Japan fossil reveals first mosasaur premaxilla, possible new species

A fossil collected more than 30 years ago in Osaka Prefecture has yielded four previously unrecognized bones, including the first confirmed premaxilla from a Japanese mosasaur, researchers announced at the 177th Annual Meeting of the Palaeontological Society of Japan on June 27, 2026. The bones were found still embedded in rock that had not been fully prepared, according to a team from Okayama University of Science, Tokyo City University, the Natural History Museum, Kishiwada City, and other institutions.

The fossils were originally discovered between 1990 and 1992 in Sobura, Kaizuka City, Osaka Prefecture, and later stored at the Natural History Museum, Kishiwada City. When scientists recently reexamined the unprepared rock, they identified four bones, including a premaxilla—the bone at the very front of the upper jaw. This is the first time a premaxilla has been confirmed in a mosasaur fossil found in Japan.

The discovery matters because the specimen also shows unusual anatomical features that do not match any previously described mosasaur, suggesting it may belong to a new species. Mosasaurs were large marine reptiles that lived in oceans worldwide during the Late Cretaceous, roughly 98 to 66 million years ago. At the time of the original discovery, researchers had far fewer mosasaur specimens for comparison, making identification difficult; decades of accumulated anatomical data enabled the team to recognize the premaxilla.

In addition to the premaxilla, the researchers examined the basisphenoid, a bone near the braincase, and found distinctive traits: short horn-like projections extending outward and the absence of a ventral median groove. These features differ from those documented in known mosasaurs, raising the possibility that the fossil represents an undescribed species.

The findings highlight the value of revisiting unprepared specimens in museum collections, which may hold important bones overlooked for decades. The work may also help scientists better understand marine ecosystems in the northwestern Pacific during the Cretaceous Period. Further research is needed to determine whether the specimen truly represents a new mosasaur species.