Scientists from multiple Japanese institutions have announced the discovery of four previously unidentified fossil bones embedded within rock samples that contained mosasaur remains. The specimens were originally collected between 1990 and 1992 in Sobura, Kaizuka City, Osaka Prefecture, and have been stored at the Natural History Museum in Kishiwada City since their initial excavation.
The most significant find is a premaxilla bone, which forms the front portion of the upper jaw. This marks the first confirmed discovery of a premaxilla in a mosasaur fossil recovered from Japan. Researchers also identified a basisphenoid bone near the braincase, which displays distinctive characteristics not previously documented in known mosasaur species. The basisphenoid features short outward-projecting structures and lacks a characteristic groove typically found in other mosasaur specimens.
When the fossils were originally discovered, scientists had limited comparative materials available for detailed analysis. In the intervening decades, the international paleontological community has accumulated substantially more mosasaur specimens and anatomical data, enabling more precise identifications. This expanded knowledge base allowed the research team to recognize the significance of bones that had remained unnoticed in unprepared rock samples for approximately three decades.
Mosasaurs were large marine reptiles that thrived in oceans worldwide during the Late Cretaceous period, approximately 98 to 66 million years ago. The unusual combination of features documented in these specimens suggests the fossil may belong to a previously unknown mosasaur species, which could provide new insights into Cretaceous marine ecosystems in the northwestern Pacific region. The findings were presented at the 177th Annual Meeting of the Palaeontological Society of Japan on June 27, 2026, at Osaka Metropolitan University.
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