Loosely resembling modern dolphins, Ichthyosaurs were distant relatives of snakes and lizards. But though numerous Ichthyosaur fossils have been found since the 19th century, paleontologists remain unsure which animal they may have evolved from.

Now, researchers say they have recovered an early Ichthyosaur fossil in southern China that fills in some evolutionary blanks. The specimen, which lived about 248 million years ago, had unusually large flippers that could have been used to walk on land. It retained other features of land-dwelling reptiles as well, including a short snout and body trunk.

The findings support the theory that Ichthyosaurs evolved from land-dwelling reptiles that returned to the ocean, said Ryosuke Motani, a paleontologist at the University of California, Davis, and the lead author of the study, which was published online by the journal Nature.

“When reptiles and mammals go marine, they typically go through a stage where they become amphibious and heavy, presumably to counter the waves near the coast,” he said. “This stage was mysteriously lacking from the fossil record of Ichthyosaurs before. Now we have this new animal to fill the gap.”