Planocephalosaurus lucasi (Heckert 2004) Carnian, Late Triassic, ~20 cm in length, was a rhynchocephalian derived from a sister to Gephyrosaurus and phylogenetically preceded the living example of this lineage, Sphenodon.
Distinct from Gephyrosaurus, the skull of Planocephalosaurus had teeth fused to the jawbones. The rostral profile was more squared off and the maxilla was longer on its dorsal rim. The postorbital portion of the skull was slightly longer. The mandible appears to have had two low coronoid processes separated by a depression. In palatal view the internal nares were narrower and further separated by a wider pair of vomers. The transverse processes of the pterygoid were turned posteriorly. The parasphenoid and basisphenoid (both beneath the brain between the quadrates) were greatly enlarged.
The scapulocoracoid were coossified. The pelvis was relatively much larger, especially ventrally.
Heleosuchus griesbachi Owen, 1876 (BMNH, NHM 2374) is poorly preserved but appears to nest close to Planocephalosaurus.