Fossils newly identified in a rock collected 30 years ago in Osaka Prefecture may belong to a previously unknown species. For roughly 30 years, four mosasaur bones remained concealed inside a rock stored at the Natural History Museum, Kishiwada City. The investigation involved researchers from Okayama University of Science, Tokyo City University, the Natural History Museum, Kishiwada City (Osaka Prefecture), and other institutions. The Osaka fossils were collected between 1990 and 1992 in Sobura, Kaizuka City, and later placed in the museum’s collection. Decades of additional mosasaur research have since produced a much larger global collection of fossils and anatomical information.