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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]
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. [2] Many modern tetrapods lack a quadratojugal bone as it has been lost or fused to other bones.
Synapsids, including mammals, have one temporal fenestra, which is ventrally bordered by a zygomatic arch composed of the jugal and squamosal bones. This single temporal fenestra is homologous to the infratemporal fenestra, as displayed most clearly by early synapsids. [2]
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] The squamosal bone lies ventral to the temporal series and otic notch, and is bordered anteriorly by the postorbital.
A schematic of an anapsid skull showing the location of major dermal bones of the upper skull, including the quadrate bone (q).. The quadrate bone is a skull bone in most tetrapods, including amphibians, sauropsids (reptiles, birds), and early synapsids.
The jugal is covered by a series of fluted grooves which are typically found around the orbits in other thalassophoneans, but Eardasaurus is the only member of the group with the ornamentation present on the jugal bones. The contact between the jugal and squamosal is step-like, with the later forming a process that overlays the jugal bone.
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The jugal is moderately deep, with a thick front prong. Like neotheropods and a few sauropodomorphs, the rear edge of the jugal bore a deep notch for the front of the quadratojugal. The jugal’s lower edge has a sharp ridge that projects straight down, an unusual configuration not seen in other dinosaurs.