- Wonderful Life by Stephen Jay Gould
Hutchinson Radius, 347 pp, £14.95, February 1990, ISBN 0 09 174271 4
The ecosystems of shallow marine waters – coral reefs, for example – are the most diverse in the modern oceans, and they have probably been so throughout the history of life. And yet they are under-represented in the fossil record. For environments that are rich in life are also rich in the means of destroying it. When a shrimp or fish dies, it is rapidly devoured by scavenging crustaceans or decomposed by bacteria: every trace of the organism is destroyed in the process. In order for a fossil to be left, the dead organism must somehow be sheltered from the grave-robbing crabs, starfish and bacteria that thrive in shallow-water environments. An animal stands a much better chance of being preserved as a fossil in an anoxic deep-water environment, where there are fewer bacteria and almost no scavengers. The fossil record for oxygen-rich shallow waters is much less complete.
The Burgess Shale is the exception. A lucky accident, or series of accidents, about 530 million years ago transported shallow-water animals by the hundreds of thousand from their normal environment into much deeper water. They were killed in the process but came to lie in a place where they could be preserved. The means was probably a mudslide: the animals, it is thought, were living beneath a cliff made of mud, and when the cliff collapsed it carried down with it an avalanche of shallow-water animals into the anoxic depths.
The Burgess Shale fossil locality is now eight thousand feet up a mountain quarry in British Columbia. Fossils had first been discovered there by a US geologist, Charles Walcott, in 1909 (as Gould convincingly shows, from Walcott’s diaries). In the next eight years, Walcott collected over eighty thousand specimens and deposited them in the Smithsonian Institution in Washington. He never found the time to work on them as thoroughly as he wished, but he did make some preliminary observations. The observations proved highly influential.
Walcott described some of the species, and suggested where they fitted into the Linnaean classification of animal life. He placed, (or, as Gould likes to say, ‘shoehorned’) all the Burgess Shale animals into already-known taxonomic groups. Of 22 arthropods, for example, he thought that eight were fairy shrimps, six were malacostracans (the group containing crabs and prawns), four were trilobites, and four others belonged in another mainly fossil group, the merostomoids (the group containing the horseshoe crab). Not only did Walcott place the Burgess Shale animals in known groups: he also suggested that they were ancestral versions of their modern relatives. After all, you might expect that if you traced life back over 500 million years, you would come to some ancestors. For over half a century, Walcott’s descriptions provided the basis for almost everything that was said about the Burgess fauna.
In 1966, the Cambridge geologist Harry Whittington decided to re-examine the Burgess fossils. His first crucial discovery was that the Burgess fossils have a three-dimensional structure, preserved within the rocks. Walcott had thought that the fossils were squashed flat on the rock surface: Whittington drilled down with a dentist’s drill and found there was more structure below. He was thus able to reconstruct the animals in much more detail. And when Whittington produced his reconstructions, the animals turned out very differently from Walcott’s. They no longer appeared to be primitive versions of modern animals. They were related to modern forms: but as separate early radiations within a broad category, and not as ancestors. For example, many of the fossils are arthropods or annelids, both of which are large modern groups. Walcott had described the Burgess Shale species as ancestral arthropods or worms: but, on Whittington’s reconstructions, the same fossils in many cases turned out not to fit into any modern sub-divisions of annelids or arthropods: they belong in their own sub-divisions. Some of the more astonishing Burgess animals do not even fit into large modern categories. They are recognisable as animals – but not as arthropods, or annelids, or chordates, or any other such group. They belong in major groups of their own, groups no longer represented in our oceans.