How to Open the Mouth: the Kinetic Skull, and the Jaw and Branchial Apparatus in Sharks

Aims:  Mammals cannot swallow if their necks and, hence, their mm. sternohyoidei et sternothyroidei are stretched. In order to understand the functional morphology of mastication and swallowing in mammals, it is useful to understand its evolutionary origin.
Material and Methods:  Five formalin-preserved specimens of the Spiny Dogfish (Squalus ancanthias) were used for dissection or to prepare sagittal or transverse serial sections.
Results and Conclusions:  The feeding apparatus of the shark Squalus acanthias as a representative of the cartilaginous fishes serves as an excellent model of that of an ancestral vertebrate with a kinetic skull, in which the chondrocranium is movable relative to the jaw apparatus. To open the mouth, the cranium is raised by the epibranchial musculature, the palatoquadrate cartilage (i.e., upper jaw) is entrained, and the mandibular cartilage is held in place and prevented from following the upper jaw by the coracomandibular muscle. At the same time, the branchial basket and oropharyngeal cavity are expanded and held in place by the coracohyoid and coracoarcual muscles, which prevent the hyoid and branchial arches from being entrained cranially by the raised chondrocranium. The mandible is not retracted during the opening of the mouth in sharks because the branchial region would have to be compressed instead of expanded. In the course of the evolutionary modifications leading to mammals, the heart and respiratory apparatus became decoupled from the feeding apparatus through the formation of a neck. The skull became akinetic through the integration of the upper jaw into the base of the cranium. The mouth is not opened any longer by lifting the cranium, but by depressing the mandible through a special muscle, the m. depressor mandibulae . The coracomandibular muscle lost its function as a stabilizer of the mandible during mouth opening and became modified into the lingual and geniohyoid muscles.