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Why the class of reptiles is divided into four orders, one of which is occupied by two species only, the tuataras? What is so special about the tuatara that makes zoologists put it in an order of its own?

The tuatara is the only living descendant of the order of reptiles known as Rhynchocephalia. This fact distinguishes it from all other modern day reptiles. Rhynchocephalians were once a much larger order of reptiles a few hundred million years ago, with a considerable number of species during the Triassic period. All except for the tuatara apparently went extinct around 60 million years ago, in the late Cretaceous period. The tuatara has been falsely called a living fossil. Though very similar to its extinct ancestors, it has developed features unique to its own modern species. As well, it has been likened to a living dinosaur, due to its extra holes in the skull, boney processes on the ribs and other anatomical features shared with prehistoric reptilians.

Class Reptilia
Subphylum Vertebrata
Phylum Chordata
Kingdom Animalia
Life on Earth