30-year-old fossil in Japanese museum may be a brand new species of mosasaur

Fossils sitting in a Japanese museum for more than three decades may belong to a previously unknown species of mosasaur. Researchers from Okayama University of Science and partner institutions announced the findings on Wednesday, reporting that specimens collected in Kaizuka, Osaka Prefecture, between 1990 and 1992 include a bone never before identified from a mosasaur fossil in Japan. The same specimens also show an anatomical difference from every known mosasaur species studied to date.

The fossils have been stored at the Natural History Museum in Kishiwada City since they were excavated in the early 1990s. At the time, technical limitations prevented detailed preparation and analysis. Recent advances in fossil cleaning methods allowed researchers to study the specimens for the first time — and what they found after three decades of waiting was more significant than expected.

A bone never before identified in Japan

The key fossil element is the premaxilla — the bone that forms the very tip of the upper jaw. In mosasaurs and their relatives, the shape and structure of this bone vary between species, making it one of the most useful features for identification. Researchers confirmed the presence of the premaxilla in the Osaka specimen and found it had never previously been documented in a mosasaur fossil from Japan.

The premaxilla was found alongside other skeletal fragments. Together, the remains suggest the animal measured approximately 20 feet (6 meters) in length — a mid-sized mosasaur for the era, smaller than giants like Mosasaurus itself but large enough to be an effective predator in the Late Cretaceous seas that once covered what is now the Kansai region of Japan.

A missing groove changes the picture

The anatomical detail that most strongly hints at a new species is a groove — or rather, its absence. Most mosasaurs carry a visible blood vessel groove on their jaw bones, a feature used routinely to classify remains. The Osaka specimen lacks it entirely.

That absence, combined with the structure of the premaxilla, points toward a unique combination of traits not seen in any currently recognized mosasaur species. If subsequent analysis confirms the distinction, Japan’s first newly identified mosasaur species could formally join the record.

The animal lived approximately 70 million years ago, during the Maastrichtian stage of the Late Cretaceous — the same period as Tyrannosaurus rex on land and some of the most fearsome marine predators in Earth’s history in the seas.

Thirty years from a creek bed to a discovery

The story behind the identification is as notable as the fossil itself. The specimens were collected during excavations in Kaizuka City between 1990 and 1992. They were recognized as mosasaur material at the time and placed into storage, but the technology needed to clean and study them properly did not yet exist.

The investigation also carries a human dimension. A young researcher contributed foundational work toward identifying the fossil before dying at the age of 17. His contributions helped build the knowledge base that made the current analysis possible. In Japan, where significant fossil discoveries have accelerated in recent years — from Hokkaido to Wakayama and now Osaka — this identification adds a new chapter to the country’s rapidly expanding prehistoric record.