Tooth Form and Function in Temnospondyl Amphibians: Relationship of Shape to Applied Stress

Temnospondyl amphibians were important members of the tetrapod fauna and the terrestrial vertebrate food chain during the Paleozoic and early Mesozoic. The “labyrinthodont” teeth of these amphibians are not simple cones; they exhibit an overall tooth form that is relatively complex and generally conserved over large temporal and phylogenetic ranges. We assess the functional morphology of these teeth and show the relationship of tooth shape to applied stress in three species of temnospondyl amphibians: the Pennsylvanian-Permian eryopid Eryops; the Middle Triassic capitosaurid Eocyclotosaurus; and the Late Triassic metoposaurid Koskinonodon. Beam theory was employed to develop strength profiles of the study teeth, which were then compared to their bending moment and shear stress profiles. These profiles are a graphic representation of the stress and strength along the longitudinal tooth axis at any position along the length of the tooth crown. This analysis showed that the teeth were best adapted to loading in the labiolingual direction, and that their strength varied as a cubic parabola, increasing from tip to base. In labial aspect the teeth have an approximately triangular shape. When such a tooth has penetrated a prey item and is loaded in the labiolingual direction, its triangular shape produces a uniformly varying load along the tooth length that results in a cubic parabola bending moment profile. Whereas no correlation was seen between tooth strength profiles and shear stress profiles, the bending moment stress profiles and tooth strength profiles were identical. Thus, shape has evolved to resist the applied bending moment while maintaining good penetration and crack propagation in hard parts of the prey items and for optimal performance during normal usage rather than to survive a relatively rare catastrophic impact.