Extinct Marine Arthropods Trilobite Had Hard Crystal Eyes; Fossilized Lenses Reveal They Were Made of Calcite

Trilobites graced the Earth over 350 million years. Although they are already extinct, there are plenty of tribolite body plans in the fossil record because their eyes were strangely made of stone, leaving the scientists puzzle.

Trilobites With Crytal Eyes Had Compound Sight

One of the first examples of a complex eye may have relied on a substance that is highly rare and not present in modern species to concentrate light. Trilobites, an extinct species of arthropods, had eyes constructed of the mineral calcite, a hard crystal. This peculiar characteristic provides insight into how these early animals detected their environment, ScienceAlert reported.

Yes, trilobites had compound eyes made up of many lensed units. The trilobite's eye design makes them very sensitive to motion, helping them detect predators, per Trilobite's Info.

These eyes comprised ommatidia, which were collections of photoreceptors and lenses. Broken pieces of the fossilized lenses can be examined, showing that calcite crystals are present.

Since pure calcite is translucent, light could theoretically pass through it, focus, and reach the photoreceptors.

Like insect vision, trilobites' vision probably had a trade-off: they couldn't see with great spatial precision but were very sensitive to motion.

These trilobite eyes came in three different varieties. The earliest and most prevalent variety is holochroal, in which the adjacent lenses were in direct touch, and a single corneal membrane covered little ommatidia.

The small lenses are each protected by a thin cornea, and the abathochroal eye is only found in members of the family Eodiscidae.

Finally, only members of the Phacopina suborder have the schizochroal eye. The lenses are bigger, farther apart, and have individual corneas.

Scientists think the holochroal eye functioned similarly to current apposition eyes, which may be found in some insects and crustaceans. The image the insect perceives is a mosaic of all the merged images because each ommatidium functions independently.

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Calcite Makes Trilobites' Eyes Tricky

But with calcite, things become a little more complicated. Calcite is a carbonate mineral with one of the strongest birefringences in nature. Having two refractive indices means that light will be split twice as it passes through calcite, with the two rays moving at various speeds to create a double image.

This is unlikely to be an issue for small ommatidia, as observed in the holochroal eye, as the ray deviation is smaller than the light-sensing organ.

The issue of birefringence is more acute for eyes with schizochroal vision. The larger ommatidia cannot alter its focus to lessen the effect because the crystal is rigid. Instead, researchers discovered that schizochroal eyes possess a double lens structure.

This indicates that the lens includes two layers with a distinct refractive index, which might adjust for birefringence, much like how the trilobites had built-in eyewear. In the 17th century, mathematicians Rene Descartes and Christian Huygens each independently developed this kind of lens, not realizing that trilobites had already done so.

According to this research, schizochroal eyes are significantly more sophisticated than previously believed. It was discovered that each lens formed a sort of "hyper-eye" by enclosing a little compound eye of its own. It's even feasible that the assumptions regarding trilobite vision are wholly incorrect.

A 2019 study suggests their crystal eyes are much less unusual than people believe and questions whether the calcitic corneas may have been byproducts of the preservation process.

Scientists still don't know why this kind of eye evolved or what advantages it offered. However, researchers studying trilobite vision now have a new method to look at it.