The
Burgess Shale
The Burgess Shale is a fossil-bearing deposit exposed in the Canadian Rockies of British Columbia, Canada. It is famous for the exceptional preservation of the soft parts of its fossils. At old (middle Cambrian), it is one of the earliest foss ...
of
British Columbia
British Columbia (commonly abbreviated as BC) is the westernmost province of Canada, situated between the Pacific Ocean and the Rocky Mountains. It has a diverse geography, with rugged landscapes that include rocky coastlines, sandy beaches, ...
is famous for its exceptional preservation of mid-
Cambrian
The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...
organisms. Around 69
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
The Ediacaran Period ( ) is a geological period that spans 96 million years from the end of the Cryogenian Period 635 million years ago (Mya), to the beginning of the Cambrian Period 538.8 Mya. It marks the end of the Proterozoic Eon, and th ...
and
Ordovician
The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start ...
periods.
These various shales are of great importance in the reconstruction of the ecosystems immediately after the
Cambrian explosion
The Cambrian explosion, Cambrian radiation, Cambrian diversification, or the Biological Big Bang refers to an interval of time approximately in the Cambrian Period when practically all major animal phyla started appearing in the fossil recor ...
. The
taphonomic
Taphonomy is the study of how organisms decay and become fossilized or preserved in the paleontological record. The term ''taphonomy'' (from Greek , 'burial' and , 'law') was introduced to paleontology in 1940 by Soviet scientist Ivan Efremov t ...
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
trilobite
Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the At ...
s.
The most famous localities preserving organisms in this fashion are the Canadian
Burgess Shale
The Burgess Shale is a fossil-bearing deposit exposed in the Canadian Rockies of British Columbia, Canada. It is famous for the exceptional preservation of the soft parts of its fossils. At old (middle Cambrian), it is one of the earliest foss ...
, the Chinese
Chengjiang fauna
The Maotianshan Shales are a series of Early Cambrian deposits in the Chiungchussu Formation, famous for their '' Konservat Lagerstätten'', deposits known for the exceptional preservation of fossilized organisms or traces. The Maotianshan Shales ...
, and the more remote
Sirius Passet
Sirius Passet is a Cambrian Lagerstätte in Peary Land, Greenland. The Sirius Passet Lagerstätte was named after the Sirius sledge patrol that operates in North Greenland. It comprises six places in Nansen Land, on the east shore of J.P. Koch F ...
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,
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
A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The continental margin ...
or in a
sedimentary basin
Sedimentary basins are region-scale depressions of the Earth's crust where subsidence has occurred and a thick sequence of sediments have accumulated to form a large three-dimensional body of sedimentary rock. They form when long-term subsidence ...
. They are known in sediments deposited at all water depths during the
Precambrian
The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the ...
(
Riphean stage
The Riphean is a stage or age of the geologic timescale from . The name Riphean is used in the Proterozoic stratigraphy of Russia and the Fennoscandian Shield in Finland. It was also used in a number of older international geologic timescales but, ...
onwards), with a notable gap in the last 150 million years of the
Proterozoic
The Proterozoic () is a geological eon spanning the time interval from 2500 to 538.8million years ago. It is the most recent part of the Precambrian "supereon". It is also the longest eon of the Earth's geologic time scale, and it is subdivided ...
.
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.
Charles Walcott, who discovered the Burgess Shale on 30 August 1909, 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.
When carbon is preserved it usually forms films of the highly cross-linked and essentially inert compound
kerogen
Kerogen is solid, insoluble organic matter in sedimentary rocks. Comprising an estimated 1016 tons of carbon, it is the most abundant source of organic compounds on earth, exceeding the total organic content of living matter 10,000-fold. It ...
, with kerogen formation from organic precursors likely to happen as the host rock is exposed to high pressures.
In addition, films of phyllicate (clay) minerals can grow ''
in situ
''In situ'' (; often not italicized in English) is a Latin phrase that translates literally to "on site" or "in position." It can mean "locally", "on site", "on the premises", or "in place" to describe where an event takes place and is used in ...
'', 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.
Oxygen in the sediment allows decomposition to occur at a much faster rate, which 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.
It seems that whilst anoxia improves Burgess Shale-type preservation, it is not essential to the process.
In addition to the organic films, parts of many Burgess Shale creatures are preserved by
phosphatisation Phosphatic fossilization has occurred in unusual circumstances to preserve some extremely high-resolution microfossils in which careful preparation can even reveal preserved cellular structures. Such microscopic fossils are only visible under the s ...
: 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
The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Iron, FeSulfur, S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral.
Pyrite's metallic Luster (mineralogy), lust ...
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
Scale or scales may refer to:
Mathematics
* Scale (descriptive set theory), an object defined on a set of points
* Scale (ratio), the ratio of a linear dimension of a model to the corresponding dimension of the original
* Scale factor, a number w ...
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
Kerogen is solid, insoluble organic matter in sedimentary rocks. Comprising an estimated 1016 tons of carbon, it is the most abundant source of organic compounds on earth, exceeding the total organic content of living matter 10,000-fold. It ...
), the counterpart's film is blue, less reflective, and more translucent.
A carbon film seems to be common to all BST deposits, although the carbon may 'evaporate' as rocks are heated, potentially to be replaced with other minerals.
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
''Wiwaxia'' is a genus of soft-bodied animals that were covered in carbonaceous scales and spines that protected it from predators. ''Wiwaxia'' fossils – mainly isolated scales, but sometimes complete, articulated fossils – are known from ear ...
''), 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
Kaolinite ( ) is a clay mineral, with the chemical composition Al2 Si2 O5( OH)4. It is an important industrial mineral. It is a layered silicate mineral, with one tetrahedral sheet of silica () linked through oxygen atoms to one octahedral ...
(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
Illite is a group of closely related non-expanding clay minerals. Illite is a secondary mineral precipitate, and an example of a phyllosilicate, or layered alumino-silicate. Its structure is a 2:1 sandwich of silica tetrahedron (T) – alumina ...
(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.
The precise mineral formation depends on the porewater (and thus rock) chemistry; the thickness of the films increases as metamorphism continues; and the minerals align with the prevailing strain. They are not present in comparable deposits with very little metamorphism.
Calcium carbonate was originally present in the carapaces of trilobites, and may have crystallized early in diagenesis in (for example) the guts of ''
Burgessia
''Burgessia'' is a genus of arthropod known from the mid-Cambrian aged Burgess Shale of British Columbia, Canada. It is relatively abundant, with over 1,700 specimens having been collected.
Description
The body had an unsegmented approximat ...
''. It may also have filled late-stage veins in the rock. The carbonate was apparently leached away
[ and the resultant voids filled with phyllosilicates.]
Pyrite
Pyrite takes the place of phyllosilicates in some BST deposits. Labile tissues are associated with framboids, as they produced many nucleation sites due to the rapid production of sulfides (perhaps by sulfur-reducing bacteria); recalcitrant tissues are associated with euhedra. 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).[
]
Other preservational pathways
Some specimens bear a dark stain
A dark stain is often associated with fossils of the Burgess Shale, representing decay fluids that were squashed out of the organism during the taphonomic process.
Occurrence
It is most famously known in (and most commonly associated with) ''Marr ...
representing decay fluids injected into the surrounding wet sediment.
Muscle can in very rare cases survive by silicification, or by authigenic mineralization by any of a range of other minerals. However, predominately soft tissues, such as muscles and gonads, are never preserved by the carbonaceous-compression preservational pathway. Phosphatisation and the presence of other enzymes means that guts and mid-gut glands are often preserved. Some bilaterally-symmetrical entities in the heads of arthropods have been interpreted as representing nervous tissue—a brain.
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, and cellular material has no preservation potential.[ However, Conway Morris and others disagree,] and non-cuticular organs and organisms have been described, including the setae of brachiopods and the jellyfish ctenophores
Ctenophora (; ctenophore ; ) comprise a phylum of marine invertebrates, commonly known as comb jellies, that inhabit sea waters worldwide. They are notable for the groups of cilia they use for swimming (commonly referred to as "combs"), a ...
(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
Chitin ( C8 H13 O5 N)n ( ) is a long-chain polymer of ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is probably the second most abundant polysaccharide in nature (behind only cellulose); an estimated 1 billion tons of chit ...
or collagen
Collagen () is the main structural protein in the extracellular matrix found in the body's various connective tissues. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whole ...
. Rather, it is kerogen
Kerogen is solid, insoluble organic matter in sedimentary rocks. Comprising an estimated 1016 tons of carbon, it is the most abundant source of organic compounds on earth, exceeding the total organic content of living matter 10,000-fold. It ...
ized. This process seems to involve the incorporation of aliphatic
In organic chemistry, hydrocarbons ( compounds composed solely of carbon and hydrogen) are divided into two classes: aromatic compounds and aliphatic compounds (; G. ''aleiphar'', fat, oil). Aliphatic compounds can be saturated, like hexane, or ...
lipid
Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include ...
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
*Calcium and phosphorus are generally associated with mid-gut glands, and aluminium is higher in the alimentary canal.
*Areas in which silicon is depleted, and aluminium and potassium concentrations elevated, have been interpreted as originally fluid-filled cavities.
Because the fossiliferous layer is so thin, it is effectively transparent to electrons at high-accelerating (>15V) voltages.
Sedimentary setting
In the Wheeler Formation
The Wheeler Shale (named by Charles Walcott) is a Cambrian ( 507 Ma) fossil locality world-famous
for prolific agnostid and ''Elrathia kingii'' trilobite remains (even though many areas are barren of fossils)
and represents a Konzent ...
, lagerstätte
A Lagerstätte (, from ''Lager'' 'storage, lair' '' Stätte'' 'place'; plural ''Lagerstätten'') is a sedimentary deposit that exhibits extraordinary fossils with exceptional preservation—sometimes including preserved soft tissues. These for ...
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.[
]
Before burial
The majority of the decay process occurred before the organisms were buried.
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 current
A turbidity current is most typically an underwater current of usually rapidly moving, sediment-laden water moving down a slope; although current research (2018) indicates that water-saturated sediment may be the primary actor in the process. T ...
s. The location at which an organism ultimately comes to rest may depend on how readily it floats, a function of its size and density. Organisms are much more randomly arranged in the Burgess Shale itself.
Turbidity current
A turbidity current is most typically an underwater current of usually rapidly moving, sediment-laden water moving down a slope; although current research (2018) indicates that water-saturated sediment may be the primary actor in the process. T ...
s 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
Debris flows are geological phenomena in which water-laden masses of soil and fragmented rock rush down mountainsides, funnel into stream channels, entrain objects in their paths, and form thick, muddy deposits on valley floors. They generally ...
. 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 framboid A framboid is a micromorphological feature common to certain sedimentary minerals, particularly pyrite (FeS2). The first known use of the term is ascribed to Rust in 1935 and is derived from the French ‘la framboise’, meaning ‘raspberry’, re ...
al (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
{{Short pages monitor