New specimens of Plio-pleistocene odobenines from the north sea give new insight on the past diversity of the genus ontocetus

The walrus (Odobenus rosmarus) is one of the most iconic mammal of the Arctic, significantly differing from all other pinnipeds by its size and prominent tusks. The walrus is the only extant member of the family Odobenidae although this family had one of the highest diversity in the fossil record within pinnipeds. Phylogenetically, the extant walrus lies in the tribe Odobenini with fossil genera such as Protodobenus, Ontocetus and Valenictus. Odobenids are characterized not only by a significant development of the upper canines and premolariform lower canines, but also an absence of enamel. Odobenines first originated in the North Pacific but some taxa then dispersed in the North Atlantic during the Early Pliocene. Over the past 17 million years, a succession of evolutionary grades have been identified from the earlier enaliarctine taxa to the extant walrus. Ontocetus is the most well-known fossil odobenine thanks to its worldwide distribution. Its oldest occurrence is in the Lower Pliocene of Japan, but it quickly spread out into the North Atlantic Ocean during the Pliocene being notably found in England, Belgium, Netherlands, the east coast of the United States or even in Morocco. Historically, five genera and eight species of Plio-Pleistocene Ontocetus-like walruses were described. The main issue being that the species have been defined on different skeletal elements. However, based on the tusks, all the specimens found in the North Sea have been found to integrate the ontogenetic and sexual variation of Ontocetus emmonsi from the U.S. East Coast. Recent studies using morphometric geometrics have highlighted that other skeletal elements could be used as a proxy to sex or morphotype in such “problematic” walruses. We used 2D geometric morphometrics to quantify morphological differences among Plio-Pleistocene populations, in order to better understand the past diversity of Ontocetus, using a total of 21 specimens, including some new Ontocetus remains from the North Sea and Japan. We identified clear morphological variations among the different Ontocetus populations with some distinct size variations as specimens from the North Sea that are
significantly larger than those from the U.S. East Coast. Moreover, in view of their similar characteristics to Odobenus, two mandibles from the North Sea recognized as Ontocetus, differ from what we known. This could suggest that those different populations of Ontocetus showed a wide spectrum of ecological adaptations.