How do tuatara differ physically from typical lizards?

Although tuatara are often described as lizardlike, they belong to a distinct order of reptiles called Rhynchocephalia, while typical lizards belong to the order Squamata. Physically, they differ from lizards in several key ways:

• Skull Structure: Unlike lizards, which have lost the lower temporal bar to allow for greater jaw mobility, tuatara possess enclosed temporal fossae (depressions) in the skull.1
• Dentition: Tuatara feature a unique, beaklike extension of the premaxillary bones that replaces the front teeth, whereas lizards typically possess conical or bladelike teeth.12
• Anatomy: Tuatara possess gastralia, or abdominal ribs, a feature not found in modern lizards.1
• Sensory Organs: Tuatara lack external ear openings, though they do possess a middle ear cavity with a stapes for conducting sound. Additionally, they have a well-developed parietal (third) eye on the top of the head, which functions as an endocrine organ to regulate hormones rather than as an organ of vision.1

While both groups share traits such as a transverse cloacal opening and the ability to shed skin, these anatomical differences highlight the tuatara's status as the sole survivor of an ancient lineage.1