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This Tyrannosaurus skull shows it was a diapsid

The temporal fenestra are anatomical features of the amniote skull, characterised by bilaterally symmetrical holes (fenestrae) in the temporal bone.

Physiological speculation associates it with a rise in metabolic rates and an increase in jaw musculature. The earlier amniotes of the Carboniferous did not have temporal fenestrae but the more advanced sauropsids and synapsids did. As time progressed, sauropsids' and synapsids' temporal fenestrae became more modified and larger to make stronger bites and more jaw muscles. Dinosaurs, which are sauropsids, have large advanced openings and their descendants, the birds, have temporal fenestrae which have been modified. Mammals, which are synapsids, possess no fenestral openings in the skull, as the trait has been modified. They do, though, still have the temporal orbit (which resembles an opening) and the temporal muscles. It is a hole in the head and is situated to the rear of the orbit behind the eye.

The presence and morphology of the temporal fenestra is critical for taxonomic classification of the synapsids, of which mammals are part.

Euryapsids have one large opening on the sides of their skulls, just like the synapsids, but eurapsids are actually diapsids, their upper temporal fenestra were lost.

Humans, despite having lost their fenestrae, are synapsids

There are four types of amniote skull, classified by the number and location of their fenestra. These are:

• Anapsida - no openings
• Synapsida - one low opening (beneath the postorbital and squamosal bones)
• Euryapsida - one high opening (above the postorbital and squamosal bones)
• Diapsida - two openings

Evolutionary, they are related like so:

• Amniota
  • Class Synapsida - mammal-like reptiles
    • Order Therapsida
      • Class Mammalia - mammals
  • Class Sauropsida - reptiles
    • Subclass Anapsida
    • (unranked) Eureptilia
      • Subclass Diapsida
        • (unranked) Euryapsida
        • Class Aves - birds

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