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. [1] 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. [1] 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. [2] 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.
Actinopterygii (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.
Early Dipnoi (lungfishes) had upper jaws fused to their braincase, which implies feeding on hard substrates. Many crossopterygian fishes had kinesis also.
![]() | This section needs expansion. You can help by adding to it. (October 2010) |
Early tetrapods inherited much of their suction feeding ability from their crossopterygian ancestors. The skulls of modern Lissamphibians are greatly simplified, with many bones fused or otherwise reduced. They have mobility in the premaxilla of the snout, allowing amphibians to open and close their nasal openings. [3] In caecilians, the gap between the parietal bone and squamosal bone enables the skull to bend, which aids the animal in burrowing. [4] Caecilians are the only extant amphibian known to exhibit streptostyly, and their quadrate bone moves even after death. [5]
![]() | This section needs expansion. You can help by adding to it. (October 2010) |
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.
Alligators and crocodiles possess highly sutured (or akinetic) skulls. This is thought to allow them to have a stronger bite. [6] [7]
Three forms of cranial kinesis exist within lizards: metakinesis, mesokinesis, and streptostyly. [8]
Different lizards possess different degrees of kinesis, with chameleons, agamids, phrynosomatids, and amphisbaenians possessing the least kinetic skulls. [9] [10]
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]
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.
The three principle types of kinesis found in Dinosaurs are:
Some show a combination of the two, such as streptostyly and prokinesis ( Shuvuuia ). 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 refers to the complex multiple jointing thought to occur in ornithopods, such as hadrosaurs. 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. Because of this, Norman and Weishampel proposed a pleurokinetic skull. 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. These motions were later proved by a microwear analysis on an Edmontosaurus jaw. [14]
Birds show a vast range of cranial kinetic hinges in their skulls. Zusi [15] recognised three basic forms of cranial kinesis in birds,
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 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 cranes, shorebirds, swifts, hummingbirds, and furnariids. 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. [15]
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]
In hares or "jackrabbits" (but not in their ancestors), there is a suture between regions in the fetal braincase that remains 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 force of impact as the hare strikes the ground. [1]
The jaws are a pair of opposable articulated structures at the entrance of the mouth, typically used for grasping and manipulating food. The term jaws is also broadly applied to the whole of the structures constituting the vault of the mouth and serving to open and close it and is part of the body plan of humans and most animals.
The quadrate bone is a skull bone in most tetrapods, including amphibians, sauropsids, and early synapsids.
The quadratojugal is a skull bone present in many vertebrates, including some living reptiles and amphibians.
A snake skeleton consists primarily of the skull, vertebrae, and ribs, with only vestigial remnants of the limbs.
Carnotaurus is a genus of theropod dinosaur that lived in South America during the Late Cretaceous period, probably sometime between 72 and 69 million years ago. The only species is Carnotaurus sastrei. Known from a single well-preserved skeleton, it is one of the best-understood theropods from the Southern Hemisphere. The skeleton, found in 1984, was uncovered in the Chubut Province of Argentina from rocks of the La Colonia Formation. Carnotaurus is a derived member of the Abelisauridae, a group of large theropods that occupied the large predatorial niche in the southern landmasses of Gondwana during the late Cretaceous. Within the Abelisauridae, the genus is often considered a member of the Brachyrostra, a clade of short-snouted forms restricted to South America.
Selmasaurus is an extinct genus of marine lizard belonging to the mosasaur family. It is classified as part of the Plioplatecarpinae subfamily alongside genera like Angolasaurus and Platecarpus. Two species are known, S. russelli and S. johnsoni; both are exclusively known from Santonian deposits in the United States.
Thalassodromeus is a genus of pterosaur that lived in what is now Brazil during the Early Cretaceous period, about a hundred million years ago. The original skull, discovered in 1983 in the Araripe Basin of northeastern Brazil, was collected in several pieces. In 2002, the skull was made the holotype specimen of Thalassodromeus sethi by palaeontologists Alexander Kellner and Diogenes de Almeida Campos. The generic name means "sea runner", and the specific name refers to the Egyptian god Seth due to its crest being supposedly reminiscent of Seth's crown. Other scholars have pointed out that the crest was instead similar to the crown of Amon. A jaw tip was assigned to T. sethi in 2005, became the basis of the new genus Banguela in 2015, and assigned back to Thalassodromeus as the species T. oberlii in 2018, though other researchers consider it a valid genus. Another species was described in 2015 based on a supposed crest fragment, but this was later shown to be part of a turtle shell.
Gerrothorax is an extinct genus of temnospondyl amphibian from the Triassic period of Greenland, Germany, Poland, Sweden, and possibly Thailand. It is known from a single species, G. pulcherrimus, although several other species such as G. pustuloglomeratus have been named in the past.
The evolution of mammalian auditory ossicles was an evolutionary process that resulted in the formation of the bones of the mammalian middle ear. These bones, or ossicles, are a defining characteristic of all mammals. The event is well-documented and important as a demonstration of transitional forms and exaptation, the re-purposing of existing structures during evolution.
Junggarsuchus is an extinct genus of sphenosuchian crocodylomorph from the Middle or Late Jurassic period of China. The type and only species is J. sloani. The generic name of Junggarsuchus comes from the Junggar Basin, where the fossil was found, and the Greek word "souchos" meaning crocodile. The specific name, "sloani" is in honor of C. Sloan, who is credited with finding the holotype.
Durophagy is the eating behavior of animals that consume hard-shelled or exoskeleton-bearing organisms, such as corals, shelled mollusks, or crabs. It is mostly used to describe fish, but is also used when describing reptiles, including fossil turtles, placodonts and invertebrates, as well as "bone-crushing" mammalian carnivores such as hyenas. Durophagy requires special adaptions, such as blunt, strong teeth and a heavy jaw. Bite force is necessary to overcome the physical constraints of consuming more durable prey and gain a competitive advantage over other organisms by gaining access to more diverse or exclusive food resources earlier in life. Those with greater bite forces require less time to consume certain prey items as a greater bite force can increase the net rate of energy intake when foraging and enhance fitness in durophagous species.
Sanajeh is a genus of late Cretaceous madtsoiid snake from western India. A fossil described in 2010 from the Lameta Formation was found coiled around an egg and an adjacent skeleton of a 50 cm (19 in) long sauropod dinosaur hatchling. This suggests that the snake preyed on hatchling sauropods at nesting sites.
Aquatic feeding mechanisms face a special difficulty as compared to feeding on land, because the density of water is about the same as that of the prey, so the prey tends to be pushed away when the mouth is closed. This problem was first identified by Robert McNeill Alexander. As a result, underwater predators, especially bony fish, have evolved a number of specialized feeding mechanisms, such as filter feeding, ram feeding, suction feeding, protrusion, and pivot feeding.
Athabascasaurus is an extinct genus of platypterygiine ophthalmosaurid ichthyosaur known from Alberta, Canada.
Aegisuchus is an extinct monospecific genus of giant, flat-headed crocodyliform within the family Aegyptosuchidae. It was found in the Kem Kem Formation of southeast Morocco, which dates back to the Cenomanian age of the Late Cretaceous epoch. The type species Aegisuchus witmeri was named in 2012 by paleontologists Casey Holliday and Nicholas Gardner, who nicknamed it "Shieldcroc" for the shield-like shape of its skull. A. witmeri is known from a single partial skull including the braincase and skull roof.
Most bony fishes have two sets of jaws made mainly of bone. The primary oral jaws open and close the mouth, and a second set of pharyngeal jaws are positioned at the back of the throat. The oral jaws are used to capture and manipulate prey by biting and crushing. The pharyngeal jaws, so-called because they are positioned within the pharynx, are used to further process the food and move it from the mouth to the stomach.
Almadasuchus is an extinct genus of crocodylomorph known from the early Late Jurassic Puesto Almada Member of the e Cañadón Calcáreo Formation of Patagonia, Argentina. It contains a single species, Almadasuchus figarii. It is known from the holotype MPEF-PV 3838, a well-preserved posterior region of the skull as well as other skull and postcranial remains. Almadasuchus was recovered from Puesto Almada, 30 m above the fish beds, dated as Oxfordian in age.
Palatodonta is an extinct genus of neodiapsid reptile known from the early Middle Triassic of the Netherlands. It was initially described in 2013 as a basal placodontiform closely related to a group of marine reptiles called placodonts, characterized by their crushing teeth and shell-like body armor. Under this interpretation, Palatodonta is transitional between placodonts and less specialized reptiles. Like placodonts, it has a row of large teeth on its palate, but while these teeth are thick and blunt in placodonts, Palatodonta has palatal teeth that are thin and pointed. A 2023 study instead classified it as a sauropterygomorph and the sister taxon to Eusaurosphargis. In other words, it is close to, but not within, Sauropterygia.
Gobivenator is an extinct genus of troodontid theropod dinosaur known from the late Campanian Djadokhta Formation of central Gobi Desert, Mongolia. It contains a single species, Gobivenator mongoliensis. G. mongoliensis is known from a single individual, which represents the most complete specimen of a Late Cretaceous troodontid currently known.
This glossary explains technical terms commonly employed in the description of dinosaur body fossils. Besides dinosaur-specific terms, it covers terms with wider usage, when these are of central importance in the study of dinosaurs or when their discussion in the context of dinosaurs is beneficial. The glossary does not cover ichnological and bone histological terms, nor does it cover measurements.
{{cite book}}
: CS1 maint: multiple names: authors list (link){{cite book}}
: CS1 maint: multiple names: authors list (link) CS1 maint: numeric names: authors list (link)Photographs of birds performing rhynchokinesis can be found here:
A very clear animation of pleurokinesis in Hadrosaurs can be found here: