About 10 million years ago on the shores of what’s now Hokkaido, Japan, ancient walrus relatives sported sharp canines but no tusks, according to findings published in PLOS ONE this week.
Nowadays, walruses (Odobenus rosmarus) are the only living members of the family Odobenidae, but the group was incredibly diverse in the past: The fossils of at least 16 genera and 20 species of extinct walruses have already been recovered. Most of them lived during the late Miocene and the Pliocene, about 8 million to 2 million years ago. A few years ago, a staff member of the Tobetsu Town Board of Education discovered the partial skeleton of a fossil pinniped (seals, sea lions, and friends) in the late Miocene Ichibangawa Formation near the Rutaka Bridge in northern Japan. The left cranium, jawbones, vertebrae and ribs, and some limb bones were in a sandstone block that at some point likely floated downstream from Aoyama dam about 1.5 kilometers to the north.
According to University of Otago’s Yoshihiro Tanaka and colleagues, the fossilized remains belonged to a never-before-seen archaic odobenid with a slender, small cranium and non-tusked, moderate-sized upper canines. While they weren’t tusk-like, they were large, sharp, and cone-shaped, measuring 86.3 millimeters long (or nearly 3.5 inches). They named it Archaeodobenus akamatsui from the Greek “archaios” meaning ancient. The species name honors Morio Akamatsu, curator emeritus of the Hokkaido Museum.
The holotype cranium of Archaeodobenus akamatsui. Tanaka Y, Kohno N/PLOS ONE.
Archaeodobenus akamatsui lived 10 million to 9.5 million years ago. Based on its relatively large canines and the well-developed crest on the back of the skull, the specimen was likely a male; his cranial sutures suggest that he was young, a subadult. The large mammal would have been between 2.8 and 3 meters long (that’s nearly 10 feet), and it weighed anywhere between 390 and 473 kilograms (860 to 1,042 pounds).
A late Miocene odobenid discovered in 2006 named Pseudotaria muramotoi lived around the same time in this same location. But it had different skeletal features. Based on a phylogenetic analysis, the two species split during the late Miocene in the western North Pacific. This rapid diversification of archaic odobenids was likely the result of geological isolation among their extinct common ancestors. In the beginning of the early late Miocene, 12.5 million to 10.5 million years ago, the sea level was dropping as shelf structures began to develop.
Teeth of Archaeodobenus akamatsui. Tanaka Y, Kohno N/PLOS ONE.
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