Survivors: The Animals and Plants that Time has Left Behind

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.

2

The Search for the Velvet Worm

New Zealand is a country that beguiles but deceives, for much of it is dressed in false colours. Although there is still some almost untouched forest on the South Island, human hands have transformed much of New Zealand in the service of forestry and sheep.

The story of these islands is one of isolation. Their origins lie within the great and ancient vanished land of Gondwana, from a time when peninsular India, South America, Africa and Australia were united together as a ‘supercontinent’. Something like 100 million years ago the nascent New Zealand separated from its parent, as Gondwana began to fragment progressively into its individual plates. These eventually forged the continents of the southern hemisphere that we would recognise today. Unravelling this story was one of the great achievements of modern science, and it is linked to some of the stories of biological survivors in this book. New Zealand may be just a small part of that story, but its own narrative is geologically complex. To a kernel of old Gondwana rocks, newer rocks have been added piece by piece because the islands have sat in a tectonically active, though isolated zone for millions of years. Volcanoes have made their fiery contribution in ash and lava, other igneous rocks have been intruded into the Alpine range as it grew, and then sediments eroded from the young mountains completed a dynamic rock record. It could be said that the geography of New Zealand has been under constant revision. But animals and plants were also carried onwards into the growing New Zealand from the ancient Gondwana days, a persistent legacy of an old continent bequeathed to a future land. Sometimes the evolutionary signal of an organism betrays a far-distant past in surprising ways.

The ancient coniferous podocarp forests that once covered much of the North Island have all but disappeared. Little patches of it hang on almost by oversight. They are dark and mysterious within; silent, but for melodic tweets from birds high up in the canopy feeding on the little fruits the trees produce. Podocarps are southern hemisphere conifers of several species that make superb and stately trees if they are allowed centuries to grow to maturity. This is too long for a healthy profit. The original forests were felled for their good timber, but were replaced in many areas by quick-growing conifers such as Californian pines deriving from the other hemisphere. Huge areas of the North Island are covered with conifer plantations. Periodically they are felled en masse and then the rolling hills are scenes of devastation, with nothing green left standing but wrecks of stumps and unwanted branches in rough piles everywhere, and small fires smouldering as if shells had exploded not long before. When I drove through such an area I was torn between recollections of battle scenes from World War One, and J. R. R. Tolkien’s descriptions of the ghastly land of Mordor in The Lord of the Rings. I suppose the latter might be more appropriate, since splendid alpine New Zealand has been repeatedly used as the location for the movie version of Tolkien’s saga. The sheep country looks like steep sheep country everywhere, and reminded me of Wales and Scotland, even to the extent of carrying scrubby patches of brilliant yellow-flowered gorse – which, of course, is a troublesome introduction from Europe. There are so many other Europeans on these islands, not just Smiths and Joneses in suburban villas, but oaks, sycamores, elderberries, and implacable ivy. They compete for space with other native trees, including the New Zealand red beech, Nothofagus fusca, with its delightfully delicate little leaves and graceful habit. I could not help feeling that a coarse and unthinking hand has been at work, interfering with the landscape, scrubbing forests out, planting weeds. This is grossly unfair to the New Zealanders, the kindest people on earth.

Podocarp trees are in a sense ‘survivors’ from the time of Gondwana. These trees are found in Australia, New Caledonia, South America, and Sub-Saharan Africa – one or two genera are even in common between New Zealand and the Andes. Gondwana may have split into its separate pieces, but the identity tags of its former inhabitants were not redesigned so easily. These Gondwanan coniferous trees, with their relatively large leaves and bright berries, do have a very special appearance, at least to a European accustomed to pines and firs with their dry-looking cones. A botanist would remind me I should really describe the berries as ‘fleshy peduncles’ because they carry exposed seeds at their tips. On the wet west coast of the South Island near Karamea, I walk into a podocarp forest where dampness rules. Everything that could be is covered in moss, epiphytes or filmy ferns. They clothe the trunks and branches of trees in a creeping, delicate and close-fitting cloak of tiny green leaves. Inconspicuous orchids are there somewhere, perched on branches, sporting small yellowish flowers, the antithesis of tropical showiness. Where light breaks through the canopy, tree ferns erupt like green fountains perched on shaggy stems, adding ebullience to the primeval atmosphere. Little brown birds with bright little eyes – tom-tits New Zealanders call them – pipe tamely from exposed twigs hoping that these clodhopping visitors might disturb insects for their supper. Trunks of the podocarp totara tree soar upwards, while the rimu – the most elegant of its family, with weeping, cypress-like branches – breaks through the canopy like drapes. The wood of this tree is so hard that the heart is still sound for working from trunks lying on the ground years after the outer layers have rotted. The more familiar southern beeches (Nothofagus) are unsuitable for major construction since they rot from the inside out, but they also have a Gondwanan signature, following closely the pattern of the podocarps. I recall that Charles Darwin observed how the natives in Tierra del Fuego ate a curious fungus looking like a cluster of yellow golf balls that grew on southern beech branches. The fungus was named Cyttaria darwinii by Miles Joseph Berkeley, the great nineteenth-century mycologist who worked out the fungal cause of the Irish potato blight. Further species of the same fungus were discovered, but they only grew on southern beeches: fungi can be choosy. The Gondwana legacy even applies to soft, edible fungi that would never stand a chance of being preserved as fossils. Biologists must have their wits about them if they are to understand the complexity of the past.