A Hadrosauroid Vertebra from the Upper Cretaceous Izumi Group, Kagawa Prefecture, Japan

Of the four main Japanese islands, Late Cretaceous dinosaur fossils have been discovered in Hokkaido, Honshu, and Kyushu. Here, we report the first Late Cretaceous dinosaur from the remaining main island Shikoku, an isolated dorsal vertebra from the upper Campanian Hiketa Formation (Izumi Group) in Sanuki, Kagawa Prefecture. The vertebra is incomplete, comprising a robust centrum with a subrectangular profile that is generally characteristic of dinosaurs. Histological examination of the centrum reveals moderately packed trabecular bone and the complete absence of pneumatic structures, which precludes referral of the specimen to Saurischia or Pterosauria. Instead, we interpret it as a hadrosauriform ornithischian based on the slightly expanded anterior articular surface of the centrum; heart-shaped outline of the intervertebral surface, resulting from its dorsally wide and ventrally narrow shape; ventral keel; and neural canal partially embedded into the centrum. As the ventral positioning of the neural canal is common to Bactrosaurus and higher taxa within Hadrosauriformes, we tentatively refer the specimen to Hadrosauroidea. This discovery provides additional evidence that hadrosauriforms had dispersed into present-day Japan by the Campanian and highlights the potential of the Hiketa Formation to provide additional evidence to improve our understanding of the dinosaur diversity at the extreme eastern continental margin of Eurasia.