New York ? A fossil find in Argentina has revealed a two-legged creature that’s the most primitive snake known, a discovery that promises to fire up the scientific debate about whether snakes evolved on land or in the sea.

The snake’s anatomy and the location of the fossil show it lived on land, researchers said, adding evidence to the argument that snakes evolved on land.

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 newly found snake lived in Patagonia some 90 million years ago. Its size is unknown, but it wasn’t more than 3 feet long, said Hussam Zaher of the University of Sao Paulo in Brazil. He and an Argentine colleague report the find in today’s issue of the journal Nature.

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

The creature, named Najash rionegrina, clearly lived on land, both because its anatomy suggests it lived in burrows and because the deposits where the fossils were found came from a terrestrial environment, said Zaher. So, if the earliest known snake lived on land, that suggests snakes evolved on land, he said.

He said that although the creature had two small rear legs, it crawled like a modern-day snake and probably used its legs only on occasion, though for what purpose is unclear.

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.