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 (snake
Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales...

s), which grip awkwardly-shaped prey (parrot
Parrots, also known as psittacines , are birds of the roughly 372 species in 86 genera that make up the order Psittaciformes, found in most tropical and subtropical regions. The order is subdivided into three families: the Psittacidae , the Cacatuidae and the Strigopidae...

s 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, who 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 mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s (the human skull
Human skull
The human skull is a bony structure, skeleton, that is in the human head and which supports the structures of the face and forms a cavity for the brain.In humans, the adult skull is normally made up of 22 bones...

 shows no cranial kinesis at all). Birds have varying degrees of cranial kinesis, with parrot
Parrots, also known as psittacines , are birds of the roughly 372 species in 86 genera that make up the order Psittaciformes, found in most tropical and subtropical regions. The order is subdivided into three families: the Psittacidae , the Cacatuidae and the Strigopidae...

s exhibiting the greatest degree. Among reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...

s, crocodilians and turtle
Turtles are reptiles of the order Testudines , characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield...

s lack cranial kinesis, while lizards possess some, often minor, degree of kinesis and snake
Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales...

s possessing the most exceptional cranial kinesis of any tetrapod
Tetrapods are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian...

. In amphibians, cranial kinesis varies, but is unknown in frogs and rare in salamander
Salamander is a common name of approximately 500 species of amphibians. They are typically characterized by a superficially lizard-like appearance, with their slender bodies, short noses, and long tails. All known fossils and extinct species fall under the order Caudata, while sometimes the extant...

s. Almost all fish have highly kinetic skulls, and teleost fish
Teleostei is one of three infraclasses in class Actinopterygii, the ray-finned fishes. This diverse group, which arose in the Triassic period, includes 20,000 extant species in about 40 orders; most living fishes are members of this group...

 have developed the most kinetic skulls of any living organism.

Joints are often simple syndesmosis
A syndesmosis is slightly movable articulation where the contiguous bony surfaces are united by an interosseous ligament, as in the inferior tibiofibular articulation...

 joints, but in some organisms, some joints may be synovial
Synovial may refer to:* Synovial fluid* Synovial joint* Synovial membrane...

, 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 facial skeleton (the nasofrontal joint, or within the nasals), e.g. birds.

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
The palate is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity. A similar structure is found in crocodilians, but, in most other tetrapods, the oral and nasal cavities are not truly separate. The palate is divided into two parts, the anterior...

 at the basipterygopterygoid joint.


The first example of cranial kinesis in the Chondricthyans, such as sharks. There is no attachment between the hyomandibular and the quadrate, and instead the hyoid arch
Hyoid arch
The second pharyngeal arch or hyoid arch assists in forming the side and front of the neck.-Skeletal elements:From the cartilage of the second arch arises*Stapes,*Styloid process,*Stylohyoid ligament, and...

 suspends the two sets of jaws like pendulums. This allows sharks to swing their jaws outwards and forwards over the prey, allowing the synchronous meeting of the jaws and avoiding deflecting the prey when it comes close.

Actinopterygian fish

Actinopts (ray finned fish) possess a huge range of kinetic mechanisms. As a general trend through phylogenetic trees, there is a tendency to liberate more and more bony elements to allow greater skull motility. Most actinopts use kinesis to rapidly expand their buccal cavity, to create suction for suction feeding.

Sarcopterygian fish

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


Early tetrapods inherited much of their suction feeding ability from their crossopterygian ancestors. The skulls of modern lissamphibians are greatly simplified.

Modern reptiles

Reptiles exhibit an extraordinary range of kinetic mechanisms, the most spectacular of which is snakes, who use highly kinetic joints to allow a huge gap; it is these highly kinetic joints that allow the wide gape and not the "unhinging" of joints, as many believe. Kinesis also prevents the "scissor effect", whereby the food item is pushed out of the mouth as the jaw occludes posteriorly to anteriorly. Typically, most modern reptile skulls are dikinetic, having both meta- and meso-kinetic joints. The mandibular bone is connected to the neurocranium via the quadrate and squamosal. The mandibulo-quadrate joint also articulates with the (palatine-pterygoid) bar which then connects to the maxilla, when the quadrate is pulled towards the skull by muscle x then the bar pushes on the base of the maxilla and causes the upper jaw to open.


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 many therapods, such as Hypsilophodon
    Hypsilophodon is an ornithopod dinosaur genus from the Early Cretaceous period of Europe. It was a small bipedal animal with an herbivorous or possibly omnivorous diet...

    , Tyrannosaurus
    Tyrannosaurus meaning "tyrant," and sauros meaning "lizard") is a genus of coelurosaurian theropod dinosaur. The species Tyrannosaurus rex , commonly abbreviated to T. rex, is a fixture in popular culture. It lived throughout what is now western North America, with a much wider range than other...

    , Massospondylus
    Massospondylus and ) is a genus of prosauropod dinosaur from the early Jurassic Period . It was described by Sir Richard Owen in 1854 from remains found in South Africa, and is thus one of the first dinosaurs to have been named...

    , Coelophysis
    Coelophysis , meaning "hollow form" in reference to its hollow bones , is one of the earliest known genera of dinosaur...

    , and Allosaurus
    Allosaurus is a genus of large theropod dinosaur that lived 155 to 150 million years ago during the late Jurassic period . The name Allosaurus means "different lizard". It is derived from the Greek /allos and /sauros...

  • Metakinesis; jointing between the neurcranium and the dermatocranium, seen in some lizards. Dromaeosaurus
    Dromaeosaurus was a genus of theropod dinosaur which lived during the Late Cretaceous period , sometime between 76.5 and 74.8 million years ago, in the western United States and Alberta, Canada. The name means 'running lizard'....

    and also Hypsilophodon
    Hypsilophodon is an ornithopod dinosaur genus from the Early Cretaceous period of Europe. It was a small bipedal animal with an herbivorous or possibly omnivorous diet...

    shows 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 (Shuvuuia
Shuvuuia is a genus of bird-like theropod dinosaur from the late Cretaceous period of Mongolia. It is a member of the family Alvarezsauridae, small coelurosaurian dinosaurs which are characterized by short but powerful forelimbs specialized for digging. The type species is Shuvuuia deserti, or...

). Many, on the other hand, have at various points been thought to show akinesis, such as sauropods, ankylosaurs, and ceratopsians. It can be very difficult to prove that skulls were akinetic, and many of the above examples are contentious.

Pleurokinesis in Ornithopods

Pleurokinesis refers to the complex multiple jointing thought to occur in Ornithopod
Ornithopods or members of the clade Ornithopoda are a group of ornithischian dinosaurs that started out as small, bipedal running grazers, and grew in size and numbers until they became one of the most successful groups of herbivores in the Cretaceous world, and dominated the North American...

s, such as hadrosaurs. Ornithopod jaws are isognathic (meet simultaneously), working like a guillotine
The guillotine is a device used for carrying out :executions by decapitation. It consists of a tall upright frame from which an angled blade is suspended. This blade is raised with a rope and then allowed to drop, severing the head from the body...

 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
Secondary palate
The secondary palate is an anatomical structure that divides the nasal cavity from the oral cavity in many vertebrates.In human embryology, it refers to that portion of the hard palate that is formed by the growth of the two palatine shelves medially and their mutual fusion in the midline...

, the force of this would not be braced. Because of this, Norman and Weishampel proposed a pleurokinetic skull. Here, there are four (or perhaps even more) kinetic parts of the skull,
  • 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.


Birds show a vast range of cranial kinetic hinges in their skulls. Zusi recognised three basic forms of cranial kinesis in birds,
  • Prokinesis, where the upper beak moves at the point where it is hinge
    A hinge is a type of bearing that connects two solid objects, typically allowing only a limited angle of rotation between them. Two objects connected by an ideal hinge rotate relative to each other about a fixed axis of rotation. Hinges may be made of flexible material or of moving components...

    d with the bird's skull
    The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...

  • 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
    A symphysis is a fibrocartilaginous fusion between two bones. It is a type of cartilaginous joint, specifically a secondary cartilaginous joint.1.A symphysis is an amphiarthrosis, a slightly movable joint.2.A growing together of parts or structures...

    . 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 in addition; in retraction the tip bends down with respect to the rest of the upper jaw.
  • 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.


Rhynchokinesis is an ability possessed by some bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

s 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
A yawn is a reflex of simultaneous inhalation of air and stretching of the eardrums, followed by exhalation of breath. Pandiculation is the act of yawning and stretching simultaneously....

, 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 crane
Crane (bird)
Cranes are a family, Gruidae, of large, long-legged and long-necked birds in the order Gruiformes. There are fifteen species of crane in four genera. Unlike the similar-looking but unrelated herons, cranes fly with necks outstretched, not pulled back...

s, shorebirds, swift
The swifts are a family, Apodidae, of highly aerial birds. They are superficially similar to swallows, but are actually not closely related to passerine species at all; swifts are in the separate order Apodiformes, which they share with hummingbirds...

s and hummingbird
Hummingbirds are birds that comprise the family Trochilidae. They are among the smallest of birds, most species measuring in the 7.5–13 cm range. Indeed, the smallest extant bird species is a hummingbird, the 5-cm Bee Hummingbird. They can hover in mid-air by rapidly flapping their wings...

s. The adaptive significance of rhynchokinesis in certain non-probing birds is not yet known. It is hypothesized that the schizorhinal skull in proximally rhynchokinetic birds reflects ancestry, but has no adaptive explanation, in many living species.

Species in which this has been recorded photographically include the following species: Short-billed Dowitcher
Short-billed Dowitcher
The Short-billed Dowitcher like its congener the Long-billed Dowitcher, is a medium-sized, stocky, long-billed shorebird in the family Scolopacidae. It is an inhabitant of North America, Middle America, and northern South America. It is strongly migratory; it completely vacates in breeding areas...

, Marbled Godwit
Marbled Godwit
The Marbled Godwit, Limosa fedoa, is a large shorebird. On average, it is the largest of the 4 species of godwit. The total length is , including a large bill of , and wingspan is . Body mass can vary from ....

, Least Sandpiper
Least Sandpiper
The Least Sandpiper is the smallest shorebird.This species has greenish legs and a short thin dark bill. Breeding adults are brown with dark brown streaks on top and white underneath. They have a light line above the eye and a dark crown. In winter, Least Sandpipers are grey above...

, Common Snipe
Common Snipe
The Common Snipe is a small, stocky wader native to the Old World. The breeding habitat is marshes, bogs, tundra and wet meadows throughout northern Europe and northern Asia...

, Long-billed Curlew
Long-billed Curlew
The Long-billed Curlew, Numenius americanus, is a large North American shorebird of the family Scolopacidae. This species was also called "sicklebird" and the "candlestick bird". The species is native to central and western North America...

, Pectoral Sandpiper
Pectoral Sandpiper
The Pectoral Sandpiper, Calidris melanotos, is a small wader. It is sometimes separated with the "stint" sandpipers in Erolia. This may or may not represent a good monophyletic group, depending on the placement of the phylogenetically enigmatic Curlew Sandpiper , the type species of Erolia...

, Semipalmated Sandpiper
Semipalmated Sandpiper
The Semipalmated Sandpiper, Calidris pusilla, is a very small shorebird. It is sometimes separated with other "stints" in Erolia but although these apparently form a monophyletic group, the present species' old genus Ereunetes had been proposed before Erolia.Adults have black legs and a short stout...

, Eurasian Oystercatcher
Eurasian Oystercatcher
The Eurasian Oystercatcher Haematopus ostralegus, also known as the Common Pied Oystercatcher, or just Oystercatcher, is a wader in the oystercatcher bird family Haematopodidae. It is the most widespread of the oystercatchers, with three races breeding in western Europe, central Eurasia,...

 and Bar-tailed Godwit
Bar-tailed Godwit
The Bar-tailed Godwit is a large wader in the family Scolopacidae, which breeds on Arctic coasts and tundra mainly in the Old World, and winters on coasts in temperate and tropical regions of the Old World...

 (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. Neognatnous 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.


In hare
Hares and jackrabbits are leporids belonging to the genus Lepus. Hares less than one year old are called leverets. Four species commonly known as types of hare are classified outside of Lepus: the hispid hare , and three species known as red rock hares .Hares are very fast-moving...

s or "jackrabbits" (but not in their ancestors), there is a suture between regions in the fetal braincase that remain open in the adult, forming what is thought to be an intracranial joint, permitting relative motion between the anterior and posterior part of the braincase. It is thought that this helps absorb the forces impacted as the hare strikes the ground.

External links

Photographs of birds performing rhynchokinesis can be found here:
A very clear animation of pleurokinesis in Hadrosaurs can be found here:
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