The Function of Labyrinthodont Teeth: Big Teeth in Small Jaws

Summary

Labyrinthodont teeth occur in crossopterygians, Lepisosteus, Devonian terrestrial amphibians, ichthyosaurs, and varanoids. That means they have evolved several times and seemingly independently. The question is why.

In biting, the tooth is exposed to compressive forces and to bending moments, which are resisted by the tooth.

For several cross-sections of two labyrinthine teeth we calculated the resistance properties: areas and moments of inertia in two planes (namely mesio-distal and bucco-lingual). The results do not deviate much, and show the ± linear increase of compressive strength, combined with a parabolic increase of bending strength.

From this we can conclude that the investigated teeth belong to predators which captured soft prey, not prey with hard shells.

Immature teeth are weaker than mature teeth, although they possess the same strength patterns. If we cut off the folds in a “young” tooth, the strength drops by 20–40%: a tooth with unfolded, ring-shaped cross-section would be much weaker. If one half of the folds is added to the remaining, peripheral part of the wall, strength increases by 8%. An old tooth, in which the plicidentine is thick, and the folds are in contact with each other, shows nearly no difference to a tooth with a smooth, unfolded wall of the same thickness. A tooth without folds would fulfill exactly the same task.

Labyrinthine teeth seem to occur in predators with large fangs, that are higher than the shallow jaw. Therefore, the high tooth cannot be fully developed in the shelter of a jaw. It has to erupt before it has reached its full strength. The arrangement of small amounts of dentine in folds provides higher resistance at an early developmental stage than would be available without folds. In a mature tooth, the folding offers neither advantages nor marked disadvantages over smooth dentine walls.