VISCERAL SKELETON (or SPLANCHNOCRANIUM)

• skeleton of the pharyngeal arches
• Fishes - skeleton of the jaws & gill arches
• Tetrapods - skeleton modified for new functions

Bony fishes

• visceral skeleton resembles that of sharks except that bone is added
• caudal ends of the cartilaginous pterygoquadrate undergo endochondral ossification & become the quadrate bones. The remainder becomes the palatine & pterygoid bones. The posterior tip of Meckel's cartilage becomes an articular bone. (See http://www.usm.maine.edu/bio/courses/bio205/bio205_08_skull_2.html)

    Feeding movements in many bony fishes -> cranial kinesis (see Figure 9.23, p.351 of text)

    Cranial kinesis:

• movement between the upper jaw and braincase
• advantages:
• provides a way to change the size and configuration of the mouth rapidly
• optimize biting and rapid feeding.
• disadvantages: lose force, difficult to optimize apposition of occlusive surfaces.

• visceral skeleton unlike that of jawed fishes
• Hagfishes - no identifiable pterygoquadrate or Meckel's cartilage

Jaw suspension of fishes

• The jaw-hyoid complex of fishes requires bracing against some support to function effectively, and the nearest one is the neurocranium (endocranium).
• Types of suspensions:
• autostyly  (below left) - hyomandibula play no role in bracing the jaws (lungfish & tetrapods)
• amphistyly (below middle) - jaws & hyomandibula both braced directly against the braincase (extinct sharks)
• hyostyly (below right) - mandibular cartilage is braced against the otic capsule; jaws braced against hyomandibula (sharks & present-day bony fishes)

Source: http://www.uta.edu/biology/restricted/3452hske.htm

TETRAPODS - With life on land (& pulmonary respiration), the visceral skeleton underwent substantial modification. Some structures were lost & others remained to perform new functions.

• Pterygoquadrate (palatoquadrate) cartilage = embryonic upper jaw cartilage
• Amphibians, reptiles, & birds - posterior end undergoes endochondral ossification & becomes the quadrate (which articulates with the articular bone of the lower jaw)

Source: http://www-users.york.ac.uk/~wjh101/hedbone/Turtle/turtle.htm

• Mammals - dentary (lower jaw) articulates with the squamosal of skull (quadrate separates from the rest of the palatoquadrate & becomes the incus of the middle ear)
• Meckel's cartilage
• Reptiles - largely ensheathed by dermal bones (as in the above turtle)
• Birds & mammals - few or no remnants in adult lower jaw (&, in mammals, the articular, formed by ossification of the tip of Meckel's cartilage, projects into the middle ear cavity & becomes the malleus)

Source: http://home.houston.rr.com/vnotes/Bones/Meckel.html

• Arch II  = Hyomandibular cartilage:
• Sharks - interposed between quadrate region & otic capsule
• Tetrapods - no longer articulates with quadrate & ossifies to become part of the stapes (columella)
• Arches III ---> V become part of hyoid apparatus
• Arches VI & VII - not present in tetrapods

Source: http://www.liberty.edu/academics/registrar/courses00/biol418pa.htm

Hyoid apparatus of tetrapods

• consists of a body & 2 or 3 horns (cornua)
• anchors tongue, provides attachment for some extrinsic muscles of larynx, & is site of attachment of muscles that aid in swallowing

Lower jaw

• may have originated as part of a visceral arch, as in sharks (mandibular cartilage)
• in bony vertebrates, mandibular cartilage is reinforced & largely replaced by a series of dermal bones

Related Links:

Head Skeleton

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