Insights into a 429-million-year-old compound eye

Brigitte Schoenemann,Euan N. K. Clarkson

doi:10.1038/s41598-020-69219-0

Brigitte Schoenemann, Euan N. K. Clarkson

Open Access

https://doi.org/10.1038/s41598-020-69219-0

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Journal: Scientific Reports	Publication Date: Aug 13, 2020
Citations: 7	License type: open-access

Affiliation: University of Cologne, University of Edinburgh

Abstract

In all arthropods the plesiomorphic (ancestral character state) kind of visual system commonly is considered to be the compound eye. 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). 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. Functionally the latter underlines the idea of a primarily calcitic character of the lens because of its high refractive properties. Perhaps the trilobite was translucent. 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. 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.

Highlights

In all arthropods the plesiomorphic kind of visual system commonly is considered to be the compound eye
For a long time it has been thought to be most unlikely that soft tissues, such as neural tissues or even receptor cells could be preserved in the fossil record
Rhabdomeres of adjacent ommatidia oriented in the same direction are combined to neuronal cartridges, enhancing the field of light capturing, as we know, for example in the neural superposition eyes of dipteran flies[71,72,73], and there are strong indications that comparable systems occur in certain beetles, craneflies, earwigs and waterbugs[74,75]

Summary

Discussion

The visual system of the excellently preserved Silurian trilobite Aulacopleura koninckii (Barrande, 1846) is revealed as a classical apposition compound eye. Rhabdomeres of adjacent ommatidia oriented in the same direction are combined to neuronal cartridges, enhancing the field of light capturing, as we know, for example in the neural superposition eyes of dipteran flies[71,72,73], and there are strong indications that comparable systems occur in certain beetles, craneflies, earwigs and waterbugs[74,75] It seems to be more probable, that the rhabdom here is uniform. The physiological expensive establishment of a screening pigment system indicates, that the small trilobite A. koninkii likely was a translucent trilobite, comparable to modern shrimps and other smaller aquatic crustaceans with translucent shells (Fig. 2j), providing an excellent camouflage in water This separation of the individual ommatidia by deep cuticular partitions, and the pigment screen protruding up to the lens (Fig. 2a,b), argue against an optical superposition eye as existing typically in shrimps and other modern decapod crustaceans. That just four million years after the appearance of S. reetae, trilobites with densely packed facets were clearly present (Holmia18), and that trilobites with crustacean-like crystalline cones and numerous, very flat lenses[17] are likewise known

Conclusion

Material and methods