Meeting Abstract

S2.6  Tuesday, Jan. 4  Masticatory motor patterns of herbivorous marsupials. CROMPTON, AW; WILLIAMS, SH*; Harvard Univ; Ohio Univ acrompton@oeb.harvard.edu

Primitive masticatory motor patterns are characterized by the activation of a set of muscles (Group I) that move the working-side jaw laterally and vertically during occlusion followed by a comparable set (Group II) that move the jaw medially and vertically. Jaw movement during occlusion is essentially vertical. In herbivorous mammals the primitive motor pattern is modified in many different ways. There is considerable variation in the muscles that are included in each group, the sequence of the onset, peak and offset of activity in individual muscles and jaw movements during occlusion in marsupial and placental herbivores. For example, koala molars increase the number of shearing facets of a typical tribosphenic molar, and the power stroke is divided into a vertical phase (Group I muscles active) and a medially directed transverse phase (Group II muscles active). Their motor program is convergent on that of alpacas (but not on goats or horses) among placental herbivores. In both species, the late firing of the balancing-side deep masseter is associated with a fused mandibular symphysis. In wombats, koala’s closest living relatives, molar occlusion consists of a single medially directed power stroke and only the working side muscles are active. In kangaroos, wallabies and potoroos, the power stroke is also divided into two phases. During the vertical phase shearing takes place between the large sectorial premolars in potoroos and the lower procumbent incisor and upper incisal arcade in kangaroos and wallabies. Group I muscles maintain the vertical orientation of the lower jaw and force the jaw laterally to prevent food separating the shearing surfaces. In kangaroos and wallabies more muscles are active during the vertical than the transverse phase. This is associated with tall shearing transversely orientated lophs on the molars.