Walruses did not always have tusks: Archaic walrus ancestors were sea-lion look-alikes

Archaeodobenus is not the only fossil walrus to be found in Hokkaido. One of the authors of this study, Naoki Kohno, published another paper in 2006 describing Pseudotaria muramotoi, another walrus relative found at the same site. Archaeodobenus and Pseudotaria are close relatives to each other, but they are indeed different species. The two species may have split from each other at some point during the late Miocene, 12-10 million years ago. The fact that the two species are from the same family and same geographic range implies that there may have been a rapid diversification of odobenids in the late Miocene.

What could have caused this quick increase in the number of odobenid species? Scientists believe that a rapid change in sea-level in the North Pacific during the late Miocene may have been the cause. A sea-level drop may have isolated two different populations of a common ancestor of Archaeodobenus and Pseudotaria which over time, caused them to evolve into different genera. When sea-level rose again, the two different genera then came back into contact with each other, but at that point, may have already been too different to interbreed.

Today, there is only one living species of the Odobenidae family, the modern walrus, Odobenus rosmarus. In the past, this family was much more diverse with at least 16 genera made up of 20 different species.

This study sheds some light on pinniped evolution. It shows that the diversification of walruses happened quickly and possibly 3-5 million years earlier than previously thought. There were very few archaic species of walrus known before this study, but scientists now believe there could still be more to discover.

Read the original research in >PLOS One.

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