by Matthew Cobb
Paleontologists sometimes use terms that are rather different from the rest of the scientific community. One of my favourites is ‘enigmatic’. This term is often and appropriately applied to the soft-bodied multicellular organisms found in the Ediacaran strata (635-542 million years ago), which immediately predate the Cambrian rocks that saw the astonishing ‘explosion’ of multicellular animal life.
Indeed, so enigmatic are the fossils found in the Ediacara that many palaeontologists refer to the organisms that left these remains as ‘the Ediacaran biota’, maintaining a strictly agnostic position as to whether the organisms were animals, plants or something else entirely.
Here’s an example of how weird these things can be: Parvancorina. These fossils are 1-2 cm in length. They could be a hold-fast (the remnants of some organism that was attached to the sea-bed), although there are things about the fossils that suggest this is not the case. Other people have suggested they may be some primitive arthropod (the lack of any sign of legs suggests to me that this is a superficial identification).
In an article published online in Nature last week, geologist Gregory Retallack of the University of Oregon put forward what he accepts is an ‘unconventional’ hypothesis. Not only does he suggest that Parvancorina, which is often identified as an animal, was in fact a fungal fruiting body, above all he suggests that these fossils are not, as is universally accepted, from marine layers, but were in fact overwhelmingly terrestrial. This would mean that life first colonised the land at least 100 million years before the currently accepted dates (mid-Ordovician).
Using a close reading of the geological evidence, he applies this categorisation to most of the iconic Ediacaran fossils and reinterprets them as organisms that lived on dry soils. (Retallack has been arguing this for some time. Here’s a link to a 1994 paper of his arguing essentially the same thing, though with less geological detail. – the key thing in the new paper is his confident assertion that the rocks were terrestrial.)
So, for example, Retallack argues that Dickinsonia is ‘more likely to have been lichens or other microbial consortia’ rather than an early invertebrate. But he is arguing here simply on the basis of the geological identification of the rocks as being terrestrial – which he accepts is cotnroversial – not on the basis of the anatomy of the fossil.
Here is Dickinsonia. I appreciate that taphonomy – the way that things decay and become fossils – can have weird effects, but I find it hard to see this as anything other than an early invertebrate.
Unusually, the appearance of Retallack’s research article was accompanied by the publication of four ‘magazine’ articles, the first of which is an Editorial, justifying the publication of the article partly on the basis that previous whacky theories (eg Jane Gray’s advocacy of an Ordovician terrestrialisation date – around 460 million years ago) have become accepted. But as we know, most of the people who argue against a widely-accepted scientific view are not like Galileo, they are simply wrong.
That’s certainly the view of two of the three other pieces that accompany Retallack’s article. Shuhai Xiao dismisses Retallack’s terrestrial interpretation and puts forward counter-arguments for Retallack’s view. One of his most telling points is made by this picture. It shows Dickinsonia fossils on a bed of rock. And if those aren’t undersea ripples, I’ll eat my geologist’s hammer (I don’t have one).
In an online article at Nature News, Brian Switek has hunted out quotes from other geologists:
“I and my colleagues are quite weary with being asked to review his material over the last ten years,” says James Gehling, a palaeontologist at the South Australian Museum in Adelaide.
Guy Narbonne, a palaeobiologist at Queen’s University in Kingston, Ontario, says that the new paper is little more than a summary of Retallack’s “long-standing views” on Ediacaran life.
He adds: “Most of us appreciated that Retallack’s lichen hypothesis was innovative thinking and tested his ideas critically, but it quickly became clear that there are simpler explanations for the features Retallack had validly noted, and most of us moved on to more promising explanations.”
Gehling is unconvinced by the new paper: “Retallack has presented not a single piece of evidence that would contradict the interpretation of the sedimentary layers involved as anything other than marine.”
He and Narbonne argue the red coloration of the rock and its weathering patterns, which Retallack presents as new evidence, could be just as easily accounted for by a marine origin.
Narbonne says that “multiple sedimentary and geochemical approaches by multiple independent laboratories worldwide have nearly universally converged on a marine origin for the Ediacara biota”.
Traces of animal behaviour in the wave-rippled Ediacaran sediments also contradict the terrestrial hypothesis, in Gehling’s view. Dickinsonia, an animal possibly related to today’s blob-like placozoans (the simplest multicellular organisms), left tracks after “spending time on one site, decaying the organic matter below, and then creeping across the mats to the next site,” he says, and the “mollusc-like” Kimberella created scratch marks on the sea floor as it grazed.
“If 60 years of published interpretations of the Ediacara biota have shown anything,” Gehling says, “it is that the Ediacara biota were a diverse array of organisms with remarkably consistent body plans found in distinct associations and most often preserved in place on fossil sea floors.”
Retallack remains unfazed. “I am expecting controversy,” he says, adding that he anticipates “the usual trajectory of grief, beginning with denial, then proceeding to mourning and acceptance” of his idea.
I’m no geologist, and as Paul Knauth, who remains open to Retallack’s suggestion rightly says in his commentary ‘We were not there when all this happened’. However, as a biologist with an interest in paleontology (I teach this stuff to second year students, and will include Retallack’s ideas in my lectures next year) I think that ‘mourning and acceptance’ might be the future for Retallack, not for the rest of us. As to what exactly the Ediacaran biota were, that remains one of the most fascinating enigmas in science.
Gregory J. Retallack (2012) Ediacaran life on land. Nature doi:10.1038/nature11777
Shuhai Xia & L. Paul Knauth (2012) Palaeontology: Fossils come in to land Nature.
“cotnroversial” An amusing new term for something that can cause arguments between professionals, which the majority of non-professional people have little hope of following.
I’m glad you addressed this. The DiscoTute and various other apologists have been trumpeting that paper as if it now means that there are no direct ancestors to Cambrian lifeforms. To them, naturally, it’s looking more and more like ‘god did it’. Quelle surprise.
the ‘fossils’ called Vernanimalcula are just random bits of geological minerliazation; they may be infills of cysts, but they tells us nothing biological and they are certainly not fossil bilaterians.
I love a new idea as much as the next person but those ripples look watery to me too. I do in fact have a geologist’s hammer (g-pick) and I do not feel it would be at risk with this bet. It is also the case that terrestrially deposited rocks are in a small minority for obvious reasons and Bayes’ Theorem would indicate that this puts the theory at further disadvantage.
A nice theory killed by facts, to paraphrase whoever it was who said something similar.
Perhaps you ere thinking of something like this: “Science is organized common sense where many a beautiful theory was killed by an ugly fact.” — Thomas Henry Huxley
3 billion years of life has always struck me as a long time before land becomes colonised.
What I get from the history of life on earth is that there is bacterial-grade life everywhere in the universe, but hardly anything more complex, and very, very little in the way of critters leaving the water/methane/whatever… stasis is a bitch!
Have you been listening to Simon Conway-Morris again?
Well, it’s not like bacteria could have left the water any time during those 3 billion years. Multicellularity had to come first. But once it did, colonizing the land happened relatively quickly.
…and the point is that first, eucaryote grade with complex organelles, and second, multicelluarity with differentiated tissues, seem to be much larger hurdles than life itself.
But again there are other factors in play. After the invention of photosynthesis, it took a couple of billion years to saturate the geochemical oxygen sinks and transform ocean chemistry. That process had to go to completion before oxygen-based metabolism could take off — and then it did take off, pretty quickly.
So those billions of years of apparent stasis don’t accurately reflect the probability or difficulty of subsequent biological developments. It’s just that some of the previous developments took a long time to exert their full enabling impact on the environment.
That’s a bit of an overstatement. The history of the increase of environmental oxygen is more complex than a simple slow rise. From the first appearance of “banded iron formations” to the last was probably around a billion or so years, but within any particular BIF there is a lot of cyclicity (that’s the “banded” part of the descriptive name) between oxidised iron compounds (typically haematite, iron III oxide, which is red) and less-oxidised iron compounds (magnetite iron II-III oxide FeO-Fe2O3 and wustite FeO, both of which are black). The banding is on a millimetre to centimetre scale, though it is not clear if the banding represents tidal cycles, “monthly” spring-neap cycles, annual cycles. (Milankovich cycles would probably be a bit on the long side, but I don’t think they’ve been really ruled out.)
These alternations of redox (reduction-oxidation) state are thought to represent the effect of altering local balances between mineral input and oxygen production.
The repeated cycling of redox stsate would have been a potent driver for the discovery of oxygen-managing metabolic techniques by non-respiring biota. Handling one level of environmental oxygen is one metabolic trick ; handling a different level is a different trick ; but to handle alternating levels needs both sets of metabolic tricks AND some “glue” chemistry and gene switching to handle the change over. You’d also need some oxygen level sensing too.
That’s a lot of biochemistry. In particular, the ability to switch from one metabolic configuration to another is an evolutionary novelty. Perhaps.
Yeah, that wasn’t stasis. It was titration.
There are Cambrian fossils of Ediacaran type found in association with Cambrian marine fauna, including trilobites. Hard to make that into a terrestrial organism.
Could you tell us more about that? I understood (as a non-biologist) that the Ediacaran fauna died out at the time the first hard-shelled organisms appeared. Where has an overlap been found? (And when was it found?)
I don’t have the reference to hand, and I do have a hangover, but ISTR that Ediacaran-type fossils have been found in several “Lagerstatten” – areas of exceptional fossil preservation – ranging from mid-Cambrian to early Ordovician.
Whenever you read that sort of thing, you should be adding mental caveats to the effect that SO FAR the fossils of organism [X] have been found in rocks of ages not younger than [Y]. But you’re looking at the end product of three low probability random events : only a small number of organisms that die will avoid being scavenged or decaying to amorphous sludge ; if your organism gets buried, only a small proportion will get mineralised within the rock (look up “taphonomy”) while most will be digested by microbes and circulating ground water into unidentifiable mineral sludge ; and finally your fossil needs to be among the relatively small proportion of rocks picked up and examined by someone who recognises the significance of the obscure scribbles on the rock. By the time that you get to the end of that chain, the probability that any specific specimen which is found is actually a fossil of the last member of that taxon that ever lived becomes quite low. That is why much “industrial” dating work is done using microfossils – you can get thousands of specimens in your series of samples from an oil well – and process and identify them in bulk – leading to statistically significant changes in the abundance of taxa and correspondingly high precision of dating.
For an example of the identification problem, people had been noticing (but not understanding) the “enigmatic” fossils in the sandstones of the Ediacara hills of Australia for decades before Reg Sprigg realised that they were really fossils and started describing them formally (I think previously they’d been thought of as some mineral concretion.) On the other hand, if you want to get your foot into the door of industrial dating, you need to start off by getting your PhD in your group of organisms of choice (so you’re already mid-late 20s) and do several years getting up to speed on that group (over 30 by now) and improving your microscope technique. Then you can start trying to get a reputation for knowing what you’re talking about and you can finally start earning a decent living. I’ve not met a competent palynologist who’s under 35. Unsurprisingly, the recruitment queues are not exactly long.
It is likely that the Ediacaran fauna suffered severe competition from newly developed organisms with hard parts, but that doesn’t necessarily mean that they died out entirely, just that they reduced in diversity, abundance and range. It is by no means impossible that they are still in existence, in modified form. Matthew mentions the Placozoa, which certainly shares some affinities with some Ediacaran forms (remembering that the “Ediacaran biota” includes a fair range of diversity and probably some quite distantly related organisms). Some of the Ediacarans are clearly segmented (Spriggina for example), and I don’t buy completely losing a complex character set like that as a credible evolutionary trajectory.
Your last question : search for and research on early Lagerstatten is continuing all the time. Simon Conway-Morris added the Sirius Passet fauna of North Greenland to the literature in the mid-80s, and is still coming up with new stuff from there every few years. The Chinese found the Doushantou (spelling?) phosphorite fauna in the mid-90s, but that is really out of logical contact with anything else being stuffed with microscopic embryo fossils from who-knows-what (and a bit of seaweed and some shrimps). The phosphorite has been exploited for many decades ; the fossils were only recognised in the mid-90s (IIRC). There’s another fauna called “Chenjiang” which the Chinese found in the late 80s and SCM is up to his gills in that too – real jaw-cracking names too! Oh, and I’m forgetting SCM’s former colleague Derek Briggs and his Shropshire sludge where he’s pioneering computerised microtomography to understand his spiky weirdnesses. To (mis- ?) quote SCM, “Oh fuck, not another new phylum!” (Sorry SCM, even if it’s not true, it’s too good a story to not repeat. Just man up and embrace it!)
Terrific answer – thanks.
We aim to please.
John, that seems like an absolutely definitive refutation of the dry-land thesis. Did people not mention this in the Nature articles?
I haven’t seen the Nature articles. But here’s a reference:
Hagadorn, J. W., Fedo, C. M., and Waggoner, B. M. 2000. Ediacara-type biotas from the Cambrian of the southwestern United States. J. Paleontol. 74:731-740.
There are others.
No, they didn’t. They were focusing more on the geology.
Quite an oversight then….
From Retallack’s paper,
“Discovery of some Ediacaran fossils in the surface horizons of palaeosols does not mean that all Ediacaran fossils everywhere were terrestrial.”
This qualification seems to insulate his thesis from all the most obvious kinds of falsification, but one could respond by agreeing that “Discovery of some Ediacaran fossils in the surface horizons of palaeosols [if they are paleosols] does not mean that any Ediacaran fossils anywhere were terrestrial.”
Marine organisms might find drowned soils quite tasty during intervals of local sea-level rise.
(I used to be in the office next to Jim Gehling’s but I’m a terrestrial vertebrate guy, so my bias exceeds my expertise on this matter)
I certainly cannot comment on evidence about whether rocks are formed from marine, freshwater, or terrestrial soils, but I would bet that the Ediacaran ‘biota’ are NOT fungi or lichens or bacteria colonies. My reason is that most of the Ediacara are bilateral in symmetry, and some have a radial symmetry. Bilateral and radial symmetry is known in bacteria colonies, and I suppose lichens and fungi, but the Ediacarans that I know of ALWAYS have those kinds of symmetry. One would have to explain why Precambrian bacteria/fungi/lichens are fossilized only IF they have bilateral or radial symmetry(!)
Several Ediacaran fossils (including the two pictured above) show bilateral symmetry, which is usually associated with animals, not fungi. The Ediacaran fossils do not show intermediate growth stages, which you’d expect to see if they were fungi or related organisms. Instead, most have well-defined and consistent sizes and shapes, which suggests that they are invertebrates.
Ripplemarks are found in many types of underwater sediments and at all depths, so on their own they rarely indicate a specific environment.
Lichens are not particularly “primitive” and are supremely adapted for terrestrial life. They may be jellylike (Collema, Leptogium, etc.) or grow inside rocks (like certain Antarctic species) and a few species actually grow in the water, but even the odd ones resemble “typical” thin, papery lichens: All of them must maximize surface area to gather as much light and moisture as possible, yet the feltlike thallus protect the vulnerable reproductive structures and algal cells from drought, wind, and UV light. It’s hard to imagine anything further removed from the compact and obviously quite sturdy Ediacaran forms.
For me the Ediacaran fossils have always suggested primitive worms and jellyfish, “proto” arthropods, and ancestral echinoderms. It’s a superficial set of assumptions for which I have little evidence other than aesthetics, so I’ve always been willing to set my ideas aside if someone offers ample and convincing evidence that the fossils are something quite different.
But this isn’t it.
I’m sure there can be some oddities that doesn’t fit the observational constraints here and there, but it doesn’t warrant continued visiting after innovative ideas have been “tested … critically”.
So I don’t know much here, but one description somewhere in the mass of blogs covering this jumped out to me. Presumably Retallack’s hypothesis is constrained by a persistent tundra like climate to predict some crack patterns and what not.
But the global glaciation is supposed to end before then. This can be a major problem for him, can it not?
FWIW, that’s an Estwing brick hammer in the pic. A lovely digging tool.
But not a “real” rock hammer, unless you only collect shale. I use a blacksmith’s hammer, not just because I’m a smith, but because I collect ultramafic igneous rocks, which would make that little Estwing just bounce and emit a wimpy “ping”!
Since I lost my nice hammer (“The Equaliser”, with the legend “All rocks are equal in the end” barely legible down the side. If you find it in a car park near Durness, gimme!), I’ve restrained myself to a lump hammer and a chisel. If I’m back packing, just the chisel. If I can’t find a lump of rock to hit the chisel with … I should hang up my hammer (which I don’t have any more, sob!)
Yes, my chisel would bounce off a good granulite. Then again, I’ve got splinters of steel in the front of my old old sun glasses which came off The Equaliser in collision with a Lewisian granulite.
Durness. Lovely place. Apart from the bloody midges.
I wouldn’t have it any other way. Midges and all. The Man-Eating Highland Midge (Culex somethingy horrendous? ) is one of the best defences against overcrowding. Without it, we’d have to start blowing bridges on the M6 or re-building Hadrian’s wall – facing the other way!
sub
Does it seem increasingly true that the way to get into Nature is to say something bizarre and iconoclastic, or something you can spin that way, regardless of scientific merit?
That’s the “magazine” part of being a journal. They’re a more or less separate editorial team from the science side – or they are in the journals that I’ve talked to staff from.
I saw this story in the media, and was hoping there’d be a post on WEIT about it (no Nature access for me at the moment, unfortunately). Great post and very interesting and informative comments. Thanks everyone.
Reblogged this on Mark Solock Blog.