A mechanism for tooth pattern reversal in sharks: The polarity switch model.

In many shark families the tooth cusps are deflected in a posterior direction. In tooth pattern reversal, one (or a few) tooth families have cusps which are deflected in an anterior direction. This tooth pattern reversal is attributed to a splitting of hypothetical tooth protogerms. After splitting, both parts of the protogerm regenerate and form complete tooth germs.A 'Polarity Switch Model' is proposed according to which tooth germs with curved cusps have an asymmetric (polarized) double gradient. The model states that, depending on where the splitting takes place within the protogerm, either both teeth have a normal posterior deflection, or the deflection of the posterior tooth is switched into the opposite direction. On the basis of the specimens available, a switch in polarity occurs only when the protogerms are injured: It does not occur when protogerms become split during the normal process of adding new tooth families.