Survey of modern counterparts of schizochroal trilobite eyes: Structural and functional similarities and differences

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Abstract
Three main characters can be identified as typical of schizochroal trilobite eyes: well‐separated optical units, the presence of doublet lenses, and correction for spherical aberration. Surveying the structure and function of some known modern counterparts of schizochroal trilobite eyes, an attempt is made to reveal the functional significance of these characters. First the anatomy and the possible functioning of three different known eye types in trilobites ‐ holochroal, schizochroal and abathochroal ‐ are briefly reviewed. Then the structure of those animal eyes are considered, which share at least one of the above‐mentioned three characteristics with the schizochroal trilobite eyes. The structural and functional similarities and differences between schizochroal eyes and their modern counterparts are compared. The derivation of schizochroal eyes from holochroal ancestors are discussed. Finally, the possible evolution of the optics of trilobite lenses is outlined. Although our review supports the earlier idea that the schizochroal ommatidia probably had extended retinal structures behind them, none of the known modern counterparts considered are so compellingly similar to the trilobite eyes that they point to a clear solution of the way these ancient eyes worked. The most convincingly similar modern structure to compare with the schizochroal trilobite eye is the Strepsipteran eye that would repay further detailed studies.