Rhynchocephalia

Rhynchocephalia is an order of lizard-like reptiles that includes only one living genus, the tuatara (Sphenodon), and only two living species. Despite its current lack of diversity, the Rhynchocephalia at one time included a wide array of genera in several families, and represents a lineage stretching back to the Mesozoic Era. Many of the niches occupied by lizards today were then held by sphenodontians. There was even a successful group of aquatic sphenodontians known as pleurosaurs, which differed markedly from living tuatara.