JAW SUSPENSIONS IN VERTEBRATES
Jaw suspension means attachment of the lower jaw with the upper jaw or the skull for efficient biting and chewing. There are different ways in which these attachments are attained depending upon the modifications in visceral arches in vertebrates.
The vertebrate skull has three major parts – neurocranium, dermatocranium and spianchnocranium. The splanchnocranium includes the visceral arches. The first or mandibular arch consists of a dorsal palatopterygoquadrate bar forming the upper jaw, and a ventral Meckel’s cartilage forming the lower jaw.
The second or hyoid arch consists of a dorsal hyomandibular which supports and suspends the jaws with the cranium, and a ventral hyoid proper. The remaining arches support the gills and are known as branchial arches. Thus, we find that splanchnocranium plays an important role in the formation of jaws in gnathostomes, and in their suspension with the chondrocranium. The method of attachment or suspension of jaws from the chondrocranium is termed jaw suspension or suspensorium. There are 5 principal variants or types of suspensoria as follows :-
- This condition was found in some earliest gnathostomes such as acanthodians. The jaws are attached to the cranium by anterior and posterior ligaments. Hyoid arch remains completely free or independent and does not support the jaws. The gill cleft in front of hyoid arch bears a complete gill and does not form any spiracle.
- This is a rather primitive arrangement found in Crossopterygii and some primitive sharks (e.g. Heptanchus, Hexanchus). The quadrate or the basal and otic processes of upper jaw (mandibular arch) are attached by ligaments to chondrocranium. Similarly, the upper end of hyomandibula (hyoid arch) is also attached to chondrocranium, while the two jaws are suspended from its other end. This arrangement makes a double suspension (amphi = both+style = bracing) since both the first and second arches participate in bracing the jaws against the chondrocranium.
- It is found in most elasmobranchs and all bony fishes. Upper jaw (palatoquadrate) is loosely attached by anterior ethmopalatine to cranium. Both the jaws are braced against hyomandibular, the upper end of which fits into auditory region of skull. Since only hyoid arch braces or binds the two jaws against cranium, this jaw-suspension is termed hyostylic. It provides the jaws a wider movement and helps in swallowing larger preys.
- This condition is found in extinct placoderms, chimaeras, lung fishes and most tetrapods (amphibians, reptiles and birds). Hyomandibular does not participate but becomes modified into columella or stapes of middle ear for transmitting sound waves. Upper jaw (palatoquadrate) is directly and intimately bound to cranium by investing dermal bones (auto = self)- The articular of lower jaw articulates with the quadrate of the upper jaw. Autostylic suspensorium is widespread and has at least 3 variation or subtypes.
- Holostylic :- In Holocephah (chimaeras), upper jaw is firmly fused with skull and lower jaw suspended form it. Hyoid arch is complete, independent and not attached to skull.
- Monimostylic :- In many tetrapods, hyomandibular forms columella and articular articulates with quadrate. However, the quadrate remains immovably attached with skull.
- Streptostylic :-In some reptiles (lizards, snakes) and birds, quadrate is loosely attached and is movable at both ends, a condition known as streptostylism.
- This type of jaw-suspension is characteristic of mammals and some consider it as a modification of autostylic suspension. Upper jaw fuses throughout its length with cranium, and hyomandibular forms the ear ossicle stapes. But articular and quadrate also become modified into ear ossicles malleus and incus, respectively. Consequently, two dermal bones, dentary of lower jaw and squamosal of skull, provide the articulation between jaw
- In agnathans the jaw suspension is in paleostylic stage in which none of the arches attach themselves directly to the skull.
- ln ganathostomes and acanthodians jaw suspension is autodiastylic in which jaws are attached to the cranium by anterior and the posterior ligaments. Hyoid arch remains completely free and does not support the jaws.
- In primitive sharks the jaw suspension is amphistylic in which the quadrate or the basal and otic processes of upper jaw (mandibular arch) are attached by ligaments to chondrocranium.similarly the upper end of hyomandibula is also attached to chondrocranium.
- In modern sharks and all bony fishes the type of jaw suspension is hyostylic,in which the upper jaw (palatoquadrate) is loosely attached by anterior ligament to cranium.both the jaws are suspended from the hyomandibular.since only hyoid arch binds the two jaws against cranium it is called hyostylic jaw
- In most tetrapods like amphibians,reptiles and birds hyomandibular does not participate but becomes modified into columella or stapes of middle ear for transmitting sound waves.
- In most lung fishes upper jaw is firmly fused with skull and lower jaw suspended from it hyoid arch is complete independent and not attached to the skull; this is holostylic type of jaw suspension.
- In many tetrapods monimostylic jaw suspension is seen ie.hyomandibular forms columella and articlar articulates with quadrates.however the quadrate remains immovably attached with skull.
- In some reptiles(lizard,snakes)ann birds the type of jaw suspension is streptostylic ie.quadrate is loosely attached and is movable at both the ends a condition known as streptostylism
- In mammals craniostylic type of jaw suspension is seen it is modification of the autostylic suspension.upper jaw fuses throughout its length with cranium,and hyomandibular forms the ear, ossicle, stapes. But articular and quadrate also become modified into ear ossicles malleus and incus.