Enlarge /. Don't think you can overtake this trilobite – he still has a good eye, you know.
Among the fossils, trilobites are rock stars. They are adorable (like stone arthropods), with a segmented shape so distinctive that it represents a common logo. But they're also fascinating because there are so many examples in the fossil record over such a long period of time that they have thrived for over 250 million years. Studying their development is partly revealing, as the chances are good of finding excellent specimens.
Brigitte Schoenemann from the University of Cologne and Euan Clarkson from the University of Edinburgh looked an exquisitely preserved specimen of trilobites in the eye and learned a lot about how the creature's eyes developed and what this tells us about evolution. And as a bonus, they conclude that this particular species of trilobite was likely translucent.
A real lens
The fossil in question comes from 429 million year old sedimentary rocks in the Czech Republic. It is a centimeter-long trilobite called Aulacopleura koninckii, which split in half when the rock layer was peeled off. The shape of the structures in one of the two eyes is clearly visible, with the parts divided between the two halves.
Like other early arthropods, trilobites had compound eyes – think of the multifaceted collection of a fly's eye. Each unit in this cluster is called an "Ommatidium". On top of each ommatidium is a lens with cone cells underneath that also help to focus the incident light. This light is passed through a stem-like “rhabdom” to reach the receptor cells that send signals to the brain. The researchers were able to identify each of these components in the fossil.
Enlarge /. Here you can see the entire compound eye cluster.
Some details of these structures have been discussed for trilobites as you don't come across a fossil every day that preserves them. In particular, the composition of the lens and cone pair is somewhat unclear, with questions about whether trilobites, using the mineral calcite, formed useful lenses, as some organisms do today. These researchers found an older (over 500 million years old) trilobite eye a few years ago and noted a lean, non-calcite lens that left the work of refraction to beefy cone cells.
This trilobite eye looks different. The cone appears tiny while the lens is considerably thicker. Even a thicker lens made of chitin isn't refractive enough to focus light underwater, but it would do the job of using calcite inside. The researchers suspect that this is the case here.
Very good isolation
Another interesting observation relates to what surrounds this entire structure. In this type of compound eye, each ommatidium must be surrounded by something that blocks light to isolate it from the neighboring ommatidia and keep each unit separate. Structural walls do this job in the fossil sample, but researchers are also seeing signs of dark pigment in these walls. (Incredibly, these pigments are stable enough to be preserved in fossils.) That seems double, but modern translucent creatures like shrimp have pigments in these walls too, since the walls themselves are not enough to block light. The researchers suspect that these trilobites were also translucent.
Enlarge /. The round structures are individual lenses on an ommatidium – a unit of a compound eye.
Overall, everything about this compound eye looks modern – "comparable to that of live bees, dragonflies and many diurnal crustaceans (living in daylight)," the researchers write. That would show how long it has been since this system evolved.
Given the length and width ratios of the lenses and ommatidia in the fossil, researchers can also use analogies to modern organisms to guess the trilobite's habitat. It is believed to have lived in well-lit, shallow waters and was active during the day, it is said. So if you could travel back 429 million years, you would be looking for A. koninckii scurrying around like a glassy, flattened shrimp.
Scientific Reports, 2020. DOI: 10.1038 / s41598-020-69219-0 (About DOIs).