A fossil collected in Osaka Prefecture, Japan, more than three decades ago has just given up a secret: four bones that had been hiding in plain sight inside the rock. Among them is the first premaxilla, the frontmost bone of the upper jaw, ever confirmed in a Japanese mosasaur fossil. Even more intriguing, the skull's unusual features do not match any known mosasaur, suggesting the remains could belong to an entirely new species.
A Surprise in Unprepared Rock
The bones were found in pieces of rock that had not been fully prepared since the original mosasaur remains were discovered between 1990 and 1992 in Sobura, Kaizuka City, Osaka Prefecture. The specimens were stored at the Natural History Museum, Kishiwada City, where researchers recently decided to take another look at the unprepared material. That decision paid off: four fossil bones were still embedded in the surrounding rock, waiting to be recognized.
When the fossils were first found, scientists had far fewer mosasaur specimens to compare them with, making identification difficult. In the decades since, researchers worldwide have built a much larger collection of comparative fossils and anatomical data. That new knowledge allowed the team, which included scientists from Okayama University of Science, Tokyo City University, the Natural History Museum, and Kishiwada City, to identify the premaxilla for the first time in a Japanese mosasaur.
Odd Skull Features Point to a New Species
The researchers also examined the basisphenoid, a bone near the braincase, and found several distinctive characteristics. The bone has short, horn-like projections extending outward, and it lacks a ventral median groove. These features differ from those seen in previously described mosasaurs, raising the possibility that the animal represents a species new to science.
Mosasaurs were large marine reptiles that swam in oceans worldwide during the Late Cretaceous, roughly 98 to 66 million years ago. The newly identified bones add to a growing picture of these ancient predators, but the unusual combination of traits sets this specimen apart.
Why This Matters for Fossil Collections
The discovery highlights the value of revisiting unprepared specimens stored in museums and research collections. Fossils that have been kept for decades can still yield surprises when examined with fresh eyes and modern knowledge. The findings, published by the research team, demonstrate that patience and reexamination can turn old rocks into new discoveries, expanding our understanding of prehistoric life in Japan and beyond.
