a hearing structure. In modern mammals, the quadrate bone evolves to become the incus, one of the ossicles of the middle ear. This is an apomorphy of the mammalian clade, and is used to identify the fossil transition to mammals. Evolution Origin The quadratojugal likely originated within the clade Sarcopterygii, which includes tetrapods and lobe-finned fish. Although a tiny bone similar in position to the quadratojugal has been observed in the placoderm Entelognathus and some early actinopterygiians (Mimipiscis, Cheirolepis), it is unclear whether this bone was homologous to the quadratojugal. A quadratojugal is absent in actinians (coelacanths) and onychodonts, but it was clearly present in Porolepiformes, distant relatives of modern dipnoans (lungfish). Many paleontologists argue that the quadratojugal was formed by a division of the preoperculum, although a few believe that it was present before the preoperculum formed. All tetrapodomorph fish had a quadratojugal, retained by their tetrapod descendants. Elpistostegalians such as Panderichthys, Tiktaalik, and other very tetrapod-like fish were the first vertebrates to have contact between the quadratojugal and jugal. Before the elpistostegalians, the jugal was small and isolated from the quadratojugal by the squamosal and maxilla. 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 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