Eumelanin — a natural pigment found for instance in human eyes — has, for the first time, been identified in the fossilized compound eyes of 54-million-year-old crane-flies. It was previously assumed that melanic screening pigments did not exist in arthropods.
“We were surprised by what we found because we were not looking for, or expecting it,” says Johan Lindgren, an Associate Professor at the Department of Geology, Lund University, and lead author of the study published this week in the journal Nature.
The researchers went on to examine the eyes of living crane-flies, and found additional evidence for eumelanin in the modern species as well.
By comparing the fossilized eyes with optic tissues from living crane-flies, the researchers were able to look closer at how the fossilization process has affected the conservation of compound eyes across geological time.
The fossilized eyes further possessed calcified ommatidial lenses, and Johan Lindgren believes that this mineral has replaced the original chitinous material.
This, in turn, led the researchers to conclude that another widely held hypothesis may need to be reconsidered. Previous research has suggested that trilobites — an exceedingly well-known group of extinct seagoing arthropods — had mineralized lenses in life.
“The general view has been that trilobites had lenses made from single calcium carbonate crystals. However, they were probably much more similar to modern arthropods in that their eyes were primarily organic,” says Johan Lindgren. Paper. paywall – 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, Per Ahlberg. Fossil insect eyes shed light on trilobite optics and the arthropod pigment screen. Nature, 2019; DOI: 10.1038/s41586-019-1473-z More.
Note that they are now wondering whether Cambrian arthropods’ eyes were really that different. Talk about stasis.
See also: Stasis: Life goes on but evolution does not happen
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