Quadratojugal bone  The quadratojugal is a skull bone present in many vertebrates, including some living reptiles and amphibians. 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 of 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 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. Crocodilians and rhynchocephalians (the latter represented solely by the tuatara, "Sphenodon") retain a quadratojugal. Turtles also seem to possess a quadratojugal. Among living amphibians, a quadratojugal is known to be present in some frogs and caecilians. However, it is notably absent in salamanders. In modern birds, the quadratojugal bone is a thin and rodlike element of the skull. Upon ossification, the jugal and quadratojugal bones fuse to form the jugal bar, which is homologous to the lower temporal bar of other diapsids.