Museums hold countless specimens that were collected decades ago and never fully studied. Occasionally, one of them is hiding a surprise. Researchers from Okayama University of Science, Tokyo City University and the Natural History Museum in Kishiwada City have now found exactly that: four previously unrecognized bones of a giant Cretaceous mosasaur, sitting inside rock collected in Osaka Prefecture more than 30 years ago.
Mosasaurs were large marine reptiles that inhabited oceans around the world during the Late Cretaceous, roughly 98 to 66 million years ago. The fossils in this study were first discovered between 1990 and 1992 in Sobura, Kaizuka City, Osaka Prefecture, and later preserved at the Natural History Museum in Kishiwada. When the team recently returned to pieces of rock that had not yet been fully prepared, they found four fossil bones still embedded in the surrounding material.
Among them, one stands out: a premaxilla, the bone at the very front of the upper jaw. It is the first time a premaxilla has been confirmed in a mosasaur fossil discovered in Japan. When the fossils were originally found, scientists had far fewer mosasaur specimens for comparison, which made it difficult to identify exactly what the bones represented. Decades of accumulated anatomical knowledge allowed the team to finally recognize it.
Even more intriguing are features of the basisphenoid, a bone positioned near the braincase. It has short, horn-like projections extending outward and lacks a ventral median groove — characteristics that differ from those documented in previously known mosasaurs. That unusual combination raises the possibility that the specimen belongs to a species that has not yet been described.
The findings illustrate how valuable it can be to revisit unprepared specimens in museum collections: fossils kept for decades may still contain important bones or evidence that went unrecognized when they were first collected. The discovery could also help scientists build a clearer picture of marine ecosystems in the northwestern Pacific during the Cretaceous. Additional research will be needed to confirm whether the animal truly represents a new species. The results were presented at the 177th Annual Meeting of the Palaeontological Society of Japan, held at Osaka Metropolitan University on June 27, 2026.
