Burgess Shale type preservation

The Burgess Shale of British Columbia is famous for its exceptional preservation of mid-Cambrian organisms. Around 40[1] 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[2] and Ordovician periods.[3]

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.

Distribution
Burgess Shale type biotas are found only in the early and middle Cambrian,[4] 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.

Preservational regime
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 onwards), with a notable gap in the last 150 million years of the Proterozoic.[5] They become increasingly restricted to deep waters in the Cambrian.[6]

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.

Walcott, the discoverer of the Burgess Shale on 30 August 1909,[7] hypothesised that the organic material was preserved by silicification.[1] 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.[1] In many cases, both were present, suggesting that the original carbon was preserved, and the process of its preservation caused clay minerals to form in a predictable fashion.[1]

When carbon is preserved it usually forms films of the highly cross-linked and essentially inert compound kerogen, with kerogen formation from organic precursors likely to happen as the host rock is exposed to high pressures.[8] In addition, films of phyllicate (clay) minerals can grow in situ, overprinting the biological tissue.[9] Chemical gradients - created by the decay process - are essential for mineral growth to continue long enough for the tissue to be preserved.[6] Oxygen in the sediment allows decomposition to occur at a much greater rate; this decreases the quality of the preservation, but does not prevent it entirely; the conventional, exceptionally preserved fossils of the Burgess Shale are supplemented by the shells of organisms which lived on and burrowed into the sediment before the exceptional preservation pathway was complete. The organisms' presence shows that oxygen was present, but at worst this "paused" the mineralisation process.[6] It seems that whilst anoxia facilitates BS-type preservation, it is not essential to the process.[10]

In addition to the organic films, parts of many Burgess Shale creatures are preserved by phosphatisation. The mid-gut glands of arthropods often host a high reactivity and concentration of phosphate, making them the first structures to be preserved; they may be preserved in three dimensions, having been solidified before they could be flattened.[11] As these structures are unique to predatory and scavenging arthropods, this form of preservation is limited to - and diagnostic of - such creatures.[11]

Another type of mineralisation that is common in Chengjiang deposits is pyritisation; pyrite is deposited as a result of the activity of sulfate-reducing bacteria organisms soon after their burial.[1]

With the exception of phosphatic preservation, individual cells are never preserved; only structures such as chitinous exoskeleton, or scales and jaws, survive. This poses little problem for most invertebrate groups, whose outline is defined by a resistant exoskeleton.[5] Pyrite and phosphate are exceptional additions to Burgess Shale type preservation, and are certainly not found in all localities. The defining preservation process is that which preserves organic film plus phyllosilicate. For this preservation to occur, the organisms must be protected from decay.[1] There are a few ways that this can happen; for instance they can be chemically protected within the sediment by phyllosilicates or biopolymers, which inhibit the action of decay related enzymes.[1] Alternatively the sediment could be "sealed" soon after the organisms were buried within it, with a reduction in porosity preventing oxygen from reaching the organic material.