An article published in the journal “Proceedings of the National Academy of Sciences” describes the discovery of the oldest compound eye yet. It belongs to a trilobite of the species Schmidtiellus reetae that lived about 530 million years ago in today’s Estonia. The researchers were able to examine the structure of its eyes and also that of a specimen belonging to another species of trilobites called Holmia kjerulfi, about 2 million years younger. Both trilobites have an eye structure similar to that of many modern arthropods.
The origin of the eyes has no evidence in fossils and in the case of compound eyes typical of arthropods, traces of quite sophisticated eyes can be seen in fossils dating back to the beginning of the Cambrian period, during the greatest diversification ever seen in the history of life on Earth. In the case of a very well preserved Schmidtiellus reetae specimen it was possible to examine its eyes to understand the similarities and differences with respect to modern arthropods, obtaining new clues about the evolution of the eyes in arthropods.
Dr Brigitte Schoenemann of the University of Cologne in Germany and her colleagues Helje Pärnaste of Tallinn, Estonia, and Euan Clarkson of Edinburgh, Scotland, carried out this research. The right eye of the specimen of Schmidtiellus reetae presents a slight abrasion that allows to see inside it. It presents the typical compound structure of arthropods composed of about 100 elementary units, called ommatidia in jargon, which are relatively distant from each other compared to those of today’s animals.
Each ommatide consists of about 8 sensory cells just like today’s ommatidia grouped around a central light-guiding receptive structure called rhabdom. It contains the visual pigments and sends the brightness of the surrounding environment to the animal’s nervous system. Unlike modern compound eyes of bees, dragonflies and many crabs, this ancient compound eye have no lens. According to Dr. Brigitte Schoenemann probably these trilobites didn’t have in their exoskeleton the layer that in other arthropods led to the formation of lenses.
The characteristics of the central rhabdom allow each element of the compound eye to have a limited field of vision and that the global vision of the eye is like a mosaic, just like in modern compound eyes. This mosaic was compared to digital images formed by pixels and with 100 ommatidia those ancient eyes’ performances were probably far from extraordinary. The Schmidtiellus reetae saw enough to perceive movements in its field of vision, from obstacles or predators.
The image above shows the fossil of Schmidtiellus reetae examined in its entirety (A), its head (B), its field of vision (C), the abraded part of its right eye (D), the lateral view of the its right eye (E), a drawing of its right eye (F), two ommatidia (G) and their schematic drawing (H).
Another examination, this time on a trilobite specimen of the Holmia kjerulfi species, showed that it had better eyes than the Schmidtiellus reetae. The difference in age of the two specimens is about 2 million years and the two species belong to different families of trilobites. The comparison between the eyes of the two species can also help to better understand the evolution of the eyes in arthropods in general.