Crystal Eyes: Trilobite Optics

Asaphus expansus (my image)

 I must give a great credit to Richard Fortey's Trilobite! Any book about paleontology which ends with an exclamation mark is always worth reading. Also, the good people at www.trilobita.de/english/eyes.html. I have adapted and extended this from a excerpt of my entry to the Bill Bryson Prize. 

Trilobites were some of the oldest, longest lived organisms on earth. They were some of the earliest complex animals and swarmed the seas for around 300 million years before dying out at the end of the Permian, around 252 million years ago. They look a little like the modern woodlouse (or pillbug, if you are from the US) and are easily dismissed as “bugs”. But they were one whole subphylum (the Trilobitomorphs) out of the five subphyla of arthropod, meaning they have equal taxonomical ranking with the crustaceans. Trilobites show us some of the experiments with animal body plans, and therefore help us understand how the body plans we now see came about or could have been. Palaeontology is to the modern zoologist as science-fiction is to the physicist, it gives us idea of how things could have been, not just what they were. It gives us evolutionary possibilities.

Tissues which can sense changes in light are some of the oldest organs in the animal kingdom, basic vision is even found in a type of algae called Volvox. As vision could not have evolved in algae then evolved directly from that into animal vision (as they are not that closely related, in evolutionary terms), a shared evolutionary ancestors of animals and algae must have had the chemicals needed for sight, and therefore may have used these chemicals for sight. The protein used in vertebrate lenses is crystallin, the genes for which pre-date vision in any life form, bacterial and all. The eye probably appeared in animals before the split of the Protostomes (including arthropods, molluscs, vertebrates and worms) and the Deuterstomes (echinoderms and vertebrates), as both groups feature some animals with similar eyes. This could have occurred between 750 and 1250 million years ago (which doesn't mean anything!), well before the Cambrian explosion and the Edicaran, when life was probably little more than very simple multicellular organisms. But we have very few fossils from this time, both because of the small numbers of creatures and because they were mostly soft bodied. Trilobites are easily dismissed due to their abundance in the fossil record, but this is their advantage. They had a hard exoskeleton and crystal eyes, meaning that even a rare species has a fair chance of being fossilized due to their hard tissues. Many abundant species have been found with soft tissues preserved, even some preserved in iron pyrite: trilobites made of fool's gold.

A trilobite with it's soft tissue preserved in iron pyrite from hudsonvalleygeologist.blogspot.co.uk/2011/04/beecher-trilobite-beds.html

Not all trilobites had vision, of course. None of the suborder Agnostida ever developed eyes (known as primary blindness). But of the eyed trilobites, there are two generally accepted categories of trilobite eyes, the typical holochroal and the more rare schizochroal eyes (a development of the suborder Phacopida), but we will come to this issue later. All trilobite eyes involved small lens, which may have given similar images that modern arthropod (compound) eyes do.

Examples of Agnostus pisiformis (my image) 

Making a lens

All eyed trilobites had hundreds of tiny lenses made of calcite (calcium carbonate crystals) which focused light onto photoreceptors. It is thanks to the hardness of the crystal lens that we even know about them today, as they are easily preserved in the fossil record. The trilobite eye is therefore one of the earliest eyes we know about, though there could have been an incredibly diverse ways of seeing in the Pre-Cambrian seas.
One of the first known trilobite to have vision is Fallotaspis which appeared around 540 million years ago; it had crescent shaped ocular lobes which would have given it quite complex vision. It is remarkable that such a complex sensual system is found in such an ancient creature, it is tempting to get poetic and think of what strange worlds these eyes must have seen.

Fallotaspis from fossilmuseum.net

Only the most pure calcite is transparent, as you can tell if you have ever seen marble, which is impure calcite. Therefore, for the calcite to be ever suitable for a lens, it must grow slowly, so few impurities can muscle their way into it's structure (though small amounts of impurities can be helpful, as I will explain later).

But just being transparent doesn't make a good lens. Calcite crystals are rhomb- shaped when cleaved. Rhombs have one major axis and three axes perpendicular to this axis. If a light ray is introduced into the sides of the rhomb, the light ray will be split in a process known as double refraction. Only when light passes down the major axis (c-axis) is it not split. When the rhomb is elongated enough in parallel to the c-axis, only the light that passes down the c-axis will pass clearly through the crystal.

An example of Icelandic Spar calcite, showing double refection from www.segerman.org/CoT.html

How they managed to build these lenses is not known, calcium carbonate is common the sea, but how they harnessed this and made crystals grow in such a specific way is not known.

Holochroal eyes

Holochroal eyes are the most common eyes in trilobites. They involve lots (hundreds or up to ~ 15000) of hexagonal, convex lenses squashed together and covered by a single cornea of calcite. If you put a photoreceptors behind this crystal, you have a single lens of a compound eye, which is what the trilobites probably did. Each lens can detect light coming in from one particular direction. If you stretch a sheath of these lenses almost completely around the eye, then the trilobite can see from tail to (so to speak) nose. They would have seen the world in an overlapping mosaic of images of the Paleozoic seas.

Schizochroal eyes

These type of eyes are found in the some of the Phacopida trilobites. They had large crescent-shaped eyes that stood out from the cheeks (part of the head or cephalon). In these eyes there were comparatively large, round but slightly tear-shaped lens, each sunken into the eye so that they were separated from the other lenses by the opaque sclera. Each lens was covered by it's own cornea, and appears to have operated independently of other lenses.

A curled up Phacops with clear Schizochroal lenses from fossilmuseum.net

But each eye had only about a hundred lenses, much fewer than the thousands of lenses of Holochroal eyes. Why? Well, it may simply be because they did not need so many lenses, due to an ingenious piece of optics. According to the normal behaviour of transparent, spherical lenses, they shouldn't have been able to see anything. When light enters a sphere, different rays travel at different distances, so the rays get bent at different degrees, a process known as spherical aberration. However, there is some impurity of Magnesium in the calcite in a varying band around the lens so that the spherical aberration is corrected. Therefore, the image from each lens should have been combined, and Phacops should have been able to see a single, continuous image, in the way we see a single image from two eyes.

An illustration showing how spherical aberration is corrected from www.trilobita.de/english/eyes.html

There were many different forms of eye in trilobites, each adapted to the particular trilobite’s behaviour. The Agnostida were blind, as they dwelt in the mud and had nothing worth seeing. Some of the earliest trilobites had simpler holochroal lenses in a long, thin eye which could only see to each side, suited to shuffling about on the sea floor. Other eyes became more baroque. The free-swimming trilobites must have been able to see upwards and downwards as well, so some like the comically bulge-eyed Opipeuter inconnivus had very large eyes to spot predator or prey coming in three dimensions. 

A 3D reconstruction of Opipeuter inconnivus from www.nickjainschigg.org/Images/Trilobite/Trilo+ExtralegsRerender.html

Another mud dweller, Asaphus kowalewski, ended up with eyes on stalks, so to spot predators that would swoop in from above.

Asaphus kowalewski from fossilmuseum.net