Walruses, Odobenidae, have a good fossil record from the middle Late Miocene (about 8 million years ago) onwards, and appear to have been a fairly diverse group of large Seals for much of that time, with the single surviving modern species, Odobenus rosmarus, being a rather modified and specialized member of the group. Fossil Walruses older than about 8 million years are rare, though the group was clearly present before this time, as the group was already quite diverse when it became abundant in the fossil record; older fossil Walruses are therefore key to understanding diversity within the group.
In a paper published in the journal PLoS One on 5 August 2015, Yoshihiro Tanaka of the Hokkaido University Museum, the Department of Geology at the University of Otago and the Numata Fossil Museum and Naoki Kohno of the Department of Geology and Paleontology at the National Museum of Nature and Science and the Graduate School of Life and Environmental Sciences at the University of Tsukuba describe a new species of fossil Walrus from a partial skull and skeleton from the 10.0-9.5 million-year-old Ichibangawa Formation of Tobetsu Town in Hokkaido.
The new species is named Archaeodobenus akamatsui, where ‘Archaeodobenus’means ‘ancient Walrus’ and ‘akamatsui’ honours Morio Akamatsu of the Hokkaido Museum for his longstanding contributions to the geology and palaeontology of Hokkaido. The species is described from a single specimen comprising a partial skull with most of the left side preserved, as well as the left mandible and several teeth, several vertebrae and ribs and an incomplete sternebra, a partial scapulae, and a nearly complete right humerus. The specimen is thought to have been a male animal between about 2.8 and 3 meters in length, and weighing about 390–473 kg.
Cranium of Archaeodobenus akamatsui, in (A) dorsal view, (B) left lateral view, (C) ventral view. Tanaka & Kohno (2015).
Archaeodobenus akamatsui clearly belongs to the Odobenidae, having a number of traits only found in Walruses, notably a laterally expanded pterygoid strut (supporting structure on the upper jaw which forms part of the palate), a bony tentorium that is closely appressed to the petrosal (part of the structure of the ear) and well developed first and second premolars. However it is otherwise distinct from other members of the group, suggesting that it represents a distinct lineage.
Reconstructed cranium of Archaeodobenus akamatsui, in (A) dorsal view, (B) left lateral view, (C) ventral view. Tanaka & Kohno (2015).
Archaeodobenus akamatsui was derived from a distinctive glauconitic (green sandstone, typically deposited in a marine environment with low oxygen levels) layer within the Ichibangawa Formation. It is the second Walrus species described from this layer, implying that two separate Walrus lineages were already present in the area at this time, and that therefore that the rapid diversification of the group had already begun. This glauconite layer is thought to represent a deposits laid down following a sudden transgression (rapid deepening of the sea) covering part of a delta environment following a period of receding sea levels over the previous million-and-a-half years.
Tanaka and Kohno reason that the falling sea levels between 12 and 10.5 million years ago would have reduced the number of coastal environments suitable for Seals on the East Asian coastline as the waters drew towards the continental shelf, leading to seas that increased in depth rapidly close to the shore. This would have caused reproductive isolation between groups of Early Walruses, with small isolated populations beginning the process of diversifying away from one-another. The sudden transgression after 10.5 million years ago would have flooded many coastal areas, creating many new shallow marine coastal environments, allowing these isolated Walrus species to greatly expand their ranges, bringing them into contact with other, now reproductively isolated, populations and triggering competition between these groups, pushing further morphological diversification within the group.
The role of eustasy in early late Miocene odobenid diversification in Hokkaido, Japan. Tanaka & Kohno (2015).
Such sea-level change events have been linked to events in Seal evolution elsewhere, most notably in South America where a sudden transgression in the Late Pliocene is thought to have triggered an extinction event among Phocids ('True' or Earless Seals) by replacing broad shallow seas covering much of what is now the Atacama Desert with deeper water environments less suitable for these animals.
Reconstruction of Archaeodobenus akamatsui. Tatsuya Shinmura/Ashoro Museum of Paleontology, in Tanaka & Kohno (2015).
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Walruses (Odobenidae) are large Seals (Pinnipedia) related to Sea Lions and Fur Seals (Otariidae). The group first appears in the fossil record about 16 million years ago towards the end of the Early Miocene. The modern Walrus (Odobenus rosmarus) is one of the largest living Seals, and is a specialized shellfish eater, with well-developed tusks used to root out Clams...
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