between the quadratojugal and maxilla. In diapsids, the quadratojugal and jugal form the lower temporal bar, which defines the lower border of the infratemporal fenestra, one of two holes in the side of the head. In early diapsids such as Petrolacosaurus and Youngina, the quadratojugal is long as in amphibians, early synapsids, and "anapsid" reptiles. It forms most of the length of the lower temporal bar. However, significant transformation of the temporal region of the skull occurs in many more "advanced" members of Diapsida, with implications for the structure of the quadratojugal. Numerous diapsids have an incomplete lower temporal bar, where the quadratojugal and jugal fail to contact each other. This leaves the infratemporal fenestra with an arch-like structure, open from below. An incomplete (or absent) lower temporal bar is first seen in the Permian genus Claudiosaurus, and is retained by most other Permian and Triassic diapsids. In many cases, the quadratojugal is lost completely. This loss occurs in several Triassic marine reptiles such as tanystropheids, thalattosaurs, pistosaurs, and plesiosaurs. Squamates, the group containing modern lizards and snakes, also lack a quadratojugal, but early squamate relatives such as Marmoretta do retain the bone. Ichthyosaurs, a group without a lower temporal bar, have a quadratojugal that is taller than it is long, stretching above (rather than below) the open infratemporal fenestra to contact the postorbital bone (rather than the jugal). Early turtles such as Proganochelys also have a tall quadratojugal, which contacts the jugal without any trace of the infratemporal fenestra. Several Triassic reptiles reacquire the lower temporal bar, albeit with the jugal forming most of the bar's length. In these reptiles, the quadratojugal is a small L- or T-shaped bone at the rear edge of the skull. Although early rhynchocephalians such as Gephyrosaurus have an incomplete lower temporal bar and a quadratojugal fused to the quadrate, later members of the group such as the modern tuatara (Sphenodon) do have a complete lower temporal bar, albeit with the quadratojugal still fused to the quadrate. All members of the group Archosauriformes, which contains archosaurs such as crocodilians and dinosaurs, have a complete lower temporal bar. This is also the case in placodonts, Trilophosaurus, some rhynchosaurs, and choristoderes. Modern birds have a quadratojugal which is assimilated into the thin, splint-like jugal. However, a separate quadratojugal is retained by several Mesozoic avialans, such as Archaeopteryx and Pterygornis. Non-avialan dinosaurs also have a separate quadratojugal.