the ischium, and the antorbital region of the skulls is between a third and a quarter of the total skull length. The family Sphenodontidae has been used to include the tuatara and its closest relatives within Rhynchocephalia. However the grouping has lacked a formal definition, with the included taxa varying substantially between analyses. The closest relatives of the tuatara are placed in the clade Sphenodontinae, which are characterised by a completely closed temporal bar. Paleobiology Rhynchocephalians were once considered to be a morphologically conservative group with little diversity. However, discoveries in recent decades have disputed this, finding a wide array of diversity within the clade. Early rhynchocephalians possess small ovoid teeth designed for piercing, and were probably insectivores. Amongst the most distinct rhynchocephalians are the pleurosaurs, known from the Jurassic of Europe, which were adapted for marine life, with elongated snake-like bodies with reduced limbs, with the specialised Late Jurassic genus Pleurosaurus having an elongated triangular skull highly modified from those of other rhynchocephalians. Several other lineages of rhyncocephalians have been suggested to have had semi-aquatic habits. Eilenodontines are thought to have been herbivorous, with batteries of wide teeth with thick enamel used to process plant material. The sapheosaurids, such as Oenosaurus and Sapheosaurus from the Late Jurassic of Europe possess broad tooth plates unique amongst tetrapods, and are thought to have been durophagous, with the tooth plates being used to crush hard shelled organisms. Evolutionary history Rhynchocephalia is estimated to have diverged from Squamata between the Middle Permian and earliest Triassic, between 270 and 252 million years ago. The oldest known remains of rhynchocephalians are indeterminate jaw fragments from the Erfurt Formation near Vellberg in Southern Germany, dating to the Ladinian stage of the Middle Triassic, around 238-240 million years old. Rhynchocephalians reached a worldwide distribution across Pangaea by the end of the Triassic, with the Late Triassic-Early Jurassic genus Clevosaurus having 10 species across Asia, Africa, Europe, North and South America. The earliest rhynchocephalians were small animals, but by the Late Triassic the group had evolved a wide range of body sizes. During the Jurassic rhynchocephalians reached their apex of morphological diversity, including specialised herbivorous and aquatic forms. The only record of Rhynchocephalians from Asia are indeterminate remains of Clevosaurus from the Early Jurassic (Sinemurian) aged Lufeng Formation of Yunnan, China. Rhynchocephalians are noticeably absent from younger localities in the region, despite the presence of favourable preservation conditions.