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Crytal Clear View

The trilobites' calcite eyes

"Trilobites could see their immediate environment with amazingly sophisticated optical devices in the form of large composite eyes, the first use of optics coupled with sensory perception in nature." - Riccardo Levi-Setti

Hollardops sp. Our fossilized arthropods show some peculiar characteristics and one of the most astounding of them can be found in the construction and capabilities of their eyes. Even in the early stages of their development, being one of the first classes of marine invertebrates that we know of, they had already developed a very advanced visual system. Most trilobites owned a sort of multi-facetted composite eyes, situated at either side of the glabella – the emphasis is placed on the wording “sort of” as they are not entirely identical to the kind of eyes that we see in recent insects and the like. In at least one of the nine orders we find trilobites showing a primary blindness, i.e. they never developed eyes in their evolutionary progress in the first place, among them the representatives of the suborder Agnostina within the order of the Agnostida. Furthermore, numerous species show a secondary blindness, i.e. a “devolution” of eyes developed at earliers stages of their evolution, in some cases leading to the complete disappearance of the visual system they previously owned. The latter can be attributed to the adaption to changing environments or specific feeding habits. Devolution can be found even in trilobite orders that are well known for their highly developed eyes like the Phacopida.

Hollardops sp. Currently there are three accepted types of trilobite eyes: holochroal, schizochroal (the image to the left portrays the schizochroal eyes of a Phacopid belonging to the genus Hollardops) and abathochroal. This third type, however, appears to be peculiar to Cambrian Eodiscina and recent investigations placed doubt on the independence of this type. Holochroal eyes are what we find in the better part of the trilobite family. Schizochroal eyes with their extraordinary optical capabilities are an 'invention' of the suborder Phacopina within the order Phacopida and can be found in this taxon only.

Talking of schizochroal eyes: I have to state very clearly that the whole lot of „schizochroal“ eyes that can frequently be found in high-priced Russian trilobite fossils (e.g. Hoplolichas, Hoplolichoides, etc.) are nothing but falsifications. These taxa undoubtedly belong to the order Lichida, not to the Phacopida, and can therefore not have had the visual system that skillful preparators seem so eager to reconstruct. As a matter of fact, the visual surface is only scarcley preserved in these trilobites. The wish of collectors to own a more or less complete fossil in all its glory has led to the extensive use of epoxy by the means of which “schizochroal” eyes are revived in places where they do not belong. All the more astonishing, that these partially faked trilobites have been sold over years at sometimes astronomic prices to unwary collectors until at least some of them realized that they had been deceived into buying expensive fabrications. Additional proof that many collectors seem to follow their hobby without going into too much detail, as long as they can get spectacular pieces for their prestigious showcases. If you think this statement to be scolding, you are absolutely right!

Now, what is the difference between (real) holochroal and schizochroal eyes?

Asaphus kowalewskii Holochroal eyes – consist of a usually large amount of closely packed convex lenses made of primary calcite, covered by a single cornea of the same material. Often, each lens comes in the form of a hexagonal prism. The number of lenses can vary between 1 and more than 15,000 individual crystals. The function seems to have come close to that of the multi-facetted eyes of recent insects, although they consisted of a different material: the purest and clearest of calcite crystals..

Schizochroal eyesconsist of a varying number (up to 700 and more) of comparably large and thick lenses, each of them covered by its individual cornea. Each lens is situated inside a conical socket and separated from its neighbouring lenses by an opaque layer of tissue, the so-called sclera, consisting of the same material that is present in the rest of the trilobite exoskeleton. This separation reaches deep into the skeletal framework of the eye and serves as a suspension for the individual corneas attached to it.

To give a better understanding of the obvious differences between the two types of trilobite eyes we would like to show a few close-ups of both holochroal and schizochroal systems as can be found in well-prepared specimens.

These pictures come from both our own collection and from images we reveiced over time for use on our website. However, these images represent but a very small portion of the whole repertoire of what a trilobite eye can look like. The morphological aspects of the eyes are just as divergent as the trilobite morpohology as a whole.

Holochroal and schizochroal eyes:

Kayerops tamnrherta
Phacops sp.

schizochroal "towering eye" in Erbenochile from the Devonian of Morocco

schizochroal eye in Kayserops tamnrherta from the Devonian of Zguilma, Morocco

schizochroal eyes in a Devonian Phacopid showing hexagonal eye sockets

Gerastos sp.
Cheirurus gibbus
Encrinurus sp.

large, holochroal eyes in a Proetid from Germany / Eifeler Kalkmulden

small, knob-like holochroal eyes in Cheirurus gibbus from the Devonian of Morocco

Encrinurus macrourus from the Silurian of Gotland: holochroal eyes on short stalks

Drotops armatus
unbeschriebener Redlichiide

schizochroal eye in Drotops armatus: convex lenses in hexagonal sockets

holochroal eyes on small stalks in a Devonian Cyphaspis sp. from Morocco

sickle-like, holochroal eyes in an undescribed Cambrian Redlichiid from Siberia

The uniqueness of the schizochroal eye rests with its construction. Individual lenses are more or less of spherical shape and comparably large. They were all designed to concentrate light into a focus. As with all such systems, spherical aberration was a problem. Spherical aberration is an optical effect that occurs due to the increased refraction of light rays when they strike a lens. It signifies a deviation of the device from the norm, i.e., it results in an imperfection of the produced image. But our trilobites would not have been trilobites, had they not found a solution to this problem ... ;-)

schizochroal lens The problem of spherical aberration - the problem of a blurry view -, was solved by trilobites like Phacops in a most peculiar way: Each individual lens is actually a doublet with two sharply marked-off lens layers of different refractive indices. The intralensar body at the bottom of each lens shows a different chemical composition. It had accumulated magnesium atoms (magnesium being the closest relative to calcium in the periodic system of elements). With enough magnesium “contamination” the refractive index of the crystal changes and can be changed as much as to correct the spherical aberration that would otherwise have hampered the optical performance of the schizochroal eye. The schizochroal lenses of Phacops were able to cover a larger area of the trilobite’s environment and its seems certain that it observed its habitat with a crytal clear view. A truly identical construction has never been seen again in the whole history of evolution and it indeed appears to be unique in its development. Late investigations into the optical devices of recent insects may have come up with comparable constructions but the trilobite eye is still one of a kind.

The following images once more portray the superficial, clearly visible differences between the holochroal and schizochroal trilobite eye.

holochroal eye schizochroal eye left: holochroal eye from CLARKSON, 1975, right: schizochroal eye from LEVI-SETTI, 1993

Note: How trilobites managed to develop such highly complex eyes has not been entirely explained. I think it is beyond dispute that they built their lenses from the same amorphous calcium carbonate that is found in abundance in the world’s oceans and which they used to build and harden their entire exoskeleton in the first place. The exact method of constructing and transforming this material into transparent individual lenses with a predefined shape has not been described to a satisfactory level. Hints may have been given by a recent study into some of today’s sea urchins featuring spines built from single calcite crystals. These sea urchins enclose amorphous calcium carbonate in an envelope of living cells and shape it into its desired form prior to crystallization. How exactly they are able to do this is yet unclear, but the solution to this problem could have a significant effect on how we can explain the secrets of the trilobite eye.

Variations in trilobite eyes

Asaphus kowalewskii As with all other aspects of the trilobite body, the eyes show a high degree of divergence in size and shape. Many of the early “primitive” trilobites featured long and thin eyes in the shape of crescents or sickles, a good example of which can be found in the Corynexochid Polypleuraspis or many of the large Redlichiida. A more conical eye shape gave a large field of vision to Phacopid trilobites like Phacops, Drotops and it relatives.

In some pelagic trilobites, the eyes were as large as to allow for a 360 degree field of vision, a very logical development, taking into account that such an animal was well advised to watch out for predators which could approach from every possible direction. The other extreme can be found in specialized taxa like Agnostus, which was completely blind. Some benthic and semi-benthic trilobites which roamed across the the sea floors in pursuit of food lost the eyes they had previously developed. Others, like the strange-looking Asaphus kowalewski that can be found in the Ordovician of Russia (see images right and above), managed to place their visual receptors on tall eye stalks in order to peek out from under the mud or algae carpet they were crawling in, looking for enemy predators or prey.

More examples can be found in our various galleries.

All line drawings on this page ©1999, 2000 by S. M. Gon III
© Photographs 2 and 3 on this page courtesy of PaleoDirect

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Last Update : 01/30/2010 5:03 PM