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| Ontocetus posti Boisville, Chatar & Kohno, 2024 DOI: 10.7717/peerj.17666 |
Abstract
Ontocetus is one of the most notable extinct odobenines owing to its global distribution in the Northern Hemisphere. Originating in the Late Miocene of the western North Pacific, this lineage quickly spread to the Atlantic Ocean during the Pliocene, with notable occurrences in England, Belgium, The Netherlands, Morocco and the eastern seaboard of the United States. Reassessment of a pair of mandibles from the Lower Pleistocene of Norwich (United Kingdom) and a mandible from the Upper Pliocene of Antwerp (Belgium) that were referred to as Ontocetus emmonsi reveals existences of features of both Ontocetus and Odobenus. The presence of four post-canine teeth, a lower canine larger than the cheek-teeth and a lower incisor confirms the assignment to Ontocetus; simultaneously, characteristics such as a fused and short mandibular symphysis, a well-curved mandibular arch and thin septa between teeth align with traits usually found in Odobenus. Based on a combination of these characters, we describe Ontocetus posti, sp. nov. Its mandibular anatomy suggests, a better adaptation to suction-feeding than what was previously described in the genus suggesting that Ontocetus posti sp. nov. likely occupied a similar ecological niche to the extant walrus Odobenus rosmarus. Originating from the North Pacific Ocean, Ontocetus most likely dispersed via the Central American Seaway. Although initially discovered in the Lower Pliocene deposits of the western North Atlantic, Ontocetus also left its imprint in the North Sea basin and Moroccan Plio-Pleistocene deposits. The closure of the Isthmus of Panama during the Mio-Pliocene boundary significantly impacted the contemporary climate, inducing global cooling. This event constrained Ontocetus posti in the North Sea basin leaving the taxon unable to endure the abrupt climate changes of the Early Pleistocene, ultimately going extinct before the arrival of the extant counterpart, Odobenus rosmarus.
Systematic paleontology
Mammalia Linnaeus, 1758
Carnivora Bowdich, 1821
Pinnipedia Illiger, 1811
Odobenidae Allen, 1880
Odobeninae Mitchell, 1968
Odobenini Deméré, 1994b
Genus Ontocetus Leidy, 1859
Type species: Ontocetus emmonsi Leidy, 1859; Yorktown Formation, North Carolina, Pliocene
Ontocetus posti sp. nov.
A species of Ontocetus that exhibits mosaic characters including some shared traits with the genus Odobenus. These diagnostic features of Ontocetus comprise a lower canine markedly larger (at least 20% larger than cheekteeth) from the cheek toothrow, presence of the lower first premolar (p1), specifically at least four lower postcanine teeth that are consisted of p1–p4, a broad and short edentulous mandibular terminus exhibiting two lower incisors (i2, i3), an elevated and slanted anterior margin, the relative size of the mandible (reaching almost 30 cm in horizontal length), a high, thin, and posterolaterally projected coronoid process, as well as a short distance between the dorsal margin and coronoid process (less than 8% of the total length of the mandible).
This species also shares certain characteristics with Odobenus, such as the fusion of the mandibular symphysis, a mandibular symphysis oval in shape and shorter in proportion to the total length of the mandible (less than 50%), a horizontal ramus with a well-curved lateral occlusal outline (also occurs in the referred mandible of Dusignathus seftoni) that accounts for the shortening of the rostrum. Additionally, a shorter space between the canine and the cheek teeth can be observed (less than 10% of the total length of the dental row), as well as between the thin cheek tooth alveoli themselves. Other shared traits encompass an underdeveloped angular process, and a flat, less developed mandibular condyle.
Etymology: The species is named in honor of Mr. Klaas Post, curator of Vertebrate Paleontology at Natuurhistorisch Museum Rotterdam, in recognition of his extensive contributions to geology and paleontology in the North Sea district, and as a token of appreciation for his continuous support, encouragement, and invaluable assistance provided to all of us throughout this study.
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| Holotype mandibles of Ontocetus posti. adult female, NMR 7472, cast of NWHCM 1996.1. Scale bar equals 5 cm. The hatched parts correspond to broken surfaces. Pictures taken and figures drawn by M. Boisville. |
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| Hypothesized sequence of cranial, mandibular, and dental character transformations during odobenine evolution toward the suction-feeding specialization. Cranial, mandibular and dental characters from Deméré (1994b) and Boessenecker & Churchill (2013). Scale bar equals 10 cm. The restored mandible of Ontocetus posti sp. nov. based on NWHCM 1996.1. The shaded areas correspond to the missing parts of the specimens. Reconstructions redrawn from R.W. Boessenecker illustrations by N. Chatar. |
Mathieu Boisville, Narimane Chatar and Naoki Kohno. 2024. New Species of Ontocetus (Pinnipedia: Odobenidae) from the Lower Pleistocene of the North Atlantic shows similar Feeding Adaptation independent to the extant walrus (Odobenus rosmarus). PeerJ. 12:e17666. DOI: doi.org/10.7717/peerj.17666
