A new species of dinosaur-era sea reptile that could live on both land and in water has been unearthed in China—the first amphibious "sea monster" ever found.

The ichthyosaur, whose discovery was announced Wednesday, fills a crucial gap in the evolution of these dolphin-like predators, which thrived in Jurassic seas about 200 million to 145 million years ago. The reptiles could grow to 65 feet (20 meters) in length, about as long as a tractor-trailer.

Scientists knew that ichthyosaurs evolved from land to the sea, since they have found fossils of both land-dwelling ancestors and the fast-swimming marine creatures—sometimes nicknamed "sea monsters."

So paleontologists suspected there must be some in-between ichthyosaurs out there. For instance, both whales and plesiosaurs,