Planocephalosaurus was one of the first sphenodonts and bore a strong resemblance to the extant tuatara, albeit much smaller, at only 20 centimetres (7.9 in) in length. The main difference was that Planocephalosaurus's teeth were fused with the cartilage, unlike the tuatara's. The creature is presumed to have fed on large invertebrates and small vertebrates