Article: Variation in the eyes of the Silurian trilobites Eophacops and Acaste and its significance

Among the compound eyes of trilobites, the most remarkable are the schizochroal type of the suborder Phacopina. As well as representing an ancient visual system of probably unique kind, schizochroal eyes show patterns of variation in lens distribution which have figured in discussions of possible dimorphism and polymorphism in trilobites species, and have been used by some authors as taxonomic characters. Eophacops musheni is a common species in the British Wenlock, and variation in the lens pattern on the visual surface is described from about 40 well preserved specimens. Adults typically have 19 or 20 files of lenses, and two cases are described of individuals with 20 files in the left eye and 19 in the right. Comparable cases occur in Acaste. Dimorphism in the visual surface of A. downingiae is doubtful. The new data on the variation of the visual surfaces of E. musheni and A. inflata indicate that visual surface morphology provides a reliable guide to species identity only in some cases. They also allow Clarkson's developmental model for the visual surface to be extended to imply that the initial length of the section of the generative zone actively producing lenses was variable, and that lens emplacement was initiated at different times relative to the descent of the generative zone in different individuals. If development of a lens was controlled by the distance from adjacent lens centres, and given that lenses are round and that emplacement began in a single horizontal row, hexagonal close packing and the development of dorso-ventral files result automatically. Cubic close packing could be produced by modifying the spacing factor in successive horizontal rows. The number of dorso-ventral files of lenses and their relative height are controlled by the length of the active section of the generative zone and its pattern of growth. The existence of individuals with eyes differing in the number of files demonstrates that file number is a consequence of a developmental programme, rather than being under immediate genetic control. Variation in the timing of termination of lens emplacement accounts for the observed variation posteriorly and near the base of the visual surface.