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.

[1] 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.

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.

[4] 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.

[5] 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.