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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 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 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.
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
Zatracheidids are distinguished by lateral (sideways) bony protuberances of the quadratojugal bone of the skull, and a large opening in the snout called the internarial fontanelle (sometimes the internarial fenestra) that is bordered by enlarged premaxillae. The skull is flattened, with small orbits or eye sockets set far
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 quadratojugal bosses on the cheek are broad and smooth with strong caudal curvature; The apex of the quadratojugal boss is rounded and unkeeled, and is situated caudally; The transverse nuchal crest on the rear skull roof is not visible in lateral, side, view; The occipital condyle is small, approximately 16% of basal skull length
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