The tuatara is genus of reptiles found only in New Zealand. The genus contains only two living species - the Brothers Island tuatara (Sphenodon guntheri) and the Northern tuatara (Sphenodon punctatus). When the tuatara was first scientifically classified in 1831 they were assumed to be lizards. In 1867 the order Rhynchocephalia (meaning “beak head”, in reference to the distinctive skull structure) was created by Albert Günther of the British Museum, for the tuatara and its fossil relatives.
The tuatara is often referred to as a living fossil due to several primitive features that they still have, though they have changed significantly since the Mesozoic era.
The brain structure and manner in which tuatara move is more similar of amphibians than reptiles, whilst their heart has the most primitive structure of any reptile and their lungs have only a single chamber and lack bronchi.
The skulls of tuataras also shows their age - it is simple in build with two openings (temporal fenestra) on each side of the skull, with complete arches, and the upper jaw is firmly attached to the skull. This makes for a very rigid, inflexible construction.
The tip of the upper jaw is beak-like and separated from the remainder of the jaw by a notch. There is a single row of teeth in the lower jaw and a double row in the upper, with the bottom row fitting perfectly between the two upper rows when the mouth is closed. This specific tooth arrangement is not seen in any other reptile. The jaws chew with backwards and forwards movements combined with a shearing up and down action. As their teeth wear down, older tuatara have to switch to softer prey such as earthworms, larvae, and slugs, and eventually have to chew their food between smooth jaw bones.
These are just some of the physical features which differentiate tuataras from squamata (lizards and snakes). If you wish to read more just check out the wikipedia page on these amazing animals [x]