Extra-long rear flippers might have aided propulsion in concert with its long finned tail. And unlike other mosasaurs, or large extinct marine reptiles, it had a dorsal fin like a shark's that would have helped it turn quickly and with precision in the water.

University of Cincinnati Associate Professor Takuya Konishi and his international co-authors described the mosasaur and placed it in a taxonomic context in the Journal of Systematic Palaeontology.

The mosasaur was named for the place where it was found, Wakayama Prefecture. Researchers call it the Wakayama Soryu, which means blue dragon. Dragons are creatures of legend in Japanese folklore, Konishi said.

"In China, dragons make thunder and live in the sky. They became aquatic in Japanese mythology," he said.

The specimen was discovered along the Aridagawa River in Wakayama by co-author Akihiro Misaki in 2006. Misaki was looking for fossils of invertebrates called ammonites when he found an intriguing dark fossil in the sandstone, Konishi said.

Misaki continued looking for ammonites before curiosity got the better of him and he returned to the dark bone. Closer examination revealed it was a vertebra, part of a nearly complete mosasaur captured in the hard sandstone.