Devonian phacopid trilobites had unique hyper-compound eyes

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Trilobites are extinct arthropods that dominated the faunas of the Paleozoic era. Since their appearance 523 million years ago, they were equipped with elaborate compound eyes. While most of them possessed appositional compound eyes, comparable to the compound eyes of many crustaceans and insects living today, trilobites of the suborder Phacopina that lived 390 million years ago developed the so-called schizochroal eyes – atypical large eyes with wide lenses and wide interspaces. Between. New research shows that these compound eyes were highly sophisticated systems – hyper-compound eyes hiding an individual compound eye beneath each of the large lenses.

Geesops Phacops, a species of trilobite that lived during the Devonian period; the animal’s eyes each consist of 200 single lenses, covering six small facets, which again form one eye each. Image credit: Brigitte Schoenemann.

“Most trilobites had compound eyes similar to those still found in insects today: a large number of hexagonal facets form the eye. There are usually eight photoreceptors under each facet,” said Dr. Brigitte Schoenemann, researcher in the Department of Zoology, Neurobiology/Animal Physiology and Biological Education at the University of Cologne, and her colleagues.

“Comparable to the image on a computer screen, which is made up of individual pixels, an image is made up of individual facets. In dragonflies, there are up to ten thousand individual facets.

“To produce a coherent image, the facets must be very close to each other and connected by neurons.”

“However, in the trilobite suborder Phacopina, the lenses visible from the outside of the compound eyes are much larger, up to 1mm in diameter and larger. In addition, they are further apart from each other.

In the new research, Dr. Schoenemann and his co-authors analyzed X-ray images taken by Wilhelm Stürmer, an amateur paleontologist and a pioneer of X-ray analyzes in fossils in the 1970s.

The researchers found that the facets in the schizochroal eyes of phacopid trilobites are fewer in number than in most trilobite eyes, but can reach diameters of 2 mm and more, and there are wide gaps in between.

They found that below each of these large lenses is a small, complete individual compound eye – so in total this results in a hyper-compound eye, with several dozen or even hundreds of compound eyes in one eye system .

Structure of the visual unit of phacopid trilobites.  Image credit: Schoenemann et al., doi: 10.1038/s41598-021-98740-z.

Structure of the visual unit of phacopid trilobites. Image credit: Schoenemann et al., doi: 10.1038/s41598-021-98740-z.

“Each phacopid had two eyes, one on the left and one on the right,” Dr. Schoenemann said.

“Each of these eyes consisted of about 200 lenses up to 1mm in size.”

“Beneath each of these lenses, in turn, at least 6 facets are put in place, each of which again constitutes a small compound eye.”

“So we have about 200 compound eyes (one under each lens) in one eye. These sub-facets are arranged in a ring or in two rings.

“Underneath was a foam-like nest that was probably a small neural network to process signals.”

According to the team, the hyper-compound eyes of phacopid trilobites may have been an evolutionary adaptation to living in low-light conditions.

“With its very complex visual apparatus, it was perhaps much more sensitive to light than a normal trilobite eye,” Dr. Schoenemann said.

“It is also possible that the individual components of the eye perform different functions, allowing, for example, the enhancement of contrast or the perception of different colors.”

“So far, such an eye has only been found in the trilobite suborder Phacopina,” she said.

“It’s unique in the animal kingdom. During evolution, this eye system did not continue, since the trilobites of the suborder Phacopina became extinct at the end of the Devonian period 360 million years ago.

The discovery is reported in a newspaper article Scientific reports.

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B.Schoenemann et al. 2021. A 390-million-year-old hyper-compound eye in Devonian phacopid trilobites. Scientific representative 11, 19505; doi: 10.1038/s41598-021-98740-z

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