The arms race takes off in the Cambrian

There are just a few areas around the world that still preserve the passage from the latest Precambrian into Cambrian times with more or less continuous sedimentation of marine deposits. One of these is in Siberia, which is an ancient and discrete entity that only later became 'welded' onto Asia. In late Precambrian times Siberia was close to the equator and broke away from the Rodinian supercontinent as it fell apart. Because of its position in low latitudes, a succession of shallow- and warm-water carbonate deposits accumulated on the margins of Siberia. These now form cliffs of limestones exposed along the banks of the Siberian rivers, which flow into the Arctic Ocean. Luckily, the occurrence of some lavas in the lower part of the succession provides us with a very accurate date for the beginning of Cambrian times - 542 ± 1.0 million years ago.

For several decades, Russian geologists and palaeontologists have been trying to tell the rest of the world how wonderful and important these rocks are for understanding the beginning of the Cambrian story. But thanks to frigid relationships and lack of communication during the Cold War, it is only relatively recently that the Siberian rocks have become widely appreciated. Meticulous measurement of the strata and layer-by-layer collecting of their fossils have revealed a remarkable history of burgeoning life in the oceans.

The first sign of life is found in crinkly laminated limestones, similar in kind to stromatolitic mounds, and similarly constructed by intertidal successions of bacterial mats. A few meandering trails are preserved across the sediment surfaces. Clearly, by this time there were also free-moving, elongate wormlike organisms some tens of millimetres long in existence. For relatively large organisms like this to be capable of sinuous movement across a sediment surface in search of food, they have to be multicelled with differentiated tissues and perhaps specialised body organs. In other words, they have to be quite complex and sophisticated in their body organisation, more so than the Ediacarans found in more ancient strata. Recently, there have been a number of claims of sinuous trace fossils being found in more ancient strata, but their organic nature is disputed.

As the carbonates pass up into sands and then muds, so the fossils change. To begin with there are tiny, millimetre-sized, hollow cones, but increasing numbers of tiny shells, plates and other weird mineralised structures of different shapes appear over the next few tens of metres. Although they are just a millimetre or so in size, some are nevertheless recognisable as the shells of sea creatures that are still familiar today, such as spiral-shelled snails, bivalved lamp shells (brachiopods), plus a lot of others, such as the extinct archaeocyathans, which are only recognisable to fossil experts. The archaeocyathans formed some of the first reef structures in shallow tropical waters, had small conical porous shells and were probably related to the sponges.

These are the fossilised remains of increasingly complicated animals that have already separated into major biological groups (technically known as phyla -singular phylum) with very different bodily organisation. As they are already separate, the question arises of how long ago they split into these fundamentally different groupings.

Historically, the biological argument was based on an understanding of anatomy, embryology and development of the different organisms. Charles Darwin was already aware of the implications of this biological argument with respect to the fossil record when he was writing The Origin of Species. The presence of separate kinds of well-differentiated organisms appearing in the earliest Cambrian strata seems to require a long Precambrian evolutionary 'gestation' and ancestry, but where are the fossils to support such a conclusion?

Darwin had a much greater problem than we do today. As we have seen, when he was preparing his theory of evolution in the mid-nineteenth century, there were no known Precambrian fossils. Darwin had to produce some very good reasons for this absence and devoted two whole chapters of The Origin to detailing why the geological and fossil record could be excused for not preserving the critical evidence for late Precambrian life. His answer was that the rock record was riddled with 'holes', gaps in successions of strata that represented significant periods of Earth Time. If no strata were preserved during a particular epoch, there would be no fossils and thus no record of the animals and plants that lived during that interval. Darwin used a book metaphor to describe the situation. He characterised the testimony of the rocks as:

a history of the world imperfectly kept, and written in a changing dialect; of this history we possess the last volume alone, relating only to two or three countries. Of this volume, only here and there a short chapter has been preserved; and of each page only here and there a few lines.

Since there was good evidence to suggest that this was in fact the case for much of the geological record, Darwin felt able to claim:

if my theory be true, it is indisputable that before the lowest Silurian stratum was deposited, long periods elapsed, as long as, or probably far longer than, the whole interval from the Silurian age to the present day; and that during these vast, yet quite unknown periods of time, the world swarmed with living creatures.

As we have seen, when Darwin was writing this, the hegemony of Murchison's Silurian 'empire' was in full swing. Sedgwick's Cambrian had been eclipsed as the oldest fossil-bearing strata and subsumed into the Silurian. Murchison claimed that life began in 'his' Silurian and such was his influence that Darwin and many other geologists happily went along with the idea. Darwin knew that his theory of evolution would not be acceptable to his old geological mentor. With Sedgwick already a potential opponent, matters could be no worse than they were.

Now that we have a fossil record extending back deep into Precambrian times and early Earth Time, there is not so much of a problem. But the question has not been completely resolved. Relatively complex multicellular organisms turn up well over a billion years ago, and we have the variety of relatively large Ediacaran creatures around 560 million years ago. So can the ancestors of complex animals such as the molluscan snails and arthropod trilobites be found among them? Although the scientists who first examined the Ediacarans tried 'shoehorning' them into these groups of living animals, the modern opinion is that this will not do. Neither the ancestors of the annelid worms, molluscs nor arthropods can be discerned with confidence among the soft-bodied Ediacarans, so where are they?

Modern genetic studies have raised the possibility that we can time the evolutionary separation of groups of organisms by measuring the genetic 'distance' between their living representatives. If we can obtain some measure of the rate of evolution in the groups and know the 'distance', then we can estimate when the original split (divergence time) occurred - providing that the rate of evolution did not change. This 'molecular clock' sounds very promising and a lot of work has been done to measure important evolutionary developments like when the early branches of the animal 'tree' first arose. The first estimates were getting on for a billion years ago, but after squeals of complaint from palaeontologists, the measures were refined and came down to a more sensible 750-700 million years ago. If this is in any way correct then it reinforces the problem with the fossil record.

Even if we allow that the first shelled creatures such as the 550-million-year-old Cloudina represent some complex invertebrate group such as the molluscs, there is still a gap of 150-200 million years in the late Proterozoic when we have little or no fossil record that can reasonably be related to the missing ancestors. The problem is compounded by the fact that they would almost certainly have been entirely soft bodied and therefore generally difficult to preserve, unless we can find some strata with exceptional states of preservation. There is some hope that further study of the 590-6oo-million-year-old Doushanto fossil embryos will reveal the presence of identifiable representatives of invertebrate groups such as the molluscs or annelids. The problem is that at early stages of cleavage and cell division one embryo cannot be distinguished from another in the fossil state. The alternative argument, followed by some palaeontologists, is that the molecular clock is wrong and that there really was a very early Cambrian 'explosion' of invertebrate life with very rapid rates of evolution and diversification.

Trace fossils provide some of the strongest supporting evidence for this argument. By their very nature, they record the activities of soft-bodied creatures and they are often well preserved in sandy deposits typical of shallow waters. A great deal of research has gone into the analysis of this very particular kind of fossil and its interpretation. Burrow form and pattern can now be quite securely related to the level of organisation of the animal that made it. For instance, the construction of upright U-shaped burrows that terminate at a seabed surface are characteristic of complex invertebrates such as polychaete worms.

Such burrows are present in some of the earliest Cambrian beach sands but not in older Proterozoic sands, suggesting that they did not evolve until the beginning of the Cambrian. Details of the fossil forms that appear successively within the earliest Cambrian strata also suggest that aspects of the evolutionary explosion are recorded. For instance, the trilobite arthropods do not appear in the oldest Cambrian layers but rather some 20 million years later, whereas small cap-like shells made of calcium phosphate that belong to a kind of lampshell (brachiopod) do occur quite early, after some 13 million years. A couple of million years later, the first shelled molluscs (for example clams and snails) appear. But our best view of the Cambrian explosion is seen far away from the Grand Canyon, in China and the Canadian Rockies.

New windows on Cambrian life

Walcott was always something of a workaholic and fortunately his wife did not seem to mind. Even their honeymoon involved geological fieldwork! Having risen to become Director of the US Geological Survey in 1894, Walcott was not only one of the most important geologists in America but one of the most important and influential scientists. His organisational and 'manhandling' skills, well honed and tempered by his years working for James Hall, led him to appointment as Secretary to the Smithsonian Institution in Washington DC from 1907 to 1927, one of the most powerful scientific organisations in the world. Although his first academic 'passion' was research, especially on Cambrian fossils, much of his time was diverted into administration and attending endless committees, so much so that his private journals are full of complaints that there were only 'odd moments spent with Cambrian brachiopods'. Nevertheless, he did manage to escape from time to time.

His geological work in the Canadian Rockies began in 1907 and involved serious hiking and riding over high passes. On August 31st, 1909 Walcott, his wife and his young son were making their way along the ridge connecting Mount Field and Wapta Mountain in British Columbia, when Walcott's trained eye spotted some fossils on the frost-shattered blocks of rock littering the mountain scree slopes. Although the muddy sediment and its fossil content was intensely flattened and slightly metamorphosed to a slaty condition, Walcott recognised that he was looking at Cambrian fossils with a curious mode of preservation that seemed quite promising.

His accidental discovery of what became known around the world as the Burgess Shale fauna opened a whole new window on life in Cambrian times. Over the next few years, Walcott returned again and again to the locality because its fossil novelties seemed inexhaustible. He excavated a small quarry into the mountainside so that he could methodically work his way through the most fossiliferous layers. In the process, he uncovered some 70,000 specimens, all of which were shipped back to the Smithsonian.

Despite his other duties, Walcott managed to describe over 100 new species from the Burgess Shale, but he recognised that it was only the tip of the iceberg. Surprisingly, the Burgess riches hidden away in the Smithsonian did not receive much further attention until the 1960s and then it was British trilobite enthusiast Harry Whittington who really began the renaissance in the study of the Burgess fossils. Whittington had been well placed at Harvard to research the Smithsonian fossils, but when he took up the famous Woodwardian

Detail of the tail of a Cambrian trilobite from the Burgess Shale showing preserved soft parts such as the typically arthropodal jointed pairs of legs and gills.

professorship in the University of Cambridge, England, he supervised a succession of research students to look in detail at the different fossil groups, how they lived and died, and what the original seabed environment was like. More recently, Canadian palaeontologists have been able to reclaim their fossil heritage and make their own investigations.

This remarkable locality with its ancient seabed muds and huge diversity of early life is now a World Heritage Site. The fossil fauna is dominated by arthropods (around 50 per cent of both species and biomass), all of which are extinct and few of which are familiar except for the trilobites. This diversity of arthropods shows well-developed ecological specialisations from burrowing to grazing seabed sediment to a variety of carnivores, including metre-sized anomalocarids that were top predators and active swimmers. It is now thought that all these animals were caught up in a submarine collapse of the seabed and

Although highly flattened the preservation of soft parts in the Burgess Shale fossils such as this trilobite provided new insights into the biology of these extinct arthropods.

Wkuaxia, a strange mollusc-like organism from the Burgess Shale which has its upper surface covered with long flat blade-like plates, presumably for protection from predators.

Pikaia, a segmented chordate-like animal originally thought to be an annelid worm.

avalanche of mud into deeper water. When it finally came to rest, a wonderful sample of Cambrian life was trapped and entombed, all jumbled up within the mud. What was a catastrophe for the biota was a stroke of good luck for palaeontologists.

Sponges, echinoderms and worms each comprise about 10 per cent of the biomass and the remaining 20 per cent is made up of a variety of other organisms. Unlike the Ediacarans, most of the Burgess animals can be assigned to major living groups. Not so long ago, it was thought that many of the Burgess animals were quirky evolutionary 'one-offs' that became extinct without issue. Much of the problem arose from the severe flattening of the fossils, which has made interpretation of their original form extremely difficult. But now we have a much better understanding thanks to finds of similar fossils elsewhere, which also demonstrate that there was nothing unusual about the Burgess animals, life really was like that in mid-Cambrian times. Dating the strata was also a considerable problem, but now a relatively secure age of around 505 million years ago places it in mid-Cambrian times, some 37 million years into the period.

One of the most interesting and enigmatic of the Burgess shale animals was a tiny (4 cm long) elongate leaf-shaped creature called Pikaia, which has some resemblance to the living lancelet Branchiostoma (previously known as Amphioxus). For over a century zoologists have recognised that the lancelet is a primitive chordate with certain features in common with all backboned animals, such as a single main nerve cord running along its back, supported by a stiffening rod called a notochord and a series of paired muscles along the left and righthand side of its body - like a fish has. Pikaia seems also to preserve such features and was hailed as a possible ancestor to all backboned animals including ourselves, but it has recently been overshadowed by some exciting new Chinese finds.

Like the Burgess Shale on the margin of Laurentia, South China also lay within the tropics in early Cambrian times but on the other side of the world, and perhaps still clustered with the Gondwanan continents of Australia and India and so on. It was in 1984 that the amazing Chengjiang deposits were discovered by Chinese geologist Hou Xian-guang. The essential difference between the Chengjiang and Burgess strata is that the Chengjiang is preserved differently, with the result that the strata are easier to split open and many of the fossils are easier to see. Also the way the deposit and its fossils accumulated is quite different.

The Chengjiang muds were being laid down in the quiet coastal waters of a shallow sea. Periodically the oxygen levels in the waters were lowered to such an extent that many of the organisms died. At other times, it seems that influxes of freshwater from the land killed off the biota. The remains were quickly covered by more mud, which helped preserve fine details, including aspects of their soft tissues as also happened in the Burgess Shale. Importantly, the Chengjiang deposit is now known to be around 525 million years old. Being 20 million years older than the Burgess Shale, the evolution of the Chengjiang fossils thus predates those of the Burgess and adds significantly to our view of life in early Cambrian times.

Like the Burgess biota, there are some 100 so far known kinds of fossils and of these 60 per cent are arthropods, but here another 30 per cent are algae and bacteria, with the remaining 10 per cent belong to a diversity of other forms, from sponges to possible vertebrates. Most of the animals lived on and in the seabed. The burrowers included lampshells (lingulid brachopods that still have living relatives) with a long rootlike structure called a pedicle, which they used for burrowing and anchorage. In addition there were predatory priapulid worms. However, the top predators were the large anomalocarid arthropods similar to those found in the Burgess Shale. Most of the other arthropods, including trilobites, were active scavengers, some moving around on the seabed, some ploughing or scratching the surface sediment for tiny particles of food. Some were active swimming predators, but below the giant anomalocarids in the 'pecking' order. There were also a variety of other swimming and floating creatures including comb jellies (ctenophores), perhaps some jellyfish and probably vertebrates.

The free-swimming primitive vertebrates and vertebrate-like animals of Chengjiang are similar in size and general form to the Burgess Shale's Pikaia, but they are also more diverse and much more abundant and better preserved than Pikaia. Hundreds of specimens have been found for each of the two genera known. The Myllokunmingia specimens preserve traces of a single fin along the back, paired muscle blocks, paired filamentous gills and paired sensory structures in the head region, which are all primitive vertebrate characters, although there appear to be no hard skeletal structures.

The animal shares a number of characters with the living hagfish. Yunnanozoon, the other genus, is more problematic. It is not clear whether the body has paired muscle blocks or segments, there are structures that might be paired gonads, and the 'throat' region (pharynx) has arched filamentous structures that could be similar to the branchial arches seen in the living lancelet. Experts are still arguing over whether it is another primitive vertebrate, an even more primitive lancelet-like chordate or a representative of a related but completely extinct group.

With regard to our deepest ancestry, the most important aspect of these discoveries is that, by early Cambrian times, evolution had already advanced very close to the origin of the vertebrates. That this should be so makes the idea of the Cambrian 'explosion' even more remarkable or, alternatively, even more implausible. The argument is not yet resolved.

Yet another gap in the Canyon record

Modern analysis of the Cambrian strata in the Canyon indicates that there was a slowly subsiding continental shelf that received a wedge of sediments carried by rivers from eroding landscapes and dumped offshore. Beyond lay a widening ocean - the Paleo-Pacific, also known as the Panthalassic Ocean. These are all characteristics of what is known as a 'passive' or 'drifting' margin, where there are no major tectonic processes such as subduction of ocean floor rocks. There is good evidence that this passive margin existed on the western margin of Laurentia from early Cambrian times and persisted for at least 200 million years, until early Carboniferous times (around 359 million years ago) and perhaps for as long as 37$ million years until the end of Palaeozoic times and the beginning of the Mesozoic Era and the Jurassic Period (around 199 million years ago).

During this 200-milli0n-year interval of Palaeozoic Earth Time, Laurentia was periodically inundated by large-scale marine floods, which at times reached

deep into the continent's interior. Each flood event lasted for tens to hundreds of millions of years and was terminated by major retreats of the seawaters (known as regressions to geologists). The receding waters exposed the recently deposited sediments to subaerial erosion and weathering, which in places removed much of what had been deposited and consequently erased that part of the stratigraphic record.

Although shallow seas invaded the southern margin of Laurentia from mid-Ordovician to late Silurian times (and are preserved today in and around New Mexico), the Grand Canyon region remained above sea level and does not preserve any record of sediments of any kind until late Devonian times. As we have seen, events on the opposite, southeastern and eastern margins of Laurentia resulted in a rich record of dramatically changing environments and life. The land was first invaded by animals and primitive plants during Ordovician times, although they were confined to freshwaters for a considerable period of time. The incredibly inhospitable nature of barren rocks and rock debris exposed to the atmosphere required a whole barrage of pre-adaptations before organisms could expose themselves on dry land.

The next marine transgression to leave any deposit in the Canyon's record began in early Devonian times and spread over much of the continent, eventually inundating the Canyon region. Renewed subsidence of the region meant that the seaways remained right through mid-Mississippian (lower Carboniferous) times, leaving a significant thickness of strata.

Million ^fears Ago 145

0 0

Post a comment