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Fish jaw

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Fish jaw

Skull diagram of the large predatory placoderm fish Dunkleosteus terrelli, which lived about 380–360 million years ago

A jaw is an opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. The term jaw is also broadly applied to the whole of the structure constituting the vault of the mouth and serving to open and close it. In most fishes the jaws are bony or cartilaginous. They oppose vertically, comprising an upper jaw and a lower jaw derived from the most anterior two pharyngeal arches supporting the gills. They usually bear numerous teeth. All vertebrate jaws, including human jaws, originated from an early fish jaw.

All jaws, including fish jaws, use linkage mechanisms. These linkages can be especially common and complex in the head of bony fishes, such as wrasses, which have evolved many specialized feeding mechanisms. Especially advanced are the linkage mechanisms of jaw protrusion. For suction feeding a system of linked four-bar linkages is responsible for the coordinated opening of the mouth and a three-dimensional expansion of the buccal cavity. Other linkages are responsible for protrusion of the premaxilla.

Linkage systems are widely distributed in animals. The most thorough overview of the different types of linkages in animals has been provided by M. Muller,[1] who also designed a new classification system, which is especially well suited for biological systems.


Fish head parts, 1889, Fauna of British India, Sir Francis Day

The skull of fishes is formed from a series of loosely connected bones. Lampreys and sharks only possess a cartilaginous endocranium, with both the upper and lower jaws being separate elements. Bony fishes have additional dermal bone, forming a more or less coherent skull roof in lungfish and holost fish. The lower jaw defines a chin.

The simpler structure is found in jawless fish, in which the cranium is represented by a trough-like basket of cartilaginous elements only partially enclosing the brain, and associated with the capsules for the inner ears and the single nostril. Distinctively, these fish have no jaws.[2]

inner ear. Finally, the skull tapers towards the rear, where the foramen magnum lies immediately above a single condyle, articulating with the first vertebra. There are, in addition, at various points throughout the cranium, smaller foramina for the cranial nerves. The jaws consist of separate hoops of cartilage, almost always distinct from the cranium proper.[2]

Skull of an Atlantic wolffish
Jaw from side and above of Piaractus brachypomus, a close relative of piranhas

In ray-finned fishes, there has also been considerable modification from the primitive pattern. The roof of the skull is generally well formed, and although the exact relationship of its bones to those of tetrapods is unclear, they are usually given similar names for convenience. Other elements of the skull, however, may be reduced; there is little cheek region behind the enlarged orbits, and little, if any bone in between them. The upper jaw is often formed largely from the premaxilla, with the maxilla itself located further back, and an additional bone, the symplectic, linking the jaw to the rest of the cranium.[3]

Although the skulls of fossil lobe-finned fish resemble those of the early tetrapods, the same cannot be said of those of the living lungfishes. The skull roof is not fully formed, and consists of multiple, somewhat irregularly shaped bones with no direct relationship to those of tetrapods. The upper jaw is formed from the pterygoids and vomers alone, all of which bear teeth. Much of the skull is formed from cartilage, and its overall structure is reduced.[3]

Lower jaw

In vertebrates, the lower jaw (mandible or jawbone)[4] is a bone forming the skull with the cranium. In lobe-finned fishes and the early fossil tetrapods, the bone homologous to the mandible of mammals is merely the largest of several bones in the lower jaw. It is referred to as the 'dentary bone, and forms the body of the outer surface of the jaw. It is bordered below by a number of splenial bones, while the angle of the jaw is formed by a lower angular bone and a suprangular bone just above it. The inner surface of the jaw is lined by a prearticular bone, while the articular bone forms the articulation with the skull proper. Finally a set of three narrow coronoid bones lie above the prearticular bone. As the name implies, the majority of the teeth are attached to the dentary, but there are commonly also teeth on the coronoid bones, and sometimes on the prearticular as well.[3]

This complex primitive pattern has, however, been simplified to various degrees in the great majority of vertebrates, as bones have either fused or vanished entirely. In teleosts, only the dentary, articular, and angular bones remain.[3] Cartilagenous fish, such as sharks, do not have any of the bones found in the lower jaw of other vertebrates. Instead, their lower jaw is composed of a cartilagenous structure homologous with the Meckel's cartilage of other groups. This also remains a significant element of the jaw in some primitive bony fish, such as sturgeons.[3]

Upper jaw

Kype of a spawning male salmon

The upper jaw, or maxilla[5][6] is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible (lower jaw), which is also a fusion of two halves at the mandibular symphysis. In bony fish, the maxilla is called the "upper maxilla," with the mandible being the "lower maxilla". The alveolar process of the maxilla holds the upper teeth, and is referred to as the maxillary arch. In most vertebrates, the foremost part of the upper jaw, to which the incisors are attached in mammals consists of a separate pair of bones, the premaxillae. In bony fish, both maxilla and premaxilla are relatively plate-like bones, forming only the sides of the upper jaw, and part of the face, with the premaxilla also forming the lower boundary of the nostrils.[3] Cartilaginous fish, such as sharks and rays also lack a true maxilla. Their upper jaw is instead formed from a cartilagenous bar that is not homologous with the bone found in other vertebrates.[3]

The jaws of male salmon are often remodelled during spawning runs so they have a pronounced curvature. These hooked jaws are called kypes. The purpose of the kype is not altogether clear, though they can be used to to establish dominance by clamping them around the base of the tail (caudal peduncle) of an opponent.[7][8]

Some fish have permanently protruding upper jawbones called rostrums. Billfish (marlin, swordfish and sailfish) use rostrums to slash and stun prey. Paddlefish, goblin sharks and hammerhead sharks have rostrums packed with electroreceptors which signal the presence of prey by detecting weak electrical fields. Sawfish and sawsharks have rostrums which are both electro-sensitive and used for slashing. The rostrums extend ventrally in front of the fish. In the case of hammerheads the rostrum extends both ventrally and laterally (sideways).

Pharyngeal jaws

Moray eels have two sets of jaws: the oral jaws that capture prey and the pharyngeal jaws that advance into the mouth and move prey from the oral jaws to the esophagus for swallowing

Pharyngeal jaws are a second set of jaws contained within an animal's throat, or pharynx, distinct from the primary (oral) jaws. They are believed to have originated as modified gill arches, in much the same way as oral jaws.

Approximately 30,000 species of fishes are known to have pharyngeal jaws. The most notable examples are the moray eels. Unlike other fishes, the pharyngeal jaws of the moray are highly mobile. This is possibly a response to their inability to swallow as do other fishes by creating a negative pressure in the mouth, perhaps induced by their restricted environmental niche (burrows). Instead, when the moray bites prey, it first bites normally with its oral jaws, capturing the prey. Immediately thereafter, the pharyngeal jaws are brought forward and bite down on the prey to grip it; they then retract, pulling the prey down the moray eel's gullet, allowing it to be swallowed.[9]

Cartilaginous jaws

The serrated teeth of a tiger shark, used for sawing through flesh
The teeth of tiger sharks are oblique and serrated to saw through flesh

The serrated teeth of a tiger shark, used for sawing through flesh

Cartilaginous fishes (sharks, rays and skates) have cartilaginous jaws, and are not attached to the cranium. The jaw's surface (in comparison to the vertebrae and gill arches) needs extra support due to its heavy exposure to physical stress and its need for strength. It has a layer of tiny hexagonal plates called "tesserae", which are crystal blocks of calcium salts arranged as a mosaic.[10] This gives these areas much of the same strength found in the bony tissue found in other animals.

Generally sharks have only one layer of tesserae, but the jaws of large specimens, such as the bull shark, tiger shark, and the great white shark, have two to three layers or more, depending on body size. The jaws of a large great white shark may have up to five layers.[11] Because sharks do not have rib cages, they can easily be crushed under their own weight on land.[12] In the rostrum (snout), the cartilage can be spongy and flexible to absorb the power of impacts.

Shark teeth are embedded in the gums rather than directly affixed to the jaw, and are constantly replaced throughout life. Multiple rows of replacement teeth grow in a groove on the inside of the jaw and steadily move forward in comparison to a conveyor belt; some sharks lose 30,000 or more teeth in their lifetime. The rate of tooth replacement varies from once every 8 to 10 days to several months. In most species, teeth are replaced one at a time as opposed to the simultaneous replacement of an entire row, which is observed in the cookiecutter shark.[13]

Tooth shape depends on the shark's diet: those that feed on mollusks and crustaceans have dense and flattened teeth used for crushing, those that feed on fish have needle-like teeth for gripping, and those that feed on larger prey such as mammals have pointed lower teeth for gripping and triangular upper teeth with serrated edges for cutting. The teeth of plankton-feeders such as the basking shark are small and non-functional.[14] While the shark is moving, water passes through the mouth and over the gills in a process known as "ram ventilation". While at rest, most sharks pump water over their gills to ensure a constant supply of oxygenated water. A small number of species have lost the ability to pump water through their gills and must swim without rest. These species are obligate ram ventilators and would presumably asphyxiate if unable to move. Obligate ram ventilation is also true of some pelagic bony fish species.[15]

The teeth of extant elasmobranchs are in several series; the upper jaw is not fused to the cranium, and the lower jaw is articulated with the upper. There are several archetypal jaw suspensions: amphistyly, orbitostyly, hyostyly, and euhyostyly. In amphistyly, the palatoquadrate has a postorbital articulation with the chondrocranium from which ligaments primarily suspend it anteriorly. The hyoid articulates with the mandibular arch posteriorly, but it appears to provide little support to the upper and lower jaws. In orbitostyly, the orbital process hinges with the orbital wall and the hyoid provides the majority of suspensory support. In contrast, hyostyly involves an ethmoid articulation between the upper jaw and the cranium, while the hyoid most likely provides vastly more jaw support compared to the anterior ligaments. Finally, in euhyostyly, also known as true hyostyly, the mandibular cartilages lack a ligamentous connection to the cranium. Instead, the hyomandibular cartilages provide the only means of jaw support, while the ceratohyal and basihyal elements articulate with the lower jaw, but are disconnected from the rest of the hyoid.[16][17][18]

Wrasse jaws

Lips of a humphead wrasse

Wrasses have become a primary study species in fish-feeding biomechanics due to their jaw structure. They have protractile mouths, usually with separate jaw teeth that jut outwards.[19] Many species can be readily recognized by their thick lips, the inside of which is sometimes curiously folded, a peculiarity which gave rise the German name of "lip-fishes" (Lippfische.)[20]

The nasal and mandibular bones are connected at their posterior ends to the rigid neurocranium, and the superior and inferior articulations of the maxilla are joined to the anterior tips of these two bones, respectively, creating a loop of 4 rigid bones connected by moving joints. This "four-bar linkage" has the property of allowing numerous arrangements to achieve a given mechanical result (fast jaw protrusion or a forceful bite), thus decoupling morphology from function. The actual morphology of wrasses reflects this, with many lineages displaying different jaw morphology that results in the same functional output in a similar or identical ecological niche.[19]


Reconstruction of the huge, predatory placoderm Dunkleosteus terrelli
Skull from side and above of Arapaima gigas, a living fossil and one of the largest freshwater fishes in the world.
Skeletal head of a bowfin, another living fossil, showing the bones in its jaw

The vertebrate jaw probably originally evolved from jawless fish in the Silurian period, and appeared in the Placoderm fish which further diversified in the Devonian. In jawless fishes a series of gills opened behind the mouth, and these gills became supported by cartilaginous elements. The first set of these elements surrounded the mouth to form the jaw. Placoderms, an extinct Class (biology) of armoured prehistoric fish which lived from the late Silurian to the end of the Devonian about 430–360 million years ago, were among the first fish with jaws. Their largest species, Dunkleosteus terrelli, measured up to 10 m (33 ft)[21][22] and weighed 3.6 t (4.0 short tons).[23] It possessed a four bar linkage mechanism for jaw opening that incorporated connections between the skull, the thoracic shield, the lower jaw and the jaw muscles joined together by movable joints.[24][25] This mechanism allowed Dunkleosteus terrelli to achieve a high speed of jaw opening, opening their jaws in 20 milliseconds and completing the whole process in 50-60 milliseconds, comparable to modern fishes that use suction feeding to assist in prey capture.[24] They could also produce high bite forces when closing the jaw, estimated at 6,000 N (1,350 lbf) at the tip and 7,400 N (1,660 lbf) at the blade edge in the largest individuals.[25] The pressures generated in those regions were high enough to puncture or cut through cuticle or dermal armour[24] suggesting that Dunkleosteus terrelli was perfectly adapted to prey on free-swimming, armoured prey like arthropods, ammonites, and other placoderms.[25]

There is ample evidence that vertebrate jaws are homologous to the gill arches of jawless fishes.[26] The upper portion of the second embryonic arch supporting the gill became the hyomandibular bone of jawed fishes, which supports the skull and therefore links the jaw to the cranium.[27] The hyomandibula is a set of bones found in the hyoid region in most fishes. It usually plays a role in suspending the jaws or the operculum in the case of teleosts.[28]

The original selective advantage offered by the jaw was not related to feeding, but to increased respiration efficiency. The jaws were used in the buccal pump still observable in modern fish and amphibians, that uses "breathing with the cheeks" to pumps water across the gills of fish or air into the lungs in the case of amphibians. Over evolutionary time the more familiar use of jaws (to humans) for feeding was selected for and became a very important function in vertebrates. Many teleost fish have substantially modified jaws for suction feeding and jaw protrusion, resulting in highly complex jaws with dozens of bones involved.

Jaws are thought to derive from the pharyngeal arches that support the gills in fish. The two most anterior of these arches are thought to have become the jaw itself (see hyomandibula) and the hyoid arch, which braces the jaw against the braincase and increases mechanical efficiency. While there is no fossil evidence directly to support this theory, it makes sense in light of the numbers of pharyngeal arches that are visible in extant jawed (the Gnathostomes), which have seven arches, and primitive jawless vertebrates (the Agnatha), which have nine.

Meckel's cartilage is a piece of cartilage from which the mandibles (lower jaws) of vertebrates evolved. Originally it was the lower of two cartilages which supported the first gill arch (nearest the front) in early fish. Then it grew longer and stronger, and acquired muscles capable of closing the developing jaw.[29] In early fish and in chondrichthyans (cartilaginous fish such as sharks), Meckel's cartilage continued to be the main component of the lower jaw. But in the adult forms of osteichthyans (bony fish) and their descendants (amphibians, reptiles,[birds and mammals) the cartilage was covered in bone - although in their embryos the jaw initially develops as the Meckel's cartilage. In tetrapods the cartilage partially ossifies (changes to bone) at the rear end of the jaw and becomes the articular bone, which forms part of the jaw joint in all tetrapods except mammals.[29]

See also


  1. ^ Muller, M. (1996). "A novel classification of planar four-bar linkages and its application to the mechanical analysis of animal systems". Phil. Trans. R. Soc. Lond. B 351: 689–720.  
  2. ^ a b Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 173–177.  
  3. ^ a b c d e f g Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 244–247.  
  4. ^ The mandible is also in some sources still referred to as the inferior maxillary bone, though this is an outdated term which goes back to at least the 1858 first edition of Gray's Anatomy, if not earlier.
  5. ^ OED 2nd edition, 1989.
  6. ^ Entry "maxilla" in Merriam-Webster Online Dictionary.
  7. ^ Witten, P. E., & Hall, B. K. (2003). "Seasonal changes in the lower jaw skeleton in male Atlantic salmon (Salmo salar L.): remodelling and regression of the kype after spawning. Journal of anatomy, 203 (5):435–450. doi:10.1046/j.1469-7580.2003.00239.x Full text.
  8. ^ Groot C and Margolis L (1991) Pacific salmon life histories page 144, UBC Press. ISBN 978-0-7748-0359-5.
  9. ^ Mehta, Rita S.; Peter C. Wainwright (2007-09-06). "Raptorial jaws in the throat help moray eels swallow large prey".  
  10. ^ Hamlett, W. C. (1999f). Sharks, Skates and Rays: The Biology of Elasmobranch Fishes. Johns Hopkins University Press.  
  11. ^ Martin, R. Aidan. "Skeleton in the Corset". ReefQuest Centre for Shark Research. Retrieved 2009-08-21. 
  12. ^ "A Shark's Skeleton & Organs". Archived from the original on April 9, 2012. Retrieved August 14, 2009. 
  13. ^ Martin, R. Aidan. "Skin of the Teeth". Retrieved 2007-08-28. 
  14. ^ Gilbertson, Lance (1999). Zoology Laboratory Manual. New York: McGraw-Hill Companies, Inc.  
  15. ^ William J. Bennetta (1996). "Deep Breathing". Retrieved 2007-08-28. 
  16. ^ Wilga, C. D. (2005) "Morphology and evolution of the jaw suspension in lamniform sharks". Journal of Morphology, 265 (1): 102-119. doi:10.1002/jmor.10342
  17. ^ Wilga, C. D., Motta, P. J. & Sanford, C. P. (2007) "Evolution and ecology of feeding in elasmobranchs". Integrative and Comparative Biology, 47 (1) 55-69. doi:10.1093/icb/icm029 Full text
  18. ^ Motta PJ and Huber DR (2012) "Prey Capture Behavior and Feeding Mechanisms of Elasmobranchs". In: JC Carrier, JA Musick and MR Heithaus (eds) Biology of Sharks and Their Relatives Second Edition, pages 153–210. CRC Press. ISBN 9781439839249.
  19. ^ a b Wainwright et al. (2005). "Many-to-One Mapping of Form to Function: A General Principle in Organismal Design?". Integrative & comparative biology 45: 256–262.  
  20. ^  
  21. ^ Ancient Fish With Killer Bite. Science News. May 19, 2009.
  22. ^ Palmer, D., ed. (1999). The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. London: Marshall Editions. p. 33.  
  23. ^ Monster fish crushed opposition with strongest bite ever. The Sydney Morning Herald. November 30, 2006.
  24. ^ a b c Anderson, P.S.L.; Westneat, M. (2007). "Feeding mechanics and bite force modelling of the skull of Dunkleosteus terrelli, an ancient apex predator". Biology Letters 3 (1): 76–79. 
  25. ^ a b c Anderson, P.S.L.; Westneat, M. (2009). "A biomechanical model of feeding kinematics for Dunkleosteus terrelli (Arthrodira, Placodermi)". Paleobiology 35 (2): 251–269.  
  26. ^ For example: (1) both sets of bones are made from neural crest cells (rather than mesodermal tissue like most other bones); (2) both structures form the upper and lower bars that bend forward and are hinged in the middle; and (3) the musculature of the jaw seem homologous to the gill arches of jawless fishes. (Gilbert 2000)
  27. ^ Gilbert (2000) "Evolutionary Embryology"
  28. ^ Clack JA (1994) "Earliest known tetrapod braincase and the evolution of the stapes and fenestra ovalis", Nature, 369, 392–394.
  29. ^ a b The Gill Arches: Meckel's Cartilage, palaeos. Retrieved 4 December 2014.

Other reading

External links

External video
Video of a slingjaw wrasse catching prey by protruding its jaw
Video of a red bay snook catching prey by suction feeding
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