Why is tuatara grouped on its own?

The tuatara is grouped on its own in the order Rhynchocephalia because it possesses unique characteristics that distinguish it from other reptiles. It has a distinct skull structure, a third parietal eye, and specific reproductive traits that set it apart from lizards and snakes, which belong to the order Squamata. Additionally, the tuatara is the only surviving member of its order, with a lineage dating back over 200 million years, highlighting its evolutionary significance. This unique combination of traits and its ancient lineage justify its classification as a separate group.