The eyes have it: Trilobites as models of ecology and evolution

Following my recent scribblings about cave bear dentition, I thought I would make the journey back to looking at extant invertebrates a step at a time. So, staying in the world of palaeonotology, but moving onto invertebrates (slightly more familiar ground), I decided to see if I could derive some more inspiration from my curio shelves. So, given that I've previously written about my Cretaceous water bug, it seemed about time I tackled that most popular of fossil invertebrates, the trilobite. Now, I've only got the one trilobite, but it's quite a good specimen, so here it is in all its glory:

My trilobite, 5cm long and showing clear segmentation plus its well preserved head on the right.

The shape of this, with a bulging and pimply glabella ('forehead') suggests it is in the genus Phacops (possibly P. rana) but not Reedops as this would have a smooth glabella. However, what brings me straight to the suborder Phacopina in the first place is its eyes (see Murray 1985 for a key to trilobite groups).

Phacops compound eye showing clearly separated lenses. This is the right eye looking from above and slightly off to the right. The pimply surface to the left is the fixed 'cheek' area known as the fixigena which appears as a lobe on either side of the glabella.

Trilobite eyes are compound like those of modern invertebrates (well, up to a point) but vary greatly, and this one is known technically as 'schizochroal'. Only found in the Phacopina, and not all of these, eyes of this type have relatively few relatively large lenses as can be seen in the photo. These lenses are well separated and if we took a section through the eye, apart from destroying my trilobite, we would see that each lens had its own cornea (outer layer - which we also have) and was separated from the others by a thick piece of exoskeleton called the sclera with the cornea extending down through this. This is not the case in most trilobites which have 'holochroal' eyes. These have small lenses which are often more numerous and may look more like our familiar 'mosaic' picture of an insect's eye. The lenses all touch each other (i.e. are not separated by sclera) and have a common cornea covering the whole surface of the eye. Lastly, a few trilobites (only the Cambrian Eodiscina) have 'abathochroal' eyes which have few, small lenses which are separated as in schizochroal forms but only have thin sclera and a cornea which stops at the surface of the sclera. Got all that? Well, I've only just worked through the anatony of these eye types and found an excellent summary on S.M. Gon's webpage 'The Trilobite Eye' which I recommend if you would like a expanded and illustrated version of my account here as well as other variant such as stalked eyes and those with inbuilt eyeshades!

Apart from my liking for all things morphological, one important aspect of these eye forms (and those trilobites that were eyeless) is what they can tell us about the life/ecology of trilobites. For example, eye loss is seen in some benthic (bottom-feeding) forms which lived in low-light conditions. So, starting with Phacops, it has fully developed eyes and can be seen as an ancestral form of genera with reduced eye development such as Cryphops which in turn evolved into the eyeless Trimerocephalus. So, we have a genus with well developed eyes which evolved into forms reducing and then losing them - a process that took a long time in human terms but occurred in the, er, blink of an eye, when looking at geological timescales (see Dawkins 1996 for more on the evolution of eyes of various types).

This loss of eyes in a benthic environment is a simple enough concept, but what about the development of schizochroal eyes in the first place? Unlike most (holochroal) trilobite eyes they are highly specialised and have no clear analogue in the modern fauna (Fortey, 2000). Firstly the lenses are crystalline, being made of calcite and are almost spherical, sometimes a little drop-shaped. These lenses have even had photographs taken through them and it is evident that sharp images could be formed and that larger 'pieces' of the trilobite's surroundings would have been visible per lens than for those with holochroal eyes. However, trying to use spherical transparent items such as marbles in a visual system doesn't work well because of 'spherical aberration' - the images become distorted, inverted, fuzzy. However, Phacops solved this problem by making the calcite impure, specifically by replacing some of the calcium atoms in calcite with magnesium and forming an internal 'bowl' in the lens which worked as a corrective structure, separating the eye into two sections of differing refractive index and allowing for the spherical aberration (Clarkson & Levi-Setti, 1975). So, although holochroal eyes would presumably have been good at detecting movement (food, predators?) as is the case in many modern invertebrates, Phacops could see chunks of detail. It is unknown exactly why this type of eye evolved, but it arose, like all other evolved structures, because it improved survival, in this case through a process known as post-displacement paedomorphosis (i.e. the retention of juvenile features - part-developed holochroal eyes are like smaller versions of schizochroal ones).

So, there we have it - Phacops evolved a visual system which has not (yet) been 'repeated', but why did I call trilobites 'models of ecology and evolution' in the title of this article? Well, the evolutionary side is well documented (despite what hordes of frankly bizarre creatonist and Intelligent Design websites might assert to the contrary - however, I won't go there...) both in the scientific literature and in popular-science publishing/broadcasting - despite the hundreds of millions of years that separate trilobites from our modern Earth, they are to some extent familiar. As for the ecology, it allows us to mix some evidence with a dash of speculation. Benthic lifestyles with low light levels led to the loss of eyes and so we can infer something of the 'lifestyles' of genera such as Trimerocephalus from their morphology as well as from the location and material in which they are found. However, in Phacops, we have some evidence of what level of detail they might have been able to see - I say might because their optic nerves are not preserved, thus their 'wiring' remains a mystery as far as I am aware, and by extension so is precisely how they perceived the world.

I'll stop there - I hope you enjoyed that, and please do watch this space for a return to the wonderful world of small beetles soon!

As in some modern invertebrates, the rear segments formed a section behind the thorax known as the 'pygidium' AKA 'the end'!

References

Clarkson, E. N. K. & Levi-Setti, R. (1975). Trilobite eyes and the optics of Des Cartes and Huygens. Nature 254: 663-667.
Dawkins, R. (1996). Climbing Mount Improbable. Viking, New York.
Fortey, R. (2000). Trilobite! Eyewitness to Evolution. Flamingo, London.
Gon, S.M. (2007). The Trilobite Eye. http://www.trilobites.info/eyes.htm [accessed 20/11/2011].
Murray, J.W. (ed.) (1985). Atlas of Invertebrate Macrofossils. Longman, London.