The best-known Burgess Shale site is the Walcott Quarry on Fossil Ridge. About 150 species of animals, algae, and bacteria from here have been described to date. The mammoth collections available to researchers – about 65,000 specimens at the Smithsonian Institution in Washington D.C., 10,000 at the Geological Survey of Canada in Ottawa, and 150,000 specimens at the Royal Ontario Museum in Toronto – form the basis for detailed studies of individual species. These collections allow researchers to study the overall ecosystem using quantitative statistical methods in ways that are not yet possible for other Burgess Shale-type deposits, which lack sufficient fossils.
Most of the fossils from the Walcott Quarry represent organisms that probably lived in comparatively deep waters at the foot of the Cathedral Escarpment. It had been thought the organisms came from shallower waters on top of the Escarpment, but recent research has suggested most of the animals lived and died where they were buried, at the Escarpment’s base.
The Walcott Quarry community is representative of a typical Burgess Shale-type community during the Cambrian. It is estimated that up to 98% of the fossils from this locality are entirely soft-bodied and would stand no chance of being preserved through normal taphonomic processes. The remainder (2%) is represented by animals with parts that were originally mineralized (such as trilobites) and thus can usually fossilize more readily. Trilobites and other “shelly” fossils are found in most typical Cambrian marine deposits around the world. The absence of soft-bodied fossils from those deposits reflects the non-preservation of organisms without hard parts, rather than their absence from the original ecosystem.
The composition of the Walcott Quarry community has been extensively studied based on fossil counts made from existing collections. The pie-charts below are derived from a subset of 50,282 specimens collected by the most recent Royal Ontario Museum field expeditions in the Walcott Quarry (the relative abundances of specimens provided in the fossil gallery are based on this number). These represent the relative abundance of specimens and the relative abundance of species within main groups of organisms (or taxa).
The relative abundance of specimens is simply the number of fossils of a particular taxon as a percentage of the total number of fossils collected. The relative abundance of species is the number of species of a particular taxon as a percentage of the total number of species combined. A single taxon might have many species, but be represented by very few specimens. Conversely, one taxon might contain just a few species, but many individual fossil specimens. By comparing the two kinds of pie-charts, palaeoecologists can study patterns of species associations.
The exact affinity of many fossils from the Walcott Quarry is still unknown. However, a large number of species can be linked (as primitive or ancestral forms) to broad groups of organisms that are still known today (see Evolutionary Significance below, and stem group, crown group concepts).
In terms of both number of species and number of specimens, animals make up the majority of the community; a few species of green and red algae, as well as some microbial colonies, are also known. The more important groups by far are the moulting animals with jointed limbs (the arthropods) and the sponges (the poriferans). Most other groups are represented by fewer specimens and species.
Finally, Burgess Shale animals can be categorized on the basis of how they obtained their food. Suspension feeders filtered particles of food out of the water. Deposit feeders gathered particles of food that settled on or in the sea floor sediment layer. Carnivorous hunters actively captured and devoured other animals, and scavengers took advantage of any dead bodies they came across. Grazers lived by munching on photosynthetic algae or cyanobacteria that grew in the dim sunlight that penetrated to the base of the Escarpment.
Palaeontologists can recreate the proportions of species in the Burgess ecosystem falling into each of these categories. The overwhelming majority of species (64%) were epifaunal, living on the sea bed (most notably the sponges), followed by the infaunal sediment-dwellers (13%) and the low-level swimming nektobenthics (12%). Swimming nektonic species were the least common (11%).
A food web reconstructs the feeding relationships between different organisms in a community.
In the Burgess Shale, the ecosystem ultimately relied on photosynthetic algae and bacteria, which used energy absorbed from sunlight in order to grow. Other organisms fed (directly or indirectly) either on algae and bacteria growing on the sea floor or on planktonic forms that sank from the waters above (see reconstruction of the Burgess Shale community above).
Animals living in or on the sea floor could filter food particles out of the water, scrounge for fragments of food on the muddy bottom, or graze directly on the algal or bacterial mats. Comparatively few animals made a living by active hunting or scavenging, but their impact on the food web would have been great.
The structure of the Burgess Shale ecosystem food web is surprisingly similar to what we see in modern marine communities, although the individual species involved are clearly quite different. This suggests that the basic feeding relationships were quickly established during the Cambrian Explosion and have remained relatively unchanged to the present.
Palaeontologists can use a variety of clues to reconstruct the diets of Burgess Shale animals. Occasionally feces, or even gut contents, are fossilized, the latter providing a direct record of an animal’s final meal.
Usually, dietary habits have to be inferred from the ecological niche an animal occupied and from specialized body structures used in feeding. Anomalocaris, for example, had large eyes, grasping limbs, swimming lobes, and tooth-like mouthparts. Together with its large size, these features strongly suggest Anomalocaris was a predator.
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.