A tarsometatarsus of a plotopterid bird from the lower Oligocene Yamaga Formation, Ainoshima, Japan

Abstract

A tarsometatarsus belonging to the family Plotopteridae (Aves, Suliformes) is described from the lower Oligocene Yamaga Formation of the Ashiya Group on the island Ainoshima in Kitakyushu, Fukuoka, western Japan. It resembles Phocavis in gracility and size but shares with Copepteryx and Hokkaidornis features such as an elongated trochlea metatarsi IV, splayed trochlea metatarsi II, and an intertrochlear fossa between the trochleae metatarsorum III and IV. Comparison with modern waterbirds indicates that the intertrochlear fossa represents an ancestral character. Intertrochlear structures exhibit variability among plotopterids, suggesting that these structures underwent distinct evolutionary modifications within each lineage. The specimen does not appear to be a referable to Copepteryx or Hokkaidornis, implying a greater morphological diversity of the family in Japan.