The function of the glabellar ‘tubercle’ inNileusand other trilobites

Abstract

The glabellar ''tubercle'' of Nileus armadillo (Dalman) is an inverted funnel-shaped thinning in the cuticle, covered by the outer cuticular layer. Its structure is consistent with a function as a light-sensitive organ, whose angular range of light receptivity complements that of the lateral eyes. Median cephalic tubercles of most other trilobites are unlike that of Nileus and are different in structure and position; hence they are unlikely to have been homologous.