BIO325H5 Lecture Notes - Lecture 3: Animal Diversity Web, Selenodont, Hypsodont
Document Summary
Tooth crowns, the part protruding from the gum line, are covered with enamel and backed by dentine. Enamel is prisms" of tightly packed hydroxylapatite crystals, a mineral form of calcium apatite: dentine also involves calcium. Enamel plus dentine can be viewed as a composite material: enamel is hard, stiff, but brittle, resistant to compression but taking tension poorly. Dentine is hard, less stiff, less brittle, takes tension better: tension is created in a tooth by an off-axis load or by shearing; compression is created as upper and lower jaw teeth occlude. The dentine gives a little and reduces the force concentration at the tip of the crack. The trick amounts to making self-rounding tips on cracks. Imagine it as it isn"t: the entire tooth made of enamel: it would be stronger, but more prone to breakage. Hypsodontteeth: high-crowned, protrude well above the gum, creating a length of tooth adapted for wear during a lifetime of grazing.
