Location: British Columbia
Age: Middle Cambrian (505 Ma)
Figures 1 and 2. The image on the left is a fossil of the chelicerate arthropod Sanctacaris (10 cm in length). Note the fine preservation of the head appendages and the clear delineation of segments along the body. This exceptional preservation of the soft tissue is characteristic of Burgess Shale fossils. On the right is a reconstruction of Sanctacaris as it appeared in life. Images from The Fossils of the Burgess Shale, Briggs, D.E.G., et al. 1994.
Taphonomy is the study of the process of how organisms become fossils. Most fossils, due to relatively poor preservation, lack the fine structure of the hard or soft tissues they once had in life. The Burgess Shale, however, is a site of exceptional fossil preservation, or a conservation lagerstätte. Its fossils are preserved as dark organic films on thin layers of fine-grained shale; soft parts, such as muscle tissue or poorly calcified skeletons, are preserved in great detail.
Figure 3. Geomorphic illustration of the Burgess Shale location. This illustration shows the scale of the shallow reef environment. A mudslide killed, transported, and buried the Burgess Shale animals. They were deposited at the base of the submarine escarpment, or underwater cliff, in deeper water that had lower dissolved oxygen concentrations. This environment, coupled with burial, contributed to the preservation and fossilization of the animals. Illustration courtesy of the Geological Survey of Canada.
The key to this high degree of preservation are the large-scale processes of rapid burial and instant death. The animals were swept down from their shallow water algal reef habitat by a turbidity flow, essentially a landslide of mud. It is possible that a tremor, earthquake (the region was probably highly tectonically active), or storm event caused a very large mass of sediment to dislodge and roll downward the cliff because of gravity, a process known as slumping. The single solid mass broke up into a large, fast-moving cloud of tiny sedimentary particles which eventually engulfed the animals, suffocating them. They were swept down to the base of the reef cliff (the base of what is now known as the Cathedral Formation) and were killed instantly in a single mass mortality event; there is no evidence that they attempted to free themselves from the sediment flow. Because of the relatively high energy of the mudslide, the animals were assembled haphazardly; they were not in life position when they eventually settled in the sediment.
The freshly killed carcasses were quickly buried in a deep water environment with little or no oxygen (anoxic) and thus were protected from scavengers and bacteria that encourage decomposition. The lack of trace fossils (tracks or burrows of living animals) strongly suggest the bottom waters were anoxic. Because of the lack of exposure, their rates of decomposition were slowed down. Note, however, that oxygen concentrations in the Burgess Shale depositional environment may have been variable. When the oxygen level was low, soft-bodied forms were preserved and the surrounding sediment was characterized by no trace fossils. When it was higher, the carcasses and surrounding sediment were 'colonized' by other life.
Figure 4. A time and space series schematic of the events leading to the burial of the Burgess Shale animals. (A) depicts the animals before the slumping event. (B) shows the sediment slump transporting the animals downward an underwater cliff or slope for a certain distance away from their original locality and habitat. (C) shows the animals, in scattered positions, in their final resting place where the process of fossilization will take place. Courtesy of Conway Morris (1986).
One hypothesis on the small-scale mechanism on why the
carcasses were so finely preserved was that fined grain clay,
which inhibits decay, permeated every 'nook and cranny' of the
carcasses. Another hypothesis was that the clay behaved like a
fluid and was 'injected' into the carcasses and then the clays
eventually stiffened, leading to the preservation of the animals'
impressions in the shale. However, it
was later shown that clay could not have entered the carcasses
during or shortly after their deposition at the base of the reef.
Rather, the clay minerals replicated the fine structures of the
animal tissue by docking onto attachment points on the tissue
and/or by directly precipitating on the tissue. The latest research
points out, however, that the 'clay particle docking model' was
statistically improbable. Rather, it was proposed that iron ions
in the water stuck to the tissues of the carcasses and inhibited
decay caused by bacteria. Both this process, as well as direct
precipitation of clay minerals onto the tissue, may have occurred.
Section author: Alexei A. Rivera
This section is part of a Fossil Lagerstätten
web site which has been built up as a result of the efforts of
the 2002-3 MSc
Palaeobiology class in the Department of Earth Sciences at
University of Bristol, as part of a course in Scientific Communication.
Department of Earth Sciences
University of Bristol
Wills Memorial Building