A new fossil discovery has revealed the most primitive snake known, a crawling creature with two legs, and it provides new evidence that snakes evolved on land rather than in the sea.

Snakes are thought to have evolved from four-legged lizards, losing their legs over time. But scientists have long debated whether those ancestral lizards were land-based or marine creatures.

The new find reveals a snake that lived in the Patagonia region of Argentina some 90 million years ago, said Hussam Zaher of the University of Sao Paulo in Brazil, who describes the find in Thursday's issue of the journal Nature. Its size is unknown, but it wasn't more than 3 feet long, he said in a telephone interview.

It's the first time scientists have found a snake with a sacrum, a bony feature supporting the pelvis, he said. That feature was lost as snakes evolved from lizards, and since this is the only known snake that hasn't lost it, it must be the most primitive known, he said.

The creature clearly lived on land, both because its anatomy suggests it lived in burrows and because the deposits in which the fossils were found came from a terrestrial environment, he said.

So, if the earliest known snake lived on land, that suggests snakes evolved on land, he said.

Little new evidence had appeared in recent years in the land-versus-sea debate, he said, and "we needed something new. We needed a new start. And this snake is definitely a new start for this debate."

While the creature still had two small rear legs, it crawled like a modern-day snake, he said. It probably used its legs only on occasion, though it's not clear for what, he said.

The creature, named Najash rionegrina, is "a fantastic animal," said Jack Conrad, a researcher at the American Museum of Natural History in New York and co-curator of an upcoming exhibit on lizards and snakes.

"It's really going to help put to rest some of the controversy that's been going around snake evolution and origins," he said. Conrad said he never took sides in the land-versus-sea debate, but "but this is starting to convince me."

Olivier Rieppel, a fossil reptile expert at the Field Museum in Chicago, called the find important and said Najash is clearly the most primitive known snake.

If snakes did evolve on land rather than the sea, their fossil record might be less complete because early fossils would have been better preserved in a marine environment, he said.

That, in turn, suggests "we may not know all the lineages of early snake evolution," he said. Maybe several snake lineages lost the legs of their lizard ancestors independently, he said.

The creature's name comes from a Hebrew word for "snake" and the Rio Negro province of Argentina, where the discovery took place.