The Cambrian Explosion, part 2: The softies of Ediacara

For a long time, geologists logically set the dawn of the Cambrian at the first fossils, at the trilobites and their friends, which we met in the first post. But as we got to know the fossils better, it turned out that the story was not that simple. Below the first fossils of trilobites and the Burgessians, there were many signs of advanced life: Tracks.

Walking traces, burials, tunnels, all the tracks creepy crawlies leave behind as they creep and crawl. Tracks appear much earlier than the fossils with shells, but they show that complex life existed. Something must have made those traces.

Today, it is the first appearance of the trace fossil Treptichnus in a section on Newfoundland that defines the base of the Cambrian, dated at 542 m.a. (million years ago).

T pedum at base of Cambrian

Treptichnus burial fossils at the base of the Cambrian on Newfoundland.
(Martin Smith, CC BY-SA 4.0, via Wikimedia Commons)

It makes sense to set the start of the Cambrian at the first proof of active organisms. But similar trace fossils of the Treptichnus genus exist below the border. In practice, the border is set at a somewhat arbitrary point during a gradual change in organisms. We now know that the trilobites and Burgessians appeared not before 521 m.a., well into the Cambrian.

Before that, it seems that evolution tinkered and experimented for around sixty million years before it finally got the constructions right and the Cambrian explosion took off. But it had a long fuse, and now we will look at what evolution did during those sixty million years – and how evolution transformed the Earth and the environment into something complex organisms could live on.

But first, some philosophical notes of caution:

When we dive back into the late part of the Precambrian and look at it with the benefit of hindsight, we should remember the words of the Danish philosopher Søren Kierkegaard:

“It is really true what philosophy tells us, that life must be understood backwards. But with this, one forgets the second proposition, that it must be lived forwards. A proposition which, the more it is subjected to careful thought, the more it ends up concluding precisely that life at any given moment cannot really ever be fully understood; exactly because there is no single moment where time stops completely in order for me to take position: going backwards.”

An insight often shortened to “Life can only be understood backwards; but it must be lived forwards”

Søren Kierkegaard (1813-1855) - 9645352684

The philosopher Søren Kierkegaard (1813-1855).
(The Royal Library, Denmark, No restrictions, via Wikimedia Commons)

Life evolved forwards, obviously. With the benefit of hindsight, we can understand what happened and the consequences. But don’t confuse consequences with intent. Why something happened was mutations and natural selection, not deliberate intention.

We must avoid the trap of looking at how evolution and geology prepared the environment and built organisms, step by step, for higher life, and conclude that it had a direction, or happened on purpose, deliberately laying the foundation for higher life.

Another trap to avoid is to think of all these experiments, the tinkering, the Ediacaran fauna, as sidetracks, meandering distractions, detours on the main road to higher life. Why did evolution not go straight ahead towards trilobites?

Both these are big misunderstandings. Evolution could not go directly or plan towards a goal, because evolution is not an organism with intelligence. It is just a mechanism, made by natural laws. Evolution cannot have sidetracks because it does not have a mainline with a destination.

Therefore, evolution may often go down what we perceive as a blind alley, and need the help of a mass extinction, a climate change, an important mutation, to, in our view, get back on track. But there was really no direction in the first place.

We humans are here because a meteorite hit the Earth and ended the dinosaurs 65 million years ago, as I wrote about here. We are results of an accident, not a goal. The K-T meteorite was not the first big rock to hit Earth, but it hit in the worst place, As such, it was a black swan, the worst of luck.

The enormous volcanism that made the Permian-Triassic and Triassic-Jurassic extinctions, paving way for the dinosaurs, were more grey swans: They came from Large Igneous Provinces – LIPs among friends – that are an irregular feature of plate tectonics. LIPs will happen again, we do not know when, but they will, and life will take a hit.

The changes that heralded the Cambrian fauna also meant the extinction of another fauna: The Ediacarans.

We have met the Ediaracans before, but there is some more to say.

Tales of the Ediacaran fauna may easily fall into the “bizarro trap”: Look at these strange fossils! Nothing like we know!

But such popular science fun facts gloss over that the Ediacarans were not a bunch of evolution clowns, but real, living organisms in an ecosystem.

There was not just one Ediacaran fauna, but three in succession, each consisting of distinct groups of organisms. For nearly 30 million years they were the citizens of the sea floor – from around 575 m.a. up to the Cambrian border at 542. Some Ediacarans persisted through the whole time, others came and went. Many are still mysteries, but some are possible precursors to arthropods, trilobites, molluscs – and may as such be regarded possibly as not “real” Ediacarans at all.

The first “Ediacaran fauna” is the “Avalon assemblage”. “Assemblage” may be an appropriate word for a community of organisms, which we do not really know what were. They are named after the Avalon peninsula on Newfoundland, which contains the oldest part of the Ediacaran period, ca 575-560 m.a. These oldest Ediacarans were also the first to be found, in 1868 by the Scottish geologist Alexander Murray. Being in the wrong place in the rock succession, they were simply dismissed as sediment concretions. The fossil terrain did not fit the map, so the terrain had to be wrong. It was only later, after the fossils from Ediacara made it clear that these had to be living organisms, that the old guard keepers of the fossil record gave in.

Ediacaran fossils Mistaken Point Newfoundland

Rangeomorph fossils at Mistaken Point, Newfoundland.
(Alicejmichel, CC BY-SA 4.0, via Wikimedia Commons)

Most of the Avalon assemblage belong to the Rangeomorphs, looking like ferns or sea lilies, which probably were anchored to the seabed and waved their fractal-geometry body like a leaf in the waves. They have no trace of a mouth or digestive system, so they probably fed by a kind of suspension filtering. Rangeomorphs could not get energy from sunlight, because they lived in a deep sea. The richest locality is the 565 m.a. old Mistaken Point, a rugged coast on the SE end of the peninsula. Mistaken Point got its name in the age before electronic and radio navigation help because, in bad weather, sailors could mistake it for the Cape Race 5 km to the east, wrecking many ships. Here, more than ten thousand fossils have been found, some in communities individuals that appear to be connected by a “root” system. The fossils were preserved by a very fine volcanic ash, which rained through the sea.

The rangeomorphs are some of the best documented, but still least understood Ediacarans, in terms of “what were they, really?”. The truth is, we don’t know, we can only make educated guesses. A big question is if they can fit into some known classification, or are sidetracks on the evolution railroad, genetic experiments that were later bypassed by the mainline train.

With the Burgess fauna, most organisms can be fit into a known classification, some easily, some with shoehorning and a generous sprinkle of speculation salt. We can at least recognize them as…something we could possibly recognize. Even the absurd Hallucigenia could be viewed as a sort of worm with strange appendices and extra spikes.

But the Ediacarans? With no sign of mouth or rear end, and nothing existing to compare to? Sure, Rangeomorphs sort of superficially, resemble sea ferns, but are so out of place and time, and different in the details, that the resemblance is like the one of a fossil of a slug and a bad fossil of a carrot. The rangeomorphs also have a fractal growth pattern that is quite different.

But some Ediacarans are possible more recognizable, as we will see in the second of the faunas: The “White Sea assemblage” is named after outcrops on the shores of the White Sea, in northwest Russia. It is more varied than the Avalon assemblage. The original Ediacarans from Australia also belong to this assemblage, including the most iconic Ediacaran: Dickinsonia.

Dickinsonia costata (Ediacaran fossil) (Ediacara Member, Rawnsley Quartzite, near-uppermost Neoproterozoic; Flinders Ranges, South Australia) 2 (15204763675)

Dickinsonia from Ediacara, Australia.
(James St. John, CC BY 2.0, via Wikimedia Commons)

1. Дикинсония - ископаемое животное эдиакарской биоты (Средний Урал)

Dickinsonia from the Urals, Russia.
(Nikolai Bobkov, CC BY 4.0, via Wikimedia Commons)

Dickinsonia was the first fossil Reg Sprigg found in Ediacara (and the first I blogged about) and he simply named it after his boss. Dickinsonia was most likely an animal, and it looks like an irregular disc with a main furrow across, and furrows radiating out from along it. Dickinsonia probably was a bottom dweller, now and then jumping, fed by osmotic grazing on the algae mats that covered the floor of the Eadiacaran sea. Some very-popular-science descriptions compare it to jellyfish, but that’s out of question. Jellyfish do not have such a radial pattern.

This radial pattern along a furrow may possibly be the first example of an invention that dominated animals forever since, everyone from apes to crabs: Bilateralism – academic for an organism which is symmetrical around its length axis. We humans are symmetrical along our spine; our right and left parts are symmetrical, save some internal skewing of soft parts like the heart and intestines. All mammals, reptiles, amphibians, fish, insects, arthropods are the same. Bilateral symmetry is so fundamental to animals that we get confused when we do not find it, like in brachiopods – and jellyfish.

This does not solve the problem of what Dickinsonia really was, or if it may be related to anything later. It only suggests that the fundamental feature of bilateralism had possibly developed. But other Ediacarans may be precursors to better known organisms.

Some of them are actually possible forefathers, of the cnidarians, the phylum of jellyfish. The next time you find a jellyfish on the beach, notice that it has a fourfold geometry; it is round, but with four rings or eyes in the center. Among the Ediacarans are some organisms with a similar geometry, the tetraradialomorphs – which look kind of like the imprint of a jellyfish, or as a disc with four arms from the center towards the rim. But whether these actually were precursors to jellyfish, or something else is speculation. They may even have been benthic, i.e. bottom dwellers. These are part of a larger group called the radialomorphs, which also include organisms with two, three, five or possibly even eight-fold symmetry, or radial, concentric rings, but no “number-segmentation”.

The five-folds are tempting to link with sea-urchins and sea-stars, which also have a five-fold symmetry (count the arms of a sea star!). Again, the similarity may be a real relationship, or a coincidence. One good reason to believe these organisms were bottom dwellers is also that the fossils of Ediacarans are often in sandstone or non-anoxic shale, and they are preserved because they made an imprint in the algae mats that covered the Ediacaran sea bottom.

Kimberella (and here) is one of the most interesting of the Ediacarans – because it may be the ancestor of mussels. Kimberella looks like a shell with a wrinkled skirt along the edge, and that is what it may be: The first shell – not of hard calcite, but a fabric of e.g. chitine, which covered the internal organism, and with the soft inner content sticking out below. If Kimberella really is the ancestor of shells and cephalopods, it probably moved around like a snail, eating its way through the algae mats. The fossils are sometimes accompanied by grazing marks and crawling trails. If Kimberella really was a proto-mussel, it must at least have had some digestive organs and a feeding apparatus (scientific for mouth).Kimberella may have started out as a worm-like, which then developed a covering layer, first of chitine, and later of calcite. Shells could curl up for snails and cephalopods, or split into two halves for mussels. Kimberella is also clearly a bilateral, which strengthens that it may be the precursor for the wide range of animals, from slugs to octopuses. Sure? Hell, no. Possible? Most certainly, my dear!

Kimberella quadrata キンベレラ クアドラタ3

Kimberella fossil. Note the undulating “skirt” around the edge.
(Masahiro miyasaka, CC BY-SA 4.0, via Wikimedia Commons)

We will look at one more Ediacaran from the White Sea assemblage, because it is a possible aspiring arthropod: .

Spriggina is found only in Ediacara in Australia, first by Reg Sprigg, and later named to his honour when the scientific world finally realized how important the finds were. Superficially, Spriggina may look like a long, slender trilobite, with an eyeless head shell. But the skin betrays. On closer look, Spriggina shows no legs, and although overall symmetric, the segments are glide shifted, meaning that the left and right sides are not pure mirrors, but one side is shifted half a segment relative to the other side, where they meet along the mid seam.

Spriggina Floundensi 4

Spriggina fossil drom Ediacara, Australia.
(No machine-readable author provided. Merikanto~commonswiki assumed (based on copyright claims)., CC BY-SA 3.0, via Wikimedia Commons)

Whether Spriggina is the ancestor of trilobites, we may never know, unless we dig up some more missing link fossils. It may also be an annelid worm, or its descendants evolved into something like the Burgessian Marella. Marella may be another trilobite ancestor, which, in case, already was a living fossil.

Sometimes, we have ideas on which phylum to put an Ediacaran into. But, because we have literally no hard data of the organisms, only soft imprints, we must rely on more or less qualified speculation.

In the words of Douglas Erwin and James Valentine in their book The Cambrian Explosion: Ediacarans are phylogentic Rorschach tests.

The Ediacaran world appears almost as a big genetic accident, and then the Creator or Evolution decided that the whole thing was a mistake and wiped it out.

But that would be a misunderstanding of the Ediacaran world. It played an important role in preparing the Cambrian explosion.

The Ediacaran sea bottom was quite different from today. It was covered by a thick, slimy mat of algae. The algae mat was both floor and food for the organisms that lived on it. But the algae mat also meant that the sediments beneath it were stable, unchurned, and with little oxygen. But as the Ediacaran progressed, more and more organisms moved down under, living in the sea bottom. It created a virtuous cycle; more burial organisms, more churning, more bioturbation as it is called in geo-lingo, more oxygen in the substrate, even more organisms. This prepared the sea bottom for the Cambrian explosion, because it enabled the big variety of organisms that live in the sea bottom, and are part of the food chain.
It also doomed the Ediacarans themselves. For with the increased bioturbation, the algae mats were eaten up and churned away, and with them went the Ediacaran fauna.

In fact, the Ediacaran fauna went downhill already from around 551 m.a. until its end at 545 m.a. This last time of the Ediacaran fauna comprise the third assemblage, the Nama assemblage. It is notably simpler than the second one, and a temporary drop in the oxygen in the sea was a possible culprit.

The Nama assemblage consists mainly of the Rangeomorphs, which lived through the whole Ediacaran, and the Emiettomorphs, which appeared halfway in the White sea assemblage. Emiettomorphs are somewhat similar to rangeomorphs in appearance, but they had not only two, but up to five “leaves” sitting on the stem.

The death of the Ediacarans is possible to explain, but what about their rise? How did they come about? That is a taller order.

There are no “proto-Ediacarans”. This may be due to a combination of preservation and fast evolution; there are after all not that many places with well preserved Ediacaran fossils, so may be no snapshot was taken during the leap. Absence of evidence is not evidence of absence, and possibly the Ediacarans shared a characteristic biology that helped their preservation. But it means that there still is a mystery to solve.

Hopefully, a Lagerstätte with the right fossils is waiting to be discovered.

In the mean time, and in the next post, we jump forward to a
group that fill in the 20-million year void in the Early Cambrian, between the Ediacarans and the well known Cambrians. These organisms were often overlooked because of their size, but now they are recognized as important, and include possible precursors to the Cambrian fauna: The small shelly fossils.

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