The Burgess fauna and flora were organisms that lived 515 million years ago in a sunlit marine reef, at a depth of 70 metres or less. More specifically, they inhabited the edge of the reef, at the top of a submarine cliff known as the Cathedral Escarpment. The Cathedral Escarpment probably formed when the edges of the reef became detached and collapsed, sliding several kilometres down the slope. At the base of the sloping Cathedral Escarpment, some 160 metres below the reef, was a basin. One day in the Cambrian, an abrupt inflow of very fine mud swept across the area, burying most of the reef, but not the edge at the top of the Cathedral Escarpment. The Burgess fauna and flora escaped this catastrophe, which saw an end to carbonate deposition on the reef - the carbonates ending up in the basin. But further inflows of fine mud were to follow, and eventually the Burgess fauna and flora were gathered. The mud flowed over the edge of the reef like ash from a volcanic eruption and carried the Burgess organisms down the face of the Cathedral Escarpment, dumping them into the basin. Here they were preserved in all sorts of positions, akin to the bodies entombed at Pompeii. Today the Burgess organisms are found fossilised, albeit flattened, in a block of rock formed from compression of the mud in the basin, above the layer of carbonate. They serve as a snapshot of a community of life that existed in the Cambrian, 515 million years ago.
What have become known as the Burgess quarries are located 5 kilometres north of Field, on Fossil Ridge. Over a million years ago, the block of rock containing the Burgess fossils was transported 160 kilometres by movement in the Earth’s crust. If it had remained in its original position, the heat and pressure of movements in the crust at that particular place might well have destroyed the Burgess fossils.
In 1999 I set out to reach Des Collins’s camp and the Burgess Shale quarry on a very grey and wet morning, in the hope that the weather would improve. It did not. But the mist actually created an enigmatic atmosphere, which somehow seemed appropriate. I knew something exceptional lay ahead, but at the same time I did not know what to expect.
The steep climb from the base of Whiskey Jack Falls is rewarded with a view through the pine trees of a small lake with the most emerald green of colours. This lake was created by glacial movement, which stirred up minerals into the body of water left in its trail. Although the rest of the path to the Burgess quarries was less steep, it was still uphill all the way, for about three and a half hours. But it was not the slope that caused the most concern, nor was it the mist. It was the snow. Not the depth of snow covering the path, but the fresh prints of bear paws, claws and all, that it had preserved. After recently bumping into a bull elk, I was quite relieved not to meet the maker of these prints during my climb.
The next lake encountered resembled a setting for one of King Arthur’s tales. The mist over the green water also covered most of the surrounding pine trees and all signs of a sky. The air was very still and the silence impressive. A very different terrain and signs of life surrounded the path from here. Beaver-like hairy marmots were playing on and around this ‘Burgess Trail’ path, which cut across an elongated mountainside that included Fossil Ridge. One of the smaller plants on the edge of the path was particularly interesting because it had leaves that were corrugated or concertinaed to give strength to the thin structure - a flat leaf would have collapsed. I will return to these leaves later in the book.
After crossing a couple of ice bridges I could eventually see the blue tents of the Collins camp in the snow, set against a backdrop of one of the larger lakes in the Rockies (appearing an intense emerald green, of course). The camp was surrounded by temporary electric fences, to keep out bears, and there were red stains in the snow. The red stains had nothing to do with bears, but were the collective red-coloured eyespots of single-celled organisms that can inhabit snow. The ‘eye’ in eyespot is not really appropriate, for these organs can only sense the direction of sunlight - they cannot produce visual images, or ‘see’. The distinction between eye and eyespot will become consequential to this book.
Now all that was left to reach the Burgess quarries was a 200-metre scramble up Fossil Ridge from the Burgess Trail path. Three quarries could be seen, but the original, and most prolific, was the Walcott Quarry. The Walcott Quarry was the site of Des Collins’s current excavations, the last in a productive series that began in 1982. This quarry takes the form of a terrace, cut a few metres into the mountainside and several metres wide. At the back of the quarry the various layers of sediment are visible, following the removal of snow, as coloured bands. Each layer was once the sea floor in the Cambrian. The back of the quarry is continually extended into the mountain by hammering iron bars vertically into the rock from above; the bars then act as levers to break the rock away. This rock, or shale, takes the form of thin sheets like slate tiles on a roof. It is painstakingly split into thinner and thinner fractions and observed for fossils, which are more reflective than the bare rock. I examined some of the fossils that had just been exposed to air for the first time in 515 million years. There was a lobster-like animal about the size of a hand, with menacing, grasping limbs and bulging eyes, and some smaller creatures with hard shells, the like of which I had never seen before. Even in the field, with the naked eye, the exquisite detail in which the Burgess fossils have preserved is apparent. It is something exceptional for a biologist to see a Burgess fossil at its original site, and this eclipsed all other amazing experiences undergone in the Rocky Mountains. The Burgess quarries are now protected from unauthorised fossil hunters by national law, which is enforced by Parks Canada wardens. This is appropriate since the fossils of the Burgess Shale are of international importance.
Scrambling back down from the quarry to the Burgess Trail path, piles of shale known as taluses are apparent. Although once discarded from the quarry for being empty, fossils are being found in abundance in the Burgess talus following further splitting. In fact taluses are becoming an increasingly useful source of fossils because the quarry itself is drying up. The talus on Fossil Ridge was left collectively by all generations of Burgess excavators, including the very first - Charles Doolittle Walcott.
A century of research
Charles Doolittle Walcott was the head scientist at the US National Museum of Natural History (Smithsonian Institution, Washington, DC) and the world’s authority on the Cambrian. From 1907 to 1924 he undertook expeditions, often with his family, to the Rocky Mountains in Yoho National Park for the purpose of collecting Cambrian and earlier fossils. Cambrian trilobites were known from this region. But during an expedition to Fossil Ridge in 1909, Walcott found Marrella, Waptia, Naraoia, Vauxia . . . in fact, amazingly, a suite of soft-bodied fossils which included many, at first sight mysterious animal forms. He made sketches of each species in his field book and attempted to make sense of these forms, which he knew shouldn’t really be there. Recognising instantly the importance of his finds, Walcott carefully packed each fossil specimen and shipped them down to his base camp by mule.
It is most unusual to find fossils with details of soft parts preserved; most fossil animals are known only from their hard parts, such as the shells of snails. Needless to say, Walcott and his family planned and carried out numerous expeditions to Fossil Ridge. In his field diary, on the day of his first major Burgess Shale discoveries, Walcott wrote unassumingly, ‘We found a remarkable group of Phyllopod Crustaceans.’ In 1910 Walcott uncovered the ‘Phyllopod Bed’, the site known today as the Walcott Quarry. This site yielded a previously unimaginable diversity of Cambrian animal forms. More than 65,000 fossils with both hard and soft parts preserved were recovered by the Walcott family and dispatched to Washington by the end of 1911. About 170 species of animals (mainly) and plants have been recognised from these Burgess Shale fossils. Walcott described over a hundred of these himself, although the phyla to which he assigned these species later became the focus of considerable controversy. His first instinct was to place the Burgess Shale fauna into animal groups that still exist today. This instinct was apparent in his original statement
, where he forced the species found during his initial discovery into the Crustacea (part of the arthropod phylum), to which today’s crabs, shrimps and woodlice (slaters) belong. This was a safe bet - less controversial than constructing new phyla, perhaps, which may have been heavier for Walcott’s colleagues to digest. After all, arthropods, with their hard external skeletons, were already known from the Cambrian in the form of trilobites. The use of living phyla continued throughout Walcott’s later treatment of his finds. Interestingly, we have gone full circle since Walcott’s times. The next wave of scientists felt that many of the Burgess species were deserved of new phyla, but others have since fitted them back comfortably into living phyla, although rather more phyla than those used by Walcott were needed.
Figure 1.7 Marrella from the Burgess Shale - fossil and three-dimensional reconstruction.
From 1924 to 1930 Percy Raymond led Harvard University summer schools to the Canadian Rockies. The Walcott Quarry was visited several times and a second quarry was excavated nearby. From this ‘Raymond Quarry’ further Cambrian finds were made, though the fossils tended to be less well preserved than those from the Walcott Quarry.
Although Walcott’s and Raymond’s original accounts received discussion, surprisingly little attention was given to the Burgess Shale fossils until the Italian biologist Alberto Simonetta began redescribing some of the Burgess species, particularly the arthropods, in 1960. Simonetta’s work revealed that there was much to gain from a re-examination of the Burgess fossils, and the first significant suggestions that the Burgess animals belong to extinct phyla were made. This introduced controversy to the subject of early multicelled animal evolution, and with controversy came a growth in scientific attention, beginning with the ‘Cambridge project’.
Harry Whittington, a world authority on trilobites, initiated the ‘Cambridge project’ in the 1960s while employed at Harvard University. Whittington initially planned to map the precise levels from where the fossils occurred within the Burgess quarries, a detail neglected by the previous excavators. While exercising these plans he found some new fossils as a bonus. The information Whittington gathered led to an understanding of the original setting of the Burgess Shale organisms and of their environmental and ecological conditions. Workers from the Geological Survey of Canada were chiefly responsible for the environmental findings of the Burgess project, and in 1966 Whittington moved from Harvard to Cambridge University, where much of the major work on the redescription of specimens and ecological aspects of the Burgess ecosystem was carried out. In 1972, Derek Briggs and Simon Conway Morris became involved in the Cambridge project. Originally students of Whittington, Briggs and Conway Morris played major roles in painting a reliable picture of the Burgess ecosystem - the community structure as a whole. This became the earliest ecosystem where the workings are understood in detail. It is one thing to know of an extensive collection of fossils from one particular site, but quite another to understand the ecological workings of the original environment. Because the Burgess environment was, in geological terms, very near to the time of the Cambrian explosion, it had great potential to interest much wider scientific circles. The stage was now set for the next phase of work on the Burgess Shale, which later transpired to be as important as the original scientific investigations.
Work on the Burgess Shale fossils led to the first major understanding of the Cambrian explosion within the community of Cambrian biologists, but for the obscure Burgess animals to attract the attention of a wider audience, and compete in the dinosaur arena, some particularly imaginative and skilful writing was necessary. This first came in the form of Stephen Jay Gould’s award-winning book Wonderful Life, published in 1989. In his book, Gould succeeded in showing the world that animals once existed on Earth that were far more bizarre than our wildest conceptions of alien life-forms. Wonderful Life captured unexpected levels of attention, partly attributed to an ingenious explanation of how we ourselves are involved in the Cambrian explosion. Gould’s curtain came down on Pikaia, a swimming worm that was the first known member (at that time) of the phylum to which we belong. If Pikaia had not survived the Cambrian period, the story goes, then we would not be here today.
Today it is generally believed that ten phyla are represented by the Burgess fauna: sponges, cnidarians (here sea pens and sea anemones), comb jellies, lamp shells, molluscs, hyoliths, priapulid worms, ‘bristle worms’ (there are also other worms in this phylum), velvet worms, arthropods, echinoderms (here including sea lilies and sea cucumbers) and chordates (to which we belong). Algae and cyanobacteria are also represented in the Burgess Shale biota, along with one or two animals that remain a mystery and have yet to be assigned to a phylum, although this does not necessarily imply that they belong to additional, extinct phyla.
Palaeontological gold
Although the Burgess Shale fauna dominated discussions on Cambrian evolution for many years, other Cambrian assemblages have been more recently discovered. The limestone shale of southern Sweden contains late Cambrian material in stones known as ‘Orsten’. This material shows mixed preservation, and includes some complete and exquisitely preserved tiny arthropods such as trilobites and ‘seed-shrimps’ or their relatives. The Orsten fossils show a type of preservation, called phosphatisation, which is also known from early Cambrian deposits of Comley in Shropshire, England.
The Canadian palaeontologist Nick Butterfield, now at Cambridge University, found Cambrian fossils in borehole samples from Mount Cap, near the Great Bear Lake in north-west Canada. Here, 525-million-year-old animals have been exceptionally well-preserved with fully resolvable structures as narrow as 100 nanometres, or one ten-thousandth of a millimetre (less than the wavelength of light). Among the fauna known from Mount Cap is a species of Wiwaxia. Wiwaxia was a primitive form of bristle worm where the ‘bristles’ were modified to become protective spines and scales. Its body was short and fat and its overall appearance was that of an armoured mouse. The Burgess Shale also contains the fossilised remains of a Wiwaxia, although a different species to the Mount Cap type. This illustrates the importance of the Mount Cap fossils. Although they don’t represent a diverse community, they contain very close relatives of the Burgess Shale fossils but are some ten million years older. This type of evidence can be used to set the date of the Cambrian explosion more precisely. Wiwaxia is also known from a similar period in the Spence Shale of Utah, USA. In fact the Mount Cap and Burgess Shale fossils now appear to belong to a broadly continuous belt of comparable early and middle Cambrian fossil assemblages extending from southern California through to northern Greenland and Pennsylvania.
Another extensive collection of Cambrian fossils is known from Chengjiang in Yunnan province, south China. The Chinese palaeontologist Hou Xianguang found the first Chengjiang specimen, an unusual trilobite, as a student in 1984. Hou, and his senior at that time, Chen Junguan, dedicated their subsequent years to the Chengjiang site. Extremely well-preserved specimens from several animal phyla were unearthed, and efforts to find further phyla continue at considerable pace today. A study of the ecology of the Chengjiang fauna is receiving the kind of attention previously reserved only for the Burgess Shale fossils. The Chengjiang palaeontologists’ ammunition has been the age of these fossils - 525 million years old - in addition to the wide diversity of animals represented. So the Chengjiang fauna predate the Burgess fauna by ten million years and reveal that a community structure similar to that we know of 515 million years ago was already in place 525 million years ago.
Cambrian fossil assemblages that can match the Burgess Shale in terms of preservation have been, and no doubt will continue to be, discovered. But the fossils of the Burgess Shale will remain a landmark in the study of evolution, not least because of their services to the Cambrian in the macrocosm of popular science. This may seem a trivial point compared with the wealth of pure science derived from these fossils, but in the modern world of science politics are as important as knowledge, and, without the Burgess Shale’s rise
to fame, the expeditions which led to findings of further Cambrian fossil sites might never have been funded or been made enticing.
The $64 million question
We now know of wonderfully preserved communities of animals where a diversity of animal phyla are represented from the Cambrian, but not before the Cambrian. As stated previously, the internal body plans of animal phyla evolved some 120 to more than 500 million years earlier (depending on who you believe). Hence, the variety of internal body plans found in animals today really was once hidden within the bodies of worms, for tens of millions of years. Now we can really understand what the Cambrian explosion is. It is the sudden acquisition, 543-538 million years ago, of hard external parts by all the animal phyla found today (except the sponges, comb jellies and cnidarians). It is the simultaneous transition from the prototype worm-shaped or soft-bodied form to complex, characteristic shapes (also known as ‘phenotypes’) within each phylum, and it happened in a blink of an eye on the geological timescale. The what of the Cambrian explosion is now understood.
In The Blink Of An Eye Page 5
