Reversal in some fossil polychaete jaws

Abstract

Scolecodonts, the jaws of polychaetes, are the fossil evidence of the rich and diverse fauna that prospered in shelf seas. Among eunicid polychaetes, the largest jaws in the jaw apparatus occur in pairs. Normally these pairs are not exact mirror images of each other. Rarely, left and right jaws are found to be reversed, the jaw in the left position having the morphology of a mirror image of the normal right jaw and vice-versa. This type of morphological reversal (reversed handedness) is recorded with a frequency that varies from one reversed jaw in 430 to one in 5,838 of normal jaws. Reversed jaws so far are documented in four species from the Upper Ordovician of Indiana, USA, and one species from the Silurian of Gotland, Sweden. The reversed morphology is thought to be a natural, genetically induced feature. The polychaetes with morphological reversal grew to normal size, but never became common.