allows the articulation of the fibula, the development of a sexual segment of the kidney, and a number of traits of the feet bones, including a fused astralago-calcaneun and enlarged fourth distal tarsal, which creates a new joint, along with a hooked fifth metatarsal. Like some squamates, the tuatara retains a parietal eye, which has been lost in the other groups of extant reptiles, the turtles and archosaurs. Rhynchocephalians are distinguished from squamates by a number of traits, including the retention of gastralia (rib-like bones present in the belly of the body, ancestrally present in tetrapods and also present in living crocodilians). The complete lower temporal bar (caused by the fusion of the jugal and quadtrate/quadratojugal bones) of the tuatara, often considered a primitive feature, is actually a derived feature among sphenodontians, with the most primitive lepidosauromorphs and rhynchocephalians having an open lower temporal fenestra. Unlike squamates, but similar to the majority of birds, the tuatara lacks a penis. This is a secondary loss, as a penis or squamate-like hemipenes were probably present in the last common ancestor of rhynchocephalians and squamates. The dentition of most rhynchocephalians is described as acrodont (the condition where the teeth are attached to the crest of the jaw bone, and lack roots), similar to those of acrodontan lizards like agamids. The term "acrodont" has also been used in reference to the absence of tooth replacement or the extent of bone growth around the teeth, causing terminological confusion. The teeth of living tuatara have no roots and are not replaced, and are extensively fused to the jaw bone. The teeth of Gephyrosaurus are pleurodont (teeth are weakly attached to the inner part of the mandible with no sockets, and replaced throughout life), like those of most squamates, and this is thought to be the ancestral condition of Lepidosauria. The most primitive sphenodontians have a combination of both pleurodont and acrodont teeth. Some rhyncocephalians differ from these conditions, with Ankylosphenodon having teeth that continue deeply into the jaw bone, and are fused to the bone at the base of the socket (ankylothecodont). Rhynchocephalians possess palatal dentition (teeth present on the bones of the roof of the mouth). In most rhynchocephalians, the teeth present on the pterygoid bone are lost, but the lateral tooth row present on the palatine bones are enlarged, and orientated parallel to the teeth of the maxilla. During biting, the teeth of the