Cranial kinesis

Animals which must exert powerful bite forces, such as crocodiles, often have rigid skulls with little or no kinesis, which maximizes their strength.

Versluys (1910, 1912, 1936) classified types of cranial kinesis based on the location of the joint in the dorsal part of the skull.

As a general trend through phylogenetic trees, there is a tendency to liberate more and more bony elements to allow greater skull motility.

Early Dipnoi (lungfishes) had upper jaws fused to their braincase, which implies feeding on hard substrates.

[5] Different groups of reptiles exhibit varying degrees of cranial kinesis, ranging from akinetic, meaning there is very little movement between skull bones, to highly kinetic.

Snakes engage in high amounts of cranial kinesis that help them perform important tasks such as eating.

Studies done in cottonmouth snakes suggests that the process of eating, as it relates to movement of the cranial bones, can be situated into three parts: hold, advance, and close.

The three principle types of kinesis found in Dinosaurs are: Some show a combination of the two, such as streptostyly and prokinesis (Shuvuuia).

Ornithopod jaws are isognathic (meet simultaneously), working like a guillotine to slice plant material which can be manipulated with their teeth.

However, because of the wedge shape of their teeth, the occlusional plane is tilted away from the centre of the head, causing the jaws to lock together and, due to the lack of a secondary palate, the force of this would not be braced.

Here, there are four (or perhaps even more) kinetic parts of the skull, As the lower jaw closes, the maxillojugal units move laterally producing a power stroke.

Zusi[15] recognised three basic forms of cranial kinesis in birds, Rhynchokinesis is further subdivided into double, distal, proximal, central and extensive.

It is hypothesized that the schizorhinal skull in proximally rhynchokinetic birds reflects ancestry, but has no adaptive explanation, in many living species.

Rhynchokinesis is not compatible with the presence of teeth in the bending zone of the ventral bar of the upper Jaw, and it probably evolved after their loss.

The evolutionary origin of rhynchokinesis from prokinesis required selection for morphological changes that produced two hinge axes at the base of the upper jaw.