Nature has tried some pretty wild approaches to life’s problems over the eons, and the same goes for vision.
One of the earliest examples of a complex eye may have relied on a rather unusual light-focusing material not found in modern organisms. The eyes belonged to an extinct group of animals called trilobites, and their eyes were made of hard crystal, a mineral called calcite — an odd little quirk that gives us a glimpse into the way these early animals perceived the world around them.
Trilobites became extinct about 250 million years ago, but existed before that about 300 million years. There are also numerous trilobite body plans in the fossil record, suggesting that these 300 million years were very successful. And because their strange eyes were made of stone, they were often beautifully preserved in the many fossils they left behind.
From this we know that trilobites, like insects, had compound eyes composed of assemblies of photoreceptor units called ommatidia, each with its own photoreceptors and lenses. Examination of fractured sections of the fossilized lenses reveals calcite crystalline material.
Pure calcite is transparent, so light could theoretically penetrate and be focused where the photoreceptors could see it. As with insect vision, there was likely a trade-off: trilobites probably did not see at high spatial resolution, but were particularly sensitive to movement.
There were three types of these trilobite eyes. The oldest and most common form is the so-called holochroal lens, in which small ommatidia are covered by a single corneal membrane and the adjacent lenses are in direct contact with each other.
The abathocroal eye is found only in the Eodiscidae family; The small lenses are each covered by a thin cornea.
Finally, the schizochroal eye is found only in the suborder Phacopina. The lenses are larger, far apart, and each has its own cornea. Scientists assume that they were probably highly specialized.
The holochroal eye most closely resembles the modern appositional eyes of some insects and crustaceans, and scientists believe they functioned in a similar way. Each ommatidium works individually and the image the insect sees is a mosaic of all the images together.
With calcite, however, things get a bit tricky. Calcite exhibits one of the strongest birefringences in nature. That means it has two indices of refraction; Light is split twice as it travels through calcite, with the two rays traveling at different speeds, creating a double image.
For small ommatidia, such as seen in the holochroal eye, this should not be a problem; The deviation of the rays is smaller than with the light-sensing organ.
For the schizochroal eyes, birefringence poses a bigger problem. Crystal is not flexible, so the larger ommatidia cannot change their focus to reduce the effect. Instead, scientists have found that schizochroal eyes have a so-called doublet lens structure.
That means the lens consists of two layers, each with a different refractive index, which could correct birefringence, almost as if the trilobites had built-in glasses. Lenses of this type were invented separately by mathematicians Rene Descartes and Christian Huygens in the 17th century, unaware that trilobites had preceded them.
Despite all our understanding of the different structures of the trilobite’s eye, we still don’t fully understand how the schizochroal eye functioned, whether it resembled an appositional eye or did something else as the different structure suggests.
A recent study showed that schizochroal eyes are far more complex than we thought, which brings us closer. It has been found that each lens covers its own small compound eye, forming a kind of “hyper eye”.
It might even be possible that we were completely wrong about trilobite vision. A 2019 study questions whether the calcite-bearing corneas may have been artifacts of the preservation process, suggesting their crystalline eyes are far less unique than many speculate.
We still don’t know why these types of eyes evolved — if at all — or what benefits they brought, but now scientists studying trilobite vision have a new way of looking at them.