Cranial kinesis revisited

Cranial kinesis is a fundamental property of the vertebrate skull, appearin g with the evolutionary origin of vertebrate jaws. Kinesis has changed grea tly during the history of vertebrates, and has been lost in a number of gro ups. Once lost, it never re-evolved. Avian kinesis was modified from that p resent in their archosaurian ancestors and is associated with a streptostyl ic quadrate as in many reptiles. All birds possess a kinetic skull which ca n be prokinetic, charadriiform rhynchokinetic or palaeognathous rhynchokine tic. The ancestral type of kinesis in living birds is unknown as is the evo lutionary history of kinesis in Mesozoic birds. The secondary and ectethmoi d articulations of the mandible are shown to be braces for the mandible whe n the jaws are opened and closed by raising and lowering the upper jaw. Two functional properties of avian kinesis are discussed. These are: (a) permi tting the two-joint dorsal adductors and perhaps the M. depressor mandibula r to contract isometrically; thereby maximizing their fiber number, minimiz ing fiber length and allowing these muscles to operate with the most effici ent energy budget; and (b) transferring the consequences of large reaction forces (biting or pecking) on the upper jaw from the dorsal part of the bra in case and brain to the stronger skull base, thereby allowing the evolutio n of a light-weight construction of the roof and walls of the brain case. T he latter functional property does not require any movement of the kinetic upper jaw. These functional properties of avian kinesis may be of greater i mportance than those consider earlier by avian morphologists.