The earliest microscopic fossil remains of probable prokaryotic organisms have been found in rocks dating back 3,500 million years. The most diverse and abundant of these are single-celled bacteria, representing one of the most ubiquitous forms of life on the planet. By around two billion years ago, eukaryotes – larger and more complicated organisms with a true nucleus and complex internal cell structures – had evolved. These organisms were the first to develop complex multicellularity and became the first individual life forms to leave fossils visible to the naked eye (macrofossils).
Some of these early eukaryote macrofossils resemble simple algal seaweeds, but are difficult to classify in living groups; others can be identified as different kinds of primitive algae or as fungi. These fossils remain extremely rare, and the first 3,500 million years of life on Earth was largely dominated by single-celled microbes lacking a nucleus.
Ample evidence of ancient microbial life has been left behind as layered rock structures called stromatolites. These formed in watery environments where the sticky biofilms secreted by microscopic filamentous cyanobacteria trapped and cemented grains of sand or mud in alternating layers. Modern stromatolites are usually found in sheltered, shallow marine environments where high salt levels in the water keep bacteria-grazing animals at bay. Over time, new layers of sediment are added, sometimes forming metre-high domes, with the cyanobacteria living only on the uppermost, youngest surface. Because they were strongly bound together by bacterial biofilms, ancient stromatolites preserve well in the rock record.
Fossilized stromatolites are found in many locations around the world, indicating the bacteria that built them were widespread and abundant from 2,500 million years ago until the end of the Ediacaran Period (542 million years ago). The rapid decline in stromatolite diversity and abundance during and after the Cambrian Period was probably due to grazing activities by newly-evolved animals and changes in environmental conditions.
The prokaryotic cyanobacteria – particularly those forming stromatolites – were largely responsible for the first free oxygen in Earth’s atmosphere, which began to accumulate around 2,500 million years ago. Cyanobacteria release oxygen as a by-product of photosynthesis, the same process plants use to convert carbon dioxide into sugars that can be used for energy. Eukaryotic organisms require oxygen, so life could not become large and complex until the oceans and atmosphere contained enough oxygen to support their evolution.
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.