Survivors: The Animals and Plants that Time has Left Behind

Their fundamental similarities make it likely that Peripatus and arthropods share a common ancestor. The arthropods seem to be more advanced in several respects: the jointed legs could only have been added when the ‘skin’ acquired its hard outer layer, and sophisticated compound eyes like those of Limulus must surely have been a later development. This is another way of saying that lobopods are probably sited on a lower level on the great tree of life, likely to have been around before the arthropods evolved. There are some scientists who would claim that they are the true ancestors of the arthropods, or even that different kinds of lobopod gave rise to different kinds of arthropods. Partly, this depends on the interpretation of the jawed animal Kerygmachela from Greenland that seems to display something of an amalgam of lobopod and arthropod characteristics. Whatever the final interpretation, these recent discoveries of Cambrian fossils provide another case of neat categories of animal classification blurring at the time of the ‘explosive’ phase of animal evolution. The story also takes us back further in time than we have been before.

Recently, additional evidence for the velvet worm’s place on the tree of life has come from the genome of the living species. Ancient fossils do not preserve DNA, which is a large and delicate molecule, readily fracturing into pieces. But by studying the molecules of living survivors from deep branches in the tree of life we are afforded a kind of telescope to see back in time. For the genetic code of DNA records another kind of history, it retains the accumulated narrative of all the changes at the fundamental molecular level that have built up slowly over time. Mutations that have been incorporated in the genome provide a kind of ancient fingerprint. But the code of life is famously huge – which means that the investigator may be obliged to seek out the particular piece of the genome that contains the information he needs. Although, as this is written, more and more organisms are having their entire DNA sequenced, this is still the prerogative of a privileged few – unsurprisingly, those like wheat or influenza that have a particular importance to Homo sapiens. For many organisms, it is more feasible to use a particular chunk of its genetic code to compare with the same chunk from a range of its potential relatives. This might be a particularly suitable gene or series of genes, for example, that do not change too rapidly to be useful through long periods of geological time. Obviously, the chosen gene has to be present in all the organisms under study. Other workers favour sequencing parts of the RNA molecule in the ribosomes that are present in the cells of all living organisms as the centres for protein synthesis. Comparing the similarity of gene sequences is one way assessing how closely (or not) organisms are related to one another. The results can be drawn up as another kind of tree, with branches drawing together the closest related species, and deeper patterns of branching inferred from still more fundamental inherited similarities. This is not as easy as it might sound from this bald description, as various kinds of ‘noise’ can obscure the signal the investigator seeks, and there are always genes that change too fast to retain meaningful signals from deep time. I need hardly add that computer programs have been designed to help out. The technical problems are not part of our story, except in so far as they have produced different ‘trees’ of relationships between organisms since the methods were first developed. Indeed, early attempts sometimes look quaint or improbable. But recent studies seem to have stabilised, and produce trees that appeal to prior knowledge and common sense, mostly by lumping together evidence from many different genes and finding the best fit. These then make a meaningful contribution to the summary trees of evolutionary history like those on our endpapers. The latest molecular analyses to treat the velvet worm and its relatives show interesting results. It places our chosen survivor as the bottom branch of a tree that includes all the arthropods above it – which must therefore have arrived later. Another name appears between the lobopods and the arthropods. This is Tardigrada (water bears), a group of tiny creatures that often live between sand grains and in other cryptic habitats. They are interesting in their own right, but they have but one known fossil, so they will not be described in detail here. Many tiny animals have no fossil record at all, but that does not mean that they did not exist in the past. The important point for us is that the molecular evidence supports the idea that lobopods are a branch even lower on the tree of life than arthropods. Those stumpy legs have walked on and on from a time even before the Cambrian. The very earliest Cambrian strata contain the traces of animals, but not their bodies. This is probably because those early animals lacked readily fossilisable hard parts, and the special conditions required to preserve the slightly younger Chengjiang fossils were not present at this particular time. No matter, for some of the tracks and trails that are preserved as fossils show clearly the traces made by arthropods of normal size digging their way into soft sediments with their numerous paired legs. It is even possible that these could have been tracks left behind by soft-bodied ‘proto’ trilobites since they are similar to tracks made by the same animals higher in the geological column; at the moment we simply do not know. But we now do know that there must have been lobopods on that same sea floor, too, stomping ever onwards. More than that, they must have been present even earlier, before the first arthropods, because both the molecules and the anatomy of the animals tell us that they preceded the jointed-legged organisms. This takes us back into the mysterious world of the Ediacaran, a period whose remains lie above the Precambrian, and below the Cambrian, before the time of abundance and variety of marine life and before the appearance of shells.*

The story of the lobopods now disappears. There are no velvet worms or indeed any kind of lobopods in strata of Ediacaran age. There has been no shortage of attempts to find them. Geologists and palaeontologists have been cracking open likely rocks for decades now. The fact is that there are no trilobites, no early horseshoe crabs, nor any old familiar biological friends to be found in Ediacaran age strata. As in The Hunting of the Snark by Lewis Carroll searchers vowed: ‘To seek it with thimbles, to seek it with care; To pursue it with forks and hope’, but to no avail. Even big hammers did not work. Instead a whole series of fossil animals have been recovered which have proved as enigmatic as they are exciting: not snarks but boojums. They are not small – some of them are bigger than a dinner plate – and neither are they uncommon if the searcher goes to the right place. The Ediacaran Period takes its name from the Ediacara Hills in the Flinders Ranges in South Australia where a diverse selection of these remarkable early fossils was first collected. They appear as impressions on fine sandstones, many looking like strange leaves or fronds. Most of them show evidence of divisions or compartments dividing up the body, but they are not simple segments, because they are usually offset from one side of the animal to the other. Similar fossils are now known from more than thirty localities all over the world: from Arctic Russia, Canada, America, Newfoundland, and Great Britain. Everyone agrees that these fossils lacked skeletons, but otherwise the experts disagree on almost everything else. Most of them would now concur that the Ediacaran animals were not obvious ancestors of the animals we know from the Cambrian onwards; they were genuinely inhabitants of a former world that did not survive. It seems only fitting that in a book about survivors I should also go to visit a world that failed to endure. The journey took me back to Newfoundland, where I had spent a year at Memorial University in St John’s when I was a young scientist. So I was travelling into my own past as well as towards a far, far deeper time.

Newfoundland is an island at the tip of eastern Canada and is itself something of a survivor. Built on the fortunes made from codfish on the Grand Banks, it has survived the great crash in the population of its most important crop. It is the textbook case for the effects of over-fishing. In the thirty years I have known the ‘rock’ (as the natives call it) I have watched with bewilderment as fishermen have laid up their boats, and an apparently endless resource has all but disappeared. The codfish has not become extinct, of course, but the decline of this otherwise unfussy fish does prove that nothing in nature can be assumed to be unassailably fecund. High-tech factory ships from outside the island indiscriminately scooping up huge quantities of fish are mostly to blame. The Newfoundlanders, ever resourceful, have now taken to oil. The name of the Come-by-Chance refinery is somehow appropriate to their persistence in the face of setbacks not of their making. The little fishing villages along the coast are known as ‘outports’, and ever since they have been required to eschew the cod, those young outport men who have not gone to Come-by-Chance have left to find work at Churchill Falls, the huge hydroelectric plant in northern Labrador, or even to become hands on the extraction of the Athabasca ‘tar sands’ on the other side of Canada. They are a breezy bunch, despite their peripatetic life, and have an unusual accent: Irish with added stretched vowels, and wheezy interpolations of interjections like ‘Jeez, my son’. The outports are all freshly painted these days, with wooden houses in cheery colours scattered up the hillsides. For the few who stay behind, there is nothing much to do except repaint the picket fences.

The drive south along the Avalon Peninsula from the capital St John’s passes several sheltered coves tucked away inside a coastline of magnificent cliffs. The geology is laid bare all along the rim of this island: the only problem is reaching it. Inland, the opposite is true; an endless forest of short conifers interspersed with scattered birch and aspen trees is interrupted only by shallow lakes called ‘ponds’ hereabouts, which are a legacy of the last ice age; the bedrock is hard to see among the scrub. As we approach the end of the Peninsula the trees get shorter and shorter, planed off by the fierce winds. Finally they crouch against the ground, as if terrified to poke up a twig. Usually the whole of this exposed area is swathed in fog, so the landscape supplies a passable setting for a vampire movie starring Vincent Price. But the day we visit it the weather is clear and sunny, with a few fluffy white clouds in a faultlessly blue sky. My companions are astonished, it was the best day they had seen in the last decade. The warden of the Reserve came from Wales, and remarked ruefully that he had chosen to work in the only place in the world with worse weather than Ffestiniog. One of the Newfoundlanders mumbles to me under his breath that the warden will be betrothed before Christmas. ‘Not a lot of single men around here’, he says, with a wink.

At Mistaken Point, a path leads for a mile across a bleak coastal heath, which is less forbidding examined closely. Berry-bearing plants hidden in the close sward bear blue-black or scarlet fruits, and bright yellow tormentil flowers smile at us along the way. Patches of Sphagnum bog support pitcher plants whose leaves trap flies and mosquitoes to compensate for the poor nutrition offered by the damp wilderness. Even wild roses are tucked into natural hollows. As we approach the sea, grasses take over to make a natural lawn. Fulmars wheel in and out, just to have a look. The path leads onto the cliffs, which are quite comfortable to clamber over in this part of the Avalon Peninsula. The sedimentary rocks of which they are composed form a series of ledges that dip at a gentle angle into the sea, forming steps that we can climb up or down to explore different strata. The rocks are dark in colour, and the more resistant beds have made natural groynes that project out into the ocean. Waves break continuously over the ledges, throwing up foam – and this on a calm day. When winter storms are raging, salt spray must blast all the exposed surfaces. It is not hard to imagine how Mistaken Point got its name. The bones of fifty ships lie offshore, waiting to be fossilised.

Each of the flat surfaces exposed on the ledges is an ancient sea floor. In 1967, a graduate student geologist called S. B. Misra at Memorial University of Newfoundland discovered the most extraordinary organic remains preserved on these stretches of petrified sediment surfaces. Only a year later an account of the finds had been published in the most prestigious scientific journal Nature, jointly with Mike Anderson, also of Memorial University. The rocks were recognised as being late Precambrian in age (this was long before the Ediacaran had been named). There was palpable excitement in the scientific community at finding such large fossils in rocks of this great antiquity, although it was not known at the time just how old they were. Misra subsequently described the original conditions under which the sediments had been deposited. There were some special features about this discovery. First, the fossils could not be safely collected. They were impressions on the exposed surfaces of a very hard but brittle rock, shot through with cracks, and often located in the middle of a great uncompromising slab. The best way to study the remains was to pour a latex solution onto the surface of the rock, allow it to dry – even that might be a challenge with the Atlantic hard by and fog always lurking in damp banks – and then take the hardened cast off to somewhere nice and warm. For scientific description it is usual to have an actual specimen on which to found a scientific name, and this should be kept in perpetuity in a public museum. This was obviously going to pose a problem, unless a public museum was constructed over the cliffs. Second, with such unusual material it is rather hard to know where to begin, since most of the usual biological pointers are absent. How does one describe an enigma, except as ‘enigmatic’? Perhaps it was a combination of these factors that stalled a full account of these remarkable fossils. Anderson took over the material when Misra went back to India, and when I met him in the late 1970s he seemed to be crippled into inaction by these admittedly difficult problems. At the same time, he put his marker down upon the fossils so that nobody else could study them. The result was that most of the Mistaken Point fossils did not receive proper descriptions and the respectability of scientific names for several decades. Guy Narbonne and his colleagues from Queen’s University, Ontario, are making good this omission even now. It is a strange fact about science that until an object or a phenomenon receives a name in some way it does not exist. Names really matter. They retrieve something from an endless chaos of anonymity into a world of lists, inventories, and classification. The next stage is to understand their meaning.

A notice at the top of the cliffs points the way (a quarter of it had blown away in the last gale) accompanied by a pinned-up sheet of paper instructing visitors to ‘remove footwear before visiting fossil bearing surfaces’. I confess that the idea of taking off one’s boots in a howling squall to safeguard fossils that had survived since the Precambrian had its funny side. In the event we are provided with a pair of rather fetching blue over-socks. Visits to the famous fossils are now strictly supervised, as the site is now part of the Mistaken Point Ecological Reserve, and quite right too. Canadians are strict about protecting their national natural heritage. There is an architect-designed Visitor Centre to explain all to those who have made the trip. I climb down onto the best surface, in my special socks, and it takes a while to identify what to look for, but once they are pointed out the fossils are obvious. Any doubt that they were of organic origin was immediately banished from my mind. The fossils are strewn over the black surface of the gently dipping former sea floor almost as if laid out for the convenience of future inspections: one here, one there. The most conspicuous look like leaves or fronds, and are about the same size as a domestic Aspidistra leaf or some other showy tropical pot plant. They are pleated within, and the closer one looks the more subdivisions inside the ‘leaf’ one begins to see. Such spindle-shaped fossils are the commonest type. There are more than a thousand of them on display under the Newfoundland sky. They were named Fractofusus misrai in 2007, four decades on from their original discovery, thereby commemorating the discoverer in perpetuity in the species name. The name Fractofusus is quite descriptive – the ‘fusus’ part refers to the fusiform (spindle-like) shape of the whole organism, and the ‘Fracto’ part to the fact that it appears to have a fractal structure. Fractals, those intriguing mathematical entities recognised by Dr Benoit Mandelbrot in 1980, are shapes that seem to repeat themselves precisely when the scale is focused down to a smaller level. So, the largest primary divisions within Fractofusus are subdivided into identical-looking smaller frondlets, and those in turn into identical-looking ‘sub-frondlets’, and so on. It seems that these Precambrian organisms favoured this kind of structure; indeed, Martin Brasier of Oxford University has shown rather ingeniously that several of the organisms at Mistaken Point can be understood as a kind of three-dimensional origami played out by folding such fractal objects in different ways. But there are also some frond-like organisms that seem to be attached to the former sea floor by a kind of disc-shaped holdfast. Charniodiscus masoni was perhaps the earliest Ediacaran species to be recognised – from Charnwood Forest in Leicestershire in England, as the generic name should make clear (like Misrai, the species name is after its discoverer). The same ‘frond’ is known from a very large number of Ediacaran localities, including several in the Ediacara Hills themselves, so it is almost totemic for this early and vanished marine world. The disc is thought to have held the organism in place while the frondose part was maintained aloft in the water current. There are several additional forms from Newfoundland that have their counterparts in Leicestershire, but since the latest reconstructions of the later Precambrian world place these areas quite close together geographically this is not as surprising as it may seem at first. Some other oddities are pointed out to me, one is a kind of plate with tumid blobs arranged all over it. It was called informally ‘the pizza’. The name reminded me that in my excitement I had not yet eaten lunch, so there I sat on an Ediacaran sea floor eating a cheese sandwich, looking out to sea on a perfect day while fulmars wheeled past on a light breeze. For a palaeontologist, it doesn’t get much better than this. I realise that whatever we eventually make of these strange fractal beings, it cannot be doubted that there was a lot of conspicuous life in the later Precambrian, but apparently no relatives of velvet worms. These special fossils position a time line in our story; they offer a calibration for evolutionary invention.

I wonder what lucky circumstances account for the preservation of the fossils. After all, they are soft bodied. They could have vanished leaving no trace. My guides tell me that the area now so often coolly fog-bound was volcanically active in those distant days. Periodic ash falls cascaded into the sea and rapidly killed off and buried the Ediacaran fauna. They point out the Charniodiscus bending over in a common direction flattened by the incoming volcanic Armageddon. I should have noticed this before. Each fossil-bearing sea floor is the record of one tragic moment for the Ediacaran animals, though it is no less than a miracle for us intelligent primates. Volcanic rocks have another property in addition to their role as natural undertakers; they yield minerals that can be used to obtain a radiometric age for the eruption. They both write the obituary and record the date. A time label of 565 million years ago has been obtained recently from an ash layer immediately above one of the best fossil-bearing beds. This is more accurate than can be achieved with many younger deposits, because datable volcanic rocks are not commonly interleaved with fossil-rich sedimentary rocks. Given that the best date for the base of the Cambrian Period is 542 million years ago, the Newfoundland rocks are only twenty-three million years older. I use the word ‘only’ advisedly; although this might seem like a long time, it is a short span in the history of the horseshoe crab or velvet worm. Even if we went back twenty-three million years from the present day we would readily recognise a world of mammals, birds, butterflies, and flowers; and our own distant ancestors were already in the trees. But the world of Mistaken Point seems to have nothing to do with the marine world familiar from Cambrian strata, with its arthropods like trilobites, together with molluscs, brachiopods, and echinoderms, ancestors of today’s sea urchins and feather stars, not to forget the distant relatives of velvet worms.

It is no wonder that an attempt to understand the Ediacaran world has attracted the attention of researchers around the globe. Some facts have become quite well established, but there remain many disputes, which is hardly surprising when considering scientific forays into such mysterious and ancient environments. In fact, the stuff of science is disagreement. If there were no disputes there would be no incentive to drive scientists out (without shoes) onto exposed Atlantic shores in order to crouch over cold wet rocks for hours on end. They want to get one step ahead in the race for the truth. However, most specialists do concur that the Ediacaran sea floor was very different from the seabed on the continental shelves today. The surface was coherent, even rubbery, due to a thin-skin veneer composed of bacterial mats. Sediments were almost cling-film wrapped, and holdfasts probably got a good purchase on this kind of surface. There is also a less universal consensus that the reason for this skin-like surface was that a range of burrowing organisms had not yet appeared to churn up the sediment. The sea floor nowadays is often a mass of so-called infaunal animals that live in the silt of the seabed and have a vital role to play in the food chain. Think of the huge flocks of waders that strut around on muddy estuaries when the tide is low, pecking down into the mud – not every dunlin has to rely on horseshoe crab eggs. Little churners and burrowers, especially marine polychaete worms, oxygenate the lower layers of the sediment as they work away. In the absence of such activity, an anaerobic layer soon develops beneath the surface, which can be recognised by the preservation of fine, horizontal layers when the sediments eventually harden into rock. Many Precambrian strata do indeed look like this – though by no means all. Sometimes the more fine-grained sedimentary surfaces betray a wrinkly skin, which is finely puckered, almost like the skin of an elephant, enabling us to visualise the gummy bacterial surface, although the minute organisms that made it are not preserved. These curious sea conditions have been ingeniously invoked to explain the preservation of many Ediacaran soft-bodied fossils. After a sudden overwhelming event – it could be a sudden slurry of sediment or a volcanic ash fall – the organisms are entombed, and a new mat then quickly grows on top of the grave sealing the dead animals in the sediment. Then the reducing conditions that inevitably ensue in the absence of wormy disturbance help to mobilise iron in the sediment in a form that migrates to make a kind of ‘death mask’ around the potential fossils before they have decayed away. The endurance of so many soft-bodied organisms certainly implies a lack of those scavengers that make short work of dead bodies in today’s oceans. As for the texture of the Ediacaran organisms, they may have lacked shells but they seem to have been membranous, possibly even quite tough. Some scientists believe that they were divided into chambers rather like an old-fashioned quilted eiderdown. Their apparently fractal structure is probably a reflection of a particular style of growing, whereby the same set of rules are repeated over and over. It may just be a simple way of growing big. However one looks at them these organisms do seem irredeemably strange.