For nearly 150 million years, they were apex predators of the ocean -- streamlined sleek-snouted carnivores called ichthyosaurs.
To the casual observer, ichthyosaurs would have looked a bit like long-nosed tuna.
Yet they were no fish.
According to theory, their forebears were land-loving reptiles which adapted to the sea after the world was struck by a mass extinction.
But for years, a key piece in this intriguing hypothesis was missing: proof that this sea-dwelling species had ever had terrestrial adaptations.
That vital evidence has now been found, US and Chinese paleontologists reported in the journal Nature on Wednesday.
After a decade-long search, scientists said they had found the first fossil of an amphibious ichthyosaur.
It had large, flexible flippers for seal-like movement on land, and a rugged bone structure to resist the battering of coastal waves.
"It is a milestone," said study co-author Da-yong Jiang, a Peking University professor.
"Before our findings, all fossils of ichthyosaurs showed them as fully marine animals."
The mystery, he told AFP by email, was that these marine reptiles seemed to have popped up from nowhere some 250 million years ago.
"It was a really puzzling clade," or family tree, said Jiang.
"There was a huge gap between these marine creatures and their ancestor -- (there was) no evidence or any information to link to its ancestor and link to the land."
The fossil, discovered in a quarry near Chaohu in China's Anhui province, is of an early ichthyosaur which lived about 248 million years ago.
About 40 centimetres (25 inches) long, it had a much shorter snout and a flexible, cartilaginous wrist -- features which give it the Latin name Cartorhynchus lenticarpus -- and thicker bones than previously-identified ichthyosaurs.
The fossil shows "the transition," said Ryosuke Motani of the University of California at Davis.
The creature probably swam near the coast and fed by sucking up worms from the sea floor, added Jiang.
The research also shed light on how life bounced back four million years after the Permian-Triassic mass extinction, the scientists said.
Sometimes called "the Great Dying," the event wiped out the vast majority of sea species and terrestrial vertebrates, according to the fossil record.
The cause has been much debated. One theory is runaway global warming, possibly caused by the escape of methane gas from the sea floor.