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A fossil find in Argentina has revealed a two-legged creature that is 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. Its size is unknown, but it wasn't more than a metre long, said Hussam Zaher of the University of Sao Paulo in Brazil. He and an Argentine colleague report the find in Thursday'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.