Quadratojugal bone
Modern examples of tetrapods without a quadratojugal include salamanders, mammals, birds, and squamates (lizards and snakes).
The squamosal and quadratojugal bones together form the cheek region[4] and may provide muscular attachments for facial muscles.
[6] 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.
[9][5] Advanced cynodonts, including the mammaliaforms, have lost the quadratojugal, with the diminutive quadrate connecting to the stapes to function as a hearing structure.
A quadratojugal is absent in actinians (coelacanths) and onychodonts, but it was clearly present in Porolepiformes, distant relatives of modern dipnoans (lungfish).
[11] 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.
[12] A quadratojugal is also missing in the caecilian-like Triassic stereospondyl Chinlestegophis[8] as well as the lysorophians, a group of long-bodied Paleozoic microsaurs.
Early synapsids such as eothyridids and caseids retained long quadratojugals and in some cases even reacquire quadratojugal-maxilla contact.
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.
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.
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).
