Arthropods typically possess two types of eyes: compound eyes, and the ocellar, so-called median eyes. Only trilobites, an important group of arthropods during the Paleozoic, seem not to possess median eyes. In a new paper, University of Cologne’s Dr. Brigitte Schoenemann and University of Edinburgh’s Professor Euan Clarkson give a review about the complexity of representation and evolution of median eyes among arthropods, and fills the gap of missing median eyes in trilobites.
Compound (a-d) and median (e-p) eyes of trilobites. Image credit: B. Schoenemann & E.N.K. Clarkson, doi: 10.1038/s41598-023-31089-7.
Compound eyes are typical eyes of euarthropods, or ‘true arthropods,’ animals with jointed limbs and hardened cuticles.
Trilobites are extinct arthropods that dominated the faunas of the oceans of the Paleozoic, and from the very beginning of their appearance in the fossil record they were equipped with very elaborate compound eyes.
During 252 million years of successful existence, they developed, especially during the Great Ordovician Biodiversification Event, different adaptations of these eyes to various habitats and most different predatory pressures.
“Arthropods typically possess two types of eyes: compound eyes, and the ocellar, so-called median eyes,” Dr. Schoenemann and Professor Clarkson said.
“Perhaps surprisingly, trilobites do not seem to have any such.”
This was the assumption until the paleontologists examined a specimen of a trilobite species called 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 color.
In a trilobite species called Cyclopyge sibilla, 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.
“In trilobites, there were three median eyes, as typical for euarthropods, not four in number, as in some earlier forms,” they said.
“These eyes consisted of cup-like ocelli, also typical for euarthropods.”
“In Aulacopleura, they lie at the front of the glabella, oriented anteriorly.”
“In Cyclopyge, which swam upside down, they are positioned on top of the glabella, consequently directed downwards.”
“The median eyes of the pelagic Cyclopyge seem to be more elaborated than those of the benthic Aulacopleura, because they seemed to have possessed a lens.”
“Both median eye-systems were found in earlier instars of trilobites, and not in adult individuals.”
According to the team, median eyes are present in trilobites at the larval stage, but lie beneath what is probably a thin, transparent carapace, which becomes opaque during fossilization.
“Both explain why they have remained undiscovered until now,” Dr. Schoenemann said.
A paper on the findings was published in the journal Scientific Reports.
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B. Schoenemann & E.N.K. Clarkson. 2023. The median eyes of trilobites. Sci Rep 13, 3917; doi: 10.1038/s41598-023-31089-7
Source : Breaking Science News