Temporal fenestra

Temporal fenestrae are commonly (although not universally) seen in the fossilized skulls of dinosaurs and other sauropsids (the total group of reptiles, including birds).

[1] The major reptile group Diapsida, for example, is defined by the presence of two temporal fenestrae on each side of the skull.

Synapsids, including mammals, have one temporal fenestra, which is ventrally bordered by a zygomatic arch composed of the jugal and squamosal bones.

[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: the synapsids (stem-mammals and mammals) and the diapsids (most reptiles and later birds).