This beak-headed reptile uses a "steak knife sawing motion" as it chews and the findings could help explain how the species has continued to adapt to a changing world over more than 200 million years, the researchers said.
"Some reptiles such as snakes are able to swallow their food whole but many others use repeated bites to break food down," said study author Marc Jones of the University College London.
"The tuatara also slices up its food, much like a steak knife," Jones was quoted as saying by LiveScience.
Scientists thought that the only animals that chewed their food up so well were mammals. This new discovery shows it doesn't take a high metabolism, which mammals have and reptiles don't, to develop complex chewing.
"The slicing jaws of the tuatara allow it to eat a wide range of prey, including beetles, spiders, crickets and small lizards," Jones said. "There are also several grisly reports of sea birds being found decapitated following predation by tuatara."
Though the reptile looks like most lizards, it belongs to a special grouping of animals in a different lineage called Sphenodontia. There are only two members of this group, which was widespread about 200 million years ago.
For their study, published in the journal The Anatomical Record, the researchers used a sophisticated computer model to show that when the Tuatara closes its mouth, its lower jaw rocks forward, turning each tooth into a steak knife, slashing through its food.
The slight give in the connection between the jaws allows them to rotate, making the tearing action even more effective.
This kind of thorough chewing is commonly found in warm-blooded animals like mammals (including humans). This new find indicates that warm-bloodedness and chewing aren't as closely linked as previously thought, since the tuataras are cold-blooded and have a slow metabolism, they said.
Extinct relatives of the tuatara probably had similar chewing mechanisms back to the time of the dinosaurs, they added.