Variation of eye-lens distribution in a new late Devonian phacopid trilobite

Distribution patterns of eye-lenses are compared from forty-two preserved visual surfaces of the new phacopine trilobite Phacops (Phacops) tafilaltensis n. sp. from the Late Devonian of Tafilalt, Morocco. In this species, variation in the lens pattern is most common on the ventral and posterior parts of the visual surface. Post-larval specimens typically have seventeen or eighteen files of lenses, but the number present is not correlated with specimen size. Several individuals display seventeen files in one eye and eighteen in the other. The smaller number of files in one eye may reflect incomplete development of that visual surface. The length of the active section of the generative zone and its pattern of growth control the number of dorso-ventral files of lenses and their relative height. Cases of asymmetry in lens distribution are observed in six individuals. The capacity of these organisms to correct both environmental or genetic stresses appears to be weak for this character. The existence of individuals with eyes exhibiting a different number of files confirms that file number is the result of a genetically controlled developmental program. In Phaeops (Phaeops) tafilaltensis n. sp., the lenses are normally arranged in a regular pattern of hexagonal close packing, their size varies within the visual surface of individual specimens and their number increases during ontogeny. Variation in the timing of lens emplacement accounts for the observed irregularities in lens distribution, i.e. in ventral and posterior parts of the visual surface. Characteristic patterns of the lens arrangement on the visual surface relate to systematic categories and probably reflect evolutionary relationships.