Temporal fenestrae are openings in thetemporal region of theskull of someamniotes, behind theorbit (eye socket). These openings have historically been used to track the evolution and affinities of reptiles. Temporal fenestrae are commonly (although not universally) seen in the fossilized skulls ofdinosaurs and other sauropsids (thetotal group of reptiles, including birds).^[1] The major reptile groupDiapsida, for example, is defined by the presence of two temporal fenestrae on each side of the skull. Theinfratemporal fenestra, also called thelateral temporal fenestra orlower temporal fenestra, is the lower of the two and is exposed primarily in lateral (side) view.

Thesupratemporal fenestra, also called theupper temporal fenestra, is positioned above the other fenestra and is exposed primarily in dorsal (top) view. In some reptiles, particularly dinosaurs, the parts of the skull roof lying between the supratemporal fenestrae are thinned out by excavations from the adjacent fenestrae. These extended margins of thinned bone are calledsupratemporal fossae.

Synapsids, includingmammals, have one temporal fenestra, which is ventrally bordered by azygomatic arch composed of thejugal andsquamosal bones. This single temporal fenestra ishomologous to the infratemporal fenestra, as displayed most clearly by early synapsids.^[2] In later synapsids, thecynodonts, the orbit fused with the fenestral opening after the latter had started expanding within thetherapsids. Most mammals have this merged configuration. Later, primates re-evolved an orbit separated from thetemporal fossa. This separation was achieved by the evolution of apostorbital bar, withhaplorhines (dry-nosed primates) later evolving apostorbital septum.^[3]

Physiological speculation associates temporal fenestrae with a rise in metabolic rates and an increase in jaw musculature. The earlier amniotes of the Carboniferous did not have temporal fenestrae, but two more advanced lines did: thesynapsids (stem-mammals and mammals) and thediapsids (most reptiles and later birds).

There are four types of amniote skull, classified by the number and location of their temporal fenestrae. Though historically important for understanding amniote evolution, some of these configurations have little relevance to modernphylogenetictaxonomy. The four types are:

• Anapsida – No openings. Theplesiomorphic ("primitive") condition exemplified byamphibians as well as some early reptiles likecaptorhinids andparareptiles.Turtles have an anapsid skull, but this was likely acquired secondarily from a diapsid ancestor.
• Synapsida – One low opening (beneath thepostorbital and squamosal bones). Amonophyletic group including mammals and their ancestors.
• Euryapsida – One high opening (above the postorbital and squamosal bones). Euryapsids are apolyphyletic group, as reptiles with euryapsid skulls lack a shared common ancestor. Euryapsids evolved from a diapsid configuration, losing their lower temporal fenestra. Examples of euryapsid reptiles includeichthyosaurs,plesiosaurs,placodonts, andTrilophosaurus.
• Diapsida – Two openings. A monophyletic group including all modern reptiles and birds. Turtles, though not diapsids in a purely anatomical sense, qualify as members of the clade Diapsida due to their likely diapsid ancestry. Some diapsids, particularly modern lizards, have an infratemporal fenestra which is open from below due to a lack of contact between the jugal andquadratojugal bones.

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