Abstract
Most knowledge about the structure, function, and evolution of early compound eyes is based on investigations in trilobites. However, these studies dealt mainly with the cuticular lenses and little was known about internal anatomy. Only recently some data on crystalline cones and retinula cells were reported for a Cambrian trilobite species. Here, we describe internal eye structures of two other trilobite genera. The Ordovician Asaphus sp. reveals preserved crystalline cones situated underneath the cuticular lenses. The same is true for the Devonian species Archegonus (Waribole) warsteinensis, which in addition shows the fine structure of the rhabdom in the retinula cells. These results suggest that an apposition eye with a crystalline cone is ancestral for Trilobita. The overall similarity of trilobite eyes to those of myriapods, crustaceans, and hexapods corroborates views of a phylogenetic position of trilobites in the stem lineage of Mandibulata.
Highlights
Most knowledge about the structure, function, and evolution of early compound eyes is based on investigations in trilobites
We describe fossil preservation of a longitudinal section through a rhabdom indicating that early euarthropod compound eyes possessed the same type of light receptors as those of some modern euarthropods
Some authors previously suggested that trilobites possessed crystalline cones, albeit without direct evidence[3,24], while others analyzed the function of the lenses without taking other putative optic elements into account[16]
Summary
Most knowledge about the structure, function, and evolution of early compound eyes is based on investigations in trilobites. Among Recent euarthropods two major types of compound eyes occur: (1) those that possess ommatidia with a dioptric apparatus comprising a cuticular lens and a cellular crystalline cone[4,5] and (2) those that have a cuticular lens with a cone-like extension, fulfilling a similar purpose of collecting light and guiding it to the retinula cells[6,7] (Fig. 1). The former type occurs in mandibulates, namely some myriapods, crustaceans, and hexapods, whereas the latter type is characteristic for chelicerate horseshoe crabs. Depending on further data on the eye type of stem lineage euarthropods, our results may corroborate the proposed close phylogenetic relationship of trilobites to Mandibulata
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