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

Abstract

The morphology of incisor and molar teeth of tammar wallabies (Macropus eugenii Desmarest) is similar to that of the archetypal grazing macropod (M. giganteus) but there are some resemblances in the wear pattern of molars to that of grazer/browsers. Incisor wear patterns show that cutting during biting is by scissor-like action of the elevated labial enamel edge of an attrition facet on each lower incisor being occluded with, and rotated supero-medially across, the buccal surface of the upper incisor arcade. With increase 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 the line of the upper jaw, with increase in body size, indicates that this index does not measure the absolute mesial movement of molars in the plane of occlusion. The estimated value of absolute mesial movement of the first upper 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 dimensions 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 tooth row, i.e. ‘mesial shift’, as well as mesial drift. However, mesial shift may not account for significant differences in rates of absolute mesial movement of upper molars with gender. With increase in body size, the caudal insertions of the masseter and temporalis and the cranial origin of the line of action of masseter all move distally along the plane of occlusion. However, a concurrent mesial movement in the cranial origin of the line of action of the temporalis may act to counter any distal movement of occlusive force along the jaw-line and to decrease the relative force of the retraction component that opposes Phase 1 occlusion.