Origin of jaws and jaw suspension
� jaws are primarily derived from the cartilage of the first visceral arch
� in primitive species such as lampreys, food was moved through the pharynx by moving the first visceral arch to create a pump-like action
� jaw formation probably evolved from selection favoring fish that utilized the first visceral arch to help it seize prey
In the first jawed fishes (Chondrichthyes) the first visceral arch was renamed the mandibular arch, which consisted of two cartilages: the palatoquadrate cartilage (upper jaw) and mandibular or Meckel�s cartilage (lower jaw) (Fig. 7.18, p. 239)
� the dorsal part of the second gill arch (hyoid arch) - called the hyomandibular cartilage - articulates with the chondrocranium
� these arches work together in different ways to create jaw movement, based on the ways in which these arches articulate with the chondrocranium (Fig. 7.8, p. 229):
Amphistylic (primitive cartilaginous fishes) - jaw is supported both by the hyomandibular and by a direct connection between the jaw and the chondrocranium
Hyostylic (elasmobranchs and most bony fishes) - upper jaw loses any major direct connection with the chondrocranium and the upper and lower jaws are supported solely by the hyomandibular
Autostylic (lungfishes and in tetrapod ancestors) - upper jaw (pterygoquadrate cartilage) articulates or is fused with the chondrocranium, lower jaw forms from the mandibular cartilage, and the jaw remains unsupported by the hyomandibular
In mammals, the pterygoquadrate cartilage is modified further to form the incus and malleus - the hyomandibular becomes part of the hyoid and the stapes