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Cranial kinesis
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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 jaws. It is usually taken to mean relative movement between the upper jaw and the braincase.
Most vertebrates have some form of a 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, which maximizes their 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 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. Snakes possess the most exceptional cranial kinesis of any tetrapod. In amphibians, cranial kinesis varies, but has yet to be observed in frogs and is 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.
Versluys (1910, 1912, 1936) classified types of cranial kinesis based on the location of the joint in the dorsal part of the skull.
Hofer (1949) further partitioned mesokinesis into
Streptostyly is the fore-aft movement of the quadrate about the otic joint (quadratosquamosal joint), although transverse movements may also be possible. Many hypothesized types of kinesis require basal joint kinesis (neurokinesis of Iordansky, 1990), that is, movement between the braincase and palate at the basipterygoid joint.
The first example of cranial kinesis was in the chondrichthyans, such as sharks. There is no attachment between the hyomandibular and the quadrate, and instead the hyoid arch suspends the two sets of jaws like pendulums. This allows sharks to swing their jaws outwards and forwards over the prey, allowing for the synchronous meeting of the jaws and avoiding deflecting the prey when it comes close.
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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 jaws. It is usually taken to mean relative movement between the upper jaw and the braincase.
Most vertebrates have some form of a 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, which maximizes their 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 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. Snakes possess the most exceptional cranial kinesis of any tetrapod. In amphibians, cranial kinesis varies, but has yet to be observed in frogs and is 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.
Versluys (1910, 1912, 1936) classified types of cranial kinesis based on the location of the joint in the dorsal part of the skull.
Hofer (1949) further partitioned mesokinesis into
Streptostyly is the fore-aft movement of the quadrate about the otic joint (quadratosquamosal joint), although transverse movements may also be possible. Many hypothesized types of kinesis require basal joint kinesis (neurokinesis of Iordansky, 1990), that is, movement between the braincase and palate at the basipterygoid joint.
The first example of cranial kinesis was in the chondrichthyans, such as sharks. There is no attachment between the hyomandibular and the quadrate, and instead the hyoid arch suspends the two sets of jaws like pendulums. This allows sharks to swing their jaws outwards and forwards over the prey, allowing for the synchronous meeting of the jaws and avoiding deflecting the prey when it comes close.