Trilobites have an additional set of eyes, new fossil research reveals

Marina Yu Phillips 기자 승인 2023.03.22 15:17 의견 0

Trillions of trilobites, a now extinct group of marine arthropods that ruled the seas for more than 270 million years, populated the oceans five hundred million years ago. There are specimens in every fossil museum, and over 20,000 species have been described thus far.

Compound eyes—eyes made up of tens to thousands of small independent units, each with its own cornea, lens, and light-sensitive cells—gave all trilobites a broad field of vision.

However, a recent fossil find showed that trilobites also possessed ocelli, or single lenses situated in the center of the forehead between the compound eyes and eyeballs with light-sensitive cells inside a cup-shaped depression (they are also known as median eyes).

The trilobite species Aulacopleura koninckii had a portion of its skull scraped off when it was inspected by the discovery's authors, Professor Dr. Euan Clarkson at the University of Edinburgh and Dr. Brigitte Schoenemann at the Institute of Zoology at the University of Cologne.

At the front of the skull, they discovered three nearly identically shaped, little, oval, dark, and unnoticeable dots of the same size. These three buildings are parallel to one another and somewhat flare out on the underneath. A homogeneous, dark brownish tint and a smooth, distinct shape define each of the three spots.

"This structure's distinct, regular look sets it apart from the random formations left behind by decay or fossilization, and it is consistent with the predicted remnants of simple, pigment-layered median eyes. The discovery supports the idea that trilobites once possessed median eyes, even if it is just one "Schoenemann, therefore.

Most extant arthropods, such as insects, spiders, and crustaceans, as well as several trilobite fossil ancestors, have median eyes. They are thought to offer extra visual information that operates apart from the compound eyes.

That trilobites did not develop such a beneficial organ seems odd (though not impossible).

Yet, earlier studies simply failed to notice the eyes because they are so difficult to see in the fossilized material.


According to Schoenemann, "These eyes are present in trilobites at the larval stage, but lie beneath what is likely a thin, transparent carapace (cuticle), which becomes opaque during fossilization," obscuring the eyes from clear view.

The trilobite Cyclopyge sibilla, which lived in the open ocean, also had three cup-shaped median eyes on the so-called glabella, the area in the middle of the forehead between the large compound eyes. These eyes were clearly more distinct and likely much more effective than those of the trilobite Aulacopleura, which lived on the ocean floor.

The discovery provides proof that the arthropod group's ocelli represent an extremely old evolutionary characteristic.

"These cup eyes are derived from the prehistoric stump-footed animals, sometimes known as velvet worms, which are tiny, legless worm-like creatures. The modern, extremely conservative arachnids still have the original median eye count of 2, which is still the case. There are four in ancestral, extremely primitive arthropods, whereas there are only three in contemporary creatures like insects and crustaceans. We now have a valuable tool for determining an arthropod's place in the evolutionary tree thanks to the number of median eyes in an arthropod "Finally, Schoenemann says.

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