Fossil insect eyes shed light on trilobite optics and the arthropod pigment screen

Johan Lindgren (), Dan-Eric Nilsson, Peter Sjövall, Martin Jarenmark, Shosuke Ito, Kazumasa Wakamatsu, Benjamin P. Kear, Bo Pagh Schultz, René Lyng Sylvestersen, Henrik Madsen, James R. LaFountain, Carl Alwmark, Mats E. Eriksson, Stephen A. Hall, Paula Lindgren, Irene Rodríguez-Meizoso and Per Ahlberg
Additional contact information
Johan Lindgren: Lund University
Dan-Eric Nilsson: Lund University
Peter Sjövall: RISE Research Institutes of Sweden
Martin Jarenmark: Lund University
Shosuke Ito: Fujita Health University School of Health Sciences
Kazumasa Wakamatsu: Fujita Health University School of Health Sciences
Benjamin P. Kear: Uppsala University
Bo Pagh Schultz: Fur Museum
René Lyng Sylvestersen: Fur Museum
Henrik Madsen: Mo-clay Museum
James R. LaFountain: University at Buffalo
Carl Alwmark: Lund University
Mats E. Eriksson: Lund University
Stephen A. Hall: Lund University
Paula Lindgren: Lund University
Irene Rodríguez-Meizoso: Lund University
Per Ahlberg: Lund University

Nature, 2019, vol. 573, issue 7772, 122-125

Abstract: Abstract Fossilized eyes permit inferences of the visual capacity of extinct arthropods1–3. However, structural and/or chemical modifications as a result of taphonomic and diagenetic processes can alter the original features, thereby necessitating comparisons with modern species. Here we report the detailed molecular composition and microanatomy of the eyes of 54-million-year-old crane-flies, which together provide a proxy for the interpretation of optical systems in some other ancient arthropods. These well-preserved visual organs comprise calcified corneal lenses that are separated by intervening spaces containing eumelanin pigment. We also show that eumelanin is present in the facet walls of living crane-flies, in which it forms the outermost ommatidial pigment shield in compound eyes incorporating a chitinous cornea. To our knowledge, this is the first record of melanic screening pigments in arthropods, and reveals a fossilization mode in insect eyes that involves a decay-resistant biochrome coupled with early diagenetic mineralization of the ommatidial lenses. The demonstrable secondary calcification of lens cuticle that was initially chitinous has implications for the proposed calcitic corneas of trilobites, which we posit are artefacts of preservation rather than a product of in vivo biomineralization4–7. Although trilobite eyes might have been partly mineralized for mechanical strength, a (more likely) organic composition would have enhanced function via gradient-index optics and increased control of lens shape.

Date: 2019
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DOI: 10.1038/s41586-019-1473-z

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