Najash rionegrina is the name given to the fossils of the most primitive snake yet found (based on phylogenetic analysis). Like a number of other Cretaceous snakes (see below), it had two hind-limbs. The fossils were found in a land-based deposit, and the robust backbone vertebrae and rear legs of the snake are adapted to a burrowing, subterranean environment. This is significant, because it suggests that snakes had a terrestrial origin, and presents new evidence in the complicated sequences that led to limb reduction and loss as snakes developed from their lizard ancestors.

The snake, the oldest snake yet found, was no more than 1.5 meters (3 feet) long, and lived during the Late Cretaceous period (ca. 90 million years ago) in what is now the Rio Negro province (hence its specific name rionegrina) of Patagonia, Argentina.

This burrowing creature had not lost its sacrum, the pelvic bone composed of several fused vertebrae, nor its pelvic girdle which are absent in modern snakes, and in all other known fossil snakes as well Other known fossil snakes with developed hindlimbs, Haasiophis, Pachyrhachis and Eupodophis—all found in marine environments— all lack a sacral region..

The discoversy seems to put to rest the suggestion, first offered by the nineteenth-century paleontologist Edward Drinker Cope, that snakes had evolved from marine mosasaurs. His suggestion seemed to receive new impetus with the discovery in the 1990s of the primitive snakes with vestigial limbs in marine sediments in Lebanon.

On a side note, this prehistoric snake was named after the biblical legged snake of Genesis, Nachash, who tempted the first family, Adam and Eve to eat from a forbidden fruit tree.

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