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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. [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.
The jugal bone is located on either side of the skull in the circumorbital region. It is the origin of several masticatory muscles in the skull. [1] The jugal and lacrimal bones are the only two remaining from the ancestral circumorbital series: the prefrontal, postfrontal, postorbital, jugal, and lacrimal bones. [2]
The quadrate bone is a skull bone in most tetrapods, including amphibians, sauropsids (reptiles, birds), and early synapsids. In most tetrapods, the quadrate bone connects to the quadratojugal and squamosal bones in the skull, and forms upper part of the jaw joint.
The jugal, or cheek-bone, is a skull bone that defines the lower border of the orbit and connects to the maxilla anteriorly and to the quadratojugal posteriorly. Its posterior end is bifurcated at its articulation with the quadratojugal, which is considered a synapomorphy of dinosaurs. [1]: 140 [25]: 12
The skull of Proterosuchus, an early archosauriform. Note the long rear branch of the downturned premaxilla and the L-shaped quadratojugal near the jaw joint. Most archosauromorphs more "advanced" than Protorosaurus possessed an adaptation of the premaxilla (tooth-bearing bone at the tip of the snout) known as a posterodorsal or postnarial process.
The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. [1] In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. [2] The bone forms an ancestral component of the dermal roof and is typically thin compared to other skull bones. [3]
A quadratojugal bone, which in modern birds is fused to the jugal, is preserved in Pterygornis. [30] The presence of these primitive features of the skull would have rendered the Enantiornithes capable of only limited cranial kinesis (the ability to move the jaw independent of the cranium).
The type specimen was a poorly preserved skull and partial skeleton designated AM 4471. Cope found that the skull of this specimen had shorter, thinner horns than those of D. magnicornis, as well as a seemingly unique feature: a large notch separating the quadratojugal from the rest of the tabular horn. [15]