The nature of the animals in the Burgess Shale biota, and their evolutionary significance, have been topics of particular interest since the discovery of the first fossils over a hundred years ago. Interpretations have changed over time, but it is clear these fossils are vital to understand how life shaped itself during the Cambrian Explosion. Interest in the evolutionary significance of the biota was revived by the discovery of other Burgess Shale-type deposits in various parts of the world and detailed re-examination of fossils from the Burgess Shale itself.
Charles Walcott originally considered the fossils of the Burgess Shale to belong to extinct categories within animal groups that are still alive today. However, many Burgess Shale animals look bizarre to modern eyes, and subsequent detailed descriptions suggested they could not be quite so easily accommodated within the definitions based on modern groups.
This was evidence for some, most notably Stephen Jay Gould that the Cambrian Explosion was a period of experimentation, with far more body plans in the Cambrian than today (thus representing greater evolutionary disparity). By chance, only some of these body plans survived to represent the phyla that we know today. If Gould was right, and the strange creatures of the Burgess Shale – referred to by Gould as “weird wonders”- truly represent lost phyla, the range of evolutionary innovations during the Cambrian Explosion would be far greater than previously thought.
More recent observations suggest the number of body plans in the Cambrian was probably no greater than in modern environments. Many of Gould’s “weird wonders” have now been re-accommodated within living animal phyla, albeit at some distance from modern groups within those phyla.
A number of bizarre forms remain difficult to classify. Part of the problem is that some of these are still poorly known – i.e., there is not enough well-preserved fossil material to describe the anatomy of the animals with certainty. Other, better-known fossils display only some of the traits associated with known groups, or possess a combination of traits that remains at odds with what we know for any living or extinct organisms. These are now interpreted as primitive animals which evolved along the lineages of established phyla or groups of phyla (see crown-group and stem-group).
Some of the most recent changes in interpretation are based on new fossil material that provides many more specimens and traits to study. A number of previously “unclassifiable” fossils (for example Odontogriphus and Nectocaris) had been known from single, poorly-preserved specimens. Hundreds of new specimens are helping to explain the nature and evolutionary significance of these animals. The use of computers (i.e., parsimony analysis) to classify organisms, and the use of the stem group and crown-group concepts, also contributed to this change. Evolutionary relationships are identified by recognizing traits that different organisms share by inheritance.
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.