THE INFLUENCE OF APPLIED STRESS ON LATE PALEOZOIC-TRIASSIC TEMNOSPONDYL AMPHIBIAN TOOTH SHAPE

Marginal tooth shape was measured and analyzed in three species of temnospondyl amphibians, the Pennsylvanian-Permian eryopid Eryops, the Middle Triassic capitosaurid Eocyclotosaurus, and the Late Triassic metoposaurid Koskinonodon. The analysis showed that while tooth size varies greatly, overall tooth form is relatively consistent and generally conserved over these large temporal and phylogenetic ranges. Generally, the base of the crown is elliptical in cross section, with the major axis in the labiolingual direction. The central portion of the crown is circular in cross section, whereas the upper portion shows an elliptical cross section, but with the major axis in the mesiodistal direction. The mesiodistally-elongate upper tooth segment often has a sharp carina to cut through the prey tissue in the direction of the toothrow, and to concentrate stress in the direction of the neighboring teeth so as to propagate a developing crack in hard parts of the prey item.

We used beam theory to develop strength profiles of the study teeth, which were then compared to their bending moment and shear stress profiles. This analysis showed that the teeth were best adapted to labiolingual loading and that their strength varied as a cubic parabola, increasing from tip to base. The labial aspect of each tooth approximates a triangle. When such a tooth penetrates a prey item and is loaded in the labiolingual direction its triangular shape produces a uniformly varying load increasing from tip to base along the tooth length, which results in a cubic-parabola bending-moment profile. While no correlation is seen between tooth strength and shear stress, the bending moment stress profiles and tooth strength profiles are identical. Thus, we hypothesize that tooth shape has evolved to resist the applied bending moment while maintaining good penetration and crack propagation in hard parts of the prey items.

A tooth may suffer catastrophic breakage due to impact on a hard object. Such an event produces a linear bending-moment profile while the observed cubic-parabola bending-moment profile is produced during normal use. Therefore, we conclude that the tooth shape of temnospondyls has primarily evolved for optimal performance during normal usage rather than to survive a relatively rare catastrophic impact.