Spathorhynchus natronicus (Berman 1977) Early Oligocene ~30 mya, ~2.5 cm skull length. Derived from a sister taxa to Sineoamphisbaena and Crythiosaurus, Spathorhynchus phylogenetically preceded Tamaulipasaurus and Bipes.

Distinct from Crythiosaurus, the skull of Spathorhynchus had an elongated snout overhanging the jaws. The jugal extended dorsaly with no connection to the squamosal at the rear of the skull. The mandible developed an elongated retroarticular process that was lateral to the supine quadrate. The nares opened ventrally, anterior to the jaws. The internal nares opened at mid skull.