The Burgess Shale

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.
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).

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.

Gould Slideshow

In 1990, noted palaeontologist Stephen Jay Gould spoke at the Royal Ontario Museum about the fossils of the Burgess Shale. While many of Gould’s interpretations have been challenged, his talk provides a snapshot of how the organisms were viewed then. (6:20)

So this is Marrella. I should say that arthropods are classified primarily by numbers of segments and patterns in their various body parts.

Photograph of Marrella
DESCRIPTION: Photograph of Marrella

And here’s Marrella, it’s an arthropod that doesn’t fit into any group. It has these two sets of spines… there it is. It doesn’t have any allegiance.

Drawing of Marrella
DESCRIPTION: Drawing of Marrella

So Whittington was puzzled when he first published on Marrella in 1971 but he went on and the next creature he studied was Yohoia.

Photograph of Yohoia
DESCRIPTION: Photograph of Yohoia

Looked like a shrimp, had been called one by Walcott, and again, as Whittington studied it with care, it just didn’t fit into any modern group. It looks like a shrimp superficially, but when you start counting the segments you don’t have anything like the crustacean body plan.

Drawing of Yohoia
DESCRIPTION: Drawing of Yohoia

For instance, up in the head you have this unique set of frontal appendages which have no homologue anywhere else in the arthropods. Whittington ended up calling them simply “the great appendages” because he didn’t know what to do with them.

Photograph of Odaraia
DESCRIPTION: Photograph of Odaraia

This is Odaraia, a creature that swims on its back and has a tail fluke that looks more like a whale than an arthropod, but again, not allied to anything.

Drawing of Odaraia
DESCRIPTION: Drawing of Odaraia

Looked vaguely like a swimming crustacean, but isn’t when you look at the segments and their patterns of the tail.

Photograph of Sidneyia
DESCRIPTION: Photograph of Sidneyia

This is Sidneyia, which was described by Walcott as a chelicerate, that is a member of the horseshoe crab, eventually the spider-scorpion group. And in some superficial sense that’s what it looks like. But in detail it isn’t.

Drawing of Sidneyia
DESCRIPTION: Drawing of Sidneyia

All chelicerates have six pairs of appendages on their head. Sidneyia has one pair. It’s not like anything… just these antennae… it’s not like anything else… it is just is what it is.

Photograph of Habelia
DESCRIPTION: Photograph of Habelia

This is Habelia, an odd creature…

Drawing of Habelia
DESCRIPTION: Drawing of Habelia

… with tubercules all over its body.

Photograph of Leanchoilia
DESCRIPTION: Photograph of Leanchoilia

This is Leanchoilia, my personal favourite for elegance, but not among the survivors.

Drawing of Leanchoilia
DESCRIPTION: Drawing of Leanchoilia

Again, these odd great appendages, as Whittington calls them, with their whiplash endings.

Photograph of Aysheaia
DESCRIPTION: Photograph of Aysheaia

This is Aysheaia.

Drawing of Aysheaia
DESCRIPTION: Drawing of Aysheaia

Now, this creature is probably an onychophore, that is it is a member of a modern group symbolized by the genus with the wonderful name Peripatus, which is a not very well known group, but it’s thought to be possibly intermediary between annelids and arthropods and may be the ancestor of the insect group. So here we may have a creature that is truly related to one of the surviving groups of arthropods.

Desmond Collins, former ROM Curator, holding fossil
DESCRIPTION: Desmond Collins, former ROM Curator, holding fossil

And here is a form that Des Collins found and initially gave a field name, following paleontological tradition…

Photograph of Sanctacaris
DESCRIPTION: Photograph of Sanctacaris

… he called it “Santa Claws”. And eventually named it Sanctacaris, which means much the same thing. Now again, does it look any different than the ones I just showed you?

Drawing of Sanctacaris
DESCRIPTION: Drawing of Sanctacaris

Would you have picked out this creature for success? Could you have predicted that this, by virtue of superiority would go on? Yet it looks as though Sanctacaris really is a chelicerate.

There are six pairs of appendages in the right place on the head so this animal may be at least a cousin to one of the successful lineages. Again, would you have known? Could anyone have known?

Photograph of Opabinia
DESCRIPTION: Photograph of Opabinia

This is OpabiniaOpabinia, I think, should stand as one of the great moments in the history of human knowledge.

Drawing of Opabinia
DESCRIPTION: Drawing of Opabinia

Because Opabinia, which was described as an arthropod, a shrimp-like creature, by Walcott, who shoehorned it into modern groups as he always did. Opabinia was the first creature re-studied by Whittington that broke the conceptual dam, so to speak, and gave insights into this new world.

Technical drawing of Opabinia
DESCRIPTION: Technical drawing of Opabinia

Because Whittington began his studies in the early 1970s on Opabinia thinking it would be an arthropod. He realizes, as Walcott did not, that there was some three-dimensionality in these creatures, that they were not just films on the rock.

Photograph of Opabinia
DESCRIPTION: Photograph of Opabinia

That he could therefore dissect through and find structures underneath. So he said “Now I can resolve this, I’ll dissect through the body and find the appendages underneath which will prove its arthropod nature. He dissected through and he found nothing. There are no appendages.

Technical drawing of Opabinia
DESCRIPTION: Technical drawing of Opabinia

And as he reconstructed Opabinia, he came to understand it is not an arthropod, it is some bizarre creature of its own unique anatomy. And in publishing a monograph on Opabinia in 1975 I think you have the breakthrough point in the new interpretation of the Burgess Shale.

Drawing of Opabinia
DESCRIPTION: Drawing of Opabinia

Here is Marianne’s picture of Opabinia, a bizarre creature with five-count them, five-eyes, this vacuum-cleaner like nozzle with a food-collecting device in front, this bellows-like apparatus behind, followed by a tail. I don’t know what it is. It’s just weird.

Photograph of Nectocaris
DESCRIPTION: Photograph of Nectocaris

This is Nectocaris, a peculiar creature that looks like a chordate behind, combined with a fin ray…

Drawings of Nectocaris
DESCRIPTION: Drawings of Nectocaris

… and more like an octopod in the front. Who knows?

Photograph of Dinomischus
DESCRIPTION: Photograph of Dinomischus

This is Dinomischus, a peculiar, stalked, stemmed creature…

Drawing of Dinomischus
DESCRIPTION: Drawing of Dinomischus

… with no known affinity to anything else.

Photograph of Odontogriphus
DESCRIPTION: Photograph of Odontogriphus

This is Odontogriphus, literally meaning “the toothed mystery” a good name.

Drawings of Odontogriphus
DESCRIPTION: Drawings of Odontogriphus

A flat, gelatinous, annulated creature with a row of tooth-like structures surrounding a mouth and a pair of sensory palps.

Photograph of a fossil originally interpreted as a jellyfish
DESCRIPTION: Photograph of a fossil originally interpreted as a jellyfish

Walcott described three separate genera which he allocated, as was his wont, according to the shoehorn, into three conventional groups.
This animal he called a jellyfish and called Peytoia.

Photograph of a fossil originally interpreted as a sea cucumber
DESCRIPTION: Photograph of a fossil originally interpreted as a sea cucumber

This creature he called a sea cucumber and called Laggania.

Photograph of a fossil originally interpreted as a fossil shrimp
DESCRIPTION: Photograph of a fossil originally interpreted as a fossil shrimp

And this, which had been described before and looks like the body of an arthropod, he called (it had been named before) Anomalocaris, meaning “the odd shrimp”. Well I think that you’ve guessed it already.

Fossils of a complete Anomalocaris
DESCRIPTION: Fossils of a complete Anomalocaris

It turns out that all three go together. They form a single creature which is one of the weirdest of all the odd animals of the Burgess.

It’s also the largest Cambrian organism. Some specimens are almost a metre in length.

The so-called jellyfish is the mouth of this creature, working on a circular, nutcracker principle rather than the jaw of vertebrates principle.

The Anomalocaris itself turns out to be one of a pair of feeding appendages, and the so-called sea cucumber is the body of the whole animal.