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| The Burgess Shale | |||||||||||||||||
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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 and Ordovician periods.[2]
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.
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Burgess Shale type biotas are found only in the early and middle Cambrian,[3] 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.[1]
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.[4] They become increasingly restricted to deep waters in the Cambrian.[5]
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, 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.[6] In addition, films of phyllicate (clay) minerals can grow in situ, overprinting the biological tissue.[7] Chemical gradients - created by the decay process - are essential for mineral growth to continue long enough for the tissue to be preserved.[5] Oxygen in the sediment allows deposition 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.[5] It seems that whilst anoxia facilitates BS-type preservation, it is not essential to the process.[8]
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.[9] As these structures are unique to predatory and scavenging arthropods, this form of preservation is limited to - and diagnostic of - such creatures.[9]
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.[4] Excellent preservation of hard part surface features can be discerned on some of the Burgess fossils - periodic surface features resembling diffraction grating patterns with periods in the order of a few tenths of a micron have been detected in some Burgess shale species such as Marrella, Wiwaxia and Canadia suggesting that these animals displayed an iridescence. [10]
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.[1]
Phyllosilicates are present in many Burgess Shale fossils. Because the composition of the minerals varies with different organ types within the fossils, some palaeontologists think that the phyllosilicates must reflect mineralised remnants of clay minerals that formed very early, during the decay process.[11][12] However, another school of thought suggests that the phyllosilicates formed much later, because they don't just form where decay products would be expected - for instance they are found to replace calcite shells and mineral veins.[7][12] This rings true with graptolitic mudstones, where phyllosilicates are found to occur as a result of low grade metamorphism.[12] The precise chemical structure of the phyllosilicate preserved depends on how resistant the tissue is to crushing and decay.[12] This theory doesn't rule out the possibility that early-forming minerals improved the quality of preservation observed.[12] However it does look unlikely that aluminosilicate films represent mud squashed in to fossils; rather they likely accumulated in voids,[13] presumably created as soft tissue slowly decayed or was converted to gasses.[14]
The fossils usually comprise of a reflective film; when the part bears an opaque, silvery film composed of organic carbon, the counterpart's film is blue, less reflective, and more translucent.[7]
Some specimens bear a dark stain representing decay fluids injected into the surrounding wet sediment. (See Dark stain.)
Calcite is often not preserved, perhaps reflecting acidic conditions of burial. It may be replaced by aluminosilicates.[7]
Muscle can in very rare cases survive by phosphatisation.[15] However, predominately soft tissues, such as muscles and gonads, are never preserved by the carbonaceous-compression preservational pathway.[16] Phosphatisation and the presence of other enzymes means that guts and mid-gut glands are often preserved.
Otherwise it is cuticle that is most consistently present. Butterfield argues that only recalcitrant tissue (e.g. cuticle) can be preserved as a carbonaceous compression,[17] and cellular material has no preservation potential.[16] However, Conway Morris and others disagree,[18] and non-cuticular organs and organisms have been described, including the setae of brachiopods[19] and the jellyish ctenophores (comb jellies).[20]
The mineralogy and geochemistry of the Burgess Shale is completely typical of any other Palaeozoic mudstone.[12]
Elemental distribution is unevenly spread through the organic remains, allowing the original nature of the remnant film to be predicted. For example:
Because the fossiliferous layer is so thin, it is effectively transparent to electrons at high (>15V) accelerating voltages.[23]
In the Wheeler Formation, lagerstatte occur predictably at periodic sea level high-stands.[24] They formed on an oxygenated sea floor, and are associated with mud-slides or turbidity current events.[24]
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.[25]
The majority of the decay process occurred before the organisms were buried.[26]
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.[27] The location at which an organism ultimately comes to rest may depend on how readily it floats, a function of its size and density.[27] Organisms are much more randomly arranged in the Burgess Shale itself.[27]
Turbidity currents have also been posited as the depositional system for the Burgess Shale, but mud-silt flows seem more consistent with the available evidence. Such "slurry flows" were somewhere between a turbidity current and a debris flow.[28] Any such flows must have enveloped free-swimming as well as bottom-dwelling organisms.[29] In either case, additional processes must have been responsible for the exceptional preservation.[28] One possibility is that the absence of bioturbation permitted the fossilisation,[28] but some Burgess Shale fossils contain internal burrows, so that can't be the whole story.[30] It is possible that certain clay minerals played a role in this process by inhibiting bacterial decay.[28] Alternatively, reduced sediment permeability (a result of lower bioturbation rates and abundant clays) may have played a role by limiting the diffusion of oxygen.[28]
The mineralisation process began to affect the organisms soon after they had been buried.[26] Organisms' cells rapidly decayed and collapsed, meaning that a flattened two dimensional outline of the three dimensional organisms is all that is preserved.[4] Pyrite began to precipitate from seawater trapped within the sediment forming lenses of framboidal (raspberry-shaped under magnification) crystals.[28]
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.[31][26] There is no evidence for these mats in the higher stratigraphic units of the Burgess Shale Formation, so they can't be the whole story.[28] However, cyanobacteria do appear to be associated with the preservation of the Emu Bay Shale, which was deposited beneath an oxygen-rich water column; by growing over carcasses, microbial mats held their soft tissue in place and allowed its preservation.[32] It is possible that the sediments were not always anoxic, but that burrowing was prevented in oxic intervals by a high deposition rate, with new material provided faster than burrowers could keep up with.[28] Indeed, a growing body of research indicates that sediment oxygenation is not related to preservation quality; the Burgess Shale itself appears to have been consistently oxic[25] and trace fossils are sometimes found within body fossils.[33]
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[7]), while the Chengjiang rocks have been deeply affected by weathering.[1] The Burgess Shale has been vertically compressed by at least a factor of eight.[34]
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,[4] although such deposits are now being found.[35] Burgess-shale type Konzervat-lagerstätten are statistically overabundant during the Cambrian compared to later time periods, which represents a global megabias.[36] The mode of preservation is more abundant before the Cambrian substrate revolution, a development in which burrowing organisms established a foothold, permanently changing the nature of the sediment in a fashion that made soft-part preservation almost impossible. Consequently, the quantity of post-Cambrian Burgess Shale-type assemblages is very low.[5] The number of pre-Cambrian assemblages is limited primarily by the rarity of soft-bodies 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.[5] Just like microbial mats, environments which could produce this mode of fossilisation became increasingly restricted to harsher and deeper areas, where burrowers could not establish a foothold; as time progressed, the extent of burrowing increased sufficiently to effectively make this mode of preservation impossible.[5]
However, Burgess Shale type biotas do in fact exist after the Cambrian (albeit somewhat more rarely). Other factors may have contributed to the closure of the window at the end of the Amgan (middle Mid Cambrian), with many factors changing around this time. A transition from an icehouse to a greenhouse world has been associated with an increase in storm intensity, which may have hindered exceptional preservation.[37] Other environmental factors change around this time: Phosphatic units disappear; and there s a stem change in organisms' shell thickness.[37]
The mode of preservation preserves a number of different faunas; most famously, the Cambrian "Burgess Shale type faunas" of the Burgess Shale itself, Chengjiang, Sirius Passet and Wheeler Formation. However, different assemblages are also preserved, such as the microfossils of Riphean (Tonian-Cryogenian age) lagerstätten.[38]
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