The fossil record of snakes is poor because of their small size and the fact that they are delicate and do not readily become fossils. However, during the late Cretaceous period between 112-94 million years ago, early snake fossils have been identified. It is thought they evolved from lizard ancestors. Some snakes have vestigial hind limbs--this means they are no longer used for that purpose, however, they are used as anal spurs during mating. Some have remnants of a pelvic girdle, which is found in reptiles possessing limbs. After the dinosaur extinction, snakes as well as mammals experienced an adaptive radiation period. Some fossil evidence points to snakes evolving from burrowing lizards. Subterranean species with bodies adapted to burrowing and very streamlined in shape evolved. Other evidence suggests that snakes may have been related to aquatic reptiles from the Cretaceous period which they in turn, may have had varanid lizard ancestors. Possibly their transparent eyelids may have been an adaptation to a marine habitat. Snakes belong to the Reptile Order called Squamata, along with lizards. They were the most recent of Reptiles to evolve although the fossil record of snakes is incomplete and hypotheses are still being debated.
Snakes, which are members of the reptilian order squamata, are descended from legged reptiles whose fossils date them from the Jurassic. The fossil record indicates that snakes may have lost their legs in two stages; Cretaceous borrowing reptiles such as Najash rionegrina lacked front legs, but did have rear ones.
Eventually snakes became more streamlined, and adapted further to the subterranean environment by losing the rear legs and external ears. Snakes also have transparent eyelids, which are another possible adaptation to living among rocks or dirt.