In The Blink Of An Eye (6 page)

Figure 1.5
The Ediacaran animals
Tribrachidium
,
Mawsonites
and
Parvancorina
.

At first sight, many of the Ediacaran organisms look like species living today. But some of these similarities may be explained by our old stumbling block - convergence. Take
Dickinsonia
, for instance. The Precambrian species
Dickinsonia
appeared elliptical in its shape from above. It grew to about a metre in length yet was less than 3 millimetres thick. Several hundred specimens of
Dickinsonia
have been collected from the Ediacaran Hills - it must have been quite common. It is also known from northern Russia, and presumably inhabited a large proportion of the Late Precambrian globe.
Dickinsonia
shows dividing lines radiating from a central region to the edges of the animal. If these lines are interpreted as segment-dividers, a body made up of separate, connected segments is conceived. Then
Dickinsonia
could be assigned to the phylum of segmented worms, to which living bristle
worms, earthworms and leeches belong. But there is another possibility even if we continue down the segmentation path. If the ‘segmentation' of
Dickinsonia
evolved as a means of increasing its body size it could belong to another phylum, because the equivalent character in segmented worms evolved in response to soft layers of sand, to facilitate burrowing. So for what purpose did the ‘segments' of
Dickinsonia
evolve? There is more evidence that the fossil record can provide towards this question, but it doesn't involve fossils of the animals themselves.

Despite the number of fossilised specimens known of
Dickinsonia
, no fossilised burrows have been found that could have accommodated this species. Yet burrows are known to preserve as fossils - such marks left by ancient animals are known as trace fossils. But surface trails rather than burrows are the characteristic trace fossils of the Ediacaran epoch. This suggests that some Ediacaran animals crawled around on the sea floor and none burrowed into it. Now we can cross segmented worms off the Precambrian list, at least in their segmented forms, with their burrowing lifestyles. So
Dickinsonia
was not a segmented worm.

Figure 1.6
The Ediacaran animal
Dickinsonia costata
.

Recently, researchers have been able to unravel the ancient ancestors of Precambrian animals by comparing the squiggles and swirls that mark their trails with the wriggling trails left by worms and other soft-bodied animals today. Thus, problems that had been unsolved following years of anatomical study are now being resolved.

Signs of internal features of the Ediacaran animals are not evident, but theories relating them to jellyfish and sea pens (cnidarians) have become accepted as serious propositions. Indeed, we have found probable embryos of jellyfish (and sponges) in Chinese rocks 570 million years old. This would place the phylum Cnidaria in the Precambrian with a diversity of external forms or body shapes. Also, it is now believed that many of the Ediacaran fossils represent animals from more derived phyla, albeit before they possessed their characteristic shapes of today. This will be addressed later.

The gap in geological time that once separated the Ediacaran fossils from the next suite of fossils to be found, and once provided evidence that the Ediacaran organisms were a ‘failed' first attempt at evolving animals and consequently died out, has been filled. Now it is known that the Ediacaran organisms lived right up to the next major event in animal evolution. Not only that, but the last six million years of their existence appears to have been the period of greatest Ediacaran diversity. No one knows why they died out, although this probably has a good deal to do with the sudden, ‘blitzkrieg' appearance of the next dominant forces on Earth. It is time now to consider the Cambrian explosion.

The Cambrian explosion, which post-dated the Ediacaran fauna, is a milestone in evolution that can be matched in significance only by the beginning of life itself. It paved the way for the emergence of the vast
diversity of life found today, whether in Australia's Great Barrier Reef or Brazil's tropical rainforests. It involved a burst of creativity, like nothing before or since, in which the blueprints for the external parts of today's animals were mapped out. Animals with teeth and tentacles and claws and jaws suddenly appeared. Explanations for this grand event do not have such a deep history as theories on the origin of life, only because the Cambrian explosion is a recent realisation. Darwin, among others, became puzzled by the sudden appearance of hard-shelled fossils at the beginning of the Cambrian period, about 543 million years ago, and by their apparent lack of evolutionary ancestors. Darwin and his contemporaries hypothesised that early forms of each animal phylum did not fossilise or became entombed in old rocks that were unsuitable to provide preservation as fossils. But as we have seen, Darwin had only micro-evolution to work with. Now we have so many examples of well-preserved sedimentary rocks (suitable for fossil preservation) from before the Cambrian that it is no longer reasonable to claim that conditions only became suitable for preservation during the Cambrian. Today's view of the fossil record invokes a Cambrian ‘Big Bang' in the evolution of external body parts from soft, worm-like forms.

The Cambrian is a relatively brief period in the history of the Earth yet outstanding in the history of life. Spanning just forty-three million years it was a period of monumental change. The earliest hard-shelled fossils that Darwin pondered over were later revealed to have appeared even more suddenly. They were narrowed down to the Cambrian period on the discovery of the fossils of the Burgess Shale. These abundant fossils, of the fauna and flora communities that existed 515 million years ago, have been the subject of many spirited scientific discussions and are deserving of their place in the history of science. They have been regarded either as the predecessors of today's fauna and flora or as enigmatic species that belong to phyla that did not survive the Cambrian. We now prefer the interpretation that the Burgess Shale organisms can be accommodated within today's thirty-eight phyla (actually a few of these are extinct today).

Although barely cutting through the distant haze, transcendental mountains are prominent over the otherwise featureless landscape viewed from Calgary in Alberta, south-western Canada. One is instantly drawn to those geological wonders, the Rocky Mountains, and the attraction becomes stronger as they are approached via the Trans-Canadian Highway. Banff National Park is the first port of call on entry to the Rocky Mountains. Everywhere there are mountains that could be termed spectacular even when compared to any other mountain range. It is the steepness of the mountainsides, their jagged peaks, endless variety of shapes and their ‘contour lines' running in conflicting directions that makes this place so unique. The ‘contour lines' swirl around the landscape, continuously pulling the eye in different directions and towards different focal points. These lines are actually the boundaries of sediment layers, laid down millions of years ago by sediment in the sea settling out on to the bottom, forming a new sea floor. So although a thousand metres or two above ground today, the rock that constitutes these mountains began its history underwater. As the Earth's plates moved around throughout geological time, and slowly crashed into each other, something had to give. The rocks that now form the Rocky Mountains were one of those things. They were forced up from below the water and into the air, and their chaotic movements produced the uneven patterns of ‘contour lines' seen on the mountains today.

Continuing west along the Trans-Canadian Highway, and staying within the Rockies, one enters British Columbia and Yoho National Park. The small mining town of Field lies at the foot of Mount Stephen, famous for its Cambrian trilobite fossils. The rusting iron shacks of Field are gradually being replaced by wooden bungalows and small motels that blend into the coniferous surroundings. This is now a useful base for serious mountain walkers. But there is something unique about Field. Some of the best preserved, complete Burgess Shale fossils are on display at the information centre here. These fossils separate Field from the rest of the Rockies and inspire thoughts that there is something very special about this place. The fossil display is the work of Des Collins,
a palaeontologist at the Royal Ontario Museum in Toronto, eastern Canada. Each year Des Collins and his team of helpers and students rent one of the utilitarian wooden bungalows in Field. On my visit to this bungalow in July 2000, I was directed to Des Collins's new and more temporary base camp, at the site of the Burgess Shale quarry.

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.