Abstract
The modern walrus, Odobenus rosmarus, is specialized and only extant member of the family Odobenidae. They were much more diversified in the past, and at least 16 genera and 20 species of fossil walruses have been known. Although their diversity increased in the late Miocene and Pliocene (around 8–2 Million years ago), older records are poorly known. A new genus and species of archaic odobenid, Archaeodobenus akamatsui, gen. et sp. nov. from the late Miocene (ca. 10.0–9.5 Ma) top of the Ichibangawa Formation, Hokkaido, northern Japan, suggests rapid diversification of basal Miocene walruses. Archaeodobenus akamatsui is the contemporaneous Pseudotaria muramotoi from the same formation, but they are distinguishable from each other in size and shape of the occipital condyle, foramen magnum and mastoid process of the cranium, and other postcranial features. Based on our phylogenetic analysis, A. akamatsui might have split from P. muramotoi at the late Miocene in the western North Pacific. This rapid diversification of the archaic odobenids occurred with a combination of marine regression and transgression, which provided geological isolation among the common ancestors of extinct odobenids.
Highlights
A partial skeleton of a fossil pinniped was collected from the late Miocene Ichibangawa Formation in Tobetsu Town, Hokkaido, Japan (Figs 1 and 2)
The blade-like dorsal margin of the spinous process of the axis (Character 91) in A. akamatsui is recognized as a derived condition that unites this taxon with later diverging odobenids such as the modern walrus, Odobenus rosmarus and the Pliocene extinct walrus, Valenictus chulavistensis (SDSNH 36786: San Diego Natural History Museum) of the subfamily Odobeninae as the potential synapomorphy for all the later diverging odobenids such as Imagotaria, Pontolis and “dusignathines” as well as the monophyletic odobenines
Archaeodobenus akamatsui gen. et sp. nov. represents the second taxon of the archaic odobenids from the early late Miocene Ichibangawa Formation, Hokkaido, northern Japan
Summary
A partial skeleton of a fossil pinniped was collected from the late Miocene Ichibangawa Formation in Tobetsu Town, Hokkaido, Japan (Figs 1 and 2). This skeleton includes a partial cranium, mandibles, anterior vertebrae and some appendicular bones. The third specimen has some unique characters differentiation it from the previously known specimens of P. muramotoi from the same locality area and other species in the Odobenidae.
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