To study the direction of jaw movements in the koala from wear facets on the molar teeth by scanning electron microscopy, gold coated epoxy resin replicas from the right maxillary and mandibular tooth quadrants were examined from 12 koala skulls. The progressive development and location of facets, the orientation of striae on them and directional data were recorded and transferred from electron micrographs to superimposable transparencies.
Polished facets with laterally oriented striations developed on the cristids and cristae progressively into dentine, where Greaves' effect indicated that the direction of the chewing stroke was labiolingual. Polished and pitted facets, aligned and striated in the parasagittal plane, occurred on the smooth interactive enamel surfaces of maxillary and mandibular cusps.
Labiolingual transit of the crislids over the cristae, with a slight anteromedial shift, was inferred to be the predominant chewing stroke on the working side with no contralateral balancing contact. A propalinal isognathous movement in which successive cusps made contact was also deduced.
Previous concepts of koala chewing and tooth wear were confirmed and amplified, and these may have application to studies of extinct marsupial jaw mechanisms.
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