How has the tuatara survived all this time? Apparently other species have diversified significantly over the last 100 million years or so — there are 9,000 species of birds, for example — but the tuatara has only 2 species:
Alfaro and his colleagues analyzed DNA sequences and fossils from 47 major vertebrate groups and used a computational approach to calculate whether the “species richness” of each group was exceptionally high or low. The research allows scientists to calculate for the first time which animal lineages have exceptional rates of success.
… The tuatara, which lives in New Zealand and resembles lizards — although it is actually a distant cousin — has only two species. “In the same period of time that produced more than 8,000 species of snakes and lizards, there were only two species of tuatara,” Alfaro said.
That’s pretty amazing. Forest & Bird have a fact sheet for kids that tells us more about the 2 species of tuatara:
In 1989 Dr Charles Daugherty, a professor at Victoria University in Wellington, discovered that there were two species of tuatara, Sphenodon punctatus and Sphenodon guntheri. …
The most common species is known simply as tuatara (Sphenodon punctatus, Cook Strait tuatara). There are around 50,000 of them living on Stephen’s Island in the Marlborough Sounds. …
There are only about 400 adults of the second species Sphenodon guntheri, Brother’s Island tuatara (known as Gunther’s or Brothers tuatara).
[Via : Tuatara Fact Sheet.]
The tuatara in my photo above came from Stephen’s Island, so I guess it’s a Sphenodon punctatus. I took the photo a couple of years ago when a number of tuatara were released into the Karori Sanctuary. I was very privileged to be able to see the animal from only a few centimetres away, and even to briefly touch it. Now they live quietly behind fences.
Wikipedia has a useful article with more information about tuatara.