What Did Trilobites See With? Lenses Made of Stone
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Trilobites built the lenses of their eyes from calcite, a mineral, which is why the optics survive as fossils and can be examined directly. Some of those designs correct for aberrations in ways that took human opticians until the seventeenth century.
Lenses that fossilise
Almost every animal builds its eye lenses from protein, which decays, so the optics of extinct animals are normally lost and must be inferred from the surrounding structures. Trilobites are the exception, having built lenses from calcite, the same mineral as their exoskeleton, and crucially having grown each lens as a single crystal oriented with its optical axis pointing along the line of sight. That orientation matters because calcite splits light into two rays when light passes through it at other angles, which would produce a double image, so the alignment is not incidental but a requirement for the eye to work. Because the lenses are mineral they preserve unchanged, and a specimen hundreds of millions of years old can be examined optically.
The main designs
Three arrangements are recognised and they differ fundamentally:
- •Holochroal eyes, with many small lenses packed together under a single covering membrane, which is the commonest form
- •Schizochroal eyes, with fewer, larger lenses each separately mounted with its own covering
- •Abathochroal eyes, with small separately mounted lenses, known from a restricted group
- •Lens counts range from a handful to many hundreds in a single eye
- •Some species are entirely blind, having lost eyes in lineages living in deep or muddy settings
- •Eye position and size indicate whether a species lived on the seafloor, burrowed or swam
The correction nobody expected
The larger lenses of the schizochroal design are the remarkable ones. A simple thick lens suffers from spherical aberration, focusing light from its edge at a different distance than light through its centre, which blurs the image, and the standard correction is to combine two materials or to shape the surfaces in a particular way. Examination of these fossil lenses found an internal boundary between calcite of slightly different composition, shaped as a wavy surface that corrects exactly this defect. The geometry matches solutions published by Descartes and Huygens in the seventeenth century, arrived at mathematically, and the animals were using them roughly four hundred million years earlier. The work describing this was published in the 1970s and remains one of the more startling results in palaeontology.
The problem of a stone eye
Building a lens from mineral solves the preservation problem for palaeontologists and creates several problems for the animal, which is worth thinking through. A calcite lens is rigid, so its focus cannot be adjusted by changing shape the way a soft lens can, which means the eye is fixed-focus and the animal cannot accommodate between near and far. The lens is part of the exoskeleton, so it must be shed and regrown at every moult along with everything else, leaving the animal effectively blind while the new cuticle hardens. Calcite is dense, which adds weight to the head. And the crystal orientation must be controlled precisely during growth, which is a demanding piece of biological mineralisation. That no other group adopted the solution suggests the costs generally outweigh the benefits.
What they were looking at
The optics constrain what the animals could see and the reconstructions are informative. Large eyes on stalks or set high on the head indicate an animal watching above itself, consistent with a life on or in the seafloor keeping watch for predators. Eyes set to give a full circle of vision around the horizon occur in several forms. The separately mounted lenses of the schizochroal design may have given depth perception, since neighbouring lenses viewing the same object from slightly different positions supply the necessary information. Blindness recurs in lineages inhabiting deep water or sediment. Comparing eye designs across the group tracks the appearance of vision-driven predation in the early oceans, which is part of the argument that the evolution of eyes drove the rapid diversification of animals in the Cambrian.
The takeaway
Lenses built from calcite rather than protein survive as fossils and can be examined optically, with each lens grown as a single crystal aligned to avoid splitting the image. Holochroal and schizochroal designs differ in lens size and mounting. The larger lenses contain an internal boundary correcting spherical aberration, matching solutions published by seventeenth century opticians.