Burgess Shale-type preservation

The Burgess Shale of British Columbia is famous for its exceptional preservation of mid-Cambrian organisms. Around 69 other sites have been discovered of a similar age, with soft tissues preserved in a similar, though not identical, fashion. Additional sites with a similar form of preservation are known from the Ediacaran and Ordovician periods. These various shales are of great importance in the reconstruction of the ecosystems immediately after the Cambrian explosion. The taphonomic regime results in soft tissue being preserved, which means that organisms without hard parts that could be conventionally fossilised can be seen; also, we gain an insight into the organs of more familiar organisms such as the trilobites. The most famous localities preserving organisms in this fashion are the Canadian Burgess Shale, the Chinese Chengjiang fauna, and the more remote Sirius Passet in north Greenland. However, a number of other localities also exist. Burgess Shale-type biotas are found only in the early and middle Cambrian, but the preservational mode is also present before the Cambrian. It is surprisingly common during the Cambrian period; over 40 sites are known from across the globe, and soft-bodied fossils occur in abundance at nine of these. Burgess Shale-type deposits occur either on the continental slope or in a sedimentary basin. They are known in sediments deposited at all water depths during the Precambrian (Riphean stage onwards), with a notable gap in the last 150 million years of the Proterozoic. They become increasingly restricted to deep waters in the Cambrian. In order for soft tissue to be preserved, its volatile carbon framework must be replaced by something able to survive the rigours of time and burial. Charles Walcott, who discovered the Burgess Shale on 30 August 1909, hypothesised that the organic material was preserved by silicification. When the shale was redescribed in the 1970s, it was possible to take a more experimental approach to determining the nature of the fossils, which turned out to be mainly composed of carbon or clay minerals.