what taxonomists call a "wastebasket taxon". Williston proposed the Sphenodontia to include only tuatara and their closest fossil relatives in 1925. However, Rhynchocephalia is the older name and in widespread use today. Sphenodon is derived from the Greek for "wedge" (σφήν, σφηνός/sphenos) and "tooth" (ὀδούς, ὀδόντος/odontos).Tuatara have been referred to as living fossils, due to a perception that they retain many basal characteristics from around the time of the squamate–rhynchocephalian split (240 MYA). Morphometric analyses of variation in jaw morphology among tuatara and extinct rhynchocephalian relatives have been argued to demonstrate morphological conservatism and support for the classification of tuatara as a 'living fossil', but the reliability of these results has been criticised and debated. Paleontological research on rhynchocephalians indicates that the group has undergone a variety of changes throughout the Mesozoic, and the rate of molecular evolution for tuatara has been estimated to be among the fastest of any animal yet examined. However, a 2020 analysis of the tuatara genome reached the opposite conclusion: That its rate of DNA substitutions per site is actually lower than for any analysed squamate. Many of the niches occupied by lizards today were formerly held by rhynchocephalians. There was even a successful group of aquatic rhynchocephalians known as pleurosaurs, which differed markedly from living tuatara. Tuatara show cold-weather adaptations that allow them to thrive on the islands of New Zealand; these adaptations may be unique to tuatara since their sphenodontian ancestors lived in the much warmer climates of the Mesozoic. For instance, Palaeopleurosaurus appears to have had a much shorter lifespan compared to the modern tuatara. Ultimately most scientists consider the phrase 'living fossil' to be unhelpful and misleading. A species of sphenodontine is known from the Miocene Saint Bathans Fauna. Whether it is referable to Sphenodon proper is not entirely clear, but is likely to be closely related to tuatara. Species While there is currently considered to be only one living species of tuatara, two species were previously identified: Sphenodon punctatus, or northern tuatara, and the much rarer Sphenodon guntheri, or Brothers Island tuatara, which is confined to North Brother Island in Cook Strait. The specific name punctatus is Latin for "spotted", and guntheri refers to German-born British herpetologist Albert Günther. A 2009 paper re-examined the genetic bases used to distinguish the two supposed species of tuatara, and concluded they only represent geographic variants, and only one species should be recognized. Consequently,