[4][5] Modern snakes are thought to have evolved from either burrowing or aquatic lizards during the mid-Cretaceous period, and the earliest known fossils date to around 112 Ma ago.

[6] Scolecophidia Najash rionegrina Dinilysia patagonica Anilius Cylindrophis Uropeltinae Anomochilus Madtsoiidae Loxocemus Macrostomata Below is different phylogenetic overview of ophidians, following the study by Caldwell et al.

[10]: 11 [11] Pythons and boas—primitive groups among modern snakes—have vestigial hind limbs: tiny, clawed digits known as anal spurs, which are used to grasp during mating.

The axial skeleton of the snakes' common ancestor, like most other tetrapods, had regional specializations consisting of cervical (neck), thoracic (chest), lumbar (lower back), sacral (pelvic), and caudal (tail) vertebrae.

Early in snake evolution, the Hox gene expression in the axial skeleton responsible for the development of the thorax became dominant.

[12] Many researchers believe that snakes share a common marine ancestry with mosasaurs, a suggestion advanced in 1869 by Edward Drinker Cope, who coined the term Pythonomorpha to unite them.

[11][19] According to this hypothesis, the fused, transparent eyelids of snakes are thought to have evolved to combat marine conditions (corneal water loss through osmosis), and the external ears were lost through disuse in an aquatic environment.

Fossilized snake remains are known from early Late Cretaceous marine sediments, which is consistent with this hypothesis; particularly so, as they are older than the terrestrial Najash rionegrina.

[clarification needed] The cladogram below, based on Palci and Caldwell (2010), suggests that snakes are more closely related to Dolichosaurids than mosasaurs proper.

Anguidae Xenosaurus Shinisaurus Heloderma Lanthanotus Varanus Aigialosauridae Mosasauridae Coniasaurus Dolichosaurus Pontosaurus Aphanizocnemus Adriosaurus Acteosaurus Ophidia