The discovery of traces of a sensory cell system in the compound eyes belonging to an extinct group of ancient marine arthropods known as trilobites is presented in the journal Scientific Reports. The find reveals new insights into the early evolution of vision, and especially of the arthropod visual system.
The trilobites are known to have well-developed compound eyes. Although the lenses and visual surfaces of these eyes tend to preserve well, the nature of the underlying sensory structures has remained unknown because soft tissues do not usually fossilize. Using high-resolution computer tomography and synchrotron radiation analysis, Brigitte Schoenemann and Euan Clarkson analyzed the fossilized compound eyes of a group of 400-million-year-old trilobites. The original internal structures were preserved by the ‘seeding’ of a mineral film by bacteria over the surface of the eyes. The authors discovered sensory structures in the eyes, in the form of traces of cells that linked the biomineralized lenses to the sensory receptor cells.
The findings indicate that the trilobites had apposition eyes, a type of multifaceted compound eye found in most living arthropods. The trilobite eye is also shown to resemble the compound eye of the horseshoe crab Limulus, which is generally regarded as a ‘living fossil’ and which may have retained this ancient visual system.