion is narrower than long but broadens anteriorly. It is limited posteriorly by the fossa for the premaxillary sac of the premaxilla, while medially it has a small contact with the medial portion of the maxilla on the dorsal face of the rostrum (Figs 1B, 2A, 3A and 5). Within the vomerian groove, the rostral supplementary bone is bordered by the medial portion of the maxilla, but the edge of the groove is formed by the rostral bone itself. The lateral suture is directed anteroposteriorly and is located just medial to the tusk alveoli. The anterior portion is not observable on MNHN SAO 202 but it is in relatively good condition on the left side of SMNK PAL 2492. It is also relatively narrow and forms a strip of bone overlying the premaxilla, tapering ventrally, and reaching (or almost reaching) the ventromedial border of the premaxilla at the anterior edge of the palate. Its medial suture is with the vomer. These supplementary bones are in fact wedged (as far as the dorsal portion is concerned) between the tusks laterally and the vomer and maxilla medially. They are partly responsible for the breadth of the snout in O. leptodon and tend to separate the proximal extremities of the tusks and alveoli (i.e. the premaxillae). As a probable consequence of their presence, the tusks are less divergent distally than in O. peruvianus and the anterior border of the palate is Ushaped in O. leptodon rather than more V-shaped as in O. peruvianus . Apparently, as is observed on the left premaxilla of SMNK PAL 2492, the rostral bones were anterodorsally protruding, forming an elevated prominence at the apex of the snout. This condition is not observable on MNHN SAO 202 since the bones have been partly eroded in their apical portions. These supplementary bones could be ossa prenasalia, cartilage bones which appear late in development (mainly in postnatal life) and which are not uncommon in mammals. They are paired or unpaired bones which articulate with the nasals or premaxillae and which are always related to the presence of a rhinarium ( Rhynchocyon , Sus , Talpa , Chiroptera) or a proboscis (Starck, 1967). In the case of O. leptodon the supplementary bones present at the anterodorsal apex of the snout could be related to the thick and powerful upper lip used by Odobenocetops to forage on the bottom in search of food (de Muizon, 1993a;b; de Muizon et al ., 2002). They could also indicate of the presence of a very sensitive tactile organ, with or without vibrissae (see de Muizon, 1993a), much more developed in O. leptodon than in O. peruvianus . It is likely that strong muscles for the upper lip of O. leptodon were attaching on the (?)prenasalia. However, it is noteworthy that the prenasalia of other mammals are generally not tightly articulated with the premaxillae and/or the nasals, and not distinctly paired. If these supplementary bones are not prenasalia, another alternative which could explain the condition of O. leptodon is the presence of septomaxillae. These bones are present in crossopterygians, lissamphibians, squamates, in all nonmammalian therapsids, and in all the nontherian mammals except multituberculates. The septomaxilla is absent in marsupials and placentals except in Xenarthra (Wible, 1990; Zeller et al ., 1993). However, we are reluctant to interpret the supplementary bones of O. leptodon as septomaxillae, since the study of the development of the nasal capsule of odontocetes (Klima & Van Bree, 1985; Klima et al ., 1986; Klima, 1987) does not show any indication that such bones are present at any developmental stage in this group. A third alternative is to regard the supplementary bones of O. leptodon as neomorphic elements different from prenasalia and septomaxillae, possibly related to the muscle attachment of the lip, which was probably much stronger than in O. peruvianus . TUSKS The tusks of O. leptodon are similar to those of O. peruvianus (Figs 1A,B). They are much better preserved in SMNH PAL 2492 than in the ho : Published as part of 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, pp. 423-452 in Zoological Journal of the Linnean Society 134 (4) on pages 425-432, DOI: 10.1046/j.1096-3642.2002.00015.x, http://zenodo.org/record/4687167 : {"references": ["de Muizon C, Domning DP, Parrish M. 1999. Dimorphic tusks and adaptive strategies in a new species of walrus-like dolphin (Odobenocetopsidae) from the Pliocene of Peru. Comptes-rendus de l'Academie des Sciences, Paris, Sciences de la Terre et des Planetes 329: 449 - 455.", "de Muizon C, Bellon H. 1980. L'age mio-pliocene de la Formation Pisco (Perou). Comptes Rendus de l'Academie des Sciences, ser. D 290: 1063 - 1066.", "de Muizon C. 1981. Les vertebres fossiles de la formation Pisco (Perou). 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