Opisthiamimus gregori (DeMar, Jones and Carrano 2022, Late Jurassic) nests close to Plancephalosaurus in the LRT. It is known from four specimens, which together preserve a nearly complete skeleton.

Heleosuchus griesbachi (Owen 1876, Late Permian, BMNH, NHM 2374) is poorly preserved but appears to nest close to Planocephalosaurus.

Planocephalosaurus lucasi (Heckert 2004) Carnian, Late Triassic, ~20 cm in length, was a rhynchocephalian derived from a sister to Late Permian Helosuchus.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. The hind limbs were much smaller and more gracile beginning a trend that terminated with Late Jurassic Opisthiamimus (above).

Carroll RL 1985. A pleurosaur from the Lower Jurassic and the taxonomic position of the Sphenodontids.

DeMar DG, Jones MEH and Carrano MT 2022. A nearly complete skeleton of a new eusphenodontian from the Upper Jurassic Morrison Formation, Wyoming, USA, provides insight into the evolution and diversity of Rhynchocephalia (Reptilia: Lepidosauria). Journal of Systematic Palaeontology. 20 (1): 1–65.

Fraser NC and Sues H-D 1997. In the Shadows of the Dinosaurs: early Mesozoic tetrapods. Cambridge University Press, 445 pp. Online book.

Heckert AB 2004. Late Triassic microvertebrates from the lower Chinle Group (Otischalkian-Adamanian: Carnian), southwestern U.S.A. New Mexico Museum of Natural History and Science Bulletin 27:1-170.

Meyer H 1831. IV Neue Fossile Reptilien, aud der Ordnung der Saurier.

wiki/Planocephalosaurus
wiki/Opisthiamimus
wiki/Pleurosaurus