This in turn is a consequence of the need to be able to create a suction during suckling.
Ancestry also plays a role in limiting or enabling cranial kinesis. Significant cranial kinesis is rare in mammals (the human skull shows no cranial kinesis at all). Birds have varying degrees of cranial kinesis, with parrots exhibiting the greatest degree. Among reptiles, crocodilians and turtles lack cranial kinesis, while lizards possess some, often minor, degree of kinesis and snakes possessing the most exceptional cranial kinesis of any tetrapod. In amphibians, cranial kinesis varies, but is unknown in frogs and rare in salamanders. Almost all fish have highly kinetic skulls, and teleost fish have developed the mostkinetic skulls of any living organism.
Joints are often simple syndesmosis joints, but in some organisms, some joints may be synovial, permitting a greater range of movement.