Читать книгу Survivors: The Animals and Plants that Time has Left Behind - Richard Fortey - Страница 8

Turn seawards off Route 1, Delaware, and a century rolls away. A small road soon leaves the commercial strip and the fast food joints behind, as it moves onto flat fields and marshland that still supports scattered, pretty villages lined with white-painted picket fences. This is how much of America’s east coast used to be. The Little Creek Inn is a grander building altogether: a large, foursquare, wooden Victorian farmstead on three floors with shutters at the windows and a fine portico, and inside all polished wood and turned banisters. In the spacious drawing room of the Inn, anticipation is buzzing. The hosts, Bob and Carol Thomas, are serving iced tea to a crew of enthusiasts all dedicated to travelling back much further in time than a mere century or so. This is the chance to come face to face with life as it was millions of years ago. Glenn Gauvry, the local expert, is waving around models of an ancient animal. A small TV crew is there to straighten out their facts before they get down to filming. Two young women biologists have travelled from Canada to see for themselves an event that only happens when the conditions are just right in late May. I am there with my notebook and a fluttering heart. All of us are impatient for darkness to fall.

Deep in the night along the shores of Delaware Bay the horseshoe crabs are stirring. The tide is now high and there is no moon. Darkness rules, but even in the feeble starlight the overwhelming flatness of the countryside can be made out, except along the rim of the bay where old sand dunes have built up a levee providing foundations for a scattering of wooden beach houses, which loom against the night sky. A path passes between them onto a sandy beach that stretches away into the darkness in a long gentle arc. The shoreline seems to heave with gentle movements.

First, I notice some very odd sounds. There is a general hollow clattering, a tapping and grinding sound, somewhat like that made by knocking coconut shells together (once used on the radio to imitate horses’ hooves) but altogether less rhythmic, and with a kind of underlying push. Then, as my eyes get used to the darkness, low shelly mounds the size of inverted colanders can be seen slowly pushing and jostling all along the shore and perhaps six metres up onto the sands. Their bumping and clambering together is the source of those tap-tapping percussive sounds. The flash of an infrared torch reveals more details. The head-shield of the horseshoe crab is domed upwards and carries a few weak spines; at its back end a hinge marks a jointed boundary with a second large plate, spiny at the edge, which can flap downwards; and beyond that again projects a stout triangular spike as long as the head, which can waggle up and down. Here at Kitt’s Hummock more crabs are gathered on the mud flats seaward of the sands waiting their turn: strange, green-black, slowly animated lumps. Further offshore again in the shallow seawater tail spikes project briefly above the gentle waves like raised radio antennae and are gone, showing where still more horseshoe crabs vie with one another to get their place on the sand. There are evidently thousands upon thousands of these large animals gathered together in some sort of compulsive collusion.

One horseshoe crab lies upturned on the sand. Its tail spike waggles feebly, quite unable to perform the task of turning the body back over again. Five pairs of legs twitch ineffectually in a vain attempt to achieve the same end. I find it impossible to resist the temptation to right the poor animal. It is easy to grasp it by the edges of the head-shield. Once righted again those spindly legs allow the crab to trundle slowly away. Its behaviour seems at once strangely determined, but also apparently random, like the slow progress of a confused old lady on a Zimmer frame.

Now I see that many of the largest crabs are digging in the sand, their limbs working away beneath the carapace. Some have become almost completely buried, and, although I can detect a kind of deep scrabbling from these animals, they do not seem to be worried by their self-inflicted interment. Other slightly smaller crabs crowd on top of the buried animals. The scrabblers are the females of the species burying their eggs in the sand, while the smaller ones on top are males, competing to fertilise the eggs with their sperm (milt). I realise that there is some kind of order to the apparent mayhem on the beach. A proportion of the horseshoe crabs are paired off, with the lighter male desperately hanging off the tail end of the female, having got a purchase by using his special claspers. However, this right of occupation does not deter other males from having a go at mounting the same burdened female. There is enough of a gap behind the head-shield for some of their interloping milt to have a chance. Much of the clinking noise is a consequence of tussles for dominance. So this gathering of crabs is really an orgy, and an orgy that runs for dozens of miles along the strand, all thickly bordered with scrabbling, lustful animals. As for the poor exhausted females, gravid and overprovided with mates, the moist sand stops their gills drying out, and they may eventually struggle back to the sea when the laying is done – although many do not. Bits and pieces of their carcasses litter the shore.

I have a better chance to scrutinise the horseshoe crabs closely during the day, although most of them have returned to the sea by sunrise. Coastal Delaware is a land of marshes, with gentle wetlands dominated by the reed, Phragmites, and the cries of wading birds always in the wind. The landscape reminds me of the East Anglian coast in England. Creeks wind their ways inland from the sea, and terminate in small picturesque harbours like Leipsic, where a few fishing boats are tied up to stout piers, with white-painted clapperboard houses landward of the stage. Sambo’s is a restaurant with a view of the creek, and well known for its edible crabs, which are consumed on simple tables covered with newspapers. Eating in Sambo’s is an audible experience with everybody bashing lunch out from shells. It is a place of crunching and squishing and little conversation. Some of the shucked piles are prodigious. There is nothing on the menu about horseshoe crabs. The nearby villages of no more than one or two streets include neat little houses dating back to the 1880s, which is ancient by American standards. Delaware car number plates bear the legend ‘The first state’, acknowledging the fact that it was the first to sign up to the Declaration of Independence. Like several other early American states, but unlike the majority, it is tiny. Nowadays, vehicles on the main roads shoot past; but a mile or two from the freeways little has changed from post-colonial days. I warmed to it immediately.

After the crustaceous lunch, a visit to Port Mahon shows a few stragglers still on the beach at midday, providing a chance to get close. A large female’s carapace is about 45 cm across. In the sunlight I can readily see that the creature has nearly semicircular eyes set to either side in the midst of the head-shield, topped by sharp spines, rather like the perky eyebrows I associate with clerics of a certain age. Under high magnification it would be apparent that the eyes are composed of many tiny lenses – they are what are known as compound eyes, similar to those of houseflies or bees. The whole animal is a dull pinkish-to greenish-grey colour, the kind of colour I used to get as a kid when I mixed all my powder paints together. The front of the head-shield is subtly bowed upwards about the middle. The tail (or telson) has a triangular cross section, and it makes a stoutly elegant termination to the animal. The middle part of the body is defined on its top surface as a kind of convex median lobe over about half its length: this is where the muscles that power the legs are lodged. The leading edge of the head-shield is thickened into a prominent marginal rim that is prolonged backwards into short spines; this part of the body needs to be strong to butt into the sands and mud that line the floor of Delaware Bay. On the shore there are several beached crabs lying on their backs, waving their legs at the sky. They bend almost double along their middle hinges, but their best efforts still fail to turn them over (it would be different under water). If they remained on their backs, greater black-backed gulls would soon come along to peck them to pieces. Before setting them aright, I have a chance to see how delicately each of the paired jointed limbs under the head-shield carries a set of pincers at its tip. I am reminded of the manual toolkit owned by the eponymous hero of the movie Edward Scissorhands. They are indeed picky little tools. Nearer the front end of the head-shield, where the carapace is doubled back from the top surface into a blunt point, a very delicate set of pincers at the centre of the animal and close to the mouth, looks just the thing to feed titbits towards the innards. The bases of the legs are really quite stout and equipped with blunt spines that face one another along the midline of the animal: they can be used like nutcrackers to crack shellfish if needs be. I begin to understand how these creatures can grab a living from the waters of Delaware Bay. Behind the legs are a few pairs of flattish flaps that cover up intricately folded book lungs. Like every marine animal the horseshoe crab needs to breathe dissolved oxygen, and as long as this breathing apparatus can be kept moist under its protective covers the crab can survive on land. Hence the female can endure her risky excursion to lay her eggs in the sand. The shore may be an unwelcoming nursery, but might still be preferable to a sea where every cubic metre holds a thousand twitching antennae sensing free food. It is time to turn our crab over to allow it to trundle away. It heaves itself along like a battered tank: slowly and undignified, as if to signal ‘I have survived endless battles, and survival is all’. As it performs its lurching exit to the sea, it leaves a track behind on the muddy sand surface. The paired imprints of the limbs are prominent; even the tips of the pincers leave their doubled marks. And the tail, dragged behind, leaves a groove between, as a child might scribe with a stick clumsily trailed across the strand.

Horseshoe crabs are not really crabs at all; indeed, they are only very distantly related to crabs in so far as both kinds of animals propel themselves through the sea on spindly jointed legs. Animals with useful appendages of this articulated kind are known as arthropods (from the Greek: jointed legs). They are classified together in Phylum Arthropoda, a vast animal group that includes all the living insects, as well as spiders, millipedes, and a host of marine ‘bugs’ of all kinds. Crabs are crustaceans, along with lobsters, shrimps, and woodlice (pillbug to some). Horseshoe crabs are no more crustaceans than are butterflies. They do not have the flexible antennae or ‘feelers’ adapted to sensing the environment that are a common property of Crustacea and insects: these delicate organs both feel and touch, and smell. Instead, in the horseshoe crab the head appendages are modified at the front into a pair of useful pincers, or chelicerae, which I had observed in my stranded animal lying on its back. The significance of this apparently small feature will become apparent. The scientific name of the horseshoe crab is Limulus polyphemus (I shall need to use scientific names throughout this book). By day the beach throngs with feeding wading birds: thousands of them skitter nervously away from human intruders in animated, piping, fluttering waves, always beyond reach. Like most waders, many of them are dressed in shades of brown and grey, but the different statures of several species are obvious even to an inexperienced birdwatcher. Small, short-billed sandpipers throng on short legs; slightly larger pale-bellied sanderlings dash along the water’s edge; taller, long-billed dowitchers elegantly stride among them. The iconic species for the area is the red knot, which has a dramatic cinnamon-coloured belly when in breeding plumage. All wader species – and there are many more in the crowd – are united in rapt attention along the shoreline, pecking and probing incessantly at the ground, like chickens fed in a yard on the best grain. They are undoing the work of all those heaving masses of horseshoe crabs the previous night, gorging on the green, millet-seed-sized eggs the female crabs sought to sequester beneath the sand. For the red knot the eggs provide vital refuelling, as this particular population started its migration near the tip of South America. By the time they arrive in Delaware Bay on their way to the Arctic the birds may have lost half their bodyweight and they are starving. The crab eggs must taste like the best caviar. The birds would not survive without those countless horseshoe crabs performing their spectacular mass mating ritual. These inelegant invertebrates are completely unaware of the gift they are providing to an animal many millions of years their evolutionary junior.

Birds always attract devotees, and naturalists’ concerns for the welfare of the red knot probably accounts in turn for their anxiety about the state of the horseshoe crab population. If that were to fail, then so would the long migration of the attractive waders. A recent census estimated that there could be as many as seventeen million horseshoe crabs in the Delaware Bay area, and that concerns about their decline may have been exaggerated. Since the horseshoe crab has a range that extends along the shore north to Maine and south to the Yucatán Peninsula in Mexico the population is assuredly larger still, although the densest concentration of individuals is probably where I saw the heaving multitudes at Kitt’s Hummock and Pickering Beach. Delaware Bay is also where the mature crabs grow largest at maturity. Since there has, indeed, been a decline in red knot numbers the cause must lie somewhere else in its complex migration story. The weakest link in an ecological chain is always the critical one.

It had always been a dream of mine to see throngs of jostling horseshoe crabs reach the climax of their life cycle. For more than three decades at the Natural History Museum in London I studied fossils of trilobites. This once important group of sea animals went extinct something like 260 million years ago, when the world was a very different place. Trilobites had once swarmed in all the ancient oceans, but now their remains have to be patiently collected by splitting open the rocks that have entombed their shelly remains. Like horseshoe crabs, trilobites are arthropods: animals with jointed legs and all the muscles and tendons tucked inside an exoskeleton. However, unlike horseshoe crabs, trilobites did not survive the mass extinctions that redesigned the biological face of our planet. It is astonishing to learn from unchallenged fossil evidence that relatives of Limulus were contemporaries of trilobites. That nocturnal scrimmage on the beach in Delaware might have happened many millions of years before; I might even have been listening to sounds that had been rehearsed in Palaeozoic times. There were relatives of the horseshoe crabs in the sea long before other arthropods, such as insects and spiders, had ventured onto land, or before crustaceans – shrimps, crabs and lobsters – had taken up the central roles in the ecology of the ocean they enjoy today. So it would not be incorrect to describe the animals thronging along Delaware Bay as primeval. Indeed, many scientists believe that Limulus is the closest living relative of trilobites themselves. Would the head-shield of the giant Cambrian trilobite Paradoxides, a fossil 510 million years old, have felt the same to the touch as the beached Limulus I restored to the sea in eastern America early in the twenty-first century? Like a horseshoe crab, a trilobite would surely have contemplated me through compound eyes set within its head-shield; its eyes are preserved in detail as fossils. Trilobite legs would have scraped against my mammal flesh with just the same spikiness as Limulus. It would have crept and it would have crawled, brother under its external skin to the hordes on Delaware Bay.

1. A Silurian trilobite, Calymene blumenbachii, from the limestone quarries of Dudley, West Midlands, England.

So a visit to Delaware is to me rather like a visit to the holy city of Rome to a Catholic. Naturally, I had to meet the Pope. The Pontiff of horseshoe crabs is Carl Shuster, in his tenth decade still a giant of a man: craggy faced, walking without the aid of a stick, with lively eyes beneath towering eyebrows, memory and curiosity undimmed, and only betraying his years by his deafness. Like all field biologists he wears a coarsely chequered thick shirt and blue jeans, hitched up with a stout belt. He was brought up during the Great Depression, when he had to run a farm, so he is himself a survivor, like the horseshoe crabs to which he has devoted his life. His father was a mathematician who gave succour to penniless intellectuals during the tough years, while young Carl raised asparagus, chickens and strawberries. He brought together all the current knowledge about his favourite animal in his book The American Horseshoe Crab. He is accompanied in Delaware by his former student, a man of boundless enthusiasm, Glenn Gauvry (himself an implausibly youthful sixty years), who coordinates much of the research on Limulus around the Bay area. Those volunteers who help with counting the crabs during their nocturnal orgies receive a handsome little pewter lapel pin as a record of their collaboration in the conservation project. Naturally, the pin features a horseshoe crab. It signifies membership of one of the more exclusive clubs in the world.

Carl Shuster and his colleagues established the biological facts about horseshoe crabs that allow naturalists to understand how they fit into the ecology of the Atlantic coast. Limulus is a typical arthropod in that it must moult in order to grow, shedding its old coat as a kind of pale ghost of its former self, and growing a new and larger external covering. With a little vigilance it is possible to find one of the cast ‘shells’ on the beach: they are almost as light as tissue paper, for the animal recycles what it can. The newly emerged horseshoe crab is capable of moving immediately. In this it has the advantage over other marine arthropods in the area; freshly moulted blue crabs, for example, are virtually motionless until their new ‘shell’ hardens. They often hide away. Younger horseshoes resemble the older ones apart from being a little spinier. The surprisingly tough, but flexible exoskeleton is made of a chitinous material, similar to that forming the wings of beetles. A typical horseshoe crab takes ten years to reach sexual maturity, after which it does not moult again, but heads to the beach for reproduction. Only these mature animals partake in the littoral orgy, which is why one does not see any little limulids scuttling between the adults. They are not demanding animals: a fully mature animal might go for months without eating. When her time comes, a female may well lay 80–100,000 eggs, and enough of these survive the depredations of wading birds to secure the future generations. The greenish eggs are laid in the sand in batches of four to six thousand in spheres about the size of a golf ball; the female makes repeated visits on successive tides to complete her duties. Females can be recognised by the scars left behind by the mounting males: up to fifteen males may have a chance of fertilising the eggs of any female. Nonetheless, only about thirty-three eggs out of a million survive to adulthood. This means that at various stages of its life Limulus provides a lot of food for other animals. Loggerhead turtles are an important predator on the crabs, even when they are adults. Turtles, too, are animals with a long geological history, so they may have had eons to make something nutritious out of horseshoe crabs, which now seem all sinew and horn and little enough meat. The crabs themselves can survive on molluscs and carrion and almost any kind of scraps, but the strong inner parts of the legs can also crush thick shells if needs be. Although they seem to lurch in an ungainly way on land, under water the crabs are more streamlined and can move quite fast, even sculling on their backs. They can easily right themselves if they need to. In short, they are tough, jack-of-all-trades kind of creatures, built to last. They remind me in a way of a Volkswagen Beetle that I once owned (a beetle being another arthropod, of course) that carried on carrying on even though its coach-work was full of rusty holes, its suspension was down almost to the tarmac, and it often fired on only three cylinders.

This analogy particularly came to mind when I saw a badly damaged horseshoe crab still trundling gamely onwards, even with a great hole punched right through its head. Looking over the beach more carefully there seemed to be a lot of these war veterans: lumps out of the thorax, broken tail spikes – clearly, it must take a lot to finish these creatures off. Glenn Gauvry pointed out to me what a great advantage for reproductive success this resilience would furnish. Such endurance is possible because the blood of Limulus polyphemus has exceptional clotting powers; the animal does not bleed to death because its blood coagulates and ‘walls off’ damaged areas. And the blood is blue. Does not the horseshoe crab begin to seem ‘curiouser and curiouser’, as Alice would have said? The blood of Limulus is blue because it is fundamentally different from that flowing in red-blooded creatures, like you and I and the kangaroo. Whereas we have haemoglobin as our oxygen-carrying pigment, which includes the element iron as an indispensable component, the horseshoe crab carries a copper-based molecule called haemocyanin to do a similar job. In nature, copper often comes with such a blue colour tag. The molecular structure of both these vital molecules is now known in detail, as is the way they move oxygen through the tissues, although this is not directly part of our story. But the Limulus narrative would not be complete without exploring the extraordinary coagulating properties of its blood a little further, because this affects the very survival of the species.

This discovery was made in 1956 by Fred Bang of the Marine Biological Laboratory at Woods Hole. He noticed how Limulus blood clotted dramatically when infected by a particular bacterium. Subsequent research showed that the crab’s blood had an extraordinary sensitivity to a vast range of micro-organisms that are found almost everywhere in nature – known as gram-negative bacteria. A few cubic centimetres of seawater may contain hundreds of thousands of these tiny organisms. Since some of these bacteria are also agents of disease in humans, this property was of immediate interest. It seems that a hypersensitivity to microbial enemies helps to protect the crabs in their natural habitat – as soon as the bacteria enter a wound their defences were up. Limulus has a very diffuse blood system compared with ours, with the interior of the animal bathed in blood, and lacking the defined circulation of veins, arteries, and capillaries we are used to in humans and other vertebrates. In horseshoe crabs the job of defending the works of the animal is given to one type of protective cell, the amoebocyte, which contains a mass of granules capable of promoting clotting. When a gram-negative bacterium is in its vicinity an amoebocyte will react by rupturing and then the granules are released. A clot follows and the infection is sealed off. Now we know why dented and holed crabs can totter on regardless. They have had hundreds of millions of years to come up with an effective response to some of their most dangerous and invisible enemies.

Twelve years later in 1968, Bang and his colleague Jack Levin had managed to prepare and extract the active principle ‘Limulus amoebocyte lysate’ (known as LAL) that clots human blood plasma when exposed to gram-negative bacteria. This is an extremely useful substance in medical diagnosis, readily used for the detection and measurement of the poisonous toxins (called endotoxins) belonging to the appropriate bacteria. Poisonous endotoxins are released into the host organism (you, me, or a horseshoe crab) when the bacterial cell wall ruptures. LAL is a highly sensitive chemical able to detect minute quantities of the offending substances. The LAL test is now widely applied, having been sold on to pharmaceutical companies for commercial manufacture. It has to be prepared close to the Limulus populations, but is then exported around the world. This means that there is a tremendous demand for the horseshoe crab’s blue blood. What had saved it from harm for millions of years now made it a desirable commodity.

The influence of this new industry has been the subject of some debate. Carl Shuster told me that recent estimates say that there may be as many as seventeen million adult crabs in the Bay. By 2003, some three million crabs were harvested for the pharmaceutical trade – an unsustainable quantity. The bird people were worried about the fate of the red knot and its fellow waders. Now the number has been reduced, and the technique for obtaining blood has been adapted so that it does not require the animals to be killed: they have become blood donors! Four companies have the right to what is called ‘bleed and release’, and this method has been applied to some 600,000 crabs per annum. Maybe it is possible to get the LAL and not threaten the horseshoe crabs, after all. Meanwhile, some ornithologists are sceptical, and feel that the companies, as beneficiaries of the local species, should put something back into safeguarding the regional marine habitat for the benefit of all parties, crabs included. Given the long time taken for the crabs to reach sexual maturity there is always a lag before the effects of a population slide can be observed when the summer high tide hits the beach. But there are certainly some very vigilant people on the case.

There was another threat to the welfare of the horseshoe crabs. They were harvested in great numbers to use as bait to catch the giant marine snail or whelk, known up and down the Atlantic coast as conch (Busycon). Big, pinkish conch shells are a familiar item in every seaside knick-knack shop; put to the ear they allow the listener to ‘hear the sea’. Roadside stalls sell the shells for a modest price in the small villages around Delaware Bay. The molluscs inside the shells have a loyal following among connoisseurs of seafood. The only time I tried them I found the flesh very tough. A crab split in two is a pungent treat for the big snails, however, and fishermen along the coast from Delaware to New Jersey are well aware of the power of this lure. The US Fisheries Commission limited the number of crabs to be used to 100,000 in New Jersey, and later introduced a moratorium, but some fishermen moved northwards to Massachusetts or even Maine (the northern limit of the crabs’ distribution) to continue their trade. A method of sustainable fishing must be worked out, and there are hopeful signs. On Delaware Bay, for example, the use of a different kind of trap employing mesh bags has cut bait use by 50 per cent. One cannot help feeling that the horseshoe crabs deserve to prosper unhindered. There may seem to be endless crowds of them jostling for a space on the sand, but the example of the American passenger pigeon comes to mind: countless millions of the birds were slaughtered in the nineteenth century until the species finally became officially extinct in 1914. How dreadful to contemplate the thought that an animal that has survived in readily recognisable form from the early days of the dinosaurs might become extinct in the cause of being a special garnish on a plate of fruits de mer. It is fortunate that its blue blood is so valuable.

Limulus polyphemus is not alone. There are three additional, Asian species of living horseshoe crabs, but none of them can be found in anything like the profusion of the North American form. Tachypleus tridentatus has been given protected status in Japan, where it lives on the Seto Inland Sea. Its shallow-water habitat there is under threat, and increasing industrialisation on the Seto Sea seems unlikely to spare it. Various attempts have been made to conserve the crab, but none of them has solved the problem of making this ancient animal at home in the modern world. Nonetheless, it has an important place in Japanese history, as its form is believed to have inspired classical samurai masks, and brave warriors were supposed to be reborn in the guise of horseshoe crabs. The contemporary artist Takeshi Yamada has made a modern interpretation of masks based upon the ancient icon. A related species, Tachypleus gigas, is fairly widespread in eastern Asia. I saw the fourth species, Carcinoscorpius rotundicaudatus, while I was on fieldwork in Thailand more than a decade ago. As its second, species name would indicate to those with a smattering of Latin, this particular type has a rounded tail spike compared with other horseshoe crabs. When I first set eyes upon Tachypleus, the poor crab was in one of those tanks in a restaurant where delicacies are displayed before consumption, along with several sad-eyed fishes and a dying lobster. I was puzzled by what there could possibly be to eat on the horseshoe crab, because I knew from dissecting its American relatives that they are not exactly meaty. If the species in the tank was indeed a relative of trilobites this might be my first and last chance to taste something resembling my own speciality. When the cooked item finally arrived I felt a twinge of conscience, for it transpired that large, yolky eggs hidden under the head-shield provided the delicacy, which was only available while the gravid females came inshore. I felt that I was consuming the next generation just to satisfy my curiosity. The eggs came served in a thin sauce on a mountain of noodles. They had a rather overwhelming rancid-fishy taste; I am not anxious to repeat the experience.

2. A growth series of juvenile Tachypleus tridentatus, the Asian species of horseshoe crab, collected from tide strand lines in Deep Bay estuary, Hong Kong. (The scale is a 15 cm/6 in ruler.)

As for the place of the horseshoe crab in the tree of evolution, the Latin name of the horseshoe crab I saw in southern Thailand might offer a clue. Carcinoscorpius means ‘crab scorpion’, and that is what these curious creatures are: superficially crab-like relatives of the scorpions. They are the most primitive living examples of a great group of arthropods that includes all living spiders and mites as well as scorpions, and several less familiar kinds of animals. This group is referred to as chelicerates, after those special appendages – chelicerae – located on the head-shield, which they all share in one form or another. All the relatives of the horseshoe crab I have mentioned live on land. But the cradle of life was the sea, and Limulus and its relatives take us back to the far, far distant days when the land surface was barren of larger organisms.

In the darkness along Delaware Bay the scratching percussion of the crabs provides an unmusical accompaniment on an imaginary journey backwards in time: to an era well before mammals and flowering plants; a time before the acme of giant reptiles, long before Tyrannosaurus; backwards again through an extinction event 250,000,000 years ago that wiped nine-tenths of life from the earth; and then back still further, before a time of lush coal forests to a stage in the earth’s history when the land was stark and life was cradled in the sea; a time when a myriad trilobites scuttled in the mud alongside the forebears of the horseshoe crabs. The trundling, heaving, inelegant not-so-crabs along Delaware Bay are messengers from deep geological time.

A palaeontologist would naturally want to track the history of the horseshoe crabs back into the distant past. A few years ago I visited the famous quarries in Bavaria, southern Germany, where the Solnhofen Limestone of Jurassic age, 150 million years old, had been excavated. Great opencast pits scour the gently rolling countryside revealing thin slabs of cream-coloured limestone, where each bed represents the former sediment surface. The limestone provides the perfect fine-grained stone for the manufacture of lithographic printing plates; this is still a popular medium with artists today, but had an even greater use in the past for graphic illustration. The Germans called this kind of rock ‘plattenkalk’, which is an appropriate name because if a fossil turns up it will be laid out on the surface of the slab like a fish on a very flat plate; and some of the fossils are, indeed, those of fishes. The most famous fossils from the Solnhofen Limestone are skeletons belonging to the early bird Archaeopteryx lithographica, complete with feathers, but they are very rare – only one turns up in an average decade. Some other fossils are quite abundant, like those of little sea lilies (Saccocoma). The Solnhofen Limestone is thought to have accumulated in a warm lagoon, or a series of lagoons, not far from a biologically diverse land habitat, but with periodic influxes of waters from the sea. From time to time the lagoon became salty enough from evaporation to poison living organisms, and its deeper parts were depleted in oxygen sufficiently to deter scavengers. The result is the outstanding preservation of delicate animals. When sticky mud was exposed, animals could get trapped upon it, such as delicate flying reptiles or dragonflies. Operating together, these special conditions preserved a huge cross section of Jurassic life. One of these animals is a horseshoe crab called Mesolimulus walchi. It really is remarkably similar to our living Limulus polyphemus. At first glance it looks as if it had just wandered in from Delaware. One has to look hard to notice that its marginal spines are longer than in our living blue-bloods, and there are a few other minor differences. Nobody could doubt that this species, too, trundled through the shallows, nor that it carried its eggs under its head-shield. To that showy new upstart – a feathered bird – it may already have seemed archaic.

Up to this point I have avoided describing the horseshoe crab as a ‘living fossil’. This is not only because I am chary about using a phrase that is a paradox and an oxymoron rolled into one, but also because it is a misleading description. Charles Darwin himself was cautious when he introduced the term in the phrase quoted at the start of this book. Despite what I have just said about Mesolimulus it is not exactly the same as Limulus. Consider everything we have learned about our living horseshoe crab. It is woven deeply into an ecology that is utterly different from that in the Jurassic. Millions of birds of many species depend on the horseshoes’ eggs every year, whereas its old relative was probably irrelevant to the life cycle of what is often called ‘the first bird’. Limulus has adapted to many changes of circumstances: new predators, new climates, and now humankind. It is a winner in the lottery of life, and not just because of its long family tree. ‘Living fossil’ seems to imply a negative judgement somehow, as if the poor old organism was just about tottering along on its last legs, having hardly changed in tune with a changing world, awaiting an inevitable end. A similar misplaced judgemental tone is often applied to dinosaurs. ‘We mustn’t be dinosaurs! We must change with the times!’ is a mantra of commerce. The dinosaurs were actually superbly efficient animals, and their extinction was most likely a combination of external factors (a drastic meteorite impact is favoured by many) that had nothing to do either with their virtues or lack of them. They were animals of the wrong size living in the wrong places at the wrong time. Bad luck! Meanwhile, the living fossils trundled on through the crisis because … well, we will come to that.

Modifications are happening at the genomic level all the time. There really is no such thing as ‘no change’; the very flexibility of the DNA molecule is what has kept natural selection on its toes for thousands of millions of years. Nor is change in DNA necessarily related directly to any change in the appearance of an animal. Many mutations accumulate in the large fraction of the genome that apparently does not do much work in the specification of proteins, or initiating developmental changes, or any of the other vital, active stuff. These mutations might well be irrelevant to the kind of changes in shape or colour that indicate the appearance of new species. A living fossil may indeed have accumulated many changes at the molecular level that have not even been expressed in its surface appearance, which is the phenotype that has to face the world. Fluctuations in gene frequency are the stuff of life, but they don’t map one-to-one on skeletons and limbs, which are the usual stuff of fossils. So a little caution in terminology is wise.

There is also a temptation to think of the living fossil as if it were a true, surviving ancestor. When the coelacanth fish was discovered it was presented in the popular press as ‘old fourlegs’ as if it were just about to march onto land on its stumpy fins as a thoroughgoing tetrapod. Not only does this scenario happen to be wrong, but the likelihood of any such ancestor surviving unchanged to the present day through many millions of years is also exceedingly remote. Time, chance, and competition will see to it that change is inevitable. What can be said without demur is that the ancient survivor and its other living – and more evolutionarily advanced – relatives will have shared a common ancestor, and that the features of the living fossil will be closer to those of that ancestor. The discovery of ancient fossils more or less similar to the survivor will date the appearance of the whole animal group to which they belong, and point up the changes that must have happened through geological time along the subsequent branches of the evolutionary tree. The survivors from the early days carry with them a package of information revealing primitive morphology, development, and biochemistry that can illuminate histories that would otherwise be hidden from us. Fossils never preserve blue blood. The ‘living fossils’ may not be the ancestor, but they are survivors carrying a precious legacy of information from distant days and vanished worlds.

Hence Limulus allows us to understand something about deep branches in evolution. It is far from unique. If every descendant species had simply replaced its predecessor, the history of life would be like one of those patients described by Oliver Sacks who live perpetually in the present day, constantly erasing the memories of yesterday. Fortunately, life is not like that. Deep history is all around us. In the life of the planet, the latest model does not always invalidate the tried-and-tested old creature. Groups of organisms that originated long, long ago, in very different worlds, have been able to evolve and adapt alongside their more recent cousins and second cousins. The story of life is almost as much about accommodation as it is about replacement. To look at a living horseshoe crab is to see a portrait of a distant ancestor repainted by time, but with many of its features still unchanged. This book reflects my interest in living survivors from the geological past and what they can tell us about the course of evolution. I have spent the last few years seeking out animals and plants that have helped to illuminate our understanding of the history of life. Wherever possible, I have visited these organisms in their natural habitats; none has proved less than fascinating. Observing how they survive today has allowed me a glimpse of their biology and provided clues about the reasons for their longevity. I have carefully selected the old timers I visited because I wished to understand their biology in depth; I have had passing encounters with several more. A few organisms proved too rare or inaccessible for me to discover personally – the coelacanth comes to mind – and then I have relied on the accounts of others. I shall relate many of these case histories to those of their fossil relatives, which is only to be expected of a palaeontologist. This will illuminate the vital fourth dimension – time. I soon discovered that there were too many potential candidates for inclusion, and I am obliged to mention some of them only briefly. I believe it is better to deal with a smaller number of organisms in detail than swish around vaguely with a broad brush. My specialist friends will probably complain that I have left out their particular favourite beast or weed, and my answer is that these survivors have lasted so long that they will almost certainly still be around for someone else to champion in the future.

Consider scorpions, for example. In some ways they are as impressive as horseshoe crabs as survivors. I have met them several times in my fossil collecting career, usually hiding beneath a log or a rock, for many of them stalk their prey at night and stay out of sight by day. In the Oman desert I once disturbed a huge, black knobbly scorpion that came running at me with its tail-sting erect, while I backed off in the opposite direction, gibbering foolishly. My Omani companions laughed and told me that its sting (in the tail of course) was relatively mild. A few hours later I lifted up a rock slab and nestled beneath it was a small yellowish scorpion with a flattened, side-wound sting. I was about to poke at it with my geological hammer when my companions tugged at my arm. ‘Don’t touch, it’s a real killer!’ The smaller, insignificant-looking creatures can often be the most deadly. The spike on Limulus’ tail and the sting on the scorpion’s are closely related structures, and indeed both animals belong to the same great arachnid group. The scorpion learned the trick of arching over the very end of its body to dart poison into enemy or prey. Encased within an external skeleton (cuticle) that prevents evaporation with outstanding efficiency, the scorpion has been able to live far away from water; some species specialise in surviving in the driest places on earth. Scorpions started out as sub-aqueous creatures like Limulus, and only later did they acquire the skill of living on land. Back in Devonian times, 400 million years ago, their relatives, the sea scorpions (eurypterids) were the largest invertebrate predators ever to have lived, some as long as a man. There are fossils from the Carboniferous age that really do look like living scorpions, at least to the non-specialist. They, too, have faced out extinction events that have blasted greater and more glamorous animals. The scorpion is built into mythology as a sign of the zodiac; it features on Roman mosaics, and in the Bible (1 Kings 12:11): ‘my father hath chastised you with whips, but I will chastise you with scorpions’. So one could argue that the scorpion’s connection to human culture is more pervasive than that of the horseshoe crab. My choice of organisms has been guided by the place they occupy in the tree of life, rather than by their innate charisma or significance in folklore and culture. The horseshoe crabs earn a special place in this natural history because their relatives root down to the beginning of the diversification of animals. The early Permian Palaeolimulus is clearly a horseshoe crab, for all that it predated the great extinction that put paid to most species at the end of the Palaeozoic Era, 250 million years ago. Limulitella is present in 242-million-year-old (Triassic) strata dating after the great trauma, evidence of their survival. Those distinctive tracks left by the tips of the legs, and the trail of the tail, that I observed in Delaware have been found as fossils even in the absence of the body itself. There were horseshoe crabs crawling among the coal swamps of the Carboniferous (Pennsylvanian), a little better segmented perhaps than the beasts on Delaware Bay, but carrying the distinct signature of their ancestry. In 2009 my Canadian colleague, David Rudkin, announced the discovery of the oldest typical horseshoe crab in rocks of Ordovician (approximately 450 million years) age, thus taking these simple arthropods back before all the major extinction events that have rocked the Phanerozoic biosphere. Whatever the magic ingredient for survival is, the horseshoes clearly have it in spades. ‘The meek shall inherit the earth’ may be an appropriate motto for their longevity.

3. Gustav Vigeland’s sculpture of a survivor, the scorpion, Frogner Park, Oslo, Norway.

Let me describe these early days in more detail. I have already remarked that the horseshoe crabs had set out upon their distinctive path before the first land plants had advanced upon harsh and barren shores; although recent discoveries suggest that a few simple plants may have already ventured onto mud flats. These had probably not yet been followed by insects or spiders. There were fishes already, of a primitive cast, but they had no ambitions then to invade the land, although a few species may have nudged into waters that were not fully marine. The seas abounded with trilobites, which occupied every ecological zone from shallow shores to ocean deeps. These prolific arthropods must have been many times more abundant than the early relatives of the horseshoe crabs. They evidently had an advantage at the time. This may have been the evolution of their robust dorsal ‘shell’ of calcite, which allowed them to develop spines, armour and a tough anchor for muscles, as well as an ability to roll up into tight, impregnable balls when threatened. Trilobites soon learned an array of different feeding habits; some were predators, some ate soft mud, others swam in the open seas. They died out some 255 million years ago. By contrast, the relatives of Limulus may have stayed conservatively on the sea floor as scavengers and predators. The horseshoe crabs on Delaware Bay have an exoskeleton of chitin, which is a natural polymer that is quite tough and flexible, although no substitute for stony calcite. But the horseshoe crab has turned what might have been a weakness to advantage by developing an exceptional immune system. Survival in the long term may depend on more subtle features than armour alone.

It is possible to trace the horseshoe crab story still further back, into the Cambrian Period more than 500 million years ago, to a time when many of the major types of animals converge towards their common ancestors. The Cambrian was an interval of unprecedented evolutionary activity and I shall describe its special features in more detail in the next chapter. Early relatives of Limulus have been identified in Cambrian strata, but they include some species that look a little different from their hardy survivors. Some of them also look more like early trilobites, such as Olenellus, a form with a big head-shield surrounded by a narrow rim; one might expect a family resemblance if they are indeed closer to a common origin. There are important differences, too. Where the limbs of trilobites are known, they are similar all along the length of the animal: the paired limbs are each split into a walking leg carrying a comb-like branch near its base that in all likelihood functioned as a gill. They are not subdivided into different ‘packages’ in different parts of the body separating walking and feeding appendages in front from gills behind, as they are in Limulus. To add to this, all trilobites had typical ‘feeler’ antennae near the front of the head, and none had the strange chelicerae. This may not be so important, since having antennae seems to have been a general property of primitive arthropods. It might just be that the trilobites still retain this one characteristic more primitive than Limulus and its allies, but they could yet have descended from a common ancestor. Trilobites are abundant fossils on account of their easily preserved calcite hard parts. Fossils of unmineralised animals are altogether more unusual. Spectacular recent discoveries of fossils of soft-bodied animals preserved within Cambrian rocks have been made in China and Greenland. These have revealed an almost embarrassing variety of undoubted arthropods early in the Cambrian. Some of them might seem to bridge the differences between Limulus and its allies and the trilobites, but for every feature that points one way, there seems to be another that suggests something else. For more than a decade now palaeontologists have argued about how these fossils should be classified, and about the only thing they all agree upon is that the Cambrian threw up many animals with curious combinations of characteristics that were probably winnowed out by subsequent evolution. It is not, perhaps, so surprising that ‘mixed up’ animals lived at this Cambrian time, because all the arthropods were not genetically far apart then – they would have had the subsequent 500 million years to box themselves into more separate evolutionary compartments. In these early days the destiny of one animal to become a crustacean, say, and another a chelicerate was not easy to anticipate.

When scientists are confronted by conundrums of this kind, they usually turn to computers. There are now sophisticated computer programs that deal with the problems of determining relationships between animals. They work by identifying the particular arrangement of the creatures analysed on a branching tree that most succinctly accounts for the features they share with their fellows. The most significant resemblances in morphology should result in organisms being classified together on a single branch. Like so many computer methods, the inner workings of the process are staggering in their complexity, so that for a big problem like analysing the Cambrian arthropods millions of potential arrangements of trees embracing the animals under study will be inspected and rejected. My own appreciation of what goes on inside these machines is thoroughly naïve, and I cannot suppress a vision of thousands of cards being shuffled into piles like a supercharged game of Patience until the answer ‘comes out’. The end product is a diagrammatic tree (technically, a cladogram) that can look enticingly simple. I should add that the way the summary ‘tree of life’ on the endpapers is drawn is not like a cladogram, but it does incorporate the results of many individual cladistic analyses. Like all computer methods, the latter are subject to the familiar caveat of RIRO (Rubbish In Rubbish Out), but the fact that they have been so widely used indicates that they have helped with thorny 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 slates in exposing a sea floor perhaps 470 million years old; time enough to erode three mountain ranges, granite and all. Many of the trilobites are preserved on site in a private museum that the owner has generously dedicated to conserving these fossil remains. A dozen different species are on display there; to one in thrall to the past like me it is an extraordinary experience to see these grey bodies covering every wall. It has something of the feel of a picture gallery, and I have to remind myself that these were once scuttling animals as intent on their business as any living horseshoe crab.^*

Many of the large slabs show assemblages of just one species together, and the individuals are all large and of similar size. They are often complete bodies, which suggests they are entire animals that have been killed rather than, say, the ‘cast-offs’ left behind after moulting, when bits and pieces might be expected, arranged higgledy-piggledy. There are examples where the bodies partially overlap. All this is very like the mating congregations of horseshoe crabs along Delaware Bay. Imagine if some catastrophe had killed and preserved the crabs at the height of their nuptials. A volcanic eruption might fit the bill; this would bury and kill the animals at exactly the same time. After eons passed the sediments would have hardened into rock and the crabs would be fossils; compaction of the sediment would also have flattened down the buried beasts. Some fossil specimens would still partly overlap their partners, frozen in the act. The younger animals would have been elsewhere, so they are not represented among the fossils. It is a plausible scenario, even a tempting one. We have to add an extra complication, because something additional had happened to the Portuguese trilobites during their long sojourn in the rock. The whole mass of slates of which they have become a part has been squeezed in a tectonic vice that has twisted some specimens out of true until they look a little lopsided. Others have been stretched somewhat, and as a result claims about the ‘longest ever trilobite’ have to be treated with caution.

A more critical examination of the evidence identifies some important differences between Delaware Bay today and Ordovician Portugal. The most obvious of these is that the giant trilobites were clearly not gathered upon a beach. They were overwhelmed and killed on the sea floor. Local Portuguese geologists believe that the Ordovician animals lived in a marine basin with poor oceanic circulation, so that deeper layers could become stagnant. The congregated trilobites might have been overcome by a phase when oxygen dropped to lethal levels: after all, even trilobites needed to breathe. Such anoxia is not much of a problem in Delaware Bay. The trilobites could still have been gathering for reproductive purposes, of course. They might have even been safer from predators in the deeper basin. But then it makes us uneasy to think of the trilobites depositing their eggs in such an inhospitable place – unless anoxic events were so rare as to have little effect on their long-term survival. Then there is the fact that a number of the slabs seem to show a mixture of species. Could they have been gathered together for some purpose other than mating? Unlike Limulus, a freshly moulted ‘soft shelled’ trilobite would have been vulnerable until it grew a new hard carapace. Maybe these congregations were huddled together for mutual protection away from the prying eyes of predators. With these ambiguities in the picture the case for a direct comparison with the behaviour of modern horseshoes begins to seem weaker. A sceptic would say that it is simple minded to expect similar habits to endure for hundreds of millions of years. Perhaps so, but common problems in nature often come up with comparable solutions, the more so if the organisms concerned are related. Those trilobite examples with marginally overlapping bodies might merit further examination, since they do recall the struggle for mating among the horseshoe crabs, and it is more than a guess that eyes in these animals were particularly attuned to seeking out mates. I still like to think that the crystal eyes of trilobites may have had similar lustful intent.