Location: British Columbia
Age: Middle Cambrian
FLORA AND FAUNA
About 140 species of animals in 119 genera are represented in the Burgess Shale. It contains well-preserved representatives from a multitude of animal phyla, including some 'weird wonders' - animals which, depending on the interpretation of the particular scientist, may not be easily classifiable into any living phyla. About 86 percent of the animal genera and about 98 percent of the individual animals were soft-bodied; because they lacked the hard parts which are most frequently found in the fossil record as teeth, shells, or bones, they would not normally survive the decay process. This means that the Burgess Shale and related sites offer a unique biological picture into a time when animals diversified, or even exploded, into unique, fundamental body plans (Baupläne).
Figure 1. Percentage histogram of Burgess Shale animal phyla. Black shaded sections of the bars represent well-mineralized skeletons and white regions of the bars represent soft-bodied biota. 'Miscellaneous' contains incertae cedis, or taxa that cannot be assigned to any known phylum. From Conway Morris (1986).
Arthropods clearly dominated the fauna, though sponges, various worm-like phyla (annelids and priapulids), brachiopods, echinoderms, chordates, and molluscs were also present. Reconstructions of the life positions of the Burgess Shale fauna can often give clues on Cambrian palaeoecology, or the relationships of extinct animals in their habitat or environment. Most of the animals in the Burgess Shale were benthic, meaning that they dwelled in substrate in the bottom of the water. However, analyses of the community structure show that diverse modes of life were already in place as early as the Middle Cambrian. Animals were divided into active and passive suspension feeders (filtered water by creating currents or using pre-existing currents, respectively, to strain nutrients and plankton), deposit feeders (ingested sediment and any food particles in it), scavengers, and active predators. The animals also show ecological tiering, being free swimmers or living on the bottom. Bottom dwellers could be further classified into organisms that burrowed into the sediment or crawled on the surface.
An often-neglected aspect of the Burgess Shale is its microscopic fossils, or microfossils. Single-celled organisms, such as bacteria, blue-green algae, and various other planktonic forms that float in the water column, have been recovered. Larger, macroscopic fossils of algae with a higher grade of complexity (i.e., possessing many cells in their bodies) have also been found and are often indicators of the environment of the Burgess Shale; their abundance patterns imply that the environment of the Burgess Shale was divided in different zones.
EVOLUTIONARY SIGNIFICANCE OF THE BURGESS SHALE
The types of organisms in the Burgess Shale can give scientists clues on the 'big picture' of natural history. This is important because it documents how evolution works across very long periods of time. Some researchers have argued that the tree of life was initially analogous to a large branching bush or lawn, with much greater range of form or morphology than that of today. Extinction events 'pruned' this bush, so today there is actually a smaller range of morphology in animals.
To assess the differences between organisms, the amount of diversity and disparity are estimated. Diversity is simply counts of taxa (sets of organisms related together by certain characteristics). In other words, it is the number of species, genera, families, and so forth in the traditional taxonomy of Linnaeus. Disparity, on the other hand, describes the extent of morphological differences between various taxa. This is essentially the organization of anatomical form that defines the larger sets of organisms.
Figure 2. Several hypotheses of the evolution of morphological diversity, or disparity. (a) Traditional model of gradually increasing disparity over time; (b) Model of maximum disparity early in evolutionary history and eventual stabilization; (c) Model of long-term gradually increasing disparity of cryptic deep Precambrian animals and followed by rapid stabilization; (d) Model of short-term gradually increasing disparity of cryptic late Precambrian animals followed by eventual stabilization. Image courtesy of M.A. Wills of the University of Bath, UK.
Despite the incompleteness of the fossil record, it is agreed upon that Cambrian diversity is lower than that of today. But it has been shown that the disparity of organisms in the Burgess Shale was nearly equal to that of modern organisms. This has been done using a variety of independent methods, including empirical and theoretical approaches. This means that in only the first 10 percent of the history of multicellular life, around 80 percent of modern disparity was reached, demonstrating the rapidity of the evolution of form.
The Burgess Shale has also stimulated interest in the concept of contingency, which means that small, chance events in natural history can potentially trigger drastic changes in the future. In other words, if one or a few of the Burgess Shale animals went extinct at the 'wrong' time and others took its place, and if the tape of the history of life was then replayed, we may see a totally different assemblage of creatures today. For example, humans would not exist if their chordate ancestors failed to flourish and survive. This intriguing hypothesis, so far, has not yet been rigorously tested and remains a controversial topic among palaeontologists.
Section author: Alexei A. Rivera
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