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The calcitic lenses in the eyes of Palaeozoic trilobites are unique in the animal kingdom, although the use of calcite would have conveyed great advantages for vision in aquatic systems. Calcite lenses are transparent, and due to their high refractive index they would facilitate the focusing of light. In some respects, however, calcite lenses bear evident disadvantages. Birefringence would cause double images at different depths, but this is not a problem for trilobites since the difference in the paths of the ordinary and extraordinary rays is less than the diameter of the receptor cells. Another point, not discussed hitherto, is that calcite fluoresces when illuminated with UV-A. Here we show experimentally that calcite lenses fluoresce, and we discuss why fluorescence does not diminish the optical quality of these lenses and the image formed by them. In the environments in which the trilobites lived, UV-A would not have been a relevant factor, and thus fluorescence would not have disturbed or confused their visual system. We also argue that whatever the reason that calcite was never again used successfully in the visual systems of aquatic arthropods, it was not fluorescence.

Why did the UV-A-induced photoluminescent blue–green glow in trilobite eyes and exoskeletons not cause problems for trilobites?