Dr Clare Torney (UK).
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.