The Skeleton: Part 1, The Skull



The oldest fossil reptiles lack the enlarged openings in lateral skull posterior to the orbit that are seen in fossils of many later reptile groups.

The groups which lack these openings (temporal fenestrae) are called anapsids. Turtles are among the anapsids, as are many extinct reptile groups.


Diapsids have two temporal fenestrae behind the orbit, one superior and one inferior. Groups in this category include the dinosaurs, crocodilians, birds, tuaturas, lizards and snakes.


Synapsids have one temporal fenestra behind the eye. It is situated below the postorbital bone, in a position more like the lower of the two fenestrae in diapsids. The synapsid reptiles are all extinct, but mammals are postulated to be descended representatives of the synapsids.


The euryapsids are extinct. They include the ichthyosaurs and the plesiosaurs. They also had one fenestra behind the eye, but its position differed from the synapsids in that was above the postorbital and not below. Its location being similar to the upper fenestra of diapsids.





















Divisions of the skull

Neurocranial division

Dermatocranial division

Splanchnocranial division

Sphenoid as a composite bone

Temporal as a composite bone

Occipital as a composite bone

Lamprey head skeleton

Shark and Ratfish head skeleton

Head skeleton of bony fishes

Neurocranial ossification in bony fishes

Dermal bones and scales in bony fishes

Dermal bones and the fish/amphibian link

Anapsids, diapsids, synapsids and others

Diapsids variations

Reduced dermal bone

Secondary palate formation

Hyomandibular expression

Quadrate and articular expression

Occipital condyle number

Sample Questions