Life reconstruction of the mosasaur specimen discovered in Osaka Prefecture, Japan. Credit: Reconstruction by Satoshi Kawasaki, Okayama University of Science

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. When researchers recently examined the unprepared specimen, they uncovered fossils unlike those of any mosasaur previously described, raising the possibility that the remains belong to an unknown species.

The investigation involved researchers from Okayama University of Science, Tokyo City University, the Natural History Museum, Kishiwada City (Osaka Prefecture), and other institutions. Their finds included a premaxilla, the bone at the very front of the upper jaw. It is the first confirmed premaxilla from a mosasaur fossil found in Japan.

Mosasaurs were enormous marine reptiles that inhabited oceans around the world during the Late Cretaceous, approximately 98 to 66 million years ago. The Osaka fossils were collected between 1990 and 1992 in Sobura, Kaizuka City, and later placed in the museum’s collection. By returning to rock that had never been fully prepared, researchers recovered four bones that had remained trapped within the surrounding material.

A hidden jawbone changes the picture

The newly recognized premaxilla is one of the investigation’s most important discoveries. When the fossil was first collected, researchers lacked enough comparative mosasaur specimens to determine which bones were present or where the animal fit within the group.

Decades of additional mosasaur research have since produced a much larger global collection of fossils and anatomical information. Those advances gave researchers the evidence needed to identify the bone as a premaxilla.

Skull anatomy hints at a new species

Another important clue came from the basisphenoid, a bone positioned near the braincase. The fossil has short, horn-shaped projections extending from its sides and lacks a central groove along its underside.

These traits differ from those documented in previously described mosasaurs. Although more analysis is required, the unusual anatomy indicates that the specimen could represent a species that has not yet been formally recognized.

Museum collections still hold discoveries

The findings demonstrate why unprepared fossils preserved in museums and research collections can remain scientifically valuable decades after they are collected. Reexamining such material can uncover overlooked bones as new research and better comparative evidence make identification possible.

The fossils may also provide additional information about marine ecosystems in the northwestern Pacific during the Cretaceous Period. Researchers will continue studying the remains to determine whether they belong to a new mosasaur species.

Meeting: The results were presented at the 177th Annual Meeting of the Paleontological Society of Japan, held at Osaka Metropolitan University on June 27, 2026.

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