Brigitte Schoenemann
Among fossils are trilobite rock stars. They are adorable (as stone arthropods go), with a segmented shape distinctive enough to be a plain logo. But they are also fascinating, because there are so many examples in the fossil record over such a long period of time, seen that they have flourished over 250 million years. Studying their evolution is partly enlightening, as odds are good for excellent specimens.
Brigitte Schoenemann of the University of Cologne and Euan Clarkson of the University of Edinburgh looked into the eyes of one beautifully preserved trilobite specimen, and they learned a lot about how the creatures’ eyes evolved and what that says about evolution. And as a bonus, they conclude that this particular trilobytic species was probably transparent.
A real lens
The fossil in question comes from 429 million-year-old sedimentary rocks in the Czech Republic. It is called a centimeter long trilobite Aulacopleura koninckii that split in half when the rock layer peeled apart. The shape of the structures in one of the two eyes is nicely visible, with bits split between the two halves.
Like other early arthropods, trilobites had compound eyes – remember the multi-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 underneath, which also helps to focus incoming light. That light is brought down by a steel-like “rhabdom” to reach the receptor cells that send signals to the brain. The researchers were able to determine each of these components in the fossil.
Brigitte Schoenemann
Some of the details of these structures have been discussed for trilobites, as it does not run over a fossil that preserves them every day. Most notably, the makeup of the lens and cone pair is a bit unclear, with questions about whether trilobites formed useful lenses with the mineral calcite, as some organisms do today. These researchers found an older (more than 500 million years old) trilobite eye a few years ago and noted a lean, non-calcite lens that showed the refractory work on fleshy cone cells.
This trilobite eye looks different. The cone appears to be minkul, while the lens is considerably thicker. Even a thicker lens made of chitin is not refractive enough to focus light under water, but it would be up to the task with calcite inside. The researchers think that is the case here.
Beautiful insulation
Another interesting observation relates to what is surrounding this whole structure. In this type of composite eye, each ommatidium must be enclosed in something that blocks light to isolate it from the adjacent ommatidia, thus keeping each unit separate. Structural walls can be seen pursuing this orbit in the fossil specimen, but the researchers also saw signs of dark pigment in those walls. (Unfortunately, these pigments are stable enough to be stored in fossils.) That seems duplicative, but modern transparent critters such as shrimp also have pigments in those walls because the walls themselves are not enough to block light. That, the researchers suggest, could make these trilobites transparent as well.
Brigitte Schoenemann
In general everything about this composite eye looks modern – “comparable to that of living bees, dragonflies and many diaries [daylight-dwelling] Cancer, ”the researchers write. That would show just how long ago this system evolved.
Considering things like the length-width proportions of the lenses and ommatidia in the fossil, the researchers can also use analogies to modern organisms to also guess at the habitat of the trilobite. It probably lived in well-lit, shallow waters and was active during the day, they say. So if you could travel 429 million years back, this is where you should start your search A. koninckii, roaring round like a glass, flat shrimp.
Scientific Reports, 2020. DOI: 10.1038 / s41598-020-69219-0 (About DOIs).
