Excerpt from the linked summary:^1

>Trilobites, prehistoric sea creatures, had so-called median eyes, single eyes on their foreheads, in addition to their compound eyes, research conducted by Dr Brigitte Schoenemann at the University of Cologne’s Institute of Zoology and Professor Dr Euan Clarkson at the University of Edinburgh has now found out.

>Such single eyes are found in all arthropods and also in many relatives of the extinct trilobites. They are usually small cup eyes (ocelli), sometimes even equipped with lenses, and not unlike human eyes.

>Schoenemann and Clarkson examined a specimen of the trilobite 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 colour.

> 

>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.

>In their article, the researchers also consider why the median eyes of trilobites have escaped detection until now: “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 fossilisation.

>Both explain why they have remained undiscovered until now,” she added. Thus, the researchers have solved the mystery of missing middle eyes in trilobites.

^1 New eyes discovered in trilobites, 17 Mar. 2023, https://portal.uni-koeln.de/en/universitaet/aktuell/press-releases/single-news/new-eyes-discovered-in-trilobites

^2 Schoenemann, B., Clarkson, E.N.K. The median eyes of trilobites. Scientific Reports 13, 3917 (2023). https://doi.org/10.1038/s41598-023-31089-7