Copepteryx

Remains of large, flightless suliformes in Japan are known since the 1970s. In 1979, Storrs L. Olson and Hasegawa Yoshikazu identified them as those of plotopterids, but their abundance and diversity complicated their identification as distinct species.[2] In 1996, the two first species endemic from Japan were described by Olson and Hasegawa. Both species were identified as belonging to the same genus, Copepterix. The type species, C. hexeris, was described after a partially articulated skeleton, KMNH VP 200,006, collected in 1977 by Hasegawa himself on Ainoshima, in rocks dated from the Late Oligocene of the Ainoshima Formation. As paratypes were considered another associated skeleton from the Yamaga Formation, a fragmentary tibiotarsus and a mandible from the Ainoshima Formation, a femur from the Asagai Sandstone Formation, and a tarsometatarsus from the Shioda Bed. In the same publication was also described another, larger species of Copepterix, C. titan, collected in 1983 by Ikeuchi Hideo in Ainoshima, with the left femur KMNH VP 200,004 as holotype.[3] In 2008, Sakurai, Kimura and Katoh removed the Shioda Bed tarsometatarsus from the genus, on the basis of its fragmentary nature disabling the possibility to compare it either to Copepteryx or to the then-newly erected genus Hokkaidornis, and were critical of the referral of specimens collected outside of Ainoshima to Copepteryx.[4] In 2009, Okazaki Yoshihiko referred to the genus an additional furcula, discovered in the Late Eocene to Early Oligocene of the Kishima Formation on Hikoshima.[5] In 2019, Ohashi Tomoyuki and Hasegawa Yoshikazu assigned to the genus an additional left coracoid from the Yamaga Formation.[6]

Fossil sternum from Kitahata Karatsu, Japan

Copepteryx was a large-sized diving plotopterid bird, roughly similar to the contemporaneous giant penguins from the South Pacific, such as Waimanu.[7] The sternum shared several similarities with pelecaniformes, and an estimated number of five ribs were articulated with it. The furcula, strongly aligned with the sternum, was large and robust and shared its strong ovoid articulation with the coracoid with most modern pelecaniformes. The coracoid was itself typical of plotopterid, with an elongated shaft and a distinctively modified scapular end. The poorly preserved scapula was similar to Tonsala. The pelvis, mostly reconstructed from impressions, shared the butterfly shape of its iliac shields with the Sulidae. The pre and post-acetabular parts of the sternul were similar-sized.[3]

The wings were, as in all plotopterids, heavily specialized for underwater propulsion and in general shape and usage much different from all other known pelecaniformes. The proximal end of the humerus was round-shaped like that of Tonsala, while the distal end was flattened and very similar to that of unrelated Alcidae like Pinguinus and Mancalla. The radius was short and similarly flattened, while the small ulna supported pits where the quills were attached, as in Tonsala. The anatomy of the elongated metacarpal was typical of that of wing-propelled diving birds. The leg bones shared more similarities with those of Aninghidae than those of other suliformes, although the tarsometatarsus was much shorter and more typical of those of Phalacrocoracidae.[3]

• Biology of Marine Birds (Marine Biology) by E. A. Schreiber and Joanna Burger