Primordial `hyper-eye' discovered

Date:
September 30, 2021
Source:
University of Cologne
Summary:
Trilobites of the suborder Phacopina had a unique eye in which
about 200 large lenses in each eye spanned at least six individual
facets, each of which in turn formed its own small compound eye.



FULL STORY ==========================================================================
An international research team has found an eye system in trilobites of
the suborder Phacopina from the Devonian (390 million years B.P.) that
is unique in the animal kingdom: each of the about 200 lenses of
a hyper-facet eye spans a group of six normal compound-eye-facets,
forming a compound eye itself. In addition to the hyper-facetted eyes,
the researchers, led by zoologist Dr.

Brigitte Schoenemann at the University of Cologne's Institute for
Didactics of Biology, identified a structure that they believe to be a
local neural network which directly processed the information from this
special eye, and an optic nerve that carried information from the eye
to the brain. The article, 'A 390 million-year-old hyper-compound eye in Devonian phacopid trilobites,' has been published in Scientific Reports.


========================================================================== Trilobites are arthropods that once inhabited the world's oceans and
became extinct about 251 million years ago. The discovery was made when Schoenemann and her colleagues examined X-ray images taken by radiologist
and amateur paleontologist Wilhelm Stu"rmer in the 1970s. Stu"rmer had
already believed the filaments under the trilobite eyes to be nerves,
or a light guiding system.

Schoenemann also found markings by Stu"rmer on the images designating
the six subfacets. However, scientists at the time did not believe his interpretations.

Now, however, the re-examination of the images and verification with
modern computed tomography succeeded in confirming his conjectures.

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. 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 Phacopinae, 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. Until now, scientists had not
been able to explain this because space is wasted where light could be captured. Since a small cup sits under the lens, they assumed that at
the bottom of the capsule was a small retina comparable to that of humans.

Dr Schoenemann's analysis of Wilhelm Stu"rmer's 40-year-old X-ray archive
now suggests a different interpretation: a hyper-compound eye. Each
phacopid had two eyes, one on the left and one on the right. 'Each
of these eyes consisted of about 200 lenses up to 1 mm in size,' said Schoenemann. '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,' the zoologist added. The filaments Stu"rmer found in fact
did turn out to be nerves leading from the eyes to the trilobite's
brain. Further examination with modern computer tomography confirmed
these structures.

Wilhelm Stu"rmer was the head of the X-ray department at Siemens and
an avid paleontologist. With a VW bus equipped as an X-ray station,
he drove from quarry to quarry to X-ray fossils. Among other things, he discovered structures called filaments under the animals' eyes, which
he thought were fossils of soft tissues, especially optic nerves. 'At
that time, the consensus was that only bones and teeth, the hard parts
of living things, could be seen in the fossils, but not the soft parts,
such as intestines or nerves,' Schoenemann explained.

Stu"rmer's heir gave the zoologist his archive. But the
hobby-paleontologist had not only correctly identified the optic nerve,
she notes: 'On an X-ray negative, there was an arrow in red pen pointing
to the structure of the six lower facets under a main lens. This probably indicated that Stu"rmer had already recognized the hyper-compound eye.' At
the time, however, scientists assumed that nerves did not fossilize, nor
that light guides existed in natural optical system. Optical fibres were
not discovered until the 1980s in the compound eyes of a deep-sea crab.

The trilobite's 'hyper-eye' may have been an evolutionary adaptation to
life in low light conditions, Schoenemann believes. With its highly
complex visual apparatus, it may have have been much more sensitive
to light than a normal trilobite eye. 'It is also possible that
the individual components of the eye performed different functions,
enabling, for example, contrast enhancement or the perception of different colours,' the biologist said. So far, such an eye has only been found
in the trilobite suborder Phacopinae: 'This is unique in the animal
kingdom,' she concluded. In the course of evolution, this eye system was
not continued, since the trilobites of the suborder Phacopinae died out
at the end of the Devonian period 360 million years ago.

========================================================================== Story Source: Materials provided by University_of_Cologne. Note: Content
may be edited for style and length.


========================================================================== Related Multimedia:
* Phacops_geesops,_a_trilobite_from_the_Devonian_age.

========================================================================== Journal Reference:
1. B. Schoenemann, E. N. K. Clarkson, C. Bartels, W. Su"dkamp, G. E.

Ro"ssner, U. Ryck. A 390 million-year-old hyper-compound eye in
Devonian phacopid trilobites. Scientific Reports, 2021; 11 (1)
DOI: 10.1038/ s41598-021-98740-z ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210930101416.htm

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