Burgess Shale-type preservation

Burgess Shale-type biotas are found principally in the early and middle Cambrian, but the preservational mode is also present before the Cambrian (e.g. Lantian biota) and through into the Ordovician (e.g. Fezouata). 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==

showing soft-part preservation 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 age onwards), with a notable gap in the last 150 million years of the Proterozoic. They become increasingly restricted to deep waters in the Cambrian. hypothesised that the organic material was preserved by silicification. In addition, films of phyllicate (clay) minerals can grow in situ, overprinting the biological tissue. The decay process creates chemical gradients that are essential for mineral growth to continue long enough for the tissue to be preserved. 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 concentration of high reactivity phosphates, making them the first structures to be preserved; they may be preserved in three dimensions, having been solidified before they could be flattened. As these structures are unique to predatory and scavenging arthropods, this form of preservation is limited to—and diagnostic of—such creatures. 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. 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. 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. 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. 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. ==What is preserved==

Carbon The fossils usually comprise a reflective film; when the part bears an opaque, silvery film composed of organic carbon (kerogen), the counterpart's film is blue, less reflective, and more translucent. Phyllosilicates Butterfield sees carbonaceous compressions as the main pathway of Burgess Shale-type preservation, but an alternative has been proposed. The fossils actually comprise aluminosilicate films (except for some localized carbonaceous regions, such as the sclerites of Wiwaxia), and Towe, followed by others, suggested that these may represent the mechanism of exceptional preservation. Orr et al. emphasize the importance of clay minerals, whose composition seems to reflect the chemistry of the underlying, decaying, tissue. It seems that the original carbon film formed a template on which aluminosilicates precipitated. Different phyllosilicates are associated with different anatomical regions. This seems to be a result of when they formed. Phyllosilicates primarily form by filling voids. Voids formed in the fossils as the carbon films were heated and released volatile components. Different types of kerogen—reflecting different initial conditions—mature (i.e. volatilize) at different temperatures and pressures. The first kerogens to mature are those that replace labile tissue such as guts and organs; cuticular regions produce more robust kerogens that mature later. 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. Once the rock is heated and compressed further, to the gas window, illite (rich in K/Al) and chlorite (rich in Fe/Mg) start to form; once all the available K is used up, no further illite forms, so the last tissues to mature are replicated exclusively in chlorite. or by authigenic mineralization by any of a range of other minerals. and cellular material has no preservation potential. and non-cuticular organs and organisms have been described, including the setae of brachiopods and the jellyfish ctenophores (comb jellies). The mineralogy and geochemistry of the Burgess Shale is completely typical of any other Palaeozoic mudstone. Variation between BST sites Preservation in the Chengjiang is similar, but with the addition of a pyritization mechanism, which seems to be the primary way in which soft tissue was preserved. Different BST deposits display different taphonomic potentials; in particular, the propensity of entirely soft-bodied organisms (i.e. those without shells or tough carapaces) to preserve is highest in the Burgess Shale, lower in the Chengjiang, and lower still in other sites. == How it is preserved ==

Normally, organic carbon is decayed before it is rotted. Anoxia can prevent decay, but the prevalence of bioturbation associated with body fossils indicates that many BS sites were oxygenated when the fossils were deposited. 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. The chemistry of the clay particles that buried the organisms seems to have played an important role in preservation. The carbon isn't preserved in its original state, which is often chitin or collagen. Rather, it is kerogenized. This process seems to involve the incorporation of aliphatic lipid molecules. ==Elemental distribution==

Elemental distribution is unevenly spread through the organic remains, allowing the original nature of the remnant film to be predicted. For example: • Silicon is more abundant in cuticular material • Aluminium and potassium are higher in the eyes Because the fossiliferous layer is so thin, it is effectively transparent to electrons at high-accelerating (>15V) voltages. ==Sedimentary setting==

In the Wheeler Formation, lagerstätte occur predictably at periodic sea-level high-stands. They formed on an oxygenated sea floor, and are associated with mud-slides or turbidity current events. Brine seeps 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. trying to escape a brine seep at the bottom of the Cathedral Escarpment ==Before burial==

The majority of the decay process occurred before the organisms were buried. Any such flows must have enveloped free-swimming as well as bottom-dwelling organisms. In either case, additional processes must have been responsible for the exceptional preservation. One possibility is that the absence of bioturbation permitted the fossilisation, but some Burgess Shale fossils contain internal burrows, so that can't be the whole story. It is possible that certain clay minerals played a role in this process by inhibiting bacterial decay. Alternatively, reduced sediment permeability (a result of lower bioturbation rates and abundant clays) may have played a role by limiting the diffusion of oxygen. ==During burial==

The mineralisation process began to affect the organisms soon after they had been buried. Organisms' cells rapidly decayed and collapsed, meaning that a flattened two-dimensional outline of the three-dimensional organisms is all that is preserved. Pyrite began to precipitate from seawater trapped within the sediment forming lenses of framboidal (raspberry-shaped under magnification) crystals. ==Post burial==

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. There is no evidence for these mats in the higher stratigraphic units of the Burgess Shale Formation, so they cannot be the whole story. and trace fossils are sometimes found within body fossils. 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. For instance, the Burgess Shale itself endured cooking at greenschist-level temperatures and pressures (250–300 °C, ~10 km depth/ 482-572 F, ~6.2 miles), while the Chengjiang rocks have been deeply affected by weathering. {{cite journal | journal=Geology | title=Cambrian Burgess Shale–type deposits share a common mode of fossilization | last1 = Gaines | first1 = Robert R. | last2 = Briggs | first2 = Derek E.G. | author-link2 = Derek Briggs | last3 = Yuanlong | first3 = Zhao | year=2008 | doi=10.1130/G24961A.1 | volume=36 | pages=755–758 | issue=10 The Burgess Shale has been vertically compressed by at least a factor of eight.{{cite book | chapter= Trilobites with Appendages from the Middle Cambrian, Burgess Shale, British Columbia | last1 = Whittington | first1 = H.B. | title = Evolution and morphology of the Trilobita, Trilobitoidea and Merostomata | series = Fossils and Strata | year=1975 | volume=4 | pages=97–136 | doi= 10.18261/8200049639-1975-06 ==Closing the taphonomic window==

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, Burgess Shale-type Konzervat-lagerstätten are statistically overabundant during the Cambrian compared to later time periods, which represents a global megabias. Although burrowing reduced the number of environments that could support Burgess Shale-type deposits, it alone cannot explain their demise, and changing ocean chemistry—in particular the oxygenation of ocean sediments—also contributed to the disappearance of Burgess Shale-type preservation. The number of pre-Cambrian assemblages is limited primarily by the rarity of soft-bodied organisms large enough to be preserved; however, as more and more Ediacaran sediments are examined, Burgess Shale-type preservation is becoming increasingly well known in this time period. 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. Other environmental factors change around this time: Phosphatic units disappear, and there is a stem change in organisms' shell thickness. ==Faunas==

The mode of preservation preserves a number of different faunas; most famously, the Cambrian "Burgess Shale-type fauna" of the Burgess Shale itself, Chengjiang, Sirius Passet, and Weeks, Wheeler, and Marjum formations (all three in western Utah, U.S.A.). However, different faunal assemblages are also preserved, such as the microfossils of Riphean (Tonian-Cryogenian age) lagerstätten. ==References==