A team of paleontologists recently looked into the analysis of a fossil that has preserved the trilobite eye, 429 million years old. Its ocular structure, the researchers point out, was similar to that of modern bees. Also, the animal was probably translucent.
One of the greatest success stories in evolution is the trilobites. These creatures, which first appeared about 521 million years ago, proliferated for tens of millions of years before finally becoming extinct about 252 million years ago. At that time, Earth was then experiencing the worst extinction in its history.
In addition, trilobites also keep very well. Their fossils are therefore highly valued by researchers as they allow us to probe the way life behaved in the oceans almost half a billion years ago.
The analysis of a new specimen allows us to see it a little more clearly (it is the case to say it). Details of the study are published in Scientific Reports.
A perfectly preserved eye
Some fossil specimens, especially representatives of the genus Aulacopleura, have retained their eyes. One of them, studied by Brigitte Schoenemann from the University of Cologne (Germany), and Euan Clarkson from the University of Edinburgh (Scotland), also presented a pair… With the difference that one of the two was cracked.
The researchers, therefore, took this opportunity to study the internal structure of this very ancient eye. Such an opportunity is rare as the preservation of cellular structures, such as those in both eyes, requires rapid burial in an oxygen-poor environment.
The fossil in question, of the species Aulacopleura koninckii, lay in sedimentary rock 429 million years old in the Czech Republic. It is about a centimeter long.
The same eyes as a bee
Like other primitive arthropods, this trilobite had multifaceted eyes. Each unit is called ommatidium, on which there is a lens. Conical cells below allowed the incoming light to be concentrated before transporting it to the receptor cells, which then took charge of sending signals to the brain. The researchers were able to distinguish each of these components in this fossil.
Overall, everything about this compound eye looks “modern,” notes Brigitte Schoenemann, who compares it to that of bees, dragonflies, and many diurnal crustaceans. Proof, once again, that this style of vision evolved a long, long time ago, since the Cambrian Explosion.
“That 200-faceted eye was surely very good at distinguishing obstacles, prey and other predators,” she says, such as cephalopods.
Given the relationship between lens size and physics-dictated light availability, this specimen’s small eyes would have performed better in bright habitats, she believes. Thus, the animal was most likely active during the day and probably lived in shallow water.
A translucent creature
Another interesting observation: in this kind of compound eye, each ommatidium must be enclosed in a structure that allows it to be isolated from neighboring structures. No light source should indeed pass through it. Thus, structural “walls” allow each unit to remain distinct.
These barriers can be seen in this fossil specimen, but upon closer inspection, the researchers also observed small, dark pigments inside. We know that modern translucent creatures like shrimp exhibit this type of pigment to further block the transfer of light between each ommatidium. Thus, the researchers suggest that these trilobites could also have been translucent.