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Skull of a generalized cichlid, showing a lateral view of the oral jaws (purple) and the pharyngeal jaws (blue)[1]
Dorsal view of the lower pharyngeal and oral jaws of a juvenile Malawi eyebiter showing the branchial (pharyngeal) arches and ceratobrachial elements (arch bones). The white asterisk indicates the toothed pharyngeal jaw. Scale bar represents 500 μm.[1]

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.[2][3]

Cartilaginous fishes, such as sharks and rays, have one set of oral jaws made mainly of cartilage. They do not have pharyngeal jaws. Generally jaws are articulated and oppose vertically, comprising an upper jaw and a lower jaw and can bear numerous ordered teeth. Cartilaginous fishes grow multiple sets (polyphyodont) and replace teeth as they wear by moving new teeth laterally from the medial jaw surface in a conveyor-belt fashion. Teeth are replaced multiple times also in most bony fishes, but unlike cartilaginous fishes, the new tooth erupts only after the old one has fallen out.

Jaws probably originated in the pharyngeal arches supporting the gills of jawless fish. The earliest jaws appeared in now extinct placoderms and spiny sharks during the Silurian, about 430 million years ago. The original selective advantage offered by the jaw was probably not related to feeding, but to increased respiration efficiency—the jaws were used in the buccal pump to pump water across the gills. The familiar use of jaws for feeding would then have developed as a secondary function before becoming the primary function in many vertebrates. All vertebrate jaws, including the human jaw, evolved from early fish jaws. The appearance of the early vertebrate jaw has been described as "perhaps the most profound and radical evolutionary step in the vertebrate history".[4][5] Fish without jaws had more difficulty surviving than fish with jaws, and most jawless fish became extinct.

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 the three-dimensional expansion of the buccal cavity. The four-bar linkage is also responsible for protrusion of the premaxilla,[6] leading to three main four-bar linkage systems to generally describe the lateral and anterior expansion of the buccal cavity in fishes.[6][7] The most thorough overview of the different types of linkages in animals has been provided by M. Muller,[8] who also designed a new classification system, which is especially well suited for biological systems.

Skull

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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 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.[9]

Cartilaginous fish, such as sharks, also have simple skulls. The cranium is a single structure forming a case around the brain, enclosing the lower surface and the sides, but always at least partially open at the top as a large fontanelle. The most anterior part of the cranium includes a forward plate of cartilage, the rostrum, and capsules to enclose the olfactory organs. Behind these are the orbits, and then an additional pair of capsules enclosing the structure of the 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.[9]

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.[9]

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.[9]

Oral jaws

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Lower

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Oral jaw from side and above of Piaractus brachypomus, a close relative of piranhas

In vertebrates, the lower jaw (mandible or jawbone)[10] 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.[11]

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.[11] Cartilaginous 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 cartilaginous 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.[11]

Upper

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The upper jaw, or maxilla[12][13] 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.[14] 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.[14]

Some fish have permanently protruding upper jawbones called rostrums. Billfish (marlin, swordfish and sailfish) use rostrums (bills) 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. Sawsharks and the critically endangered sawfish have rostrums (saws) which are both electro-sensitive and used for slashing.[15] The rostrums extend ventrally in front of the fish. In the case of hammerheads the rostrum (hammer) extends both ventrally and laterally (sideways).

  • Fish with rostrums (extended upper jawbones)
  • Sailfish, like all billfish, have a rostrum (bill) which evolved from the upper jawbone
    Sailfish, like all billfish, have a rostrum (bill) which evolved from the upper jawbone
  • The paddlefish has a rostrum packed with electroreceptors
    The paddlefish has a rostrum packed with electroreceptors
  • Sawfish have an electro-sensitive rostrum (saw) which is also used to slash at prey
    Sawfish have an electro-sensitive rostrum (saw) which is also used to slash at prey

Jaw protrusion

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Teleosts have a movable premaxilla (a bone at the tip of the upper jaw) and corresponding modifications in the jaw musculature which make it possible for them to protrude their jaws outwards from the mouth. This is of great advantage, enabling them to grab prey and draw it into the mouth. In more derived teleosts, the enlarged premaxilla is the main tooth-bearing bone, and the maxilla, which is attached to the lower jaw, acts as a lever, pushing and pulling the premaxilla as the mouth is opened and closed. These protrusible jaws are evolutionary novelties in teleosts that evolved independently at least five times.[16]

The premaxilla is unattached to the neurocranium (braincase); it plays a role in protruding the mouth and creating a circular opening. This lowers the pressure inside the mouth, sucking the prey inside. The lower jaw and maxilla (main upper fixed bone of the jaw) are then pulled back to close the mouth, and the fish is able to grasp the prey. By contrast, mere closure of the jaws would risk pushing food out of the mouth. In more advanced teleosts, the premaxilla is enlarged and has teeth, while the maxilla is toothless. The maxilla functions to push both the premaxilla and the lower jaw forward. To open the mouth, an adductor muscle pulls back the top of the maxilla, pushing the lower jaw forward. In addition, the maxilla rotates slightly, which pushes forward a bony process that interlocks with the premaxilla.[17]

Teleosts achieve this jaw protrusion using one of four different mechanisms involving the ligamentous linkages within the skull.[18]

Lips of a humphead wrasse
The sling-jaw wrasse has the most extreme jaw protrusion of all fishes.
Slingjaw wrasse protruding its jaw – YouTube
  • Mandibular depression mechanism: The depression of the lower jaw (mandible) pulls or pushes the premaxilla into protrusion via force transmission through ligaments and tendons connected to the upper jaws (e.g. Cyprinus, Labrus).[18] This is the most commonly used mechanism.
  • Twisting maxilla mechanism: The depression of the mandible causes the maxilla to twist about the longitudinal axis resulting in the protrusion of the premaxilla (e.g. Mugil).[18]
  • Decoupled mechanism: Protrusion of the premaxilla is accomplished through elevation of the neurocranium causing the premaxilla to move anteriorly. Movements of the neurocranium are not coupled with the kinematics of the upper jaw (e.g. Spathodus erythrodon),[18][19] allowing for more versatility and modularity of the jaws during prey capture and manipulation.
  • Suspensorial abduction mechanism: The lateral expansion of the suspensorium (a combination of the palatine, pterygoid series, and quadrate bones) pulls on a ligament which causes the premaxilla to protrude anteriorly (e.g. Petrotilapia tridentiger).[18][19]

Some teleosts use more than one of these mechanisms (e.g. Petrotilapia).[18]

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.[20] Many species can be readily recognized by their thick lips, the inside of which is sometimes curiously folded, a peculiarity which gave rise to the German name of "lip-fishes" (Lippfische).[21]

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.[20]

The most extreme jaw protrusion found in fishes occurs in the slingjaw wrasse, Epibulus insidiator . This fish can extend its jaws up to 65% the length of its head.[22] This species utilizes its quick and extreme jaw protrusion to capture smaller fishes and crustaceans. The genus this species belongs to possess one unique ligament (vomero-interopercular) and two enlarged ligaments (interoperculo-mandibular and premaxilla-maxilla), which along with a few changes to the form of cranial bones, allow it to achieve extreme jaw protrusion.

Pharyngeal jaws

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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 distinct from the primary (oral) jaws. They are contained within the throat, or pharynx, of most bony fish. They are believed to have originated, in a similar way to oral jaws, as a modification of the fifth gill arch which no longer has a respiratory function. The first four arches still function as gills. Unlike the oral jaw, the pharyngeal jaw has no jaw joint, but is supported instead by a sling of muscles.

Pharyngeal jaw of an asp carrying some pharyngeal teeth

A notable example occurs with the moray eel. The pharyngeal jaws of most fishes are not mobile. The pharyngeal jaws of the moray are highly mobile, perhaps as an adaptation to the constricted nature of the burrows they inhabit which inhibits their ability to swallow as other fishes do by creating a negative pressure in the mouth. 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.[23]

All vertebrates have a pharynx, used in both feeding and respiration. The pharynx arises during development through a series of six or more outpocketings called pharyngeal arches on the lateral sides of the head. The pharyngeal arches give rise to a number of different structures in the skeletal, muscular and circulatory systems in a manner which varies across the vertebrates. Pharyngeal arches trace back through chordates to basal deuterostomes who also share endodermal outpocketings of the pharyngeal apparatus. Similar patterns of gene expression can be detected in the developing pharynx of amphioxus and hemichordates. However, the vertebrate pharynx is unique in that it gives rise to endoskeletal support through the contribution of neural crest cells.[24]

Cartilaginous jaws

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Cartilaginous fishes (sharks, rays and skates) have cartilaginous jaws. The jaw's surface (in comparison to the vertebrae and gill arches) needs extra strength due to its heavy exposure to physical stress. It has a layer of tiny hexagonal plates called "tesserae", which are crystal blocks of calcium salts arranged as a mosaic.[25] 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.[26] In the rostrum (snout), the cartilage can be spongy and flexible to absorb the power of impacts.

In sharks and other extant elasmobranchs the upper jaw is not fused to the cranium, and the lower jaw is articulated with the upper. The arrangement of soft tissue and any additional articulations connecting these elements is collectively known as the jaw suspension. 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.[27][28][29]

Teeth

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Inside of a shark jaw where new teeth move forward as though on a conveyor belt
See also: Shark tooth and Animal tooth development

Jaws provide a platform in most bony fish for simple pointed teeth, however, there are many exceptions. Some fish like carp and zebrafish have pharyngeal teeth only.[30][31] Sea horses, pipefish, and adult sturgeon have no teeth of any type. In fish, Hox gene expression regulates mechanisms for tooth initiation.[32][33]

While both sharks and bony fish continuously produce new teeth throughout their lives, they do so via different mechanism.[34][35][36] Shark teeth are embedded in the gums rather than directly affixed to the jaw as in some fish.[37] Shark teeth form within the jaw move outward in rows until they are eventually dislodged in a manner similar to a conveyor belt.[38] Their scales, called dermal denticles, and teeth are homologous organs.[39] 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, although few studies have been able to quantify this. In most species of bony fish, teeth are replaced one at a time as opposed to the simultaneous replacement of an entire row. However, in piranhas and pacus, all the teeth on one side of the jaw are replaced at a time.[40]

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 and whale sharks are very small.[41][42]

  • Cartilaginous jaws and their teeth
  • Jaw reconstruction of the extinct Carcharodon megalodon, 1909
    Jaw reconstruction of the extinct Carcharodon megalodon, 1909
  • The thornback ray has teeth adapted to feed on crabs, shrimps and small fish.
    The thornback ray has teeth adapted to feed on crabs, shrimps and small fish.
  • The shortfin mako shark lunges vertically and tears flesh from prey
    The shortfin mako shark lunges vertically and tears flesh from prey
  • Tiger shark teeth are oblique and serrated to saw through flesh
    Tiger shark teeth are oblique and serrated to saw through flesh
  • The prickly shark has knife-like teeth with main cusps flanked by lateral cusplets
    The prickly shark has knife-like teeth with main cusps flanked by lateral cusplets

Examples

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Salmon

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Open mouth of a salmon showing the second set of pharyngeal jaws positioned at the back of the throat
Kype of a spawning male salmon

Male salmon often remodel their jaws 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 establish dominance by clamping them around the base of the tail (caudal peduncle) of an opponent.[43][44]

Cichlids

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Dorsal view of right-bending (left) and left-bending (right) jaw morphs[45]

Fish jaws, like vertebrates in general, normally show bilateral symmetry. An exception occurs with the parasitic scale-eating cichlid Perissodus microlepis. The jaws of this fish occur in two distinct morphological forms. One morph has its jaw twisted to the left, allowing it to eat scales more readily on its victim's right flank. The other morph has its jaw twisted to the right, which makes it easier to eat scales on its victim's left flank. The relative abundance of the two morphs in populations is regulated by frequency-dependent selection.[45][46][47]

In cichlids generally, the oral and pharyngeal teeth differ with different species in ways that allow them to process different kinds of prey. Primary oral jaws contain teeth which are used to capture and hold food, while pharyngeal jaws have pharyngeal teeth which function as a chewing tool.

Lower jawbone with molariform teeth (Ctenochromis horei)
Lower jawbone with conical teeth (giant cichlid)

This allows for different nutritional strategies, and because of this, cichlids are able to colonize different habitats. The structural diversity of the lower pharyngeal jaw could be one of the reasons for the occurrence of so many cichlid species. Convergent evolution took place over the course of the cichlid radiation, synchronous with different trophic niches.[48] The pharyngeal jaw apparatus consists of two upper and one single lower plate, all of which have dentations that differ in size and type.[49] The structure of the lower pharynx is often associated with the species of food of the species.[50]

In order to crack shellfish considerable force must be generated, which is why cichlids that feed on molluscs (e.g. the cichlid bass, Crenicichla minuano), have molariform teeth and a strengthened jawbone bone. To grab and bite prey not armoured with shells, predators need conical, bent back teeth.[51] Herbivorous cichlids also have structural differences in their teeth. Cichlids that specialise in algae (e.g. Pseudotropheus) tend to have small conical teeth. Species that feed on pods or seeds require large conical teeth for chewing their food.[52]

Other

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Stoplight loosejaw
Relative to its size the stoplight loosejaw has one of the widest gapes of any fish
Closeup of jaw
The pelican eel jaws are larger than its body.

Stoplight loosejaws are small fish found worldwide in the deep sea. Relative to their size they have one of the widest gapes of any fish. The lower jaw has no ethmoid membrane (floor) and is attached only by the hinge and a modified tongue bone. There are several large, fang-like teeth in the front of the jaws, followed by many small barbed teeth. There are several groups of pharyngeal teeth that serve to direct food down the esophagus.[53][54]

Another deep sea fish, the pelican eel, has jaws larger than its body. The jaws are lined with small teeth and are loosely hinged. They open wide enough to swallow a fish larger than the eel itself.

Distichodontidae are a family of fresh water fishes which can be divided into genera with protractile upper jaws which are carnivores, and genera with nonprotractile upper jaws which are herbivores or predators of very small organisms.[55]

Evolution

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Vertebrate classes
Spindle diagram for the evolution of fish and other vertebrate classes.[56] The earliest classes that developed jaws were the now extinct placoderms and the spiny sharks.
See also: Evolution of fish

The appearance of the early vertebrate jaw has been described as "a crucial innovation"[57] and "perhaps the most profound and radical evolutionary step in the vertebrate history".[4][5] Fish without jaws had more difficulty surviving than fish with jaws, and most jawless fish became extinct during the Triassic period. However studies of the cyclostomes, the jawless hagfishes and lampreys that did survive, have yielded little insight into the deep remodelling of the vertebrate skull that must have taken place as early jaws evolved.[58][59]

The customary view is that jaws are homologous to the gill arches.[60] 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. 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.[61] 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.[62]

↑ Skull diagram of the huge predatory placoderm fish Dunkleosteus terrelli, which lived about 380–360 million years ago
↑ Reconstruction of Dunkleosteus terrelli
↑ Spiny shark

It is now accepted that the precursors of the jawed vertebrates are the long extinct bony (armoured) jawless fish, the so-called ostracoderms.[63][64] The earliest known fish with jaws are the now extinct placoderms[65] and spiny sharks.[66]

Placoderms were a class of fish, heavily armoured at the front of their body, which first appeared in the fossil records during the Silurian about 430 million years ago. Initially they were very successful, diversifying remarkably during the Devonian. They became extinct by the end of that period, about 360 million years ago.[67] Their largest species, Dunkleosteus terrelli, measured up to 10 m (33 ft)[68][69] and weighed 3.6 t (4.0 short tons).[70] 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.[71][72] 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.[71] 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.[72] The pressures generated in those regions were high enough to puncture or cut through cuticle or dermal armour[71] suggesting that Dunkleosteus terrelli was perfectly adapted to prey on free-swimming, armoured prey like arthropods, ammonites, and other placoderms.[72]

Spiny sharks were another class of fish which appeared also in the fossil records during the Silurian at about the same time as the placoderms. They were smaller than most placoderms, usually under 20 centimetres. Spiny sharks did not diversify as much as placoderms, but survived much longer into the Early Permian about 290 million years ago.[73]

The original selective advantage offered by the jaw may not be related to feeding, but rather to increased respiration efficiency.[74] The jaws were used in the buccal pump still observable in modern fish and amphibians, that uses "breathing with the cheeks" to pump 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), in 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.[75]

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.[76] 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.[76]

See also

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References

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Further reading

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Fish jaw

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from Grokipedia
The fish jaw refers to the paired skeletal elements forming the in jawed vertebrates (gnathostomes), derived from the mandibular (first) , which enables the capture, manipulation, and ingestion of food through opening and closing mechanisms. In bony fishes (osteichthyans), the upper jaw typically comprises the and , while the lower jaw consists of the dentary and angular bones, articulating at the quadratoarticular to facilitate protrusion, depression, and adduction. These structures are supported by cartilaginous precursors like Meckel's (lower jaw) and the palatoquadrate (upper jaw), with associated elements such as the hyomandibula from the hyoid arch aiding in jaw suspension and movement. Fish jaws exhibit remarkable diversity in form and function, adapted to specific feeding strategies and habitats. Mouth positions include terminal (forward-facing for mid-water predators), superior (upward for surface or ambush feeders), and inferior (downward for bottom-dwellers), influencing prey capture efficiency. Jaw protrusion, a key innovation in percomorph fishes, allows the premaxilla to extend forward, enhancing reach and suction during strikes. The adductor mandibulae muscle complex, divided into facialis and mandibularis segments, provides the primary force for jaw closure, with variations across teleost orders reflecting phylogenetic adaptations for biting, grinding, or raking. Pharyngeal jaws, located in the throat, further process food with toothed elements in many species. The jaws represents a pivotal in history, originating around 450 million years ago from the serial pharyngeal arches of ancestral chordates, transforming supports into feeding apparatus. In early gnathostomes like placoderms (Silurian-Devonian, ~444-416 million years ago), the joint first formed between dorsal (palatoquadrate) and ventral (Meckelian) elements of the mandibular arch, enabling active predation over filter-feeding in jawless ancestors (agnathans). Subsequent diversification in actinopterygians (ray-finned fishes) and sarcopterygians (lobe-finned fishes) involved via gene duplications (e.g., Col2α1 for , SCPP family for bone) and modifications like hyoid-mediated depression in osteichthyans, replacing ancestral pectoral linkages. Developmental patterning, conserved across vertebrates, relies on cell migration into arches, regulated by for anterior-posterior identity and signaling pathways like endothelin-1 for dorsal-ventral polarity.

Anatomy

Cranial Bones and Cartilages

In bony fishes, the forms the primary structural framework of the , consisting of endochondral bones derived from the chondrocranium and dermal bones that roof and protect the and sensory organs. The , located anteriorly, supports the and olfactory structures, while the paired frontal bones form the anterior portion of the dorsal skull roof, often bearing sensory canals. Posteriorly, the paired parietal bones contribute to the dorsal roof, integrating with the otic and occipital regions to encase the and . These bones collectively provide a rigid platform for attachment and protect neural tissues during feeding activities. In cartilaginous fishes, the skull is predominantly composed of the chondrocranium, a cartilaginous scaffold that remains unossified in most , enclosing the , olfactory sacs, and otic capsules. Key components include the trabeculae cranii, paired rod-like structures forming the floor of the ethmoid and supporting the anterior braincase, and the ethmoid plate, a transverse cartilage connecting the trabeculae to the nasal capsules. The parachordal cartilages fuse posteriorly to form the basal plate, separating the otic capsules from the , while the otic capsules house the . This cartilaginous structure maintains flexibility yet sufficient rigidity for sensory integration. The jaws articulate with the cranium at specific points that ensure precise movement and load distribution. In bony fishes, the upper jaw's palatoquadrate ossifies into the pterygoid and quadrate , with the quadrate forming the primary articulation socket for the lower jaw's articular , derived from Meckel's . The hyomandibular , originating from the hyoid arch, connects the suspensorium to the otic region of the cranium, stabilizing the jaw apparatus during occlusion. In cartilaginous fishes, the unossified palatoquadrate and Meckel's directly articulate with the chondrocranium at ethmoid and otic regions, with the hyomandibula providing additional suspension via ligaments to the basal plate. The plays a critical role in jaw stability and transmission during feeding by anchoring adductor muscles and distributing mechanical loads across the feeding apparatus. Cranial geometry, including the robust ethmoid and otic regions, minimizes deformation at articulation points, allowing efficient transfer of bite s from the adductors to prey without compromising structural integrity. This stabilization is essential for both predatory strikes and prey manipulation, as the cranium acts as a fixed fulcrum against which levers operate. Ossification patterns differ markedly between actinopterygians and chondrichthyans, reflecting their divergent skeletal strategies. In actinopterygians, the undergoes extensive from the chondrocranium, supplemented by of dermal bones like the frontals and parietals, resulting in a fully bony, lightweight yet strong structure. In contrast, chondrichthyans exhibit minimal , with the chondrocranium remaining largely cartilaginous and only select areas, such as tesserae in the rostrum, undergoing for reinforcement. These differences enhance rigidity in bony fishes for diverse feeding ecologies while preserving flexibility in cartilaginous forms.

Oral Jaws

The oral jaws of constitute the primary anterior structures responsible for capturing prey, forming the mouth's gape through a paired, bilaterally symmetric arrangement that mirrors the overall of vertebrates. These jaws articulate with the cranium via suspensory elements, providing structural support for feeding activities. In bony fish (Osteichthyes), the upper jaw is primarily composed of two dermal bones: the premaxilla, which forms the anterior tip and bears teeth in many species, and the maxilla, a larger posterior element that contributes to the jaw's lateral margin and often supports additional dentition. These bones form via intramembranous ossification as dermal elements associated with the palatoquadrate cartilage, an embryonic precursor derived from the mandibular arch. In contrast, cartilaginous fish (Chondrichthyes), such as sharks and rays, retain a largely cartilaginous upper jaw formed by the palatoquadrate cartilage, which includes distinct regions like the quadrate (for articulation) and orbital processes (for cranial attachment), lacking extensive bony replacement. The lower jaw, or mandible, exhibits similar bilateral symmetry and is constructed around Meckel's cartilage, a persistent cartilaginous rod that serves as the core scaffold in both bony and cartilaginous fish. In bony fish, this cartilage is enveloped by dermal bones such as the dentary (anterior and tooth-bearing) and angular (posterior lateral), along with the endochondral articular bone (medial, derived from Meckel's cartilage for joint formation), often fusing into the anguloarticular, collectively forming a robust mandibular complex. Cartilaginous fish maintain Meckel's cartilage as the primary structural element of the lower jaw, with minimal ossification, allowing flexibility in jaw deformation during feeding. The jaws articulate at the quadrate-articular joint, where the quadrate region of the upper (or palatoquadrate) connects to the articular (or Meckel's posteriorly) of the lower , enabling hinge-like opening and closing motions essential for prey . This , supported by ligaments, maintains jaw stability while permitting wide gape angles, a key adaptation in diverse aquatic environments.

Pharyngeal Jaws

Pharyngeal jaws represent a secondary set of jaws located within the of many fish species, derived from modified elements of the gill arches, including the hyoid arch and branchial arches that originally functioned in respiration. These structures augment the primary oral jaws by enabling further manipulation of captured prey in a posterior position. In bony fish, the pharyngeal jaws exhibit a distinct bilateral structure, with the upper pharyngeal jaw typically formed from pharyngobranchials and associated elements of the second, third, and fourth gill arches, while the lower pharyngeal jaw consists of the paired ceratobranchials of the fifth gill arch. These bones are often toothed and supported by robust musculature, including levators and retractors, allowing for independent movement relative to the oral jaws. This configuration is most prominent in teleosts, where the jaws form a functional unit capable of adduction and protraction. The primary functions of pharyngeal jaws involve post-capture processing of food, including crushing and grinding tougher prey items to break them down mechanically, as well as transporting softened boluses toward the through coordinated retraction and elevation movements. In teleosts, these actions occur sequentially after oral capture, facilitating efficient by separating prey seizure from breakdown. Pharyngeal jaws are a characteristic feature of ray-finned bony fishes (), particularly well-developed in teleosts, but they are absent or significantly modified in other fish groups, such as cartilaginous fishes (), where branchial arches retain primarily respiratory roles without forming dedicated secondary jaws. In non-teleost bony fishes, the structures may be less specialized, with reduced independence from gill functions.

Mechanics and Function

Jaw Suspension Types

Jaw suspension types refer to the structural arrangements by which the upper (palatoquadrate) connects to the cranium and supporting elements in fishes, influencing overall jaw mobility, stability, and functional capabilities. These variations evolved as adaptations to diverse feeding strategies, with the hyoid arch playing a variable role in suspension. Four primary types are recognized: autostylic, amphistylic, hyostylic, and holostylic. In autostylic suspension, the palatoquadrate attaches directly to the without hyoid involvement, providing a rigid connection that prioritizes stability over extensive movement. This type is characteristic of lungfishes, such as and Neoceratodus, where the hyomandibula is reduced or absent, and the joint forms via squamosal and prearticular elements. The robust articulation supports suction-based feeding in low-oxygen environments but limits lateral jaw excursion. Amphistylic suspension features dual attachments of the palatoquadrate to both the cranium and the hyoid arch, balancing stability and moderate mobility. This configuration occurs in primitive elasmobranchs, including hexanchiform sharks like , where the hyoid arch braces the upper jaw alongside direct cranial ligaments. It enables enhanced jaw kinesis for biting and initial prey capture, facilitating ram-feeding behaviors in deep-sea habitats. Hyostylic suspension, the most widespread type, connects the palatoquadrate primarily via the hyomandibula to the cranium, with minimal direct cranial attachment, allowing significant protrusion and flexibility. Prevalent in bony fishes and derived elasmobranchs like carcharhiniform sharks, the hyomandibula acts as a supportive , articulating with the palatoquadrate and otic region. This setup improves mechanical leverage during feeding, supporting a range of modes from to biting and enabling efficient prey manipulation across varied diets. Holostylic suspension involves complete fusion of the palatoquadrate to the cranium, eliminating hyoid contribution and maximizing structural integrity. Found in holocephalans such as chimaeras ( spp.), the upper integrates seamlessly with the braincase, while the lower articulates directly below. This fused design enhances bite for crushing hard prey like mollusks but restricts dynamic movements, optimizing for specialized benthic feeding. These suspension types differentially impact leverage and feeding efficiency: rigid forms like autostylic and holostylic provide high transmission for durophagous diets, whereas mobile hyostylic and amphistylic arrangements amplify protrusion and , broadening ecological niches in predatory fishes.

Protrusion and Movement

Jaw protrusion in fishes is primarily achieved through the sliding of the palatoquadrate relative to the cranium, facilitated by ligaments such as the ethmopalatine and ascending processes of the , which allow the upper jaw to extend forward rapidly during prey capture. This mechanism positions the mouth closer to the prey, enhancing efficiency without relying on excessive forward body movement. In many , the intramandibular further enables independent movement of the dentary and angular bones, contributing to precise protrusion trajectories. Epaxial muscles, located dorsal to the vertebral column, play a crucial role in jaw elevation by contracting to lift the and transmit force through the hyoid apparatus, thereby facilitating upper protrusion. Conversely, hypaxial muscles, situated ventrally, drive jaw depression by depressing the lower and hyoid, expanding the buccal cavity to generate . These antagonistic muscle groups ensure coordinated motion, with epaxials often providing the primary power for rapid strikes in suction-feeding teleosts. Kinematic chains in jaws involve the hyoid linkage system, where the ceratohyal connects the hyoid arch to the , transmitting motion from hyoid depression to mandibular . This model, including the interhyal and symplectic bones, amplifies displacement and velocity, allowing efficient energy transfer across the complex during feeding sequences. Variations in linkage geometry across modulate the speed and extent of opening, optimizing for different prey types. Energy transfer during jaw strikes occurs via muscular contractions that propagate through skeletal linkages, converting slow muscle shortening into high-velocity jaw movements with peak forces up to several times body weight in ram-suction feeders. Force generation is enhanced by the elastic properties of ligaments and tendons, which store and release to accelerate protrusion, achieving strike speeds exceeding 100 body lengths per second in some species. This biomechanical efficiency minimizes metabolic cost while maximizing capture success.

Feeding Mechanisms

Fish employ diverse feeding mechanisms that leverage their jaw structures to capture, process, and ingest prey, adapting to aquatic environments where fluid dynamics play a critical role. These strategies include ram feeding, suction feeding, biting and tearing, and filter-feeding, often coordinated between oral and pharyngeal jaws to optimize efficiency. Ram feeding involves the fish propelling itself forward toward the prey with its mouth open, relying on inertial suction generated by jaw opening rather than active muscle-powered expansion. In this mechanism, the jaws remain relatively static, allowing the forward momentum to overcome the bow wave ahead of the prey and facilitate capture without significant buccal expansion. This approach is particularly effective for fast-swimming predators, complementing other methods by reducing the need for precise pressure gradients. Suction feeding, the most prevalent mechanism among fishes, creates negative pressure through rapid three-dimensional expansion of the buccal cavity, drawing water and prey into the mouth within 4–40 milliseconds. Jaw depression and protrusion are integral, with the lower jaw lowering via hyoid linkages and the upper jaw extending forward to position the mouth closer to the target, enhancing flow speeds and capture success. Powered by epaxial and hypaxial muscles, this process generates instantaneous outputs up to 800 W kg⁻¹, transforming skeletal levers into fluid dynamics that accelerate prey toward the gape. Protrusion briefly aids by aligning jaws with prey during the strike. Biting and tearing mechanisms enable to grasp and dismember solid prey through direct physical contact with the oral jaws, often requiring robust jaw adductor muscles for high bite forces. In biting, the jaws close rapidly to secure elusive or armored items, with adaptations like larger gapes and stronger skeletal elements allowing penetration of tough exteriors. Tearing follows via lateral head shakes or body twists, using the jaw's leverage to shear flesh or separate portions, as seen in species with bladed that amplify cutting efficiency during aquatic predation. Filter-feeding adaptations position the jaws to engulf large volumes of water laden with , using protrusion to form a pipe-like that directs flow across rakers for particle separation. This employs , where water passes parallel to porous structures while particles are retained by inertia and redirected toward the , with angles of 45–74° optimizing the oval gape for efficient semi-cross-flow dynamics. Mesh sizes in rakers, ranging from 0.007 to 0.148 mm², selectively capture microbes without clogging, enabling sustained ram-assisted . Coordination between oral and pharyngeal jaws forms a sequential feeding cycle, where the oral jaws initiate capture via or , followed by pharyngeal jaws processing ingested material. Synchronized with hyoid depression and opercular abduction, this involves bidirectional intraoral waterflows that reposition prey centrally, allowing pharyngeal jaws to intercept and transport it to the through inertia-driven . In like cyprinids, periodic backflows resuspend particles, ensuring efficient sorting and reducing escape of food during the compressive phase.

Dentition

Tooth Morphology

Fish teeth exhibit diverse morphologies adapted to specific feeding strategies, primarily categorized into conical, molariform, and villiform types. Conical teeth, often pointed and robust, are designed for piercing and holding prey, as seen in predatory species like and pike, where they facilitate impaling soft-bodied organisms. Molariform teeth, flattened and broad, function in crushing hard-shelled prey such as mollusks or crustaceans, commonly found in species like that process -covered corals. Villiform teeth, resembling small brushes or velcro-like patches, aid in grasping and rasping food items, particularly in fish that scrape or ingest particulate matter, such as (Acanthuridae). The distribution of these tooth types varies between oral and pharyngeal jaws, reflecting functional specialization. Oral jaws typically bear larger, more prominent teeth for initial prey capture, such as conical forms in piscivores, while pharyngeal jaws often feature smaller, densely packed teeth like villiform or molariform for secondary processing, as in the grinding of ingested material in teleosts. This separation enhances efficiency, with oral focused on acquisition and pharyngeal on manipulation. Structurally, fish teeth consist of an outer layer of enameloid, a highly mineralized tissue similar in function to but softer than mammalian enamel, surrounding a core of that encases the pulp cavity containing nerves and blood vessels. Enameloid provides durability against wear during feeding, while offers flexibility to absorb impacts, adaptations evident in with high bite forces. often display additional modifications, such as serrations on conical teeth for tearing flesh or recurved hooks to prevent escape of prey, enhancing lethality in strikes by like .

Tooth Attachment and Replacement

In fish, teeth are primarily attached to the jaw bones through , a direct fusion between the tooth base and the underlying without intervening , which is the primitive and most common mode in actinopterygians. This attachment provides stability during feeding, with variations including complete (Type 1) where the entire base mineralizes fully to the , or partial forms with collagenous zones allowing limited mobility (Types 2-4). Pedunculated attachment occurs in some teleosts, where teeth sit on a pedicel—a or dentinous extension from the that forms the tooth base and anchors it via a crimped , enhancing replaceability. Acrodin, a hypermineralized enameloid cap at the apex, contributes to attachment strength by integrating with the underlying , though its primary role is in wear resistance. Fish exhibit polyphyodonty, characterized by continuous tooth replacement throughout life, facilitated by the dental lamina—a specialized epithelial invagination that generates successive tooth rows. The dental lamina originates from the oral epithelium and produces tooth buds in a patterned manner, forming linear rows along the jaws; in teleosts, it often remains active as a shallow groove, enabling lifelong regeneration. Tooth row formation begins with an initiator tooth that induces adjacent buds via signaling molecules, establishing the alternating replacement pattern typical of most fish dentitions. Replacement cycles are continuous, with new teeth developing lingual to functional ones and erupting as predecessors are shed, often in a conveyor-belt fashion where rows advance forward en bloc. In elasmobranchs like , this system involves multiple oblique tooth files migrating toward the margin, resorbing old teeth via osteoclasts before new ones functionalize, allowing rapid renewal. Oral teeth generally replace faster than due to higher wear exposure; for instance, in Pacific , teeth replace at an overall rate of about 3.6% of the per day (average duration 27 days), with oral jaws at 3-4% per day and lower pharyngeal jaws up to 4.8% per day in some positions.

Variations Across Fish Groups

Bony Fish Jaws

Bony fish, or osteichthyans, exhibit jaw structures that primarily ossify through a combination of and processes, reflecting their dual cartilage and dermal origins. occurs in elements derived from Meckel's cartilage, such as the medial retroarticular process of the , where hypertrophic chondrocytes are replaced by bone following cartilage mineralization. In contrast, forms dermal bones like the dentary through direct mineralization of mesenchymal tissue without a cartilaginous precursor, as observed in the mediolateral regions adjacent to Meckel's cartilage in . This bimodal enables the robust yet flexible architecture essential for diverse feeding strategies in bony fishes. A hallmark in advanced teleosts is the protrusible , which allows the upper to extend anteriorly by up to 21% of body length in some , rapidly closing the gap to elusive prey during feeding. This mechanism relies on an elongated ascending process of the , coupled with ligamentous connections to the ethmoid and , enabling kinematics for precise protrusion. Jaw protrusion has increased phylogenetically over the past 100 million years, particularly within spiny-rayed acanthomorphs, enhancing hydrodynamic efficiency and capture success. In neopterygians, evolutionary modifications include the reduction and translocation of certain palatoquadrate-derived structures, such as the quadrato-maxillar process, which shifts from supporting attachment to forming the cartilaginous coronoid process on the lower . This reconfiguration liberates the from rigid palatoquadrate constraints, promoting greater mandibular adduction torque and upper jaw mobility compared to palaeopterygians. Such reductions in associated bones facilitate the kinetic, protrusible jaws characteristic of teleosts, optimizing feeding versatility. Percomorphs, a diverse within advanced teleosts, feature elaborated pharyngeal jaws that include a protractible upper element and fused lower tooth plates, forming a robust apparatus for prey processing. This pharyngognathy, marked by a muscular sling and single lower unit, evolved independently at least six times in percomorph lineages, enabling efficient mastication of varied diets from to hard-shelled . The mechanism involves coordinated depression and retraction via specialized muscles like the levator posterior and retractor dorsalis, distinguishing it from simpler pharyngeal structures in basal bony fishes. Sarcopterygians (lobe-finned fishes), the other major clade of osteichthyans, display jaw structures with greater retention of cartilaginous elements and variations in compared to actinopterygians. In coelacanths like , the jaws exhibit amphistylic suspension, with the palatoquadrate and hyoid arch connecting the to the , and partial of dermal bones alongside persistent . Lungfishes feature autostylic jaws, where the palatoquadrate fuses directly to the cranium, lacking a functional hyomandibula, and often lack a , with the jaw joint formed by the squamosal. These configurations support crushing or grasping functions suited to their habitats, differing from the protrusible, kinetically mobile jaws of ray-finned fishes.

Cartilaginous Fish Jaws

In cartilaginous fish, or chondrichthyans, the jaw apparatus is primarily composed of two cartilaginous elements derived from the mandibular arch: the palatoquadrate cartilage forming the upper jaw and Meckel's cartilage forming the lower jaw. The palatoquadrate is a robust, elongate structure that articulates with the cranium anteriorly via the ethmoid process and posteriorly with Meckel's cartilage through a double articulation involving a lateral process and medial fossa, enabling precise jaw closure. Meckel's cartilage, in contrast, is a bow-shaped element fused medially at a symphysis, featuring a dorsal sulcus for tooth support and ventral surfaces for muscle attachment, which collectively provide the framework for mandibular adduction and depression. Unlike bony , where these cartilaginous precursors ossify into distinct dermal and endochondral s, chondrichthyan jaws retain a largely cartilaginous composition throughout life, lacking full . Reinforcement occurs through mineralization in the form of tesserae—small, polygonal blocks of calcified tissue arranged in a pattern, often with prismatic layers of in the subperichondral zone, which enhance mechanical strength without true formation. This tessellated calcified cartilage (TCC) is particularly prominent in the jaw margins, providing a tiled rind that resists compressive forces during feeding. The dominant mode of jaw suspension in most chondrichthyans, particularly elasmobranchs like and rays, is hyostylic, where the palatoquadrate is braced against the hyoid arch via the hyomandibula, allowing significant mobility and protrusion of the upper from the cranium. This suspension type facilitates diverse feeding strategies, such as biting and , by decoupling the jaws from rigid cranial attachment and enabling expansive gape. Holocephalans (chimaeras) exhibit a derived autostylic condition with the palatoquadrate fused directly to the , but hyostyly prevails across the broader group. Specialized labial cartilages further enhance jaw functionality in many sharks, consisting of 2–5 pairs of elongated, rod-like or broad skeletal elements positioned laterally to the jaws within the labial folds, spanning the gape to support the and modulate oral volume. These cartilages, often S-shaped or segmented, vary by —for instance, up to five pairs in orectolobids—and contribute to inertial feeding by aiding mouth expansion, though they are reduced or absent in ram-feeding species like lamnids.

Jawless Fish Context

Jawless fish, or agnathans, represent the most basal extant vertebrates and lack true jaws, a defining feature that distinguishes them from all later-diverging groups. Modern agnathans include lampreys (Petromyzontiformes) and (Myxiniformes), which feed using specialized structures rather than hinged mandibular mechanisms. Lampreys employ a suctorial oral disc armed with keratinous and a piston-like velar apparatus to create and rasp tissue from hosts or prey, while utilize paired tooth plates on a basal element combined with a similar piston-driven protraction-retraction cycle, often aided by body knotting for leverage during scavenging or predation. These adaptations highlight the absence of jaws in living agnathans, relying instead on muscular and cartilaginous elements for food acquisition. In agnathans, the pharyngeal region features a branchial basket composed of unjointed bars derived from cells, which supports the gills and forms the primary skeletal framework of the viscerocranium. This structure, observed in lampreys where arches fuse into a continuous basket, exhibits dorsoventral patterning mediated by conserved genes such as , Hand, and , mirroring aspects of jawed pharyngeal development. The branchial basket's segmented yet unhinged nature is considered a precursor to the elements in gnathostomes, where the first differentiated into upper and lower components. Fossil agnathans, particularly ostracoderms like cephalaspids and thelodonts from the and periods, further illustrate this baseline anatomy; while lacking true jaws, some possessed oral armor plates or invading odontodes around the mouth, hinting at early specializations for feeding that prefigured articulation. The and development of agnathan pharyngeal structures provide critical context for jaw origins, suggesting that jaws evolved through modification of an ancestral branchial system rather than de novo. This serial homology implies that the first arch's transformation into a functional involved genetic co-option, such as the incorporation of Bapx1 and , absent or differently expressed in agnathans. By examining these jawless forms, researchers gain insight into the primitive vertebrate condition, underscoring how regionalization enabled the of jawed vertebrates.

Specialized Examples

Salmonid Adaptations

Salmonids, including species like (Salmo salar) and Arctic charr (Salvelinus alpinus), exhibit jaw adaptations that support their anadromous life cycles, transitioning between freshwater rearing, marine piscivory, and freshwater spawning. These modifications enhance feeding efficiency across habitats and life stages, with oral s facilitating prey capture during oceanic and pharyngeal structures aiding in processing. Seasonal remodeling ensures functional versatility, particularly in males during . A prominent in male salmonids is the development of the , an elongated, hook-like extension of the lower during the spawning season. This structure forms through rapid , with skeletal needles of chondroid extending the dentary tip, averaging 10.9 mm in length and 7.3 mm in height in pre-spawning grilse. The kype aids in male-male competition and nest digging but impairs feeding, as males cease eating during upstream migration. Hormones such as 11-ketotestosterone drive this elongation, correlating positively with jaw growth during maturation. In juveniles entering the pelagic ocean environment, salmonid oral jaws become streamlined, with elongated dentaries and a more acute angle to optimize piscivory on mobile prey like small . This morphology, observed in pelagic morphs of Arctic charr, increases mouth cavity efficiency for capturing evasive targets in open water, contrasting with the compact jaws of benthic forms. Such adaptations align with the shift to a -based diet post-smoltification, enhancing hydrodynamic efficiency during high-speed pursuits. Pharyngeal teeth in salmonids, located on the lower fifth , feature conical, pointed morphology suited for piercing and gripping soft-bodied prey such as and small . These teeth work in concert with the tongue-bite apparatus, where the basihyal pushes prey against vomerine teeth for initial manipulation before pharyngeal processing via raking or chewing motions. This setup allows handling of soft, elusive items without reliance on robust oral , supporting diets dominated by gelatinous or fleshy organisms during early life stages. These jaw changes are inherently seasonal and stage-specific, with resorption of the in post-spawning kelts via activity reducing its length by about 25% and height by 22%, remodeling it into for post-spawning recovery. Juvenile jaws elongate during smolt transformation for marine entry, while non-reproductive phases revert to baseline forms for sustained feeding. This plasticity, driven by environmental cues and hormones, underscores salmonid resilience across migratory phases.

Cichlid Diversity

, especially those in the , demonstrate extraordinary rapid evolution of jaw structures, enabling a wide array of trophic specializations within short geological timescales. The pharyngeal jaws, which process food after capture, show significant modifications tailored to specific diets. For scraping, species like Labeotropheus trewavasae from possess grinding-adapted pharyngeal jaws with dentition suited to rupturing algal cell walls, facilitating efficient processing of attached epilithic during repeated bite-suction cycles on rock surfaces. In contrast, insectivores such as Teleocichla species exhibit velocity-modified pharyngeal jaws optimized for minimal processing of soft, evasive prey like insects, allowing quick transport with low force requirements. Piscivores, exemplified by Rhamphochromis sp. from , feature robust pharyngeal jaws capable of crushing or shearing tougher prey like small fish, supported by forceful contractions that complement high-power suction capture. These adaptations arise from the functional decoupling of oral and pharyngeal jaws, which reduces evolutionary constraints and promotes independent diversification for prey processing. Variations in oral jaw protrusibility further enhance feeding efficiency, particularly for -based capture. In , maximum jaw protrusion ranges from 1.4% to 9.1% of standard length, with longer premaxillary ascending processes correlating strongly with greater protrusion distance (phylogenetic independent contrast r = 0.72, P < 0.0001), enabling precise positioning near prey. This trait is crucial for feeding, as it increases fluid acceleration around elusive items, improving engulfment success. For instance, in Astatoreochromis alluaudi, a basin species, diet-induced leads to modifications in dentition when feeding on versus hard prey like snails, with papilliform teeth for soft diets supporting effective processing after oral scraping. Such variations allow to exploit diverse microhabitats, from open water to attached substrates. The genetic underpinnings of this jaw diversification involve key developmental genes, with bmp4 playing a pivotal role in shaping mandibular morphology. Allelic variation and differential expression of bmp4 account for over 30% of phenotypic variance in closing mechanics across populations, correlating with species-specific patterns evident by seven days post-fertilization. For example, higher bmp4 expression promotes robust suited to biting in algae scrapers like Labeotropheus fuelleborni, while lower levels favor gracile structures for in species like Metriaclima zebra. This gene's influence facilitates adaptive shifts without deep regulatory overhauls, contributing to the rapid observed in cichlids. Parallel evolution of jaw structures is evident across the , where independent radiations in Lakes Tanganyika, , and Victoria have produced similar trophic forms despite geographic isolation. In these systems, species flocks show convergent jaw morphologies, such as protrusible oral in suction feeders and robust pharyngeal in crushers, driven by shared selective pressures on similar diets and habitats. For instance, algae-scraping adaptations with grinding pharyngeal have evolved repeatedly in rock-dwelling lineages across all three lakes, highlighting how standing and local adaptation underpin this parallelism.

Other Notable Cases

Butterflyfishes (family Chaetodontidae) exhibit highly specialized protrusible jaws that form tube-like structures for precise feeding on polyps. The intramandibular joint, an additional flexion point between the dentary and articular bones, enables extreme jaw protrusion and mouth expansion, allowing these fish to scrape and extract tissues efficiently while minimizing damage to the substrate. This adaptation, which evolved after the , supports corallivory in approximately 60 of the 86 Chaetodon species, decoupling jaw morphology diversification from gut length evolution to enhance trophic versatility. Moray eels (family Muraenidae) possess a unique second set of pharyngeal jaws, derived from modified gill arches, that function as structures for prey transport. These jaws protract forward into the oral cavity to grasp prey with sharp, recurved teeth, then retract to pull it posteriorly into the , compensating for the eels' limited suction feeding capacity in confined habitats. This dual-jaw mechanism, observed in sequences where pharyngeal jaws engage in 88% of transports, enables morays to large prey items, such as up to 5 cm long, through 3-5 cycles of alternating oral and pharyngeal bites. In anglerfishes (order Lophiiformes), the bioluminescent lure, or esca, at the tip of the (a modified ray) integrates with mechanics by positioning prey directly within the strike zone of the enlarged mouth. The esca's controlled movements, often mimicking prey or using bacterial for glow, attract victims in low-light environments, triggering a rapid closure facilitated by diverse arrangements and protrusibility levels. This lure-jaw synergy supports predation across habitats, with ceratioid showing a functional continuum from forceful bites to fast, weak snaps. Deep-sea anglerfishes, particularly ceratioids like those in the genus Linophryne, feature extreme elongation, with teeth extending up to 25% of head length, optimized for predation on scarce, large prey. This morphology reduces jaw protrusibility to enable quick, trap-like closures in the , where the expanded morphospace—2.1 times larger than in shallow-water relatives—allows many-to-one mappings of form to diet. Such adaptations reflect rapid evolutionary diversification, enabling generalist feeding despite specialized structures.

Evolutionary Development

Origin from Gill Arches

The jaws of gnathostomes are widely regarded as having evolved from the anterior pharyngeal () arches of an ancestral jawless , a transformation supported by comparative anatomical and developmental studies. In extant jawless fish such as lampreys and , the pharyngeal arches function primarily in support and suction-based feeding, lacking the specialized articulation seen in jawed forms. Hox gene expression patterns provide key evidence for the serial homology between jaws and gill arches, as these genes pattern the anterior-posterior identity of pharyngeal segments in a collinear manner across vertebrates. In gnathostomes, the first pharyngeal arch (mandibular) is characteristically Hox-negative, allowing its specialization into the upper (palatoquadrate) and lower (Meckel's) jaw elements, while the second arch (hyoid) expresses Hox group 2 genes, mirroring the nested expression in posterior gill arches. This Hox code, conserved from zebrafish to mammals, underscores the evolutionary co-option of branchial arch patterning for jaw formation, with disruptions in Hoxa2 expression leading to homeotic transformations of jaw structures into gill-like elements. Fossil evidence from placoderms, the basalmost jawed vertebrates, illustrates the primitive jaw configuration derived from gill arches. Arthrodires, such as the buchanosteid specimen V244 from ~400-million-year-old Australian deposits, preserve ossified palatoquadrate and Meckel's cartilages with distinct articular surfaces and denticle rows for occlusion, indicating a functional biting apparatus adapted from anterior arch elements. These structures in placoderms exhibit a transitional morphology, with the palatoquadrate fused to the braincase and gnathal plates resembling unjointed gill bars, supporting the hypothesis that jaws initially enhanced prey capture over ancestral filter-feeding. Embryological development further elucidates this origin, with cranial cells playing a pivotal role in forming elements through targeted migration to the pharyngeal arches. In embryos, cells delaminate from the dorsal during and migrate in streams to populate the first two arches, where they differentiate into mesenchymal condensations under the influence of signaling pathways like BMP and , yielding cartilaginous precursors to the skeleton. This process is conserved in gnathostomes but absent in jawless vertebrates, where contributions are limited to non-articulated head supports, highlighting the 's innovation in enabling . The transition from cyclostome-like suction feeding to gnathostome involved the of a novel , marked by the expression of genes such as nkx3.2 and hand2 in the first arch, which are downregulated in jawless forms. This genetic shift allowed the anterior arches to articulate independently, facilitating closure and shear forces for predation, as evidenced by comparative studies of velar apparatus versus gnathostome mandibular mechanics. In primitive gnathostomes, this adaptation likely provided a selective advantage in resource partitioning during the Silurian-Devonian radiation.

Jaw Diversification in Jawed Fish

The diversification of jaws in jawed (gnathostomes) began building on their origin from modified arches and accelerated during the period (416–359 million years ago), a time known as the "Age of Fishes" due to an explosive radiation of forms. (cartilaginous , including early sharks and rays) emerged in the , with diverse morphologies appearing by the Middle Devonian, while (, encompassing actinopterygians and sarcopterygians) followed suit, diversifying rapidly alongside them. This explosion involved the of specialized mechanics for predation, coinciding with ecological opportunities in marine and freshwater environments, though a major bottleneck occurred at the end- Hangenberg (359 Ma), which eliminated many lineages but spurred post-extinction radiations of surviving chondrichthyans and actinopterygian . Recent analyses as of 2025 highlight macroevolutionary role reversals in early radiations, with contrasting patterns of jaw-shape between ray-finned (slow rates, low disparity) and later sarcopterygians, underscoring the pivotal role of jaws in driving diversity despite environmental shifts. Further jaw diversification involved structural simplifications and losses of ancestral elements, enhancing functional efficiency in descendant lineages. For instance, in fishes (a major of actinopterygian osteichthyans), the spiracle—a vestigial derived from the hyoid arch—became reduced or entirely lost, correlating with the reorganization of the mandibular and hyoid arches to support advanced jaw suspension and opercular ventilation. This loss, observed across neopterygian s, eliminated the spiracle's minor respiratory role and freed developmental resources for jaw protrusibility and feeding, contributing to the clade's dominance in diverse aquatic habitats. Adaptive radiations of structures were closely tied to habitat-specific selective pressures, driving innovations that expanded feeding niches. In percomorph (spiny-rayed teleosts), the evolution of highly protrusible upper —extending up to 21% of body length in modern forms—facilitated precise strikes on elusive prey in complex environments like coral . This trait, which increased from minimal protrusion in ancestors to widespread elaboration by the Eocene, enhanced suction forces and bite accuracy, fueling the of acanthomorphs and their occupation of shallow benthic marine habitats, including ecosystems where prey mobility demanded such precision. More recent innovations, such as lateral motion in acanthuroid (e.g., surgeonfishes), enable enhanced dexterity for algal , with rotations up to 62° contributing to high bite rates and the ecological dominance of herbivorous lineages since the Eocene. However, a fundamental tradeoff exists between protrusion and large tooth size, as these innovations are functionally incompatible; this constraint has shaped the diversification of feeding mechanisms by limiting certain trait combinations across gnathostome lineages, as revealed by kinematic analyses in 2025. Molecular evidence underscores the genetic underpinnings of these diversifications, revealing conserved pathways co-opted for complexity. The ectodysplasin-A (eda) and its receptor edar, part of a signaling pathway regulating ectodermal appendages like scales, are expressed in dental cell types on both oral and pharyngeal jaws across jawed fishes, linking scale formation to and development. This ancient network, predating origins, was repurposed in gnathostomes for odontogenesis on new structures, with eda variations contributing to morphological divergence in craniofacial traits, as seen in adaptive radiations where scale and phenotypes evolved in tandem under similar selective regimes.

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