A famous fauna vanished with a whimper, not a bang
SOME discoveries are so unusual it takes years, decades and sometimes centuries to understand their full significance. One such discovery is the fossil bed known as the Burgess Shale, which contains a record of bizarre creatures that lived 505m years ago. It was discovered in the Canadian Rockies over a century ago and was popularised in 1989 in a book, “Wonderful Life”, by Stephen Jay Gould, an American palaeontologist. It has long been believed that the curious fauna that lived there vanished in a series of extinctions because the fossil record ends. But that no longer appears to be the case.
The Burgess Shale came soon after a period of time known as the Cambrian explosion, when most major groups of complex animals arose over a surprisingly short period. Before 560m years ago, most living things were either individual cells or simple colonies of cells. Then, and for reasons that remain a mystery, life massively diversified and became ever more complex as the rate of evolution increased. An unusual feature of the Burgess Shale is that it is one of the earliest fossil beds to contain impressions of the soft parts of animals. One of palaeontology’s dirty little secrets is that fossils are not so much a record of past life, as a record of hard-bodied past life–the things that have bones and shells.
Although the fossil bed was discovered on a mountain, when these animals were alive they dwelt under an ocean, the bed of which was later thrust up to create the Rockies. Nobody knows exactly why they were so well preserved. One possibility is that the creatures were buried quickly and in conditions that were hostile to the decomposition of soft body parts by bacteria.
Those that first worked on the Burgess Shale, unearthing 65,000 specimens over a 14-year period up to 1924, assumed that the fossils came from extinct members of groups of animals in existence today. This turned out to be misleading because many of the creatures are so unusual that they are still difficult to classify
Opabinia, for example, grew to about 8cm (3 inches), had five eyes, a body that was a series of lobes, a fan-shaped tail and ate using a long proboscis. The proboscis had a set of grasping claws on the end, which were used to grab food and stuff it into Opabinia’s mouth. Nectocaris, meanwhile, looked a little bit like a leech, with fins and tentacles. Weirdest of all was Hallucigenia, described by Simon Conway Morris, of the University of Cambridge. With its multiplicity of spines and tentacles, little about Hallucigenia made sense. Like an abstract painting, at first nobody could work out which way up it went, which hole food went into and which hole food came out of. (The picture on the previous page shows what is currently believed to be the correct orientation.)
Palaeontologists had long thought that many of the Burgess Shale animals are examples of experiments in evolution. In other words, entirely new forms of life that did not survive or lead to other groups or species. Hallucigenia, ironically, turned out to be the exception that proved the rule. It is now thought to be an ancestor of the modern group of arthropods, which includes everything with a hard exoskeleton, from flies and butterflies to centipedes and crabs. Many other strange creatures turn out to be familiar faces in disguise.
Now another misconception has been quashed. Writing in Nature, Peter Van Roy of Yale University and his colleagues suggest that the sudden absence of such crazy soft-bodied fossils does not indicate a mass extinction, but merely an end to the unusual local circumstances that caused the creatures to be preserved. In an area of the Atlas Mountains of Morocco the researchers have found another diverse (and sometimes bizarre) assemblage of soft-bodied organisms from a period not long after the Burgess Shale. One discovery includes something that may be a stalked barnacle. This suggests that the evolution of such complex life went on uninterrupted. For its part, the Burgess Shale continues to produce an astonishing array of undefinable creatures faster than palaeontologists can examine them. The world still has plenty to learn about this wonderful life.