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.
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.
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.
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.
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.
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.
So Whittington was puzzled when he first published on Marrella in 1971 but he went on and the next creature he studied was 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.
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.
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.
Looked vaguely like a swimming crustacean, but isn’t when you look at the segments and their patterns of the tail.
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.
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.
This is Habelia, an odd creature…
… with tubercules all over its body.
This is Leanchoilia, my personal favourite for elegance, but not among the survivors.
Again, these odd great appendages, as Whittington calls them, with their whiplash endings.
This is 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.
And here is a form that Des Collins found and initially gave a field name, following paleontological tradition…
… 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?
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?
This is Opabinia. Opabinia, I think, should stand as one of the great moments in the history of human knowledge.
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.
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.
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.
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.
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.
This is Nectocaris, a peculiar creature that looks like a chordate behind, combined with a fin ray…
… and more like an octopod in the front. Who knows?
This is Dinomischus, a peculiar, stalked, stemmed creature…
… with no known affinity to anything else.
This is Odontogriphus, literally meaning “the toothed mystery” a good name.
A flat, gelatinous, annulated creature with a row of tooth-like structures surrounding a mouth and a pair of sensory palps.
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.
This creature he called a sea cucumber and called Laggania.
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.
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.