CRANIAL KINESIS IN VERTEBRATES

CRANIAL KINESIS IN VERTEBRATES

Abstract

Cranial kinesis is defined as the essential movement of the bones of the skull about each other as well as the movement in the joint lower and upper jaw. This paper, we are going to discuss the evolution of cranial kinesis among the vertebrates, functions, or significance or biomechanics of cranial kinesis. The paper also discusses the cranial kinesis specifically in fishes, snakes, and dinosaurs. The evolution of cranial evolution is based on functional demand and construction restrictions and facilitated the development of cranial kinesis systems. There exist two main functions of cranial kinesis; facilitate the functioning of jaws and enable feeding in vertebrates.

Introduction

Cranial kinesis is defined as the essential movement of the bones of the skull about each other as well as the movement in the joint lower and upper jaw. Birds as an excellent example of vertebrates have fluctuating cranial kinesis degrees with parrots having the highest degree. Among crocodilians, reptiles, as well as turtles, have cranial kinesis while lizards, on the other hand, have some degree of cranial kinesis. Snakes have the highest degree of cranial kinesis among the tetrapod (Holliday & Lawrence, 2008).

Ancestry plays a significant role in enabling or limiting the process of cranial kinesis.  Cranial kinesis is not common in mammals; for example, the human skull does not show any form of cranial kinesis.  Birds, on the other hand, are having considerable variation in the cranial kinesis degrees. Amphibians have varying cranial kinesis, but it is not available in frogs and is not common in salamanders.  Among the fish family, they possess a high degree of cranial kinesis. Teleost fish is known for having the most cranial kinetic developed skull among all living organisms.   Joints are regularly of simple syndesmosis intersections, while in some living organisms, some of their bones are synovial, allowing for higher movement range. In this paper, we are going to discuss the evolution of cranial kinesis among the vertebrates, functions, or significance or biomechanics of cranial kinesis. We will also discuss the cranial kinesis specifically in fishes, snakes, and dinosaurs.

Evolution of cranial kinesis

Some groups of vertebrates such as lizards, snakes, and sphenodon have significant diversity in the structure of cranial bones which makes their cranium to become an ideal test case for examining the functional trade-off role as well as constructional restrictions in complex integrated networks. Several theories have been established to show the origin and development of cranial kinesis among the vertebrates. For instance, feeding of vertebrates on large, robust, and hard preys led to an increased demand for higher and efficient performance in nutrition. The increase in bite force can be done by increasing the mass of the muscles of the jaw adductor. It can also be improved through changing the physiology or architecture of the muscles, for example, pennate muscles, which contains short fibers or expressing various types of fiber. Finally, it can increase through changing the mechanics of the system of the lever. Unless vertebrates shorten the jaw system out the bar and ensure they bite closer to the fulcrum, the rise in the force of muscle and strength of biting will induce higher loads on the elements of cranial structure that requires to withstand large high forces. Therefore, an increase in the power of biting and mass of muscle is expected to be related to the rise in the heftiness of the cranial skull. Hitherto, the systems of mechanical systems maximized for the force production that can never be optimized for speed as a result of lever mechanics constraints as well as the physiology of the muscle. Due to this, fast preying species, the elusive prey, are said to have elongated snouts, elongated jaw opening in the levers, and long parallel fibered muscles (Anthony et al.,2007).

Not only demands for function but also constructional constraints have facilitated the evolution of cranial kinesis. For instance, the space that is taken adductors of the jaw can never be used for promoting sensory organs or the system of central nervous if the overall skull capacity remains unchanged. In some lizards, a robust dominance of particular methods of sensory may be available and may be positioned in specific selective pressures of the system of cranial kinesis. For instance, mammals may heavily rely on chemosensation have lengthened,bifurcated tongues which have a relative tine surface area of the language and also have long scouts.  Thus, the animal’s tongue can never be used again for prey transport, as well as alternative modes of transportation that need to be developed. For the snakes, this leads to a well –known cranial kinetic jaw system used to move prey using the oral cavity. Likewise, in animals, which are nocturnal, such as olfactory senses and geckos, visual dominate and their eyes are more extensive as compared to other existing lizards. The increase in the sizes of the eyeball is highly correlated with the loss of supratemporal and postorbital bar in these vertebrates. Losing these cranial elements results in functional consequences for the strength and stability of the animal’s skull and later affect the use of this system during nourishing(Anthony et al.,2007).

In some other animal groups, the deduction of the cranial kinesis elements appears to be correlated to the habits of crevice-dwelling.  It is assumed that mobile skulls and flats heads of cordylines, lacertids, and dinosaurs enable animals to segment themselves to what refers to crevices as an anti-predation approach.  Therefore, pressures on selection lead to the same functional demands of having a cranial kinesis system and may also have a similar role implication in various groups (Anthony et al.,2007).

In summary, the evolution of cranial kinesis has been contributed by constructional constraints and functional demands. The space that is taken adductors of the jaw can never be used for facilitating sensory organs or the system of central nervous if the overall skull capacity remains unchanged. This has forced for the creation and development of a cranial kinesis system that facilities the movement of the joint between upper and lower jaws of the skull. Also, the feeding of vertebrates on large, robust, and hard preys led to an increased demand for higher and efficient performance in nutrition. The increase in bite force can be done by increasing the mass of the muscles of the jaw adductor. It can also be improved through changing the physiology or architecture of the muscles, for example, pennate muscles, which contains short fibers or expressing various types of fiber.

Functions/biomechanics/ significance of cranial kinetics

Cranial kinesis function is self-explanatory, and the main function is to facilitate the efficient function of the jaw. The gap that exists between the lower and upper jaws of the skull meets the prey and provides forces directly to the prey, which involves multiple teeth.  In snakes, for example. The lower and upper non-permanent bars are lost, and jaws at the lower side are unfused in the middle line to ensure more movement in most of the skull bones.  The independence of the movements enables them to provide accommodations for large prey, even bigger than their heads.  With no cranial kinesis, animals can give powerful bite forces, e.g., crocodiles possess rigid skulls which have no or little kinesis, for optimal strength (Williams and Barrett, 2009).

Cranial kinesis is highly correlated with the feeding of vertebrates. Animals swallowing bigger prey as a whole like snakes, which grip clumsy shaped foodstuffs like parrots which eat nuts which in most times feed on water using suction feeding mechanism usually have very strong kinetic skulls with several mobile joints in them. For the cases of mammals that contain kinetic skulls apart from hares, kinesis deficiency is highly connected to the secondary palate, which disallows relative movement.  This leads top consequences of requirement to develop a suction at the time of suckling.

Cranial kinesis in fish

The earliest example of cranial kinesis in fish was discovered in chondrichthyans like sharks.  There exists no link between quadrate and hyomandibular, and in its place, the hyoid arch hangs the two jaws as pendulums.  This enables shark to swipe their jaws forwards and outwards over the prey. This allows a synchronous meeting between the jaws hence preventing the deflection of the victim as it comes close to it. Actinopterygian fish have a vast range of cranial kinetic systems. As an overall trend via phylogenetic trees, there exists a behavior to liberate extra bony elements to enable higher motility of the skull. The majority of the actinopts utilize kinesis to quickly expand their buccal cavity to facilitate the creation of suction, which is used for feeding. Sarcopterygian fish possess upper jaws, which are linked with the braincase, which infers feeding on substrates, which are hard to consume. The majority of the fishes of crossopterygian also possess kinesis.

Most of the bony fishes have major two sets of jaws, which are mainly bone. The key oral jaws close and open the mouth, while the second set, which is made up of pharyngeal jaws are located at the posterior of the throat. Oral jaws of the fish are used to manipulate and capture prey through crushing and biting. The pharyngeal jaws are used to process food and transfer it to the stomach from the mouth because it is located to the pharynx. Cartilaginous fishes like rays and sharks possess a set of oral jaws which are primarily made up of cartilage.  They have no pharyngeal jaws. Generally, these jaws are uttered and do battle with vertically, involving the lower and upper jaw which bears numerous well-ordered teeth. Cartilaginous fishes develop various sets and substitute teeth as they wear by transferring new teeth crosswise from the surface of the medial jaw in the conveyor-belt style.  Teeth are always replaced several times also in the majority of bony fishes but not like in cartilaginous fishes, and the new generated tooth explodes only post falling out of the old one.

Cranial kinesis of jaws in fish originate from the pharyngeal arches, which supports the gills of fish with no jaws. The first jaw appeared in currently extinct spiny sharks and placoderms at the time if Silurian that is about 430 million years back.  The primary choice benefit provided by the jaw was not connected to the feeding mechanism, but it was used to increase the efficiency of respiration. The jaws were used to pump water in the buccal pump across the fish’s gill.  The familiarity in the feeding use of jaws developed as a secondary role of jaws before becoming the primary function in most vertebrates.  All jaws of vertebrates, including those of humans, emerged from the earliest jaws of fish.  The display of the old jaws of vertebrates has been expressed as the most thoughtful and quick evolutionary step that has taken place in the history of vertebrates. Fish which do not have jaws face a difficult life in surviving than those possessing jaws, and most of the fishes without jaws are getting extinct.

Jaws in fish use linkage mechanisms and the connections are complex and universal in bony fishes head like wrasses, which developed several expertized devices of feeding. Notably, most improved are jaw protrusion linkage mechanisms.  A system of connected four-bar connections for the suction feeding mechanism is used to coordinate mouth opening as well as the expansion of the buccal cavity in three-dimension.

Cranial kinesis in snakes

Most of the reptiles, including snakes, have established joints with their skull that allow them to slightly move a single part of the jaw relative to the other. The space of this movement occurs within the head of the fish refers to kinesis. Kinesis allows an animal to raise the mouth gape, and hence its adaption is for swallowing massive objects. Seemingly, some of the enormous carnivorous theropod dinosaurs like allosaurus possess a joint that is located between parietal and frontal bones in the skull’s roof.  All superorder Lepidosauria reptiles such as tuatara, lizards, and snakes possess kinetic heads. They are different from dinosaurs in a joint located at the skull floor, which occurs at the occasion of the pterygoid and basisphenoid bones in super order lepidosaurs (Kardong,1995).

The lepidosaurian’s skulls became more kinetic as new groups of evolution.  Sphenodontia, which includes sphenodon( living tuatara), as well as antecedents, which are Rhynchocephalia, possessed only a basisphenoidal-pterygoid joint.  Lizards, which is also in the class of reptiles together with snakes, lost their lower non-permanent bar, thus liberating the quadrate bone and permitting the higher movement of the lower jaw, which is hinged next to the quadrate.  In snakes which is belongs to the class of reptiles, the trend ends at the most cranial kinetic skull in vertebrates.  The snake’s skull has an ancestral pterygoid-basisphenoidal joint, which is a high movable quadrate, and it gives the higher flexibility of the lower jaw. The upper jaw can rotate its longitudinal axes and move it both backward and forward. Most of the species of snakes have a skull’s roof hinge located between frontal and nasal bones that facilitates the slight raising of the snout.

Cranial kinesis in dinosaurs

There exist three main kinds of cranial kinesis in dinosaurs, and these are; streptostyly, metakinesis, and prokinesis. Streptostyly is kinesis that involves backward and forward quadrate movement, which is also common in snakes, birds, and lizards. In dinosaurs, this type of kinesis is present in Ankylosaurs and also in many other theropods, for example, coelophysis, allosaurus, and tyrannosaurus. It is also present in masospondylus and hypsilophodon. Metakinesis is a kinesis type that connects between dermatocranium and neurocranium, which is also seen in specific lizards. Hypsilophodon and Dromaeosaurus indicates a metakinetic joint type. Prokinesis is a type of kinesis with a joint in the area of the face like in modern birds and snakes. It is available in several dinosaurs.  Some dinosaurs show a combination of two types of kinetics, such as prokiness and streptostyly.  Others have several points of akinesis, such as ankylosaurs, ceratopsians, and sauropods(Casey  &Lawrence, 2007).

Many dinosaurs have displayed various forms of kinesis, such as prokinesis, pyrokinesis, and streptostyly. The foundation of kinesis conclusion usually has been including the availability of apparently synovial intracranial joints i.e., basal and octic joints, as well as sliding joints. None of the study done in the past that have reviewed the proof of underlying these conclusions, and the role mechanisms and kinesis evolution in dinosaurs have not clearly explained(Casey  &Lawrence, 2007).

Conclusion

In conclusion, the critical movement of the bones of the skull concerning each other as well as the change in the joint lower and upper jaw. Both functional demand and constructional constraints have explained the evolution of cranial kinesis. Constructional restrictions and technical requirements have contributed to the development of cranial kinesis. The space that is taken adductors of the jaw can never be used for facilitating sensory organs or the system of central nervous if the overall skull capacity remains unchanged. Functions of cranial kinesis are to enable the functioning of the jaws and also facilitate feeding mechanisms in vertebrates. The gap that exists between the lower and upper jaws of the skull meets the prays and provides forces directly to the prey, which involves multiple teeth. Cranial kinesis is highly correlated with the feeding of vertebrates. Animals swallowing bigger prey as a whole like snakes, which grip clumsy shaped foodstuffs like parrots which eat nuts which in most times feed on water using suction feeding mechanism usually have solid kinetic skulls with several mobile joints in them.  Dinosaurs, fish, and snakes possess cranial kinesis, which enables them to feed as well as the functioning of their jaws.

REFERENCES

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Casey M.H  &Lawrence M.(2007). Cranial kinesis in dinosaurs: intracranial joints, protractor muscles, and their significance for cranial evolution and function in diapsids. 1073-1088

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Kardong, Kenneth V. (1995). Vertebrates: Comparative anatomy, function, and evolution. Wm. C. Brown.

Williams, V., and  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.