Pachyrhachis problematicus (Haas 1979) Early Cenomian, Late Cretaceous, ~100 mya, ~1.5 m in length was a primitive snake that retained vestigial hind limbs. It was derived from a sister to Adriosaurus, Dinilysia and Tetrapodophis (see below).
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 fused to the jugal. The postorbital and squamosal disappearted. The supratemporal was larger and took over the duty of anchoring the quadrate. 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.
See an animation demonstrating the crushing of the Pachyrhachis skull here.