Ontocetus emmonsi Leidy, 1858

Boessenecker, Sarah J., Boessenecker, Robert W. & Geisler, Jonathan H., 2018, Youngest record of the extinct walrus Ontocetus emmonsi from the Early Pleistocene of South Carolina and a review of North Atlantic walrus biochronology, Acta Palaeontologica Polonica 63 (2), pp. 279-286 : 282

Ontocetus emmonsi Leidy, 1858

Fig. 2 .

Material.— CCNHM 1144 , a complete left canine, collected by Matthew Swilp (Shamokin, Pennsylvania, USA) in the 1990s from the?Waccamaw Formation exposed in the now-inundated Austin sand pit, Ridgeville, South Carolina, USA. A second tusk of Ontocetus emmonsi (SC 98.51.1) was referred by Kohno and Ray (2008) but not figured or described and is not considered further in this study .

Description. — CCNHM-1144 is a complete, well-preserved left canine (linear length 369 mm; anterior curvilinear length 345 mm; posterior curvilinear length 477 mm), which is markedly curved posteriorly (radius of arc of curvature 197 mm; Fig. 3 ). The tusk evenly tapers distally and bears a blunt tip with a posterolingual oval-shaped wear facet (35 mm long and 19 mm wide). Though expected, there is no obvious dentine exposed in the wear facet. The tusk is oval in cross-section and compressed transversely (transverse basal diameter 67 mm; anteroposterior basal diameter 90 mm). Two deep longitudinal furrows are present labially, the posterior furrow being slightly deeper and extending further distally; these furrows become diffuse distally, and terminate 54 mm from the apex. Though mostly curved in the parasagittal plane, the tusk is slightly twisted about its longitudinal axis from proximal to distal so that the tip is rotated and slightly reverted lingually. Longitudinal furrows are also present lingually, though they are diffuse and less deeply incised, terminating only 239 mm from the apex. The position of the gum line is unclear. However, subparallel anteroposterior scratches are present lingually but are absent from the proximal 150 mm of the tusk. If these were produced in vivo (e.g., sediment abrasion during benthic feeding) it would suggest the gum line was positioned at about 150 mm from the proximal end of the tusk. External annular growth rings are not visible under low angle light.

The pulp cavity is open and forms the widest part of the tusk, indicating continuous growth. The shape of the pulp cavity is oval, and the depth is 82 mm; the globular orthodentine core is developed within as a proximally tapering cone. Individual bead-like dentine globules are visible proximally within the pulp cavity, confirming assignment of CCNHM 1144 to Odobenini ( Deméré 1994a).

The tusk lacks evidence of post-mortem abrasion; irregular, circular-oval crater-like pits (2–16 mm diameter, 0.7–1 mm deep) are evenly distributed proximo-distally along the tusk and several occur inside the pulp cavity. Pitting is more common on the labial side and often coalesced so that large areas are missing cementum. The margins of the pits are smooth and lack evidence of fracturing, resembling enamel cupping reported in dolphin teeth by Loch et al. (2013).

Remarks.—CCNHM 1144 is identifiable as Ontocetus emmonsi owing to its strong curvature, marked distal tapering, transverse compression, and deeply incised longitudinal furrows ( Kohno and Ray 2008: 45). This specimen has a transverse width:anteroposterior diameter ratio of 0.74, which is relatively wide for Ontocetus emmonsi ( Fig. 3 ). However, the deeply incised labial grooves and small radius of the arc of curvature ( Fig. 3 ) clearly differentiate this specimen from Valenictus ( Deméré 1994b) and Odobenus ( Kohno and Ray 2008) , and are characteristic features identifying this specimen as Ontocetus emmonsi .

Figures

Fig. 2. Right canine tusk of odobenid walrus Ontocetus emmonsi (CCNHM 1144) from the Lower Pleistocene?Waccamaw Formation, Austin Sand Pit, South Carolina, in lingual (A), proximal (B), anterior (C), and labial (D) views.

Fig. 3. Scatterplot of ratio of proximal transverse width/anteroposterior length versus radius of the arc of curvature for tusks of Odobenus and Ontocetus, including CCNHM 1144. Original plot modified from Kohno and Ray (2008: fig. 27).