Cranial kinesis

Cranial kinesis is the term for significant movement of skull bones relative to each other in addition to movement at the joint between the upper and lower jaw. It is usually taken to mean relative movement between the upper jaw and the braincase.

Most vertebrates have some form of kinetic skull. Cranial kinesis, or lack thereof, is usually linked to feeding. Animals which must exert powerful bite forces, such as crocodiles, often have rigid skulls with little or no kinesis, for maximum strength. Animals which swallow large prey whole (snakes), which grip awkwardly shaped food items (parrots eating nuts), or, most often, which feed in the water via suction feeding often have very kinetic skulls, frequently with numerous mobile joints. In the case of mammals, which have akinetic skulls (except for perhaps hares), the lack of kinesis is most likely to be related to the secondary palate, which prevents relative movement. 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 most kinetic 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.

...
Wikipedia