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upstart writes in with an IRC submission for nutherguy:

Fabulous fossil preserves eyes of 429-million-year-old trilobite:

[...] trilobites fascinating because there are so many examples in the fossil record over such a long period of time, given that they thrived for over 250 million years. Studying their evolution is enlightening in part because odds are good for finding excellent specimens.

[...] The fossil in question comes from 429-million-year-old sedimentary rocks in the Czech Republic. It's a centimeter-long trilobite called Aulacopleura koninckii that split in half as the rock layer was peeled apart. The shape of the structures in one of its two eyes is nicely visible, with bits split between the two halves.

Like other early arthropods, trilobites had compound eyes—think about the many-faceted cluster of a fly's eye. Each unit in that cluster is called an "ommatidium." At the top of each ommatidium is a lens, with cone cells beneath it that also help focus incoming light. That light is carried down through a stalk-like "rhabdom" to reach the receptor cells that send signals to the brain. The researchers could make out each of these components in the fossil.

[...] Most notably, the makeup of the lens and cone pair is a little unclear, with questions about whether trilobites formed useful lenses using the mineral calcite, as some organisms do today. These researchers found an older (over 500-million-years-old) trilobite eye a few years ago and noted a meager, non-calcite lens that left the refractive work to beefy cone cells.

This trilobite eye looks different. The cone appears to be minuscule, while the lens is considerably thicker. Even a thicker lens made of chitin isn't refractive enough to focus light underwater, but it would be up to the task with calcite inside. The researchers suspect that's the case here.

Journal Reference:
Brigitte Schoenemann, Euan N. K. Clarkson. Insights into a 429-million-year-old compound eye [open], Scientific Reports (DOI: 10.1038/s41598-020-69219-0)

Original Submission