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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 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 [4] and may provide muscular attachments for facial muscles. [5]
Also the jugal bone is pneumatised. The lacrimal is I-shaped. It has an ascending branch forming the vertical rear edge of the crest; due to the triangular cross-section this branch is inclined towards the midline of the skull. The upper outer side of this branch forms a rectangular boss.
Skull diagram in top down and side-on views. Unlike the vast majority of lizards, the tuatara has a complete lower temporal bar closing the lower temporal fenestra (an opening of the skull behind the eye socket), caused by the fusion of the quadrate/quadratojugal (which are fused into a single element in adult tuatara) and the jugal bones of ...
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.
The jugal continues back and forms the lower margin of the orbits, bearing a crest and a depression on its outer surface. The jugal extends nearly to the end of the cranium where it contacts the quadratojugal, nearly forming part of the lower jaw articulation. The quadratojugal is a slim element squeezed in between the jugal and the quadrate ...
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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.