The Ediacaran fauna, a group of extinct species that lived between 571 and 541 million years ago, has been an evolutionary anomaly. Its fossil record contains multicellular organisms, but they are just plain weird, bearing little resemblance to present-day metazoan (multicellular) animals.
The two species of “dickinsoniids” shown below, for example, lack a mouth or gut (possibly having external digestion instead), are bilaterally asymmetrical, and bear a pattern of body “quilting” that isn’t seen in present-day animals or definite early metazoans like worms:
Here’s Dickinsonia, studied in the paper we’re discussing today. It’s about 7.5 cm long, or three inches, so it’s fairly large:
Here’s a dickinsoniid in the genus Andiva, also studied in the present paper:
What are these things? Controversy has centered on whether they were a whole kingdom of life different from any that we know today (a group that went wholly extinct), or, in contrast, perhaps the ancestors of modern day animals—or at least the relatives of modern day animals. Scientists have guessed that they might be either lichens or giant protozoans. (Yes, protozoans can get this large; some are nearly ten inches long!) This is important to resolve because the “Cambrian explosion” that gave rise to many modern groups of animals began about 541 million years ago, and we want to know if there were animal precursors before that, and what they were. We also want to know whether the Ediacaran fauna really does represent an entire group of creatures that disappeared without issue.
A new paper in Science by Ilya Bobrovskiy et al. (reference at bottom with free Unpaywall link, free pdf here) establishes fairly securely that Dickinsonia and related anomalous species do indeed seem to be metazoan animals rather than members of a separate large group that went extinct entirely. The telling data involves biochemical analysis of the thin films of organic matter that cover the fossils and was presumably produced by the fossils. These fossils were 558 million years old, which, if they were animals, would make them the oldest known metazoans.
To make a long story short, Bobrovskiy and colleagues collected specimens of dickinsoniids from sandstones of the White Sea region of Russia and removed the small (three micron thick) organic mat covering the fossils. Great care was taken to avoid contamination, and they also analyzed the sandstone around, above, and below the fossil to see if the peculiar organic profile they found was associated with the fossil itself. It was, and it also suggested that the fossils were animals, not lichens or giant protozoans.
The telling chemicals were 27 carbon steroids—cholesteroids—which were present as 93% of chemicals in the mat atop the fossils, but only 11-12% of the surrounding sandstone (probably coming from algae or other plants). The fossils, moreover, were almost entirely missing a class of chemicals, ergosteroids, that characterize lichens. And the chemical signature of these fossils didn’t much resemble that of the modern giant protists, either.
There was, however, one twist to the findings: the “isomers” (chemically identical molecules of different handedness or arrangement) of the cholesteroids in these fossils were mostly of a single handedness (the “5β” form), while that of more recent and genuine animal fossils have a more even mixture of right- and left-handedness. This is puzzling, and the authors have no real explanation. This may suggest that even if these fossils were related to modern animals, they were distantly related, having a unique metabolism. They may, then, have branched off from modern animals, with the dickinsoniids and other Ediacaran fauna having gone extinct without descendants. Further, analysis of Andiva doesn’t show the same elevation of cholesteroids, though it does show a preponderance of 5β forms.
So this isn’t as compelling a demonstration as I had wished, but it still shows that these things were probably metazoans and not creatures related to modern lichens or protozoans. The authors conclude this:
Molecular fossils firmly place dickinsoniids within the animal kingdom, establishing Dickinsonia as the oldest confirmed macroscopic animals in the fossil record (558 million years ago) next to marginally younger Kimberella from Zimnie Gory (555 million years ago). However alien they looked, the presence of large dickinsoniid animals, reaching 1.4 m in size, reveals that the appearance of the Ediacara biota in the fossil record is not an independent experiment in large body size but indeed a prelude to the Cambrian explosion of animal life.
“Prelude” is a bit ambiguous, but I’ll grant that these are animals. I asked my friend Latha Menon, who has a Ph.D. from Oxford in early life studies, whether this paper was important, and I give her answer (with permission):
I do think it is an important paper. I’ve recently seen a lovely Dickinsonia specimen in a collection. You can really see how it’s like a very thin flatworm like form (or like a giant Trichoplax?) draped over the uneven ground below. Much has been written about its morphology but to find a specimen with associated organic matter and demonstrate that it is an animal from the biomarkers is very solid evidence. We have indications of simple animals earlier, from the traces found by Alex Liu and myself, and the remarkable squished Haootia specimen Martin Brasier discovered, which seems to show muscle bands. This specimen is younger and part of a more complex assemblage, but it is good to have solid evidence of animals well before the Cambrian boundary and some 40 Ma before the Cambrian explosion. Animals did not burst on the scene then; there was, as most of us have suspected, a long fuse.
Of course this does not mean that all the large forms of the Ediacaran were animals; there were probably several kinds of forms. But it does show that one ‘quilted’ form was animal and that suggests a number of other enigmatic forms in the biota were too.
Just for grins, here’s a photo, courtesy of Australian National University, showing Bobrovskiy collecting fossils in Russia. It ain’t easy!
________
Bobrovskiy, I., J. M. Hope, A. Ivantsov, B. J. Nettersheim, C. Hallmann, and J. J. Brocks. 2018. Ancient steroids establish the Ediacaran fossil Dickinsonia as one of the earliest animals. Science 361:1246-1249.
A Science News and Views piece on the paper: Summons, R. E. and D. H. Erwin. 2018. Chemical clues to the earliest animal fossils. Science 361:1198-1199.
How prone are these kinds of 5beta isomers to racemation? I bet they addressed this. I’ll look up the paper later.
Exactly what I thought
After 500 million years, without stabilization, I bet the 5-beta form is most stable.
… also – I though only animals make cholesterol.
^sigh*
“racemization”
I need a coffee.
Ediacarans – doing their bestest to make the Burgess assemblage look boring.
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As a geologist, I must say that (Bobrovskiy’s collection site) is some of the crappiest sandstone I’ve seen!
Why?
I’ma resist my puerile impulses here.
“It’s about 7.5 cm long, or three inches, so it’s fairly large”
Poor, disappointed Sonia.
I tried.
🙂
Your “Trichoplax” link is truncated and isn’t going where you want it to go.
Jeez, this is fascinating! I wonder how these animals might be related to sponges or jellyfish.
Those have been recently elevated in the animal tree. Sponges especially are proposed to have been even more complex than they are now.
I have (jokingly) allied sponges with deuterostomes, because they lack a single entrance opening but have an exit opening.
Very interesting article! Thanks for posting.
It is not revolutionary in that most (I think) experts considered the Vendeean (Ediacaran) fauna somehow metazoic.
What is baffling is that these steroid signatures survived more than half a billion years. And that it gives solid evidence the ‘Vendeeans’ were indeed close to the metazoa.
Wonderful.
Well doesn’t that just ruin it for Creationists who like to see the Cambrian Explosion (not recognizing that in geologic time explosions last millenia) as a “let there be life” ta-dah moment.
There is also the Vernanimalcula, indicated to be 580-600 million years old. They too had vague bilateral symmetry. See https://en.wikipedia.org/wiki/Vernanimalcula
What’s the advantage of bilateral asymmetry? Are there any good candidate explanations?
I read The Mountains of Madness recently.
This is definitely the footprint of an Old One.
No, just that of one toe.
Love the post. Learned about dickinsoniids and Trichoplax
When I read early life specialist, my brain read ‘early education specialist’ and thought how cool that someone working in elementary education was collaborating on this! Lol. Still very, very neat.
I assume that there were ancestors of the Cambrian phyla living alongside the Ediacarans, not particularly numerous or successful? And then the Ediacaran period came to an end with the breakup of the then existing supercontinent and a marked upswing in the number and severity of volcanic eruptions and enormous climatic change, resulting in the Ediacarans going extinct, and leaving the field free for the ancestors of the Cambrian phyla to fill numerous empty ecological niches and undergo a radiation, similar to the one the mammals enjoyed 65 mya at the K-Pg event?
You might find the small shellies interesting. It could be that the ancestors of the Cambrian biota were living next to the late Ediacaran biota in great numbers. They were just very small.
https://en.wikipedia.org/wiki/Small_shelly_fauna
What a remarkable feat to extract the chemical evidence from a fossil that is so old! 450 million years, wow. That is only about a tenth of the age Earth, I wonder what more lies hid in the past.
@3
Perhaps Ediacarans need a little dose of Hallucigenia.
@11
And I want to know why Bobrovsky is keeping vewwy quiet about all the wabbit fossils he found under the Dickensonias.
Not being a chneist I have been vary of steroids since there are prokaryote confounds – and see Jerry’s note in isomers – but I think the careful differential analysis against the sandstone background close the case. It is extremely rewarding after papers that have put Dickinsonia with a head during growth to go with apparent travel marks [I’ll give no reference at this time, and sloppy references below, since this is a hasty comment] but also put them as stem lineages to rangeomorphs [“Cambrian petalonamid Stromatoveris phylogenetically links Ediacaran biota to later animals”, https://onlinelibrary.wiley.com/doi/abs/10.1111/pala.12393 ]:
“Here we provide phylogenetic evidence to identify Ediacaran macro‐biota as animals, based on 206 new fossils of Stromatoveris psygmoglena from the lower Cambrian Chengjiang Lagerstätte. Exceptionally preserved soft‐tissue anatomy shows that Stromatoveris was a soft‐bodied, radially symmetric animal with multiple, sub‐branched petaloids and a differentiated holdfast. Photo‐referenced morphological character analysis enables phylogenetic reconstruction of a monophyletic clade designated Petalonamae, that unites Stromatoveris with iconic Ediacaran genera (Rangea, Pteridinium, Ernietta, Swartpuntia, Arborea, Pambikalbae and Dickinsonia) and is placed as sister‐group to the Eumetazoa. Therefore, based on phylogenetic bracketing within the Metazoa, the Ediacaran petalonamids are established as animals. From these findings, it follows that petalonamids remained an important component of Cambrian marine ecosystems and that the metazoan radiation can be dated to a minimum age of between 558 and 571 myr.”
This supplements current ecological study:
“A ‘simple’ model assumes that most, if not all, Ediacaran organisms shared similar basic ecologies. A contrasting ‘complex’ model suggests that the Ediacara biota more likely represent organisms from a variety of different positions on the eukaryotic tree and thus occupied a wide range of different ecologies. We perform a quantitative test of Ediacaran ecosystem complexity using rank abundance distributions (RADs). We show that the Ediacara biota formed complex-type communities throughout much of their stratigraphic range and thus likely comprised species that competed for different resources and/or created niche for others (‘ecosystem engineers’). One possible explanation for this pattern rests in the recent inference of multiple metazoan-style feeding modes among the Ediacara biota …”
[“High ecological complexity in benthic Ediacaran communities”, https://www.nature.com/articles/s41559-018-0663-7 ]
Finally, I was going to say something like Wayne Robinson’, prompted by an article speculating in something similar. For reasons of preservation (soft vs hard tissue) and mass extinction the later “explosion” now seems much like “business as usual” to me.
Oy. “a chneist” – a chemist. “Wayne Robinson'” – Wayne Robinson.
Hasty.
Also, I haven’t had time to read the two references (and I would not easily grok the ecology one) – caveat emptor.
Amazing history. Such investigations unfold like a vast, complex, mystery novel.
Fascinating stuff! I haven’t been keeping up with developments in the study of pre-Cambrian fossils so I’m a little vague on how they relate to the larger tree of life.
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Fascinating article and analysis. So, is it correct that things that are animals are more closely related to me than fungi, that is, animals are the sister taxon to fungi (give or take a clade or two)? And so, the ediacaran fauna branch after the metazoa/fungi split? is the alternative that the ediacarans branch somewhere else? if so, whhere? I find this issue fascinating and I’m amazed (or maybe skeptical?) that organic compounds like steroids could last in rock for 600 My.
Does anyone her know whether
Sperling en Vinther 2010 A placozoan affinity for Dickinsonia and the evolution of late Proterozoic metazoan feeding modes. EVOLUTION & DEVELOPMENT 12: 201-209
has been well received?
Thanks for this interesting post. I’ve never heard of these fascinating creatures. Perhaps they are aliens from outer space…just kidding.