The quadratojugal is a skull bone present in many vertebrates, including some living reptiles and amphibians.
Anatomy and function
In animals with a quadratojugal bone, it is typically found connected to the jugal (cheek) bone from the front and the squamosal bone from above. It is usually positioned at the rear lower corner of the cranium. Many modern tetrapods lack a quadratojugal bone as it has been lost or fused to other bones. Modern examples of tetrapods without a quadratojugal include salamanders, mammals, birds, and squamates (lizards and snakes). In tetrapods with a quadratojugal bone, it often forms a portion of the jaw joint.
Developmentally, the quadratojugal bone is a dermal bone in the temporal series, forming the original braincase. The squamosal and quadratojugal bones together form the cheek region and may provide muscular attachments for facial muscles.
In reptiles and amphibians
thumb|270x270px|Skull diagram of the [[rhynchocephalian Sphenotitan, with the quadratojugal coloured lilac]]
In most modern reptiles and amphibians, the quadratojugal is a prominent, straplike bone in the skull and provides structural integrity in the postorbital region of the skull. In many reptiles, the inner face of the quadratojugal also connects to the quadrate bone which forms the cranium's contribution to the jaw joint. Early in their evolution, diapsid reptiles evolved a lower temporal bar which was composed of the quadratojugal and jugal. The lower temporal bar forms the lower border of the infratemporal fenestra, one of two holes in the side of the head and a hallmark of a diapsidan skull. However, many diapsids, including modern squamates (lizards and snakes), have lost the lower temporal bar. This is an apomorphy of the mammalian clade, and is used to identify the fossil transition to mammals.
Amphibians (in the broad sense) typically had long, roughly rectangular quadratojugals that contacted the maxilla, jugal, squamosal, and quadrate. In several lineages, most of them traditionally considered "Reptiliomorpha", the jugal expands downwards to reduce the amount of contact between the quadratojugal and maxilla. This is exemplified in reptiles, which have completely lost the contact. Most urodelans (salamanders) lack quadratojugals, except the Miocene genus Chelotriton. A quadratojugal is also missing in the caecilian-like Triassic stereospondyl Chinlestegophis as well as the lysorophians, a group of long-bodied Paleozoic microsaurs. Many other microsaurs had heavily reduced quadratojugals.
Synapsids
thumb|The skull of [[Gorynychus, a therocephalian synapsid. The tiny quadrate-quadratojugal complex is labelled with q-qj.]]
In synapsids (mammals and their extinct relatives), the quadratojugal undergoes significant transformation during the evolution of the group. Early synapsids such as eothyridids and caseids retained long quadratojugals and in some cases even reacquire quadratojugal-maxilla contact. In most therapsids, including gorgonopsians, therocephalians, and dicynodonts, the quadratojugal is tiny, having lost its contact with the jugal. It usually fuses with the equally small quadrate to form the quadrate-quadratojugal complex. Oddly enough, the cynodont Thrinaxodon retains a separate quadratojugal. In other cynodonts such as Cynognathus, the quadrate-quadratojugal complex remains hidden within the skull, obscured from the side by the large squamosal bone which loosely articulates with it.
Sauropsids
Sauropsids, the group containing reptiles and birds, had completely lost the contact 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.
left|thumb|261x261px|The skull of [[Prolacerta, a relative of the Archosauriformes with an incomplete lower temporal bar. The small, crescent-shaped quadratojugal is labelled with 153-0.]]
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.