Pachyrhachis problematicus (Haas 1979) Early Cenomian, Late Cretaceous, ~100 mya, ~1.5 m in length was a primitive snake that retained vestigial hind limbs. Derived from a sister to Adriosaurus, Pachyrhachis is a phylogenetic predecessor to most living snakes, except burrowing snakes, like Cylindrophis, which nests with Heloderma, the Gila monster.
Distinct from Adriosaurus, the skull of Pachyrhachis had a tiny loose premaxilla, not bound to the maxillae. The nares covered most of the dorsal surface of the rostrum. The orbit was smaller. The jugal was virtually absent and the postfrontal descended to the maxilla. The teeth were longer and more recurved. The pterygoids and palatines produced similar teeth, all in a line. The mandible was more robust. The posterior dentary descended.
More cervicals, dorsals and caudals were present. No pectoral girdle or forelimbs were present. The hind limb and pelvis were vestiges.