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 rhychocephalians. There was even a successful group of aquatic rhychocephalians 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.