Burgess Shale-type preservation
The taphonomic regime results in soft tissue being preserved, meaning that organisms without conventionally fossilized hard parts can be seen.
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.
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.
[7] 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.
[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.
[10] The decay process creates chemical gradients that are essential for mineral growth to continue long enough for the tissue to be preserved.
[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.
Kaolinite (rich in Al/Si, low in Mg) is the first phyllosilicate to form, once the rock is metamorphosed to the oil window, and thus replicates the most labile regions of the fossil.
[19] It's not entirely clear whether pyrite is involved in the preservation of the anatomy, or whether they simply replace carbon films later in diagenesis (in the same fashion as phyllosilicates).
It seems that the reduced permeability associated with the clay particles that make up the sediment restricted oxygen flow; furthermore, some beds may have been 'sealed' by the deposition of a carbonate cement.
[33] Elemental distribution is unevenly spread through the organic remains, allowing the original nature of the remnant film to be predicted.
[37] One hypothesis for exceptional preservation is that brine seeps—inputs of water with a high ion content, probably associated with fluid flow along faults—altered the sedimentary environment.
They would enrich the area with nutrients, allowing life to prosper; the high salinity of the sea floor would deter burrowing and scavenging; and the unusual cocktail of chemicals may have enhanced preservation.
[39] While the Chengjiang fauna underwent a similar preservational pathway to the Burgess Shale, the majority of organisms there are fossilised on their flattest side, suggesting that they were swept to their final resting place by turbidity currents.
[41] One possibility is that the absence of bioturbation permitted the fossilisation,[41] but some Burgess Shale fossils contain internal burrows, so that can't be the whole story.
[41] Alternatively, reduced sediment permeability (a result of lower bioturbation rates and abundant clays) may have played a role by limiting the diffusion of oxygen.
[6] Pyrite began to precipitate from seawater trapped within the sediment forming lenses of framboidal (raspberry-shaped under magnification) crystals.
[41] Organisms may have been shielded from oxygen in the ocean by a microbial mat, which could have formed an impermeable layer between the sediment and the oxic water column.
[45] Because of the great age of Cambrian sediments, most localities displaying Burgess Shale-type preservation have been affected by some form of degradation in the following 500+ million years.
[1] For instance, the Burgess Shale itself endured cooking at greenschist-level temperatures and pressures (250–300 °C, ~10 km depth[10]/ 482-572 F, ~6.2 miles), while the Chengjiang rocks have been deeply affected by weathering.
[46] Burgess Shale-type preservation is known from the "pre-snowball" earth, and from the early to middle Cambrian; reports during the interlying Ediacaran period are rare,[6] although such deposits are now being found.
[47] Burgess Shale-type Konzervat-lagerstätten are statistically overabundant during the Cambrian compared to later time periods, which represents a global megabias.
While the post-revolution world was full of scavenging and predatory organisms, the contribution of direct consumption of carcasses to the rarity of post-Cambrian Burgess Shale-type lagerstätten was relatively minor, compared to the changes brought about in sediments' chemistry, porosity, and microbiology, which made it difficult for the chemical gradients necessary for soft-tissue mineralisation to develop.
