Burgess shale type preservation

The Burgess shale of British Columbia is famous for its exceptional preservation of mid-Cambrian organisms. Around 40 other sites have been discovered of a similar age, with soft tissues preserved in a similar, though not identical, fashion.

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 preservation is restricted to the early and middle Cambrian, but is surprisingly common; 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. 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.

Walcott, the discoverer of the Burgess shale, 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. 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.

Carbon can be preserved it forms films of the highly cross-linked and essentially inert compound kerogen, which may happen as it is exposed to high pressures.cite journal
doi = 10.1016/j.palaeo.2004.07.034
title = A New Hypothesis for Organic Preservation of Burgess Shale Taxa in the Middle Cambrian Wheeler Formation, House Range, Utah
year = 2005
author = Gaines, R
journal = Palaeogeography Palaeoclimatology Palaeoecology
volume = 220
pages = 193] In addition, films of phyllicate (clay) minerals can grow "in situ", overprinting the biological tissue.cite journal
title = Fossil Diagenesis in the Burgess Shale
year = 2007
journal = Palaeontology
volume = 50
pages = 537
doi = 10.1111/j.1475-4983.2007.00656.x
author = Butterfield, Nicholas J.; Balthasar, Uwe; Wilson, Lucy A] Chemical gradients - created by the decay process - are essential for mineral growth to continue long enough for the tissue to be preserved. 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.

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.cite journal
url = http://paleobiol.geoscienceworld.org/cgi/reprint/28/1/155
title = "Leanchoilia" Guts and the Interpretation of Three-Dimensional Structures in Burgess Shale-Type Fossils
author = Butterfield
year = 2002
journal = Paleobiology
volume = 28
pages = 155–171
doi = 10.1666/0094-8373(2002)028<0155:LGATIO>2.0.CO;2
accessdate = 2008-07-01] 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 the organisms soon after their burial.

With the exception of phosphatic presrvation, 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. A very high resolution of features can be discerned; diffraction gratings of the order of hundreds of nanometres can be detected on some Burgess Shale organisms.citation
author = Parker, A. R.
year = 1998
journal = Proceedings of the Royal Society B: Biological Sciences
volume = 265
issue = 1400
pages = 967–972
url = http://journals.royalsociety.org/index/WMHTRBY5FPJJ4UCH.pdf
publisher = The Royal Society
doi = 10.1098/rspb.1998.0385
title = Colour in Burgess Shale animals and the effect of light on evolution in the Cambrian]

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 phylosilicate. 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 phylosilicates 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

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.

Calcite is often not preserved, perhaps reflecting acidic conditions of burial.Fact|date=September 2008 It may be replaced by aluminosilicates.

Muscle can in very rare cases survive.citation
author = Budd, G.E.
year = 1998
journal = Lethaia
volume = 31
issue = 3
pages = 197–210
doi=10.1111/j.1502-3931.1998.tb00508.x
publisher = Blackwell Synergy]

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.

Before burial

The majority of the decay process occurred before the organisms were buried.cite journal
doi = 10.2110/palo.2003.P05-070R
author = CARON, JEAN-BERNARD; JACKSON, DONALD A.
journal = Palaios
volume = 21
issue = 5
pages = 451–465
year = 2006
title = Taphonomy Of The Greater Phyllopod Bed Community, Burgess Shale]

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.

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 water column.

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, ~10km depth), 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
author=Robert R. Gaines
coauthors= Derek E.G. Briggs ; Zhao Yuanlong
year=2008
doi=10.1130/G24961A.1
volume=36
pages=755]

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, although such deposits are now being found.cite journal
title = A Reassessment of the Neoproterozoic Miaohe Carbonaceous Biota in South China
author = Xiao, Shuhai; M. Steiner; A. H. Knoll.
year = 2002
journal = Journal of Paleontology
volume = 76
pages = 345–374
doi = 10.1666/0022-3360(2002)076<0347:MCCIAT>2.0.CO;2]

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.cite journal | title=Post-Cambrian closure of the deep-water slope-basin taphonomic window | author= Orr, P.J., Benton, M.J., and Briggs, D.E.G. | year = 2003 | doi = 10.1130/G19193.1 | url=http://www.gsajournals.org/perlserv/?request=get-abstract&doi=10.1130%2FG19193.1 | accessdate=2008-06-28 | journal= Geology| volume=31 | pages=769 ] 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 lagerstatten 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. 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.

Faunas

The mode of preservation preserves a number of different faunas; most famously, the Cambrian "Burgess shale type faunas" of the Burgess shale itself, Chengjiang and Sirius Passet. However, different assemblages are also preserved, such as the microfossils of Riphean lagerstatten.cite journal
title = Secular Distribution of Burgess-Shale-Type Preservation
author = Butterfield, N.J.
year = 1995
journal = Lethaia
volume = 28
pages = 1
doi = 10.1111/j.1502-3931.1995.tb01587.x]

Bias

cite journal
title = Exceptional Fossil Preservation and the Cambrian Explosion
year = 2003
journal = Integrative and Comparative Biology
volume = 43
issue = 1
pages = 166–177
doi = 10.1093/icb/43.1.166
author = Butterfield, Nicholas J.]

References