The Burgess Shale

Fossil Evidence

Mineralized Skeletons

The early record of the Cambrian Explosion is based on fossils – principally the appearance of mineralized skeletons and complex trace fossils. The typically tiny skeletal elements from this time are called “small shelly fossils.” These constitute a highly varied assortment of sclerites, spicules, tubes, and shells, suggestive of several different types of animals. Unfortunately, many of the fossils remain poorly understood and are difficult to classify within known taxonomic groups.

A selection of 13 small, shelly fossils
Early Cambrian sclerite-bearing animals. 1, Siphogonuchites. 2, Hippopharangites. 3, Lapworthella. 4, Eccentrotheca. 5, 6, Microdictyon. 7, Tumulduria. 8, Scoponodus. 9, Jaw-like elements of Cyrtochites. 10, Porcauricula, 11, Dermal element of Hadimopanella. 12, Cambroclavus. 13, Paracarinachites. Scale bars = 0.1 mm.
Early tube-dwelling animals. 1, Cloudina, one of the earliest animals with a mineralized skeleton reinforced with calcite (late Neoproterozoic). 2, Aculeochrea, with an aragonite-reinforced tube (Precambrian-Cambrian boundary beds). 3, Hyolithellus, an animal reinforcing its tube with calcium phosphate (early Cambrian). 4, Olivooides, possibly a thecate scyphozoan polyp. 5, Pre-hatching embryo of Olivooides. Scale bars = 0.1 mm.
Seven fossilized shells from the Early Cambrian
Early Cambrian shell-bearing animals. 1, Archaeospira, a possible gastropod. 2, Watsonella, a possible mollusc. 3, Cupitheca. 4, 5, Aroonia, a probable stem-group brachiopod. 6, 7, Conch and operculum of Parkula, a hyolith. Scale bars = 0.1 mm.

About 521 million years ago, trilobites made their first appearance in the Cambrian fossil record. These armoured animals had of three dorsal shelly parts – a cephalon (head), a segmented thorax (body), and a pygidium (tail section). Trilobites eventually became one of the most ubiquitous groups of invertebrate organisms in the Palaeozoic seas. They survived for almost 300 million years, and their fossils can be found from the Cambrian to the Permian periods.

An early (Lower Cambrian) trilobite species, Eoredlichia takooensis from Emu Bay Shale, Kangaroo Island, Australia. Specimen length = 6 cm.

Trace Fossils and the Cambrian Substrate Revolution

Trace fossils also become considerably more complex and diverse in Early Cambrian rocks. During the late Ediacaran, metazoans produced only simple horizontal traces on the surface of the sea floor. Starting in the Cambrian, animals began to tunnel vertically through the sediments and exhibit more varied behaviours, providing indirect evidence that mobile bilaterians with differentiated tissues and organs had already evolved.

Large slab of rock showing worm-like protuberances
Early Cambrian trace fossils. Treptichnus pedum from the Lower Cambrian Mickwitzia Sandstone, Sweden (Swedish Geological Survey, Uppsala). Scale bar = 1 cm.

The rise of these bilaterians permanently altered the nature of the sea floor, an event commonly referred to as the Cambrian Substrate Revolution.

During the Precambrian, the upper layers of mud, sand or silt on the sea floor remained relatively firm, thanks to bacterial mats that covered and stabilized the surface. These mats also served as a primary food source for Ediacaran organisms capable of grazing along the sea floor (see Kimberella). The burrowing animals of the Cambrian were able to tunnel down through the microbial mats, churning the sediment beneath and making it soupier. The burrowers may have started tunneling to access new sources of food (such as the sunken carcasses of planktonic organisms buried on the sea floor) or to escape predation by digging deep into the substrate.

Left, graphic showing undisturbed layers; Right, graphic showing disturbed layers with animals
Changes in substrate types during the Cambrian substrate revolution. Left, Precambrian Period; right, Cambrian Period.
The burrows opened up new ecological niches beneath the sea floor as water and oxygen could now get into the sediment layers. At the same time, bacterial mats were progressively destroyed and forced into more restricted habitats (i.e., in environments unfavourable for animals). This change in the substrate is thought to be partly responsible for the demise of the Ediacaran biota. Other factors (such as a change in water chemistry or an increase of predators) may also have played important roles in their extinction (see above). The revolution turned the once-uniform sea floor into a heterogeneous patchwork, opening up a variety of new niches for animals – including those of the Burgess Shale – to exploit. Burgess Shale-Type Deposits and the Burgess Shale Biota Exceptionally well-preserved soft-bodied fossils of Cambrian age were first described from the Burgess Shale over 100 years ago. Today, dozens of Burgess Shale-type deposits with comparable assemblages of fossils have been found around the world. These deposits are usually found in Lower and Middle Cambrian rock layers, but may extend as far as the early Ordovician. These deposits are characterized by a similar mode of preservation called “Burgess Shale-type preservation”. The most notable sites are those located around the original Burgess Shale locality in Canada (the Walcott Quarry in Yoho National Park, British Columbia), along with the Maotianshan Shales of China (of which the Chengjiang in Yunnan Province is the most famous). Other significant occurrences include the Kaili deposit in China and sites in the western United States of America (Spence Shale and Marjum and Wheeler Formations in Utah, Pioche Formation in Nevada), Greenland (Sirius Passet), and Australia (Emu Bay Shale).
Graphic showing locations of Burgess Shale-type deposits around the world

Utah deposits

Sirius Passet



Emu Bay

Compared to conventional fossil deposits, in which only the remains of more durable body parts are typically preserved, Burgess Shale-type deposits provide a much more complete picture of a normal Cambrian marine community. In modern marine settings, animals with mineralized body parts (shells, carapaces, etc.) account for only a minor component of the total diversity. This is also the case in most Burgess Shale-type deposits where the shelly assemblage usually represents a small percentage of specimens collected. Thus, without the fossilized remains of soft-bodied organisms, especially from the Burgess Shale, our knowledge of Cambrian ecosystems would be extremely limited.

Similarities among various Burgess Shale-type deposits around the world suggest the deep marine ecosystem was geographically uniform and evolutionarily conservative from the Lower to at least the Middle Cambrian (i.e., similar types of animal fossils are recovered through this whole interval, spanning at least 15 million years). The characteristic assemblage of organisms is often referred as the Burgess Shale-type biota.

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.