Trilobites Had An Extra Pair Of Eyes, New Fossil Discovery Shows

Five hundred million years ago, the oceans teemed with trillions of trilobites – a now extinct group of marine arthropods ruling the seas for more than 270 million years. In the last 150 years of research, over 20,000 species having been described so far and some specimens are found in every fossil collection.

All trilobites had a wide range of vision, thanks to compound eyes – eyes composed of tens to thousands of tiny independent units, each with their own cornea, lens and light-sensitive cells.

But a new fossil discovery revealed that trilobites had also so-called ocelli, simple cup eyes located in the middle of the forehead between the compound eyes (they are also known as median eyes).

The authors of the discovery, Dr. Brigitte Schoenemann at the University of Cologne’s Institute of Zoology and Professor Dr. Euan Clarkson at the University of Edinburgh, examined a specimen of the trilobite species Aulacopleura koninckii in which part of the head had been scraped off.

They found three almost identically shaped dark, inconspicuous and tiny oval spots of the same size at the front of the head. These three structures are parallel to each other and fan out slightly on the underside. All three spots are characterized by a smooth, clear outline and a uniform, dark brownish color.

“This clear, regular appearance distinguishes this structure from random formations produced by decay or fossilization, and corresponds to the expected relics of simple median eyes equipped with a pigment layer. Even if it is a single find, it supports the assumption that trilobites originally had median eyes,” so Schoenemann.

Cup eyes are found in most living arthropods (including insects, spiders and crustaceans) and also in many fossil relatives of the trilobites. They are believed to provide supplementary visual information working independently from the compound eyes.

So it seemed strange (even if not impossible) that trilobites did not evolve such a useful organ.

But previous research just overlooked the eyes, as they are indeed very hard to spot on the fossil material.

“These eyes are present in trilobites at the larval stage, but lie beneath what is probably a thin, transparent carapace (cuticle), which becomes opaque during fossilization,” hiding the eyes from direct observation, Schoenemann explained.

In the trilobite Cyclopyge sibilla, which lived in the free ocean, the researchers also found three cup-shaped median eyes on the so-called glabella, the region in the middle of the forehead between the large compound eyes, which even apparently had a lens not unsimilar to human eyes, and were thus clearly more differentiated and probably much more efficient than those of the bottom-dwelling trilobite Aulacopleura.

The discovery is evidence that ocelli are a very old evolutionary trait in the arthropod group.

“These cup eyes are derived from those of the primitive stump-footed animals, so-called velvet worms, small worm-like animals with legs. The original number of median eyes is 2, which present-day, very conservative arachnids still have. In phylogenetic, very primitive arthropods there are 4, modern animals, such as insects and crustaceans, have only 3. With the help of the number of median eyes in an arthropod, we now have an important tool to determine its position in the evolutionary tree,” Schoenemann concludes.

The study “The median eyes of trilobites” was published in the journal Scientific Reports. Material provided by the University of Cologne.