The weird and wonderful world of trilobite eyes has been subject to study since the late 1800s, but despite being scrutinised intensively over the decades, we are still left questioning how trilobite eyes actually worked due to the loss of their soft parts (that is, photosensitive cells) during the fossilisation process. The numerous strange forms that trilobite eyes come in no doubt plays a role in keeping researchers interested: from the bulging eyes of the Ordovician pelagic trilobite Carolinites to the eyes of Neoasaphus, which stand proud on stalks – trilobite eyes might seem better placed in a sci-fi movie than a palaeontology textbook. However, the study of their eyes can reveal an incredible degree of information, from details of how these extinct marine arthropods lived, to the change in chemistry and temperature of our oceans; and they can even help us understand how animals of today mineralise their (exo)skeletons.
Unlike our own eyes, which are made of soft moving parts that allow most of us to focus on whatever we choose whether it be near or far, trilobite eyes in-vivo were actually composed of the hard mineral calcite (crystallised calcium carbonate), which they also used to construct the rest of their exoskeleton (Fig. 1). Although this may come with its advantages (essentially like wearing your safety specs all the time), using calcite to form eyes poses several problems. Or, in the case of the compound-eyed trilobites, several hundred, or even thousands of problems, depending on the number of lenses in each eye (Fig. 2).
Not only does constructing an eye with a hard material such as calcite cause issues relating to focal range – as the lens has no ‘accommodation’ or flexibility to change its focal distance – but there is also the issue of double-vision that comes with constructing lenses from a ‘birefringent’ material. Birefringence of calcite causes light rays to be ‘doubly refracted’ (split into two rays), producing a real image (from ‘ordinary’ rays) and a ‘ghost’ image (from ‘extraordinary’ rays) (Fig. 3). (Light rays travelling parallel to the main mineralogical axis (the c axis) of the calcite avoid this double refraction.) Light is also bent or ‘refracted’ at curved lens surfaces where there is a change in material (for example, from sea water to calcite), which makes for a very complicated system. Very few modern animals create their eyes in this way, possibly due to the complexity of light transmission in calcite. Some species of brittle star and ostracod have calcitic components to their optical systems, but neither appears as elaborate in design as trilobites eyes (Fig. 4).
So, were the trilobites dizzy with double vision? Solving the problem of how trilobites viewed the world around them is made even more difficult by the fact that not all trilobites had the same type of eye. In fact, there are three different types of trilobite eye preserved in the fossil record: the holochrol eye; the schizochroal eye; and the abathochroal eye, reported in a single species – Pagetia (Jell, 1975), about which fairly little is known. In this respect, this article will discuss the first two of these, with a view to showing just how remarkable trilobite eyes were.
The holochroal eye
The most common trilobite eye, the holochroal eye, is similar in form to the apposition eyes of many modern insects and some crustaceans; each of the thousands of tiny (typically about 30 to 100µm) lenses are closely packed together and covered by a single common cornea (Fig. 5). The lenses vary in shape as well as size. The lenses are usually hexagonal or round and less commonly square, or sometimes octagonal, some with a flat outer surface and a curved base (plano-convex), and others having two curved outer surfaces (biconvex).
Crystallographic mapping using a technique called Electron Backscatter Diffraction (or EBSD) reveals that the lenses in holochroal eyes are formed from a single crystal of calcite, but with subtle changes in orientation across the surface, made possible by the presence of sub-crystals (Fig. 6). The orientation of lens calcite varies depending on the shape of the lenses. In plano-convex lenses, the c axis of the calcite is parallel to the lens axis in the upper section of the lens and fans out at the curved lens base (forming a ‘radial fringe’). In biconvex lenses, the crystal c axis curves outwards at both lens surfaces. This means that light entering the lenses will always be doubly refracted, but at different depths within the lens depending on lens shape – to the human eye, the resulting image would appear somewhat blurry. It is possible that this crystallographic arrangement played a role in vision but, given the relationship with lens shape, it is likely that this was an inadvertent phenomenon associated with growing calcite crystals on a convex surface.
If the holochroal eye did indeed function as an apposition eye, as hypothesised in earlier studies (for example, Clarkson et al., 2006), then trilobites may have avoided ‘seeing’ double by using pigments to shield the photoreceptors from oblique light entering from adjacent lenses (Fig. 7). This would have produced a clearer mosaic-style image, providing them with a better sense of directionality.
Of the three eye types, it is the schizochroal eye that often provokes the most interest. Not only is this visually stunning eye found in just a single suborder of trilobite (the Phacopina of the Ordovician to Devonian), but, even after hundreds of millions of years since the extinction of the Phacopid trilobites, no other animal has been found to have developed an eye similar in design, structure and composition – the schizochroal eye is truly unique.
In contrast to the holochroal eye, each lens in the schizochroal eye, which are much larger in size (typically 250 to 500µm) but significantly fewer in number (up to 770 in Dalmanites pratteni Roy, sp. nov.), is separated from its neighbours by a pillar of exoskeletal material known as the interlensar sclera. Lenses are typically packed hexagonally, forming vertical dorso-ventral files and diagonal rows, but square packing is evident in a few rare cases. Each lens is covered by its own individual cornea, appearing as an eye in its own right (Figs. 8 and 9).
To complicate matters further, each lens in the schizochroal eye is formed by a number of components, which differ in composition (Lee et al., 2007). The incorporation of magnesium in the lower part of the lens (intralensar bowl) creates what is known as an ‘aplanatic’ surface, as it differs in refractive index from the upper part of the lens (upper lens unit) – such a surface results in the bending (or refraction) of light helping to bring light rays into focus at some point beneath the lens. This type of lens – known as a ‘doublet’ – was independently designed by mathematicians Descartes (1637) and Huygens (1690), both unaware that they had been beaten by nature by a few hundred million years. Additionally, there are calcite fibres within the lenses called trabeculae, which are thought to be connected to lens growth (Kaesler, 1997). Lenses in schizochroal eyes also contain a magnesium rich ‘core’, the function of which is still unclear, although it has been speculated that this feature may have enhanced the focusing power of the lenses, further reducing blurriness or in fact giving the lenses a degree of ‘bifocality’ (Egri and Horváth, 2012).
Crystallographic mapping has shown that lenses in schizochroal eyes have the same complex crystal arrangement as biconvex holochroal lenses, with both upper and lower radial fringes (Fig. 10). The axes of the crystals fan outwards at both the outer and inner surfaces of the lens in three dimensions, creating a complex arrangement of curving trabeculae (Torney et al., 2014). This microstructure is distinct from the adjacent sclera and from the rest of the exoskeleton.
Several hypotheses have been proposed for the functioning of the schizochroal eye. Based on the arrangement of lenses in these eyes, Clarkson (1967) proposed that the eye functioned as an ‘apposition eye’ similar to many modern day diurnal insects. These have lenses focusing light onto an ommatidium (a capsule of photosensitive cells) beneath each lens, to produce a mosaic-type ‘image’.
However, the large lens size led Campbell (1975) to suggest that the ocellar-like eye of modern slow-moving terrestrial and aquatic arthropods was a closer comparison. In this type of eye, lenses lie above a layer of ‘corneageneous material’, below which Campbell believed there to have been numerous photoreceptive cells. Due to the bending of light at the aplanatic surface, only light entering parallel (or at small angles) to the lens axis would be detected by the receptors.
Schoenemann’s ‘neural superposition’ model of trilobite vision is perhaps one of the most sophisticated (Fig. 11). Schoenemann (2007) suggests that each of the lenses in the schizochroal eye acted as an ‘eyelet’, with multiple photoreceptors beneath. Given that each schizochroal eye can have up to 770 lenses, such a system could result in the formation of an exceptionally high resolution in what Schoenemann terms a ‘hyper-eye’. Schoenemann (2008) has also proposed that each of the curved trabeculae acts as an optically isolated ‘light guide’, transmitting light to thousands of tiny photoreceptors at the base of the lens (Fig. 12).
Undoubtedly more research is required to unravel the mysteries of the trilobite eye, but one thing is certain. Whichever of these hypotheses proves to be correct for schizochroal trilobite eye vision, these eyes are truly unique, providing the Phacopid trilobites with the level of vision required for them to thrive for such a substantial period of time.
The Leverhulme Trust is gratefully acknowledged for funding this study through Research Grant F/07 179/AM. Gordon Hendler, Ryan McKeller, Thijs Vandenbroucke, Euan Clarkson and Petr Budil are thanked for providing the specimens illustrated in this article.
About the author: Clare Torney carried out research into trilobite eyes as part of her PhD at the University of Glasgow. She now works as a conservation scientist for Historic Scotland.
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