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 modern
phylogenetic taxonomy. The four types are: •
Anapsida – No openings. The
plesiomorphic ("primitive") condition exemplified by
amphibians as well as some early reptiles like
captorhinids and
parareptiles.
Turtles have an anapsid skull, but this was likely acquired secondarily from a diapsid ancestor. •
Synapsida – One low opening (beneath the
postorbital and squamosal bones). A
monophyletic group including mammals and their ancestors. •
Euryapsida – One high opening (above the postorbital and squamosal bones). Euryapsids are a
polyphyletic 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 include
ichthyosaurs,
plesiosaurs,
placodonts, and
Trilophosaurus. •
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 and
quadratojugal bones. == Notes ==