Environmental significance of eye-reduction in trilobites and recent arthropods

Abstract Small phacopid and proetid trilobites lived as benthos in the trough of the Variscan geosyncline during Upper Devonian and Early Carboniferous times. The majority of these were blind or had very small, degenerate eyes, and this criterion, in addition to sedimentary evidence, has been suggested as indicative of deep-water (bathyal) conditions. Some blind trilobites also occur in contemporaneous beds in association with normal eyed species; in view of this and other factors, the environmental significance of eye-degeneration and blindness is re-examined with reference to the origin and incidence of blindness in trilobites and in present-day marine arthropods. Blindness in trilobites is always secondary and occurs not uncommonly; its high incidence may result from frequently occurring mutations providing viable species, capable of living in shallow water and feeding microphagously, but only becoming dominant in particular ecological conditions. The dominance of blind and small eyed trilobites of two distinct stocks in the Variscan geosyncline may reflect a dark or dim environment where predators were at a disadvantage, and selection pressure was minimal. The presence of some of those species in shallow water environments could result from contemporaneous migration from the trough. Erben's (1958) suggestions of the origin of blindness are considered to be tenable. Many different present-day arthropods with reduced eyes live at depths below approximately 700 m where, in clear waters, the last traces of surface light fade. At the same depths, however, many arthropods with normal or highly adapted eyes abound, presumably sensitive to bioluminiscence. Although this information is suggestive, it cannot directly be used to infer comparable depths in the Variscan geosyncline, as there is no means of assessing the transparency of water and relative light sensitivity of trilobite eyes. It may, however, help to give an order of magnitude of depth, i.e. hundreds, rather than tens or thousands of metres.