So, this is how Tuatara would have looked, before humans messed the mainland environment up. Sphenodon Punctatus and S. Guntheri. There are also subspecies, but lets not get involved with all that. Burrows Tuataras live separate from each other in well defended burrows. This is not safe for the birds, for wily Tuataras are partial to eggs and chickens in spring time.
Lower teeth fit into a groove between two rows of upper teeth.
The teeth are actually made of bone. Adult tuataras also enjoy young tuatara - in a culinary sense. The female buries up to 19 soft shell eggs in soil, but not often. Life is slow in the cold blooded reptile world and egg laying occurs about once per 4 years. Even laying in soft soil is a contested activity, for other females may dig up the eggs to lay their own.