Survivors: The Animals and Plants that Time has Left Behind. Richard Fortey
problems. According to the analyses to date, on balance the trilobites indeed do still classify within a group that also includes the horseshoe crabs. Despite all the confusion of the Cambrian it seems my crusty-shelled friends and the dogged, eternally trundling horseshoe crabs are sisters under the external arthropod skin.
They do share special features. The larva of the horseshoe crab is a pinhead-sized object long known as the ‘trilobite larva’, because it does resemble the tiny larva of many trilobites.* Both kinds of animals grow larger with each moult in similar ways, casting off their old external housing and re-growing larger premises. Then there are the compound eyes. In both trilobites and horseshoe crabs the eyes are included as part of the head-shield, rather than sticking out separately at the front on flexible stalks as they are in the majority of crustaceans. Most of us will have looked a lobster in the eyes before popping him into the pot. The lenses of the trilobites are unique in the animal kingdom, since they are made of the mineral calcite. Hard calcite makes up the hard parts of the trilobite, providing the crusty shield that covers the back of the animal known as the dorsal exoskeleton. Calcite has also been recruited to provide the material for the lenses of the eye – so they have become ‘crystal eyes’ if you will. The individual lenses are minute in many trilobite eyes (they can have several thousand), but each separate lens presumably responded to an external light stimulus, and then an optic nerve conveyed the information to the brain. Eyes with many small lenses are usually thought of as particularly sensitive to movement: a moving image progressively impinges on different lenses within the field of view. Both trilobites and Limulus have eyes that look predominantly sideways, scouting around over the sea floor where they live. Strangely enough, the eye of Limulus has been very intensively studied. Haldan K. Hartline of the University of Pennsylvania used the eye of the horseshoe crab as his experimental material to investigate the physics of animal vision. In the 1930s he was the first scientist able to record the activity of a single optic nerve fibre attached to a lens (ommatidium). Limulus has about a thousand such fibres in the eye, and we might well imagine that trilobite eyes had at least a comparable sensitivity. He later showed how different fibres in the optic nerves respond to light in selectively different ways. This opened up the route to a whole new field of physiology – and earned Hartline the Nobel Prize in 1967. Robert Barlow and his colleagues are now building further on Hartline’s research. They have attached miniature video cameras onto living animals in order to scrutinise exactly where the horseshoe crabs are looking. The eyes seem to exhibit an unsuspected sophistication. There is apparently a natural, or circadian rhythm in the sensitivity of the ocular system, which combines with other dark-adaptive mechanisms so that their sensitivity at night may be as much as a million times more acute than in the daytime. Crabs are particularly attuned to recognising potential mates, which, given the frenetic activity along Delaware Bay, is not altogether surprising. The ability of the Limulus eye to eliminate visual ‘noise’ is quite extraordinary (think of our own faltering attempts to really see very faint stars on a dark night), and Dr Barlow is currently trying to understand how this works right down at the molecular level. It is probably the case that we know as much about the visual system of this ancient arthropod as about that of any other living creature. But the more we know the more we might wonder whether this particular survivor is primitive or just exquisitely adapted. Did the trilobites have blue blood? There is no final proof one way or the other; nor can there ever be with such perishable stuff as blood. However, there are many examples of trilobites that have been severely bitten and yet have survived. They usually show a sealed-off gouge on one side. Even in the early Cambrian there were predators such as the lobster-sized Anomalocaris and its relatives that might have regarded a trilobite as a crunchy snack. Anomalocaris was a strange, but evidently raptorial arthropod with two long grasping arms and a mouth surrounded by plates. In those days of accelerated evolutionary change natural selection would rapidly have favoured any mutation that stopped a wounded trilobite from bleeding to death, and the same would have applied to any of its relatives. Since the circulation system of Limulus, and doubtless of a trilobite, is diffuse compared with our own – it more or less fills the open spaces between the other internal organs – a general clotting agent would have been at a premium. It does seem possible that the alternative way of making blood – the copper route – could have had a very long pedigree, and that the blue ichor’s ability to seal wounds and its sensitivity to infection could have helped both trilobites and horseshoe crabs to survive in a newly vicious world. This is one of those moments when palaeontologists wish they could circumvent the rules of the space-time continuum, and go back and see for themselves. As it is, we have to make do with more or less plausible guesses, in the process trying to persuade our fellow scientists that we have undoubtedly arrived at an entirely logical conclusion. History, of course, does not necessarily have to follow our own human logic, and may have surprises of its own.
Could a scene like that witnessed at the beginning of this chapter been played out by trilobites in deep geological time? It is possible. To see evidence I must take you with me to the small town of Arouca in northern Portugal. It lies at the end of a very winding drive into the hills from the old seafaring city of Porto. The prevalence of hillsides covered with eucalyptus trees in some parts of this landscape can be depressing, as these antipodeans are out of place here; but their contribution to the local economy pushes all ecological niceties to one side. Most go to pulp for paper, for these efficient trees grow faster than native species. So in another sense these eucalypts, too, are natural survivors. Every now and then bush fires flare up uncontrollably, fuelled by the volatile oils of the ‘gum trees’; black swathes along the hillsides record their ugly legacy. In the higher hills, pretty valleys contain ancient mills and farmhouses built of crudely squared-off large blocks of the grey granite that makes up the highest, bare ground in the region. In geological terms, obstinate granite is probably the longest survivor of all. Little has happened to the face of these sensible buildings since medieval times other than a dappling of face-paint provided by lichens. On the bleak granite moors nearby are burial chambers that have seen much of human history pass, but still endure. Since the time of the trilobite, whole mountain ranges comprised of this most persistent stone have been worn away grain by grain by the inexorable forces of erosion, and rendered down to sea level. Life outlasts even mountains, for the greatest survivor of all is DNA.
Arouca must be the only town in the world with a trilobite monument, which is a tall spike sitting a little uneasily in the centre of a roundabout. The small hill town is bidding to achieve European Geopark status, and part of its claim is as the home of giant trilobites, which figure prominently on the monument. To see the real thing I head off to the slate quarries above the town near the little village of Canelas. Mining has been a part of the culture in the region for a long time. The Romans were in the hills seeking gold, and old workings excavated into tough Ordovician sandstones can still be seen atop a local high spot, where a dark and slippery stairway leads down into a ferny crevice. The same sandstones preserve fossils of burrows made by trilobites digging into an ancient sea floor, providing yet another type of treasure. Nowadays, the booty is roofing slate. A mass of Ordovician slates known as the Valongo Formation over-lies the sandstones and runs across country. The slates are nearly black, and split into flat sheets that can be further split again until they make usable roofing slabs. Once prepared this way, they are very durable commodities. The slates are extracted from large quarries by blasting out huge chunks of rock, which are then carried away to a factory for further working. The flat planes along which the slate cleaves also furnish a record of Ordovician sea floors, albeit now turned almost vertically as a result of convulsions of the earth. Every now and then a slab covered with trilobites is discovered. They are, as my Spanish colleagues exclaimed without overstatement ¡espectacular! The fossils often show up pale greyish against their dark background. Under normal circumstances it is a rare event to find trilobites much bigger than a small shoe, but in this locality they are often as big as tureens. The largest trilobite in the world, perhaps a metre in length, may be lurking among unstudied collections, but specimens 70 cm long are already familiar. Not only are they large, they are also numerous. Because whole bedding planes (which represent former sea floors) are extracted, an extensive view of a tragic moment in time is occasionally recovered: it is a fossil graveyard, with bodies laid out at the moment of death, a community of cadavers. A scenario like this could not be extracted by the usual tapping of the geologist’s hammer upon a rock: it requires activity on an industrial scale. Fortunately, the quarry owner is aware of the importance of his