Antorbital fenestra

Among extant archosaurs, birds still possess antorbital fenestrae, whereas crocodylians have lost them.

The loss in crocodylians is believed to be related to the structural needs of their skulls for the bite force and feeding behaviours that they employ.

The antorbital fenestra houses a paranasal sinus that is confluent with the adjacent nasal capsule.

[3] In theropod dinosaurs, the antorbital fenestra is the largest opening in the skull.

Systematically, the presence of the antorbital fenestra is considered a synapomorphy that unites tetanuran theropods as a clade.