Ancient trilobites have a hidden third eye

Researchers from the Universities of Cologne and Edinburgh have discovered in fossil marine arthropods

previously undescribed median eyes. Such solitary eyes on the forehead are found in many modern relatives of trilobites, but have not yet been described in this ancient species.

Median eyes in fossil trilobites. Image: Brigitte Schoenemann, Euan N. K. Clarkson, Scientific Reports

Scientists examined a sample of the trilobite Aulacopleurakoninckii. They found three almost identical dark, inconspicuous and tiny oval spots of the same size on 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 an even clear contour and a uniform dark brownish color.

This clear, correct appearance distinguishes thisstructure from random formations formed as a result of decay or petrification, and corresponds to the expected remains of simple median eyes equipped with a pigment layer. Even if this is a single find, it supports the assumption that trilobites originally had median eyes.

Bridget Schönemann, co-author of the study from the Institute of Zoology at the University of Cologne

Arthropods usually have two types of eyes:compound eyes and median eyes. The main organ of vision in these species is complex compound eyes, which consist of a large number of simple eyes. Each eye perceives its own image, and the overall picture is made up of individual elements. The middle eyes, on the contrary, resemble ordinary human organs of vision.

All arthropods have median eyes, andalso in many relatives of extinct trilobites, but have not yet been described in representatives of this ancient species. Researchers believe that the median eyes evolved in the larval stages of trilobites. They were located under a transparent layer of shell, which, in the process of petrification, darkened as it grew and became impervious to light. That is why such structures have not yet been discovered.

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