Fossils of the genus are primarily known from fissure fill deposits from the Late Triassic of southwest Britain, with fragmentary remains possibly belonging to the genus also known from the Late Triassic of Texas.

The genus and the type species P. robinsonae was described in 1982 by Fraser from remains found in fissure fill deposits in Gloucestershire in southwest Britain.

[2] P. robinsonae is known from disarticulated remains covering most of the skeleton,[1][3], while P. lucasi is known from fragments of the jaws.

[3] Unlike the modern tuatara, the skull is thought to lack a complete lower temporal bar connecting the jugal and the quadrate, though a complete temporal bar may have been variably present in some adult individuals.

[1][7][8] Planocephalosaurus is thought to have been insectivorous, with it possibly also feeding on small vertebrates.