Crythiosaurus mongoliensis (Gilmore 1943) Late Cretaceous ~90 mya, was a primitive amphibaenian known only from a partial skull without a mandible. Derived from a sister to Sineoamphisbaeana, Crythiosaurus phylogenetically preceded Spathorhynchus.
Distinct from Sineoamphisbaeana, the skull of Crythiosaurus further expanded the occipital portion, further exposing it to the dorsal and lateral views. The supraoccipital became one of the largest bones in the skull. Overall the skull was narrower and longer.
It is likely that Crythiosaurus had an elongated torso with reduced limbs.