A well-known trilobite is the small Aulacopleura koninckii (Barrande, 1846 1), occurring in great numbers within a layer of mudstone of 1.4 m thickness on Na Černidlech Hill and Špičatý Hill near Loděnice in the Czech Republic (Silurian, Wenlock, Liten Formation, Motol Member, Monograptus flexilis-Zone). From the very beginning of their appearance they were equipped with compound eyes, which during the Cambrian explosion and later differentiated into highly diverse visual systems. Trilobites are extinct marine arthropods that dominated the ecosystems of the Palaeozoic. It is an example of excellent preservation, and we hope that this manuscript will be a starting point for more research work on fossil evidence, and to develop a deeper understanding of the evolution of vision. We show that this Palaeozoic trilobite in principle was equipped with a fully modern type of visual system, a compound eye comparable to that of living bees, dragonflies and many diurnal crustaceans. Functionally the latter underlines the idea of a primarily calcitic character of the lens because of its high refractive properties.
It shows the characteristic elements of a modern apposition eye, consisting of 8 (visible) receptor cells, a rhabdom, a thick lens, screening pigment (cells), and in contrast to a modern type, putatively just a very thin crystalline cone. Here we are able to show the excellently preserved internal structures of the compound eye of a 429 Mya old Silurian trilobite, Aulacopleura koninckii (Barrande, 1846). In all arthropods the plesiomorphic (ancestral character state) kind of visual system commonly is considered to be the compound eye.
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