The median eyes of trilobites

Abstract

Arthropods typically possess two types of eyes—compound eyes, and the ocellar, so called 'median eyes'. Only trilobites, an important group of arthropods during the Palaeozoic, seem not to possess median eyes. While compound eyes are in focus of many investigations, median eyes are not as well considered. Here we give an overview of the occurence of median eyes in the arthropod realm and their phylogenetic relationship to other ocellar eye-systems among invertebrates. We discuss median eyes as represented in the fossil record e.g. in arthropods of the Cambrian fauna, and document median eyes in trilobites the first time. We make clear that ocellar systems, homologue to median eyes and possibly their predecessors are the primordial visual system, and that the compound eyes evolved later. Furthermore, the original number of median eyes is two, as retained in chelicerates. Four, probably the consequence of a gene-dublication, can be found for example in basal crustaceans, three is a derived number by fusion of the central median eyes and characterises Mandibulata. Median eyes are present in larval trilobites, but lying below a probably thin, translucent cuticle, as described here, which explains why they have hitherto escaped detection. So this article gives a review about the complexity of representation and evolution of median eyes among arthropods, and fills the gap of missing median eyes in trilobites. Thus now the number of median eyes represented in an arthropod is an important tool to find its position in the phylogenetic tree.

Gene expression mapping of the neuroectoderm across phyla – conservation and divergence of early brain anlagen between insects and vertebrates

“…This opens the possibility that the last common ancestor had light sensory organs developing from the anterior-median region of the neuroectoderm, evolving into ocelli in insects and into the vertebrate eyes, respectively. Based on fossil evidence, it has been argued that the median eyes/ocellar systems represent the primordial visual system that evolved before the emergence of compound eyes ( Schoenemann and Clarkson, 2023 ). Hence, in the lineage leading to the insects, the lateral complex eyes might have developed as evolutionary novelty by activating the ancestral eye specification program at a non-homologous position.…”