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.
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.
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.
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.
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.
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.