Incisor and molar wear in the tammar wallaby (Macropus eugenii Desmarest)

The morphology of incisor and molar teeth of tammar wallabies (Macropus eug enii Desmarest) is similar to that of the archetypal grazing macropod (M. g iganteus) but there are some resemblances in the wear pattern of molars to that of grazer/browsers. Incisor wear patterns show that cutting during bit ing is by scissor-like action of the elevated labial enamel edge of an attr ition facet on each lower incisor being occluded with, and rotated supero-m edially across, the buccal surface of the upper incisor arcade. With increa se in age and body size, the cutting surface moves from anterior to lateral upper incisors, progressive wear on the inner surface of the lateral upper incisors permitting an increasing degree of incisor action coincident with medial molar movement in Phase 2 occlusion, which is similarly achieved by medial rotation of the jaw. Significant distal movement of the reference point for molar index, along t he line of the upper jaw, with increase in body size, indicates that this i ndex does not measure the absolute mesial movement of molars in the plane o f occlusion. The estimated value of absolute mesial movement of the first u pper premolar along the line of the jaw (2.45 mm year(-1)) is at the known limits of mesial drift. Studies of size-related changes in the linear dimen sions of various bony landmarks on jawbone and skull indicate that the high rate of movement may result from deposition of bone in the rear of the too th row, i.e. 'mesial shift', as well as mesial drift. However, mesial shift may not account for significant differences in rates of absolute mesial mo vement of upper molars with gender. With increase in body size, the caudal insertions of the masseter and tempo ralis and the cranial origin of the line of action of masseter all move dis tally along the plane of occlusion. However, a concurrent mesial movement i n the cranial origin of the line of action of the temporalis may act to cou nter any distal movement of occlusive force along the jaw-line and to decre ase the relative force of the retraction component that opposes Phase 1 occ lusion.