Sphenodontia

The Sphenodontia is a diverse group of reptile. It includes only one living genus, the tuatara (Sphenodon). An alternative name for the order is the Rhynchocephalia. 

They are an offshoot from the  evolutionary  line leading to the lizards and snakes. They had many families during the late  Triassic and  Jurassic periods but then declined. They are now are the least successful of the lepidosaurs.Evans S.E. & Jones M.E.H. 2010. The origin, early history and diversification of lepidosauromorph reptiles. In Bandyopadhyay S. (ed) New aspects of Mesozoic biodiversity,  Lecture Notes in Earth Sciences 132, 27-44.  

The only surviving member today is the New Zealand tuatara.

 References 

Category:Scaled reptiles