Amniote skull type, lacking temporal fenestrae (holes in the side of the skull).
Recently found only in turtles it is probably the skull type that synapsid, diapsid and euryapsid skull types evolved out of. In laymen terms this means that everything that has a spine (except Amphibians) probably evolved out of creatures with an Anapsid skull
The main disadvantage of this skull type is that temporal fenestrae allow for much more powerful muscles to close the jaws with. As this type of skull has none, the creatures possesing it have/had very inefficient jaw muscles