Devonian phacopid trilobites had unique hyper compound eyes

Trilobites are extinct arthropods that dominated the faunas of the Paleozoic era. Since their appearance 523 million years ago, they had been equipped with elaborate compound eyes. While most of them had apposition compound eyes, comparable to the compound eyes of many crustaceans and insects living today, the suborder trilobites Phacopina who lived 390 million years ago developed the so-called schizochreal eyes – large atypical eyes with wide lenses and wide spaces in between. New research shows that these compound eyes were very sophisticated systems – hyper compound eyes hiding an individual compound eye under each of the large lenses.

“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 Biology Education at the University of Cologne, and her colleagues.

“Similar to the image of 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.

“In order to produce a coherent image, the facets have to be very close to each other and connected by neurons. “

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

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

The researchers found that the schizochroal eye facets of phacopid trilobites are less numerous than in most trilobite eyes, but can grow to diameters of 2mm and larger, and that there are large spaces in between.

They found that underneath each of these large lenses is a complete individual small compound eye – so in total, the result is a hyper-compound eye, with several dozen or even hundreds of compound eyes in an ocular system.

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 around 200 lenses up to 1mm in size.”

“Under each of these lenses, in turn, at least 6 facets are configured, 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 either in a ring or in two rings.

“Underneath was a moss-shaped nest that was probably a small network of neurons to process signals.”

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

“With his very complex visual apparatus, he may have been much more sensitive to light than a normal trilobite eye,” said Dr Schoenemann.

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

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

“It’s unique in the animal kingdom. During evolution, this ocular system did not continue, as 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 paper in the review 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