Tooth enamel structure in the koala, Phascolarctos cinereus: some functional interpretations.

The purpose of this study is to determine whether the structural organization of Pattern 2 marsupial enamel in the Koala is disposed to resist wear on the sectorial crests of the molar teeth. The orientation of wear on the crests is uniformly delineated by parallel scratches on their polished surfaces. Twin blades, a leading and a trailing edge of enamel are formed on each crest by wear into dentine on which the differential wear at enamel to dentine interfaces indicates that the direction of wear is labial to lingual. 96 leading and trailing edges from 12 koala molars were examined by light and scanning electron microscopy as ground sections, polished and etched surfaces or polished and etched whole mount preparations sputter coated with gold. The results showed that the leading and trailing enamel edges are different in their thicknesses, and in the course of their rods. The rods in the thinner leading edge are angled at 25 degrees to the long axis of the tooth and cross the worn surface at 60-70 degrees. Trailing rods run at 5 degrees to the long axis to cross the worn surface at 90 degrees. The inter-rod sheets run parallel to the wear striations and thus hold the rods in palisades angled in the leading edge particularly to resist the vector of the occlusal forces in the direction of wear. Crystals in the rods emerge roughly perpendicular onto the worn surface which makes them more resistant to abrasion than those in the inter-rod substance which lie parallel to the worn surface and are more readily removed. Koala enamel on the sectorial crests is thus a simple Pattern 2 rod packing pattern but the angles of the rods and the alignment of the inter-rod substance appear to be adapted to resist occlusal forces and abrasion.