"The skull bones [of modern birds] are fused and do not show cranial sutures."
(Frank (1995). Ornithology. )
"Diapsids ("two arches") are a group of reptiles that developed two holes (temporal fenestra) in each side of their skulls, about 300 million years ago during the late Carboniferous period. Living diapsids are extremely diverse, and include all crocodiles, lizards, snakes, and tuatara. Under modern classification systems, even birds are considered diapsids, since they evolved from diapsid ancestors and are nested within the diapsid clade. While some diapsids have lost either one hole (lizards), or both holes (snakes), or even have a heavily restructured skull (modern birds), they are still classified as diapsids based on their ancestry."
"Note that the [modern] birds are clearly derived archosaurs, and in a phylogenetic classification should be included, but with their expanded brains and slight jaws (beaks), the diapsid nature of their skull has been lost"
"Thanks to the two posterior holes in the skull (the upper and lower temporal fenestrae for those in the know) we know pterosaurs are diapsid reptiles, but after this it gets really tricky, and the diapsids are a big group (it includes snakes, crocodile, dinosaurs, lizards, various marine reptiles, and all kinds of odds and sods like rhynchosaurs). Pterosaurs, rather obviously, are highly adapted for their flying lifestyle and as a result their anatomy has changed a huge amount. Not just the obviously elongate fourth finger, but the skull bones have kind of merged," (David Hone)
"Also of note is that pterosaurs lack a mandibular fenestrae, a hole in the jaw that is also diagnostic of the Archosauriformes. If pterosaurs are archosauriforms, then that means that they, for whatever reason, secondarily lost the mandibular fenestrae. This makes no sense at all, given that pterosaurs excavated their skeletons like crazy in an effort to become more lightweight."
"This condition is highly variable among modern birds, which may show one, two, or no fenestrae (22). But modern birds differ from nonavian theropods in that the dentary lining presents most of the ventral margin of the rostral mandibular fenestra both laterally and medially"
"The bird also has a postorbital bone (Fig. 3), which articulates rostrodorsally to the frontal and caudally to the squamosal. This bone bears a ventral, splint-like jugal process that does not contact the jugal bar and only partially separates the orbit from the infratemporal fenestra. The postorbital bone is absent in modern birds but is present in nonavian theropods (19). Workers have argued about this bone's presence in Archaeopteryx (7-9, 15, 16), but the presence of a postorbital in the new fossil casts doubt on interpretations that it is absent in Archaeopteryx."
"Equally interesting is the presence of a tetraradiated squamosal that, in contrast to modern birds, is not incorporated into the braincase (Fig. 3). Its rostrolateral process terminates in a bifurcated facet for articulation with the postorbital."
"The most important bird from the Liaoning deposits in China is Confuciusornis the only bird besides Archaeopteryx with enlarged claws for climbing. It also has a full set of gastralia and a postorbital bone that meets the jugal." (
"In the drawing, the “orbit” is the eye socket, “Sq” indicates the squamosal bone, one of the bones that makes up the skull, and “Po” indicates the postorbital bone."