A mosasaurus fossil from Japan was identified by researchers. This fossil is comparable in size to a great white shark and is believed to have lived around 72 million years ago in the Pacific Ocean. The mosasaurus features a very long rear fin, with a long, fluked tail. Another uniqueness lies in its dorsal fin that resembles a shark, possibly for turning quickly and precisely in the water.

This finding comes 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 sandstone.

The research team classified this specimen in the subfamily Mosasaurinae and named it Megapterygius wakayamaensis based on its discovery location. "Megapterygius" refers to "large-winged," referring to the large fins it possesses. Another prehistoric marine animal known as the plesiosaur used paddle-like fins for movement, but did not have a long steering tail.

Experts speculate that the large size of the front fins may assist in quick maneuvers, while the large back fins may provide the ability to dive or surface. Like other mosasaurs, the tail of this mosasaur is used to provide strong and rapid acceleration when hunting for prey.