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Description

A selection of extinct members of the pinniped family known as odobenids, to scale.

Despite their ungainly appearance on land, pinnipeds are incredibly graceful in the water. Seals and sealions have very different propulsion method in water: seals use their broad hind feet, while sealions swim with long pectoral fins. The third living pinniped family consists of only one species - walruses combine being able to walk on land like a sealion, but use their hindlimbs for swimming. Walruses have a long and interesting fossil record, far beyond those tusks.
Where odobenids fit in with other pinnipeds has been fraught with argument, but molecular studies are pretty clear cut that they are the sister family to the otariids (sealions and fur seals). So perhaps not surprisingly the early odobenids looked very much like sealions. They overlapped with otariids and the desmatophocids (an extinct family related to the true seals) through the North Pacific during the Middle Miocene, however it was the walruses which were the smaller species. Only during the Late Miocene did the odobenids diversified and increase in body size.
There are three main groups of odobenids: an informal conglomeration of basal species ('imagotariines'), dusignathines and odobenines. Although not immediately recognisable as walruses, the basal 'imagotariines' show skeletal features in common with other odobenids. One of the earliest 'imagotarines' was the tiny Prototaria, only the size of a common seal, with a distinctive flat skull and a short snout. Imagotaria was a larger species with bigger canines (as opposed to tusks) and also displayed extreme sexual dimorphism. Other dentition were not worn down chewing on hard items suggesting that they were dietary generalists. The most impressive odobenid was also one of the largest pinnipeds, approaching modern elephant seals in size. With a massive 60 cm skull featuring a tall crest at the rear, Pontolis had basal features but on a far more robust scale.

Dusignathines are a small group notable for a pair of forward protruding canines in both jaws. In the large Gomphotaria these teeth were four thick but short horizontal tusks. While modern walrus use suction on shellfish, Gomphotaria used the tusks to smash prey. Extreme wear on postcanine teeth shows that they were crushing up prey before ingesting.

Far more familiar are the two-tusked odobenines, which include the only living species. Walrus tusks, which can reach 1 metre in length, are used largely in male-male interactions. Territorial fighting during breeding season can be as violent as those seen in elephant seals. Secondarily they are used for hauling onto sea ice and stirring up sediment when foraging. Long tusks were only feasible with a move towards suction-feeding, and this also occurs convergently in tusked cetaceans (narwhal, beaked whales and the extinct Odobenocetops). Amongst fossil odobenids there is some variation in tusks: Protodobenus had canines barely larger than sealions, Ontocetus had forward-inclined tusks, while they were absent in Aivukus. Odobenines were the only walruses which were found in the Pleistocene, and also colonised the Atlantic Ocean earlier during the Pliocene.

Description

A selection of extinct members of the pinniped family known as odobenids, to scale.

Despite their ungainly appearance on land, pinnipeds are incredibly graceful in the water. Seals and sealions have very different propulsion method in water: seals use their broad hind feet, while sealions swim with long pectoral fins. The third living pinniped family consists of only one species - walruses combine being able to walk on land like a sealion, but use their hindlimbs for swimming. Walruses have a long and interesting fossil record, far beyond those tusks.
Where odobenids fit in with other pinnipeds has been fraught with argument, but molecular studies are pretty clear cut that they are the sister family to the otariids (sealions and fur seals). So perhaps not surprisingly the early odobenids looked very much like sealions. They overlapped with otariids and the desmatophocids (an extinct family related to the true seals) through the North Pacific during the Middle Miocene, however it was the walruses which were the smaller species. Only during the Late Miocene did the odobenids diversified and increase in body size.
There are three main groups of odobenids: an informal conglomeration of basal species ('imagotariines'), dusignathines and odobenines. Although not immediately recognisable as walruses, the basal 'imagotariines' show skeletal features in common with other odobenids. One of the earliest 'imagotarines' was the tiny Prototaria, only the size of a common seal, with a distinctive flat skull and a short snout. Imagotaria was a larger species with bigger canines (as opposed to tusks) and also displayed extreme sexual dimorphism. Other dentition were not worn down chewing on hard items suggesting that they were dietary generalists. The most impressive odobenid was also one of the largest pinnipeds, approaching modern elephant seals in size. With a massive 60 cm skull featuring a tall crest at the rear, Pontolis had basal features but on a far more robust scale.

Dusignathines are a small group notable for a pair of forward protruding canines in both jaws. In the large Gomphotaria these teeth were four thick but short horizontal tusks. While modern walrus use suction on shellfish, Gomphotaria used the tusks to smash prey. Extreme wear on postcanine teeth shows that they were crushing up prey before ingesting.

Far more familiar are the two-tusked odobenines, which include the only living species. Walrus tusks, which can reach 1 metre in length, are used largely in male-male interactions. Territorial fighting during breeding season can be as violent as those seen in elephant seals. Secondarily they are used for hauling onto sea ice and stirring up sediment when foraging. Long tusks were only feasible with a move towards suction-feeding, and this also occurs convergently in tusked cetaceans (narwhal, beaked whales and the extinct Odobenocetops). Amongst fossil odobenids there is some variation in tusks: Protodobenus had canines barely larger than sealions, Ontocetus had forward-inclined tusks, while they were absent in Aivukus. Odobenines were the only walruses which were found in the Pleistocene, and also colonised the Atlantic Ocean earlier during the Pliocene.