Plotopterum

''Plotopterum'' is an extinct genus of flightless seabird of the family Plotopteridae, native to the North Pacific during the Late Oligocene and the Early Miocene. The only described species is ''Plotopterum joaquinensis''.

History and Etymology

The future holotype of ''Plotopterum'' was discovered in 1961 in the Pyramid Hill Sand member of the Jewett Sand Formation, in the San Joaquin Valley of California, by Dick Bishop and Ed Mitchell. The remains were presented by Bishop to the Natural History Museum of Los Angeles County in 1964. In 1969, Hildegarde Howard, at the time retired from her work of chief curator of science at the museum, described the remains, LACM 8927, the upper end of a left coracoid, as a new genus and species, ''Plotopterum joaquinensis'', which she ascribed to a new, monotypic family, Plotopteridae, due to its unique characteristics and adaptations towards swimming.

In 1977, Hasegawa, Okumura and Okazaki described an almost complete bird femur, collected in 1976 in the Akeyo Formation in Honshu, Japan, and dated from the Early Miocene, as an indeterminate member of the family Phalacrocoracidae. Less than a decade later, in 1985, Hasegawa himself, with Storrs L. Olson, redescribed the japanese remains as belonging to the genus ''Plotopterum'', as ''Plotopterum'' sp.

The genus name, ''Plotopterum'', is formed from the prefix ''Plot-'', meaning "swimming", and the suffix "''-pterum''", meaning wing.

Description

The holotype associated with ''Plotopterum'', the humeral end of a left coracoid, was roughly the size of those of the extant Brandt's cormorant, but narrower and more rounded. Several of its characteristics, such as the outline of the head, the shape of the bone, the scapular facet and its adjacent shaft were described as reminiscent of cormorants and anhingas. However, other characteristics, such as the head hanging over the shaft and the shape of the triosseal region, were more typical of diving birds, like penguins and auks, unrelated groups presenting flipper-like wings well adapted for swimming. The shape of the triosseal area, swollen in its lower portion and narrowed anteroposteriorly, was presumably occupied by the pectoral tendon, and strengthened the wing when the animal was swimming. Contrary to its distant relative, the flightless cormorant, the wings of ''Plotopterum'' were not reduced by the lack of use, but were heavily specialized in swimming.

The almost complete femur tentatively attributed to the genus in 1985, MFM 1800, shared similarities with Anhingidae, and the individual it belonged to was probably smaller than those of its Oligocene relatives, approximately the size of a great cormorant.

Paleoenvironment

The Pyramid Hill Sand member of the Jewett Sand Formation, where the first remains of ''Plotopterum'' were discovered, was, during the Miocene, covered by the Pacific Ocean, and has yielded, alongside the holotype of ''Plotopterum'', numerous fossils of cetaceans, fish, turtles, and molluscs.

It has been suggested that the diversification of marine mammals occurring in the Pacific Ocean during the Late Paleogene and the Early Neogene may have concurrenced the plotopterids, and participated in their extinction. During the Early Miocene, the only forms of Plotopteridae known in the fossil record, like ''Plotopterum'', were smaller and less derived than their Oligocene counterparts, and those forms probably went extinct shortly later.

References

{{Taxonbar, from=Q114088505

Fossil taxa described in 1969
Neogene Japan
Miocene birds of North America
Plotopteridae