The classification of reptilian skulls is based on the number and position of temporal fenestrae (openings) located behind the eye orbit. These openings provide space for the attachment of powerful jaw muscles, enhancing biting efficiency and feeding adaptations.

Anapsid Skull: The most primitive type, lacking any temporal fenestrae, with a solid skull roof. Early reptiles and traditionally turtles (though debated) are considered anapsid-derived. The closed structure offers protection but restricts jaw muscle expansion, limiting bite strength.

Synapsid Skull: Characterized by a single temporal fenestra low on each side of the skull, beneath the post-orbital and squamosal bones. This arrangement allowed for stronger adductor muscles and more powerful bites. Synapsids dominated the Paleozoic era, and importantly, mammals evolved from synapsid ancestors.

Euryapsid Skull: Possesses a single fenestra positioned high on each side, above the post-orbital and squamosal bones. Found in extinct marine reptiles such as ichthyosaurs and plesiosaurs, this type likely evolved from diapsids through the closure of the lower fenestra.

Diapsid Skull: The most widespread type in modern reptiles and birds, with two temporal fenestrae (upper and lower) on each side. This design provides large surface areas for jaw muscle attachment, enabling efficient and forceful biting. Crocodilians, tuataras, lizards, snakes, and birds belong to this group. In snakes and lizards, modifications include loss of bony bars, resulting in highly flexible jaws (modified diapsid condition).

These skull types form the basis for understanding reptilian evolution and phylogenetic relationships, highlighting structural innovations that supported ecological diversity and adaptive success.