The two species of Tuatara, Sphenodon guntheri and Sphenodon punctatus are the only living members within the entire order of Sphenodontia. Way back in the day, I'm talking 200 million years back, members of this group flourished. But over time, all but these two species died out, leaving the other orders within the Reptilia class to far outnumber them not only in regards to species count, but in world domination as well. They are sometimes considered living fossils. Tuataras only live in one location in the entire world- New Zealand. Specifically, small islands off the coast of the larger two that are not heavy with predators. (Though reintroduction efforts on the mainland have been ongoing) They've been living as the last of their kind for nearly 60 million years, though molecular work has confirmed that changes have occurred in the past 200 million years, though they still retain many primitive characteristics.
While they may look like lizards... they aren't. Like I mentioned, Tuataras are not a part of the Squamata order (which houses lizards & snakes), they are their own distinct branch of Reptilia. They have dentition unlike that of any other type of animal, with two rows of teeth on the upper jaw overlapping one row on the lower. They also have a second bony ridge in their skull, which doesn't exist in lizards or snakes but can be found in crocodilians. They also have a third eye, hidden between the other two under a layer of skin and scales that scientists believe may act as a light sensor. Tuataras also live an extremely long amount of time. They don't even stop their growth until around age 35 and can live well over 100 years. In 2009, a 111 year old male and an 80 year old female successfully produced 11 offspring, which is fantastic for a species that is becoming so low in numbers. And have I mentioned their ability to live in cold weather climates? Tuataras have a lower body temperature than other reptiles and are able to function in weather as low as 40 degrees, and find anything over 80 to be unbearable, even deadly. This has allowed them to live for all this time in their colder, New Zealand homes.
image from pixdaus.com