Silurian Trilobite Had Modern Type of Compound Eye
Paleontologists have found that Aulacopleura koninckii, a species of trilobite that lived around 429 million years ago (Silurian period), was equipped with a fully modern type of visual system — an apposition compound eye comparable to that of living bees, dragonflies and many diurnal crustaceans.
Trilobites are extinct marine arthropods that dominated the ecosystems of the Paleozoic Era (542-251 million years ago).
From the very beginning of their appearance they were equipped with compound eyes, which during the Cambrian explosion and later differentiated into highly diverse visual systems.
The most basic type, and still very common among diurnal insects and crustaceans, is the apposition compound eye.
It consists of up to 30,000 individual, more or less identical receptor units, so-called ommatidia, optically isolated from each other by a set of screening pigment cells.
In a new study, University of Cologne’s Dr. Brigitte Schoenemann and Dr. Euan Clarkson from the University of Edinburgh used digital microscopy to examine apposition compound eyes of a small trilobite called Aulacopleura koninckii.
This extinct species was first described in 1846 by the French-Czech paleontologist Joachim Barrande, a pioneer of trilobite research, from specimens collected at several paleontological sites near Loděnice in the Czech Republic.
The excellently-preserved specimen studied by the study authors is 1-2 mm high and has two protruding semi-oval eyes on the back of its head, one of which has broken off.
They identified a number of internal structures that are similar to those of the compound eyes of many modern insects and crustaceans, including their ommatidia (measuring 35 μm in diameter) that contain eight light-detecting cells grouped around a transparent tube called a rhabdom.
Each visual unit is topped with a thick lens and the remains of what the paleontologists suggest is a flat crystalline cone that light passed through before being focused onto the rhabdom.
The small size of its visual units indicates that Aulacopleura koninckii lived in bright, shallow waters and was probably active during the day, as smaller diameter lenses are efficient at capturing light under bright conditions.
The presence of pigment cell barriers between visual units suggests that the trilobite had mosaic vision with each visual unit contributing a small portion of the overall image, similar to the compound eyes of many modern insects and crustaceans.
The researchers also think that Aulacopleura koninckii likely was a translucent trilobite, comparable to modern shrimps and other smaller aquatic crustaceans with translucent shells, providing an excellent camouflage in water.
“This 429-million-year-old trilobite already possessed a modern type of compound eye, and it is shown that the principles of vision in modern honeybee or dragonflies, as many crustaceans, is almost half a billion years old,” they said.
“Its excellent preservation expressly underlines the relevance and potentials of insights into the fossil record in understanding the evolution to functional principles to modern sensory systems of today.”
The team’s paper was published in the journal Scientific Reports.
B. Choenemann & E.N.K. Clarkson. 2020. Insights into a 429-million-year-old compound eye. Sci Rep 10, 12029; doi: 10.1038/s41598-020-69219-0