The Cambrian-era Stanleycaris hirpex had two protruding eyes on the side of its head and a larger eye in the center.

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Stanleycaris hirpex reconstruction – Royal Ontario Museum Sabrina Cappelli

A three-eyed animal with wing-like fins once swam through shallow seas, hunting smaller sea creatures with enhanced visual perception.

Stanleycaris hirpex lived around 500 million years ago, not long after the first eyes were discovered in the fossil record. It is the first animal with three eyes known among arthropods, which includes insects, arachnids, and crustaceans, but the researchers who described it believe there may be others with a third eye that have gone unnoticed.

S. hirpex was about the size of a human hand and had two protruding eyes on each side of its head, each with hundreds of lenses, as well as a third, much larger eye in the middle.

It probably used its advanced visual system to chase down fast-moving prey while living among finger-sized animals, according to Joseph Moysiuk of the University of Toronto in Canada.

“It kind of fits that when we see the evolution of the first predators, we also see the evolution of these complex sensory systems with different eyes, possibly performing different tasks for the organism,” he says.

Moysiuk and his colleagues recently investigated hundreds of exceptionally well-preserved S. hirpex fossils discovered in the Cambrian Burgess Shale of British Columbia’s Canadian Rockies.

Many of the 268 specimens had intact soft tissue, such as brains, nerves, and reflective materials in their visual systems. “After 506 million years, you can see their eyes gleaming in the sunlight when you split one of these rocks in the field.” “So it was pretty clear from the beginning that the organism had three eyes,” says Moysiuk.

The animals had 17 body segments, two pairs of stiff blades along the lower third of their body, and spiked claws that could rake prey right into their toothed jaws. He describes the animal as “pretty ferocious.”

Moysiuk believes that early invertebrates had a large middle eye combined with two lateral eyes, before evolving to two or more paired eyes in later species. For example, the 520-million-year-old Lyrarapax, a member of the same early arthropod group known as radiodonts, had a similar structure on its forehead that could have been an eye.

According to Moysiuk, the new discovery adds to the generally bizarre physical profile of radiodonts. Radiodonts frequently had eyes protruding from stalks and long, oddly shaped appendages.

Reference: Current Biology , DOI: 10.1016/j.cub.2022.06.027

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