Najash is an extinct genus of basal snake. 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 were adapted to a burrowing, subterranean environment. This is significant, as 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 was no more than 1.5 meters (5 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.
The discoversy does not support the hypothesis, first offered by the nineteenth-century paleontologist Edward Drinker Cope, that snakes share a common marine ancestry with mosasaurs. The marine origin hypothesis received new impetus with the discovery in the 1990s of basal snakes with vestigial limbs in marine sediments in Lebanon.
The generic name comes from the biblical legged snake of Genesis, Nachash, who tempted Adam and Eve to eat from a forbidden fruit tree.