Comparison of Fossilized Schizochroal Compound Eyes of Phacopid Trilobites with Eyes of Modern Marine Crustaceans and Other Arthropods

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Abstract
We examined fossilized compound eyes of the schizochroal design from 2 species of phacopid trilobites, Phacops rana crassituberculata and Phacops rana milleri. Lens diameters and optical axis orientations were determined for all ommatidial units in both eyes of 2 individuals of each species. The results were analyzed according to the theory of optimal compound eye design developed by Snyder (1977) and Snyder et al. (1977), for comparison with the compound eyes of modern arthropods, particularly marine species inhabiting similar environments. We determined that schizochroal eyes of these species, as well as other schizochroal eyes described in the literature, apparently do not conform to this theory, which properly applies to compound eyes having a single rhabdomal light-collecting unit for each lens. Instead, schizochroal compound eyes probably acted as an assemblage of simple eyes with extended retinas and overlapping visual fields. Such a design offers unique possibilities for visual analysis of color, form, and depth, and is fundamentally different from that of any compound eye now in existence.

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