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.[1]
Most vertebrates have some form of a kinetic skull.
Ancestry also plays a role in limiting or enabling cranial kinesis. Significant cranial kinesis is rare in
Joints are often simple syndesmosis joints, but in some organisms, some joints may be synovial, permitting a greater range of movement.
Types of kinesis
Versluys (1910, 1912, 1936) classified types of cranial kinesis based on the location of the joint in the dorsal part of the skull.
- Metakinesis is jointing between the dermatocranium and occipital segment
- Mesokinesis is jointing more rostral in the skull.
Hofer (1949) further partitioned mesokinesis into
- Mesokinesis proper, which occurs within the braincase (the frontoparietal joint), e.g., many lizards
- Prokinesis, which occurs between the braincase and the facial skeleton (the nasofrontal joint, or within the nasals), e.g. birds.
Streptostyly is the fore-aft movement of the
Fish
The first example of cranial kinesis was in the
Actinopterygian fish
Sarcopterygian fish
Early Dipnoi (lungfishes) had upper jaws fused to their braincase, which implies feeding on hard substrates. Many
Amphibians
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Early tetrapods inherited much of their suction feeding ability from their
Modern reptiles
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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.
Crocodilians
Alligators and crocodiles possess highly sutured (or akinetic) skulls. This is thought to allow them to have a stronger bite.[6][7]
Lizards
Three forms of cranial kinesis exist within lizards: metakinesis, mesokinesis, and streptostyly.[8]
- Metakinesis - Movement of the skeletal braincase relative to the rest of the skull
- Mesokinesis - Movement of the front portion of the skull relative to the back portion of the skull. The hinge where the movement occurs is present at the frontal-parietal suture.
- Streptostyly - Movement of the quadrate, where it moves in a back and forth motion, allowing the jaw to swing backwards and forwards.
Different lizards possess different degrees of kinesis, with
Snakes
Snakes use highly kinetic joints to allow a huge gape; it is these highly kinetic joints that allow the wide gape and not the "unhinging" of joints, as many believe. 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.[11] The phases document the ways in which the cranial bones shift according to the action being performed on the prey, specifically when the prey is passing through the gape. Similarly observed in the banded water snake, a prey's height acts on the maxillary and quadrate bones of the snake's skull by displacing them in a way that allows for the prey to enter the mouth more smoothly.[12]
Tuatara
The tuatara possesses an akinetic skull.[13] Some researchers think that juvenile tuatara may have somewhat kinetic skulls, and the bones only fuse later in adults.
Dinosaurs
The three principle types of kinesis found in Dinosaurs are:
- Streptostyly; forwards and back movement of the quadrate, seen in most lizards, snakes and birds. In dinosaurs, this is seen in Ankylosaurs, and possibly in many theropods, such as Carnotaurus,Coelophysis, and Allosaurus. It is also seen in Hypsilophodon and Massospondylus.
- Metakinesis; jointing between the neurocranium and the dermatocranium, seen in some lizards. Dromaeosaurus and Hypsilophodon show a metakinetic joint.
- Prokinesis; a joint in the facial area, such as modern snakes and birds. This is seen in a variety of dinosaurs.
Some show a combination of the two, such as streptostyly and prokinesis (
Pleurokinesis in ornithopods
Pleurokinesis refers to the complex multiple jointing thought to occur in
- Maxillojugal Unit
- Dentary-predentary
- Quadratojugal
- Quadrate
As the lower jaw closes, the maxillojugal units move laterally producing a power stroke. These motions were later proved by a microwear analysis on an Edmontosaurus jaw.[14]
Birds
Birds show a vast range of cranial kinetic hinges in their skulls. Zusi[15] recognised three basic forms of cranial kinesis in birds,
- Prokinesis, where the upper beak moves at the point where it is hinged with the bird's skull
- Amphikinesis. Unlike prokinesis, the narial openings extend back almost to the level of the craniofacial hinge, and the dorsal and ventral bars are flexible near the symphysis. In addition, the lateral bar is flexible near its junction with the dorsal bar. As a result, protraction and retraction forces are transmitted primarily to the symphysis via the lateral and ventral bars. During protraction, the entire upper jaw is raised and the tip of the jaw is bent up. Additionally, in retraction, the tip bends down with respect to the rest of the upper jaw.[15]
- Rhynchokinesis (see below)
Rhynchokinesis is further subdivided into double, distal, proximal, central and extensive. The older terms "schizorhynal" and "holorhynal" are generally synonymous with rhynchokinesis. In schizorhinal birds and most rhynchokinetic birds, the presence of two hinge axes at the base of the upper jaw imposes a requirement of bending within the jaw during kinesis. Bending takes different forms according to the number of hinges and their geometric configuration within the upper jaw. Proximal rhynchokinesis and distal rhynchokinesis apparently evolved from double rhynchokinesis by loss of different hinges. Extensive rhynchokinesis is an unusual and probably specialized variant. Kinesis in hummingbirds is still little understood.[15]
Rhynchokinesis
Rhynchokinesis is an ability possessed by some birds to flex their upper beak or rhinotheca. Rhynchokinesis involves flexing at a point some way along the upper beak — either upwards, in which case the upper beak and lower beak or gnathotheca diverge, resembling a yawn, or downwards, in which case the tips of the beaks remain together while a gap opens up between them at their midpoint.
Unlike prokinesis, which is widespread in birds, rhynchokinesis is only known in
Species in which this has been recorded photographically include the following species: short-billed dowitcher, marbled godwit, least sandpiper, common snipe, long-billed curlew, pectoral sandpiper, semipalmated sandpiper, Eurasian oystercatcher and bar-tailed godwit (see Chandler 2002 and external links).
Either prokinesis or some form of rhynchokinesis could be primitive for birds. 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. Neognathous rhynchokinesis, however, probably evolved from prokinesis. The evolutionary origin of rhynchokinesis from prokinesis required selection for morphological changes that produced two hinge axes at the base of the upper jaw. Once evolved, the properties of these axes were subject to selection in relation to their effects on kinesis. The various forms of kinesis are hypothesized to have evolved by simple steps. In neognathous birds, prokinesis was probably ancestral to amphikinesis, and amphikinesis to rhynchokinesis in most cases, but prokinesis has also evolved secondarily.[15]
Hares
In
See also
- Snake skull
References
- Notes
- ^ a b c d Kardong, Kenneth V. (1995). Vertebrates: Comparative anatomy, function and evolution. Wm. C. Brown.
- S2CID 15142387.
- ISSN 1618-1077.
- S2CID 5998531. Retrieved 4 December 2022.
- PMID 16351979.
- ISSN 0952-8369.
- )
- S2CID 19505409.
- ISSN 0028-2960.
- ISBN 978-1-4899-1753-9
- JSTOR 1443913.
- S2CID 13080210.
- )
- ^ Williams, V. S; P. M Barrett; M. A Purnell (2009). "Quantitative analysis of dental microwear in hadrosaurid dinosaurs, and the implications for hypotheses of jaw mechanics and feeding" (PDF). Proceedings of the National Academy of Sciences. Retrieved 2010-05-22.[permanent dead link]
- ^ doi:10.5479/si.00810282.395. Archived from the original(PDF) on 2009-07-17. Retrieved 2010-05-27.
- Bibliography
- A functional and evolutionary analysis of rhynchokinesis in birds by Richard L Zusi, Smithsonian Institution Press, 1984.
- Chandler, Richard (2002) PhotoSpot - Rhynchokinesis in waders British Birds Vol 95 p395
External links
Photographs of birds performing rhynchokinesis can be found here:
- [2] - a short-billed dowitcher
- [3] - a marbled godwit
- [4] - a least sandpiper
A very clear animation of pleurokinesis in Hadrosaurs can be found here: