A research team consisting of Yasuaki Takano (Master's student) from Okayama University of Science Graduate School, Shoji Hayashi (Associate Professor) from the same university, Yasutoshi Nakajima (Associate Professor) from Tokyo City University, Satoshi Utsunomiya (working at Panasonic Electric Works Co., Ltd.), Yuma Asai (Master's student) from the University of Tsukuba Graduate School, Miho Kazama (Curator at Kishiwada Natural History Museum), and advisor Katsunori Watanabe, re-examined a mosasaur fossil discovered and excavated in Sobara, Kaizuka City, Osaka Prefecture, between the end of 1990 and 1992 (see Figure 2). This fossil contained parts that had remained untreated and encased in rock at the time of discovery. The research team proceeded with cleaning work, carefully removing the rock surrounding the fossil, and examined it in detail, including the bone fossils that became newly observable.

The skull is a particularly important part for distinguishing animal species. In particular, the bone that forms the tip of the upper jaw is a part that has been confirmed for the first time in mosasaurs in Japan, and is one of the major achievements of this study (see Figure 3). At the time of discovery, there were few mosasaur specimens that could be compared, so it was not possible to definitively determine which species this fossil belonged to. However, in recent years, research on mosasaurs has progressed all over the world, and the number of comparable specimens and information has increased.
As a result, it was discovered that this fossil has characteristics that differ from previously known mosasaurs. In particular, it was confirmed that the bones near the brain have short, horn-like projections extending horizontally on both sides, and that the grooves usually seen are absent (see Figure 4).
These characteristics suggest that this mosasaur may be different from previously known species. Further detailed research is needed, including the possibility that it is a new species.

Approximately 30 years after its discovery, advancements in cleaning techniques and mosasaur research have yielded new clues, which reaffirms the importance of the untreated specimens held by the Kishiwada Natural History Museum and also provides insights into what kinds of creatures lived in the coastal regions of the Northwest Pacific.
The results of this research were presented at the 177th Annual Meeting of the Palaeontological Society of Japan, held on June 27, 2026, at the Sugimoto Campus of Osaka Metropolitan University.