Masticatory motor programs in Australian herbivorous mammals: diprotodontia.

Movement of the jaw during molar occlusion is determined by the sequence of activity in the adductor muscles and this sequence is one way to define a masticatory motor program. Based on the similarity of molar structure, it is probable that the American opossum and the early Tertiary mammals that gave rise to all Australian marsupials probably shared a common "primitive" masticatory motor program. The distinct and various patterns of movement of the jaw in the major groups of Australian marsupial herbivores (diprotodontids) are achieved by both subtle and substantial shifts in the timing of the primitive sequence. All diprotodonts divide jaw movements during occlusion into a vertical Phase Im and horizontal Phase IIm, but the number of muscles involved and the level of activity associated with each phase varies considerably. In macropodids (potoroos and kangaroos) Phase Im dominates; in wombats Phase IIm dominates and in koalas the two phases are more evenly divided, with a more equal distribution of muscles between them. The motor program of koalas parallels that of some placental ungulates, while both macropodids and wombats have motor programs unique among mammals.