A well-preserved partial scapula from Japan and the reconstruction of the triosseal canal of plotopterids

The discovery of a well-preserved cranial end of a plotopterid scapula from the Early Oligocene Jinnobaru Formation in southwestern Japan has provided a fine example of its bone structure and has enabled the reconstruction of the triosseal canal (canalis triosseus) of the unique extinct penguin-like bird. It is believed that plotopterids performed penguin-like underwater propulsion using wings that were similar to those of penguins. Until this discovery, the lack of well-preserved plotopterid scapulae hindered reconstruction of the canalis triosseus, which is an important structure for the wing-upstroke. We reconstructed a composite model of the canalis triosseus based on the new scapula. The reconstructed size of the canal is as large as that in Emperor Penguins (Aptenodytes forsteri), suggesting that the bird had a large and powerful m. supracoracoideus, which is the essential muscle for the powered upstroke required for wing-propelled diving. Plotopterids likely have had the same functional requirement as penguins, the powerful wing-upstroke in the water. They must have also been capable swimmers. This scapula accounts for the structural difference between plotopterids and penguins in terms of the canalis triosseus. The large canalis triosseus of plotopterids was composed of the elongated acromion of the scapula, while penguins have a long processus acromialis claviculae for the same function.