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Imagotaria

Imagotaria

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Encyclopedia
Imagotaria is an extinct monotypic
Monotypic
In biology, a monotypic taxon is a taxonomic group with only one biological type. The term's usage differs slightly between botany and zoology. The term monotypic has a separate use in conservation biology, monotypic habitat, regarding species habitat conversion eliminating biodiversity and...

 genus of walrus
Walrus
The walrus is a large flippered marine mammal with a discontinuous circumpolar distribution in the Arctic Ocean and sub-Arctic seas of the Northern Hemisphere. The walrus is the only living species in the Odobenidae family and Odobenus genus. It is subdivided into three subspecies: the Atlantic...

 with the sole species Imagotaria downsi. Fossils of Imagotaria are known from the early late Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...

 of California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...

 (ca. 10-12 million years ago).

Description


The 1.8 metres (5.9 ft) long pinniped
Pinniped
Pinnipeds or fin-footed mammals are a widely distributed and diverse group of semiaquatic marine mammals comprising the families Odobenidae , Otariidae , and Phocidae .-Overview: Pinnipeds are typically sleek-bodied and barrel-shaped...

 more closely resembled in its overall shape a sea lion
Sea Lion
Sea lions are pinnipeds characterized by external ear-flaps, long fore-flippers, the ability to walk on all fours, and short thick hair. Together with the fur seal, they comprise the family Otariidae, or eared seals. There are six extant and one extinct species in five genera...

 rather than a walrus. Unlike the extant walrus (Odobenus rosmarus), Imagotaria did not possess elongate, ever-growing tusks, but instead bore enlarged canines (with respect to other pinnipeds).

Imagotaria is a example of a primitive walrus that does not grossly appear similar to a modern walrus. However, the walrus family (the Odobenidae
Odobenidae
Odobenidae is a family of Pinnipeds. The only living species is walrus.In the past, however, the group was much more diverse, and includes more than ten fossil genera.- Taxonomy :All genera, except Odobenus, are extinct.*Prototaria...

) is a more inclusive group, that includes walruses without tusks (e.g. Imagotaria), walruses with upper and lower tusks (the subfamily Dusignathinae), and walruses with upper tusks like the extant walrus (subfamily Odobeninae, tribe Odobenini). It is possible to classify these pinnipeds as walruses because they share many other skull
Skull
The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...

 features (besides tusks) as well as many skeletal features, all of which indicate common ancestry.

Palaeobiology


The teeth of Imagotaria indicate that its feeding ecology was markedly different from that of modern walrus, and more similar to that of less specialized pinnipeds like seals
Pinniped
Pinnipeds or fin-footed mammals are a widely distributed and diverse group of semiaquatic marine mammals comprising the families Odobenidae , Otariidae , and Phocidae .-Overview: Pinnipeds are typically sleek-bodied and barrel-shaped...

, fur seals, and sea lions. Conical, unworn teeth and the lack of a vaulted palate indicate that Imagotaria did not feed on molluscs like modern walrus. Modern walruses do not use their teeth to chew molluscs like sea otters do. Instead, they hold a clam in their lips, and the vaulted palate allows them to use their tongue as a powerful piston to suck the soft parts right out of the clam shell. The shell is then dropped to the seafloor, never entering the oral cavity.

Additionally, fossils of Imagotaria (and the earlier Neotherium
Neotherium
Neotherium mirum is an extinct species of walrus. It was smaller than living forms and it did not have long tusks. Males were larger than females....

, ca. 15 million years ago) demonstrate that early walruses had, by the middle and late Miocene, already developed extreme sexual dimorphism
Sexual dimorphism
Sexual dimorphism is a phenotypic difference between males and females of the same species. Examples of such differences include differences in morphology, ornamentation, and behavior.-Examples:-Ornamentation / coloration:...

(males and females having different body sizes). It is unclear whether extreme sexual dimophism is ancestral to all pinnipeds, or if it has been independently acquired in multiple pinniped lineages.