New fossils reveal extinct trilobites had a hidden third eye

Turns out that prehistoric sea creatures had single eyes on their foreheads in addition to their compound eyes.
Nergis Firtina
Fossil trilobite imprinted in the sediment.
Fossil trilobite imprinted in the sediment.


A new study led by the universities of Cologne and Edinburgh revealed that the extinct marine arthropod genus known as trilobites may have had a hidden third eye, occasionally even a fourth or fifth.

As per the results, the researchers concluded that previously overlooked eyes whose form and function could help improve archaic arthropods' evolutionary classification.

Dr. Brigitte Schoenemann of the University of Cologne's Institute of Zoology and Professor Dr. Euan Clarkson of the University of Edinburgh suggested trilobites were ancient sea creatures that had single eyes on their foreheads in addition to their compound eyes. All arthropods and numerous relatives of the extinct trilobites have single eyes like these, the release says.

They resemble human eyes in that they typically have small cup eyeballs (ocelli), sometimes even with lenses. Despite 150 years of investigation, the so-called median eyes, which are common to all arthropods, had not yet been identified in trilobites. The researchers believe these eyes were typical of the animal's larval stage.

New fossils reveal extinct trilobites had a hidden third eye
Trilobites Scavenging On The Seabottom.

Researchers also discovered median eyes

The carapace's translucent coating, which turned opaque during the fossilization process, covered the solitary eyes. The ocelli were not found till now as a result of these situations. Also, the scientists discovered median eyes in other roughly 500 million-year-old arthropods.

These arthropods had varying numbers of median eyes according to their evolutionary stage. In the future, it will be possible to categorize the evolutionary status of certain arthropod species using the number and shape of the single eyes.

The two primary eye types found in arthropods are compound eyes and so-called "median eyes" or middle eyes. The latter is situated between the compound eyes in the center of the forehead. Only trilobites, a significant class of Paleozoic arthropods, lack what seems to be median eyes.

"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," said Schoenemann.

"In phylogenetics, 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,” she added.

The study was published in Nature on March 8.

Study abstract:

Arthropods typically possess two types of eyes—compound eyes, and the ocellar, so called 'median eyes'. Only trilobites, an important group of arthropods during the Palaeozoic, seem not to possess median eyes. While compound eyes are in focus of many investigations, median eyes are not as well considered. Here we give an overview of the occurence of median eyes in the arthropod realm and their phylogenetic relationship to other ocellar eye-systems among invertebrates. We discuss median eyes as represented in the fossil record e.g. in arthropods of the Cambrian fauna, and document median eyes in trilobites the first time. We make clear that ocellar systems, homologue to median eyes and possibly their predecessors are the primordial visual system, and that the compound eyes evolved later. Furthermore, the original number of median eyes is two, as retained in chelicerates. Four, probably the consequence of a gene-dublication, can be found for example in basal crustaceans, three is a derived number by fusion of the central median eyes and characterises Mandibulata. Median eyes are present in larval trilobites, but lying below a probably thin, translucent cuticle, as described here, which explains why they have hitherto escaped detection. So this article gives a review about the complexity of representation and evolution of median eyes among arthropods, and fills the gap of missing median eyes in trilobites. Thus now the number of median eyes represented in an arthropod is an important tool to find its position in the phylogenetic tree.

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