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 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 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. The sections of the jugal bar derived from the jugal and quadratojugal articulate with the postorbital and squamosal bones, respectively. This facilitates cranial kinesis, by allowing the quadrate bone to rotate during opening of the upper jaw.

Advanced cynodonts, including the mammaliaforms, have lost the quadratojugal, with the diminutive quadrate connecting to the stapes to function as 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.

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.