''Archaeodobenus'' is an extinct genus of

pinniped Pinnipeds (pronounced ), commonly known as seals, are a widely range (biology), distributed and diverse clade of carnivorous, fin-footed, semiaquatic, mostly marine mammals. They comprise the extant taxon, extant families Odobenidae (whose onl ...

that lived during the

Late Miocene The Late Miocene (also known as Upper Miocene) is a sub-epoch of the Miocene epoch (geology), Epoch made up of two faunal stage, stages. The Tortonian and Messinian stages comprise the Late Miocene sub-epoch, which lasted from 11.63 Ma (million ye ...

of what is now Japan. It belonged to the

Odobenidae Odobenidae is a family of pinnipeds, of which the only extant species is the walrus (''Odobenus rosmarus''). In the past, however, the group was much more diverse, and includes more than a dozen fossil genera. Taxonomy All genera, except '' Od ...

family, which is today only represented by the

walrus The walrus (''Odobenus rosmarus'') is a large pinniped marine mammal with discontinuous distribution about the North Pole in the Arctic Ocean and subarctic seas of the Northern Hemisphere. It is the only extant species in the family Odobeni ...

, but was much more diverse in the past, containing at least 16 genera.

Taxonomy

The first known specimen was collected in 1977 from the Ichibangawa Formation in Tobetsu Town on the island of

Hokkaido is the list of islands of Japan by area, second-largest island of Japan and comprises the largest and northernmost prefectures of Japan, prefecture, making up its own list of regions of Japan, region. The Tsugaru Strait separates Hokkaidō fr ...

. The specimen consists of a partial skull, vertebrae, and limb bones, and was made the

holotype specimen A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was Species description, formally described. It is either the single such physical example (or illus ...

of the new genus and species ''A. akamatsui'' by the Japanese palaeontologists Yoshihiro Tanaka and Naoki Kohno in 2015. The generic name consists of ''archaio-'', the Greek word for ancient, and the generic name of the walrus, ''Odobenus''; in full, "ancient walrus". The

specific name Specific name may refer to: * in Database management systems, a system-assigned name that is unique within a particular database In taxonomy, either of these two meanings, each with its own set of rules: * Specific name (botany), the two-part (bino ...

honors Morio Akamatsu, a curator of the

Hokkaido Museum opened in Sapporo, Hokkaidō is the second-largest island of Japan and comprises the largest and northernmost prefecture, making up its own region. The Tsugaru Strait separates Hokkaidō from Honshu; the two islands are connected by ra ...

Evolution

The diversity of odobenids increased during the Late Miocene and

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58marine regression A marine regression is a geological process occurring when areas of submerged seafloor are exposed during a drop in sea level. The opposite event, marine transgression, occurs when flooding from the sea covers previously-exposed land. Descript ...

and transgression, which could have geographically isolated their ancestors. ''Archaeodobenus'' was the contemporary of the odobenid '' Pseudotaria'' from the same formation, which it may have diverged from in the western North Pacific during the Late Miocene. ''Archaeodobenus'' appears to have been closer related to later odobenids such as ''

Imagotaria ''Imagotaria'' is an extinct monotypic genus of walrus with the sole species ''Imagotaria downsi''. Fossils of ''Imagotaria'' are known from the early late Miocene of California (c. 10-12 million years ago). Description The long pinniped more ...

'', ''

Pontolis ''Pontolis'' is an extinct genus of large walrus. It contained three species, ''P. magnus'', ''P. barroni'', and ''P. kohnoi''. Like all pinnipeds, ''Pontolis'' was a heavily built amphibious carnivore. ''Pontolis'' lived along the Pacific coast ...

'', the subfamily Odobeninae, whereas ''Pseudotaria'' seems to have been more basal.

Description

Unlike the modern walrus, ''Archaeodobenus'' did not have tusks but instead had canines of moderate size, and looked more like a

sea lion Sea lions are pinnipeds characterized by external ear flaps, long foreflippers, the ability to walk on all fours, short and thick hair, and a big chest and belly. Together with the fur seals, they make up the family Otariidae, eared seals. ...

. The holotype specimen appears to have been a young adult male of about in length, which would have weighed around . This is intermediate between the size of the

Steller sea lion The Steller sea lion (''Eumetopias jubatus''), also known as Steller's sea lion or the northern sea lion, is a large, near-threatened species of sea lion, predominantly found in the coastal marine habitats of the northeast Pacific Ocean and th ...

and the

South American sea lion The South American sea lion (''Otaria flavescens'', formerly ''Otaria byronia''), also called the southern sea lion and the Patagonian sea lion, is a sea lion found on the western and southeastern coasts of South America. It is the Monotypic ta ...

. Its canines were long, compared to modern walrus tusks, which can grow to up to long. ''Archaeodobenus'' can be distinguished from ''Pseudotaria'' by features such as the shape and size of the

occipital condyle The occipital condyles are undersurface protuberances of the occipital bone in vertebrates, which function in articulation with the superior facets of the Atlas (anatomy), atlas vertebra. The condyles are oval or reniform (kidney-shaped) in shape ...

(which connects with the first neck vertebra at the back of the skull), the

foramen magnum The foramen magnum () is a large, oval-shaped opening in the occipital bone of the skull. It is one of the several oval or circular openings (foramina) in the base of the skull. The spinal cord, an extension of the medulla oblongata, passes thro ...

(the opening through which the

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue that extends from the medulla oblongata in the lower brainstem to the lumbar region of the vertebral column (backbone) of vertebrate animals. The center of the spinal c ...

passes into the cranium), the

mastoid process The mastoid part of the temporal bone is the posterior (back) part of the temporal bone, one of the bones of the skull. Its rough surface gives attachment to various muscles (via tendons) and it has openings for blood vessels. From its borders, t ...

(where various muscles attach to the back of the skull), and some features in the

postcranial The postcranium ("behind the cranium"; plural: postcrania) or postcranial skeleton in zoology and vertebrate paleontology is the skeleton apart from the skull. The postcranium encompasses the axial skeleton, which includes the entirety of the verte ...

skeleton.

References

{{Taxonbar, from=Q48557894, from2=Q25436860 Miocene pinnipeds Monotypic prehistoric carnivoran genera Fossils of Japan Extinct mammals of Asia Odobenids Fossil taxa described in 1931