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Turning a Mudstone into a Shale

The Burgess Shale fossils are preserved in a type of sedimentary rock known as shale. Shale is a type of mudstone (or mudrock) that originally formed from deposits of fine mud, made mostly of clay minerals. The different fossil layers of the Burgess Shale represent different mud deposits, originally laid down in sheet-like horizontal beds ranging from a few millimetres up to several centimetres in thickness. These layers can still be seen today in the Burgess Shale.

Photograph showing layers of rock

Layers from the Phyllopod bed in the Walcott Quarry showing the original stacks of horizontal mud beds now transformed into shale. The colour difference between the layers is emphasized by weathering.

© Royal Ontario Museum. Photo: Desmond Collins.

Seafloor muds are transformed into shale (lithified) when they encounter increased temperature and pressure during their geological history. The mudstones of the Burgess Shale were exposed to even greater temperature and pressure during the formation of the Canadian Rocky Mountains.

Photomicrograph showing variation in layers of rock

Thin section through a fossil layer from the Walcott Quarry representing different depositional events shown by slight variations in size and color of the clay minerals. Height of image =1.5 cm.

© Royal Ontario Museum. Photo: Jean-Bernard Caron.

During this process, the minerals (which have flat structures) gradually tend to align with each other, forming parallel layers. For shales (including the Burgess Shale), this results in rocks that tend to split into thin sheets. The presence of a fossil in the shale creates a zone of weakness between layers, so when the rock is broken open it is more likely to split along the plane containing the fossil, leaving parts of the fossil on each facing surface (as part and counterpart).

How Old is the Burgess Shale

The Burgess Shale itself cannot be dated directly. But its age can be determined indirectly by correlating it with other deposits that have been dated using geochronologic methods (such as radiometric decay analysis). These correlations are based on shared profiles of fossils contained within the strata (rock layers).

This method is known as biostratigraphic correlation and produces a relative rather than absolute age (see below). It relies on the stratigraphic principle of superposition: that the oldest sedimentary rock layers are always found at the base of a section (so long as the section has not been deformed or overturned by subsequent tectonic activity).

This means the further you dig down, the older the rocks should be. If two different sites contain the same set of fossils (e.g. Site B and C in figure below), it is likely that they were laid down at about the same time. The relative age of the youngest (Site A) and the oldest (Site E) sites can be determined by comparing their respective fossil compositions.

Graphic showing how overlapping fossils allow different beds to be placed in time

General principles of biostratigraphy - how the relative ages of various fossil deposits can be determined based on comparisons of fossil assemblages. In this example, site E will be the oldest fossil assemblage.

To be useful for biostratigraphic correlation, a fossil species must have been abundant and widespread, so its remains are found in many locations. The organism must also fossilize well, which is why organisms with mineralized shells, like trilobites, are often used in biostratigraphy. Short-lived species are most useful, since they only leave fossils during a narrow window of time - providing greater stratigraphic accuracy.

Bathyuriscus-Elrathina Zone, named after two trilobites commonly found in the various fossil assemblages. This Zone forms part of the Burgess Shale Formation (known previously as the "Thick" Stephen Formation).

Two fossilized trilobites

Side by side comparison (to scale) between the trilobites Bathyuriscus (left, size = 3.7 cm) and Elrathina (right, size = 1.3 cm) from the Trilobite Beds on Mount Stephen and the Walcott Quarry on Fossil Ridge, respectively.

© Royal Ontario Museum. Photos: Jean-Bernard Caron.

Ultimately, palaeontologists need to compare the relative dates provided by biostratigraphy to fixed reference points in rocks that can be dated numerically using radioactive isotopes. The lack of appropriate rock types (igneous or metamorphic rocks containing radioactive elements) in the Burgess Shale rules out a direct application of these techniques. Indeed, few points in the Cambrian have been precisely dated in this way.

The best-constrained points are at the beginning (542 million years ago) and end (488 million years ago) of the Cambrian Period, with only a few reference points available between. By linking the fossils to these fixed points, palaeontologists can establish their relative ages: that is, approximately how much younger or older they are than the reference dates. Applying these methods to the Burgess Shale indicates the fossils date to the middle of the Cambrian period and are between 505 and 510 million years old.

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Plate Tectonics and the Cambrian World

The rigid outer layer of the Earth (the lithosphere) is made up of many tectonic plates ranging in thickness from 30 to 200 km. These move slowly on an underlying layer of viscous, molten rock. When moving plates collide or override, the result can be sudden earthquakes and volcanic eruptions, or the creation of new mountain ranges - pushed upward as the rigid rocks buckle under the incomprehensible pressure. Over the course of millions of years, the face of the Earth can be changed completely, with dramatic effects on climate, plants, and animals.

Earth through time up to the age of the Burgess Shale.

© Ron Blakey.

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The organisms whose remains are preserved in the Burgess Shale originally lived far off the coast of a land mass near the equator known to geologists as Laurentia (which would go on to form most of North America).

Recreation of what the Earth looked like in the Cambrian Period

Equatorial views (West and East) of the Earth, 500 million years ago. The location of the Burgess Shale is indicated by a pin.

© Ron Blakey.

Over the past half-billion years, plate tectonics has moved this parcel of land to its present location in the northern hemisphere. The same forces thrust the ancient seabed nearly three kilometres above sea level to form the Canadian Rockies. The last major uplift ended approximately 65 million years ago. Since then, the mountains have gradually been eroded away by wind, rain, rivers and ice, cutting deep valleys in the Canadian Rockies. This erosion eventually exposed the rocks containing the Burgess Shale fossils to the light of day for the first time in 500 million years.

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Palaeoenvironmental Setting

Most of the creatures whose remains we find in the Burgess Shale lived in deeper (basinal) waters, making their homes on or in the sea floor or swimming above it. The local environment would have been calm - safely below the churning surface caused by storms or hurricanes. There were probably weak currents, allowing the many suspension feeders in the community to thrive.

Most animals lived at the base of a large submarine cliff known as the Cathedral Escarpment. This formed at the outer edge of a wide, tropical platform of carbonate rock that may have extended as far as 400 kilometres (320 miles) from the shoreline.

Graphic recreating what the Burgess Shale environment would have looked like in the Cambrian

At least twelve fossil localities have been discovered at the foot of the Escarpment along a 60-kilometre belt running roughly north-south. This suggests the Escarpment might have helped optimize conditions for a rich animal community to develop and be preserved as fossils.

The Escarpment itself was about 200 metres (650 feet) high before mud and other sediments began to fill in the basin. The shape of the Escarpment may have channelled mudflows at its base, resulting in periodic deposits that enveloped and preserved the organisms living there. The presence of fossilized algae implies sunlight must have penetrated to the base of the Escarpment. As in today's marine environments, algae at the base of the food chain would have provided food for many other organisms. Periodically, the tranquil scene would be shattered by torrents of mud - burying living and dead organisms in a disorganized mass.

This process continued for perhaps hundreds of thousands of years, with successive layers of sediment eventually filling the original basin.

Recent Discoveries

A new Burgess Shale-type deposit, representing a different kind of ancient marine environment, was discovered in 2008, in Kootenay National Park (Stanley Glacier). This deposit is not associated with the protected environment found at the base of the Escarpment and the fossils are found in smaller numbers (and showing less diversity) than at localities close to the submarine cliff. This suggests that Burgess Shale-type organisms had a much wider environmental distribution than previously recognized.

It is still possible that conditions close to the Escarpment were more favourable for animal life and for their eventual preservation as fossils. In particular, the strong carbonate rock of the Escarpment may have acted like a buttress, protecting the mudstones and their fossils from the full brunt of the ensuing metamorphic processes. Farther from the Escarpment, metamorphosis appears to have had a greater effect, and probably led to the obliteration of any fossils that might once have been present.

More field work will be needed to verify these hypotheses and in particular to evaluate the range of environmental conditions in which Burgess Shale-type fossils can be found in the Canadian Rockies.

Left, a fossil that looks like a net; Right, workers in a vast mountain valley

A specimen of the sponge Diagoniella (left, size = 76 mm) from the Stanley Glacier area (right).

© Royal Ontario Museum. Photos: Jean-Bernard Caron.

On a global scale, it is now clear that Burgess Shale-type organisms are widely distributed and are not restricted to a narrow environmental setting adjacent to an undersea escarpment. But the unique positioning of the Burgess Shale may well account for the exceptional quantity and quality of fossils found there, compared with most other Burgess Shale-type deposits around the world.

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Transforming the Dead Organisms into Fossils

Graphic outlining how an organism is fossilized

The process of fossilization from time of death, burial, preservation, metamorphisation to exhumation and discovery.

Taphonomy is the science of tracing how an organism, in whole or in part, becomes a fossil. It must take into account how death, decay and manner of burial affect what, if anything, is preserved in the rocks.

When attempting to reconstruct the Burgess Shale ecosystem, researchers have to consider how readily different kinds of organisms fossilized. In order to do that, they have to understand the taphonomic processes by which Burgess Shale-type fossils formed. This is particularly important in the context of evolutionary studies, because taphonomic assumptions are often necessary to reconstruct the original anatomy of fossilized organisms; failing to understand the taphonomy may lead to a misinterpretation of some anatomical characters, or the improper classification of a fossil.

The preservation processes in the Burgess Shale environment began with occasional flows of fine-grained sediments sweeping through the area, quickly burying living and dead animals.

Exactly what happened next remains uncertain. The sediments surrounding the buried animals were apparently depleted in oxygen, which would have kept scavengers and most bacterial activity from completely devouring the remains. Although this factor alone is insufficient to produce Burgess Shale-type preservation, most scientists agree it was a necessary condition for the extraordinary fossilization.

Another hypothesis suggests clay minerals in the sediment inhibited bacterial activity and decay. Whatever the reasons, the organisms were left relatively undisturbed after death and burial, and did not decompose entirely. In many cases only the most fragile tissues (such as muscles) decayed, resulting in the collapse of tougher organic parts and a flattening of the organisms, ultimately producing compression fossils. Over time, clay minerals were compacted and aligned around the fossils, then altered into new minerals during low-level metamorphosis.

Most of the Burgess Shale fossils consist of thin films of carbon, partially replaced by clay or iron-rich mineral products (such as mica) that preserve the original contours of the animal. These appear as dark-coloured films, which often reflect light when tilted.

Left, an ovoid fossil; Right, two views of the fossil as seen through an electron microscope

Nectocaris pteryx from the Burgess Shale. Left, full view of a complete specimen (length = 5 cm). Right, Scanning Electron Microscope images of the surface of the left eye of this specimen, showing carbon films (black) on top of clay minerals oriented more or less parallel to the surface (grey). Iron oxides are also present (small white cauliflower-like structures). Square represent magnified area.

© Royal Ontario Museum. Photos: Jean-Bernard Caron.

Left, a fossil; Right, six coloured views of the fossil

Marrella splendens from the Burgess Shale. Left, full view of a small specimen (length = 0.8 cm). Right, Scanning Electron Microscopy-elemental maps of the same specimen, showing different elements in minerals composing the fossils. (The brighter the colour, the more of each element is present.) This specimen shows enrichment in Aluminium (Al), Carbon (C), Potassium (K) (all top row), a lack of Silicon (Si) and enrichment in Sulfur (S) and Iron (Fe) (all bottom row).

© Royal Ontario Museum. Photos: Jean-Bernard Caron.

This basic process seems to be the primary means of preservation for almost all Burgess Shale-type fossils around the world. There are regional differences: aspects of many Chengjiang fossils (such as limbs and antennae) were replaced by the mineral pyrite in addition to the carbon films that define the bodies of the fossils.

In addition, certain organs of some fossils are preserved by a different process called phosphatization. Structures that were naturally high in the mineral phosphate (such as some components of the digestive system) were very quickly mineralized, allowing them to retain a three-dimensional shape even as the carbon impressions were being squeezed flat.

Fossilized organism with a dark gut

Specimen of Leanchoilia superlata (size = 76 mm) from the Burgess Shale showing phosphatic preservation of gut diverticulae in three-dimensions.

© Royal Ontario Museum. Photos: Jean-Bernard Caron.

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