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 apposition 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 in between. New research shows that these compound eyes were highly sophisticated systems — hyper-compound eyes hiding an individual compound eye below each of the big lenses.
Phacops geesops, a trilobite species that lived during the Devonian period; the animal’s eyes consist of 200 single lenses each, spanning six small facets, which again form one eye each. Image credit: Brigitte Schoenemann.
“Most trilobites had compound eyes similar to those that are 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, a researcher in the Department of Zoology, Neurobiology/Animal Physiology and Biology Education at the University of Cologne, and her colleagues.
“Comparable to the image of a computer screen, which is built up from individual pixels, an image is built up from the individual facets. In dragonflies, there are up to ten thousand individual facets.”
“In order to produce a coherent image, the facets must be very close together and connected by neurons.”
“However, in the trilobite suborder Phacopina, the externally visible lenses of the compound eyes are much larger, up to 1 mm in diameter and more. In addition, they are set farther apart.”
In the new research, Dr. Schoenemann and co-authors analyzed X-ray images taken by Wilhelm Stürmer, an amateur paleontologist and a pioneer of X-ray analyses in fossils during the 1970s.
The researchers found that the facets in the schizochroal eyes of the phacopid trilobites are less numerous than in most trilobite eyes, but can reach diameters of 2 mm and more, and there are wide interspaces in between.
They found that below each of the these large lenses sits a small complete individual compound eye — so in total there results a hyper-compound eye, with several tens, in cases 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.
“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 1 mm in size.”
“Under each of these lenses, in turn, at least 6 facets are set up, each of which together again makes up a small compound eye.”
“So we have about 200 compound eyes (one under each lens) in one eye. These sub-facets are arranged in either one ring or two rings.”
“Underneath sat a foam-like nest that was probably a small neural network to process the signals.”
According to the team, the hyper-compound eyes of the phacopid trilobites may have been an evolutionary adaptation to life in low light conditions.
“With its highly complex visual apparatus, it may have been much more sensitive to light than a normal trilobite eye,” Dr. Schoenemann said.
“It is also possible that the individual components of the eye performed different functions, enabling, for example, contrast enhancement or the perception of different colors.”
“So far, such an eye has only been found in the trilobite suborder Phacopina,” she said.
“This is unique in the animal kingdom. In the course of evolution, this eye system was not continued, since the trilobites of the suborder Phacopina died out at the end of the Devonian period 360 million years ago.”
The discovery is reported in a paper in the journal Scientific Reports.
B. Schoenemann et al. 2021. A 390 million-year-old hyper-compound eye in Devonian phacopid trilobites. Sci Rep 11, 19505; doi: 10.1038/s41598-021-98740-z