A mosasaurus fossil from Japan has been identified by researchers. This fossil is approximately the size of a large white shark and is believed to have lived around 72 million years ago in the Pacific Ocean. The mosasaurus displays a very long tail fin, with a trailing long tail. Another uniqueness lies in its dorsal fin, resembling that of a shark, possibly for quick and precise turns in the water.

This finding originated from the Aridagawa River in Wakayama in 2006, discovered by Akihiro Misaki. At that time, Misaki was actually searching for invertebrate fossils, such as ammonites, but instead found an interesting mosasaurus fossil buried in a layer of sandy rock.

The researchers identified this specimen in the subfamily Mosasaurinae and named it Megapterygius wakayamaensis based on its discovery location. "Megapterygius" refers to "large winged," referring to its large fin. Another prehistoric marine animal known as plesiosaurus uses paddle-like fins for movement but does not have a long guiding tail.

Experts speculate that the large size of the front fins may assist in quick maneuvering, while the large size of the hind fins may provide the ability to dive or resurface. Like other mosasaurs, the tail of this mosasaur is used to provide strong and fast acceleration when searching for prey.