Imagine that you have been transported back around 550 million years to what is now Australia. The first thing you notice is how cold it is, as the frost on the ground crunches under your feet. The place is eerily silent but for the wind, with no birds in the air, no animals grazing or even wandering, just the sound of your own footsteps echoing around. It is a barren landscape, one which has not been touched by life. But then you start to notice some unusual colours on the ground, small patches which do not blend into the crunchy soil. As you bend down to examine them more closely you start to notice that there are different types of patches, they even look quite familiar. Then you realise: this land is not waiting for life to invade, it already has invaded. Those colourful patches are lichens, and they are wonderfully diverse.
That, at least, is the picture which Gregory Retallack has controversially put forward in a new paper in Nature. He effectively makes three grand claims: he interprets the Ediacaran organisms as lichens; he purports to have shown that the environment of deposition was terrestrial, not marine as previously thought; and that this provides a trigger for the Cambrian Explosion. The implications are that the Ediacaran organisms could not have been animal ancestors and that life was more diverse on land before it became diverse in the sea. I’m all for unusual ideas, particularly with such enigmatic organisms, but does he have the data to support him? Do his claims warrant a paper in a journal which is meant to be one of the best?
*Disclaimer* I currently don’t have access to the paper in question, so I may make claims which Retallack has addressed in his publication.
[Edit 14/12/12. When I wrote this post I had not seen the Retallack paper, which it turns out is in the Letters section of Nature, nor had I seen the response by Xiao and Knauth. I now have copies of those in my possession, so the article will be tweaked. I’ll try to make sure that it is obvious what I have added.]
Are they lichens?
Considering how confusing the Ediacaran fossils can be, recasting them as lichens is an imaginative approach which is worth looking at. Except that it isn’t new, but has been pushed by Retallack since the 90s (Retallack 1994) and dismissed by Ediacaran workers. His original claims focussed mostly on the preservation of the organisms, interpreting the lack of compaction as being due to them having a chitinous structure, and he expanded on those ideas when he focussed on Dickinsonia, claiming that their growth and decay could also be explained by them being lichens (Retallack 2007). This possible rigid structure was also key in Adolph Seilacher’s interpretations of the Ediacaran organisms as a separate kingdom (or phylum, depending on which papers you read) and later as xenophyophoran protists. Brasier and Antcliffe (2008) noted that contraction zones in Dickinsonia fossils are suggestive that the organisms were elastic and not rigid. Waggoner (1995) pointed out that there are Ediacaran age deposits which contain organic remains and are in the same lithologies as Ediacaran macrofossils, yet they are not found together, unlike with logs preserved in sandstone (which Retallack used to make his case). This would mean that the Ediacaran lichens possessed a rigid biopolymer which resisted compaction but then disappeared without a trace, which of course is problematic. Additionally, Retallack had made the assumption that the sediments above and below the organism were the same, yet the fossiliferous sandstones are often overlying thin clay beds, allowing for an impression of a soft organism to be made.
Retallack claims that Dickinsonia displays indeterminate growth and that this has more in common with lichens and a few other groups than it does with animals. Dickinsonia grew by adding new isomers to the “back” end and by expanding existing isomers, with the prevalence of each process varying with age; through ontogeny the new isomers are added at a slower rate (Sperling and Vinther 2010). How this structure (which Retallack sees as bilateral) fits with lichens is not made clear, let alone the growth pattern. Has he overlooked this? Or have I missed something in his claims?
There is a relatively simple point to make against Retallack’s lichen hypothesis: there is evidence of movement in some Ediacaran organisms. Dickinsonia and similar forms have been found with traces which have been interpreted as showing that they absorbed the microbial mat and then moved (whether they actively or passively moved is irrelevant here, as both would cause problems for a lichen interpretation). Under the lichen interpretation these must be seen as either lichens in different states of decay, or in fairy-ring arrangements, yet many overlap and match the body fossil which accompanies them. Kimberella is also considered by Retallack to be a lichen, despite its much more clear bilateral symmetry, and claims to have an explanation for the trace fossils associated with it. Many specimens of Kimberella are accompanied by Radulichnus trace fossils which show that it fed by rasping at microbial mats. Under Retallack’s idiosyncratic interpretation these are no animal traces, but the moulds of needle ice, thereby showing that the ground sometimes froze [Edit: even though they are arranged in a pattern which cannot be explained by needle ice formation]. I’m really not sure where he gets that idea from, as it just seems a bit desperate.
At least one of the fossils, Charniodiscus, is also found in environmental settings which lichens could not tolerate but we will get to that information shortly. Retallack also claimed that the variability of thickness in Dickinsonia specimens was evidence of decay before burial and that this decay had more in common with the wilting of a leaf, lichen, or mushroom, yet it may also be explained by compaction or of smothering by mats. The majority of Retallack’s claimed evidence for lichen affinities have alternative explanations and he appears to practically ignore some lines of data. As Guy Narbonne said: “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.” His lichen claim cannot be supported, but that is only one of his claims…
Were they deposited in a terrestrial environment?
Retallack’s real controversial claim is that these organisms were living on land, long before conventional wisdom would put organisms in terrestrial environments. He has presented several arguments using “state-of-the-art” techniques (according to the press releases) in order to make this particular claim. Yet again it seems that his evidence has other explanations which are consistent with the majority view. This is where I may fall short, as I am not overly familiar with the sedimentology of the sites in question, it was never my strong suit at university, and I can’t yet access his paper, but there are some points which I think are worth making.
The first thing which springs to mind when the environment of deposition is brought up is that there are wave ripples and cross stratification, indicative of a shallow marine environment. Retallack’s response? He invokes floods or lakes to explain these features which have clearly been formed due to water action. What about dessication cracks, where are those? I am wondering if Retallack explains their absence by invoking floods too. [Edited addition: I’m not sure exactly where to stick this in, but I thought it was worth repeating. Xiao’s response mentions that there are organisms with holdfasts which show signs of having been dragged by waves or currents, not possible in a terrestrial environment.]
One of his main supporting arguments is that the rocks are red, indicative of a terrestrial weathering pattern. But this is not a problem for marine deposition, not least because weathering can go on after the rocks have formed. The chemical analyses he used to show that they were palaeosols are apparently easily contaminated by more recent weathering and his claim that the angular, interlocking nature of the sand grains which he claims shows that they were wind-blown does not negate a near-shore origin (sand grains are often transported for miles before eventual deposition). Most of his argument does depend on demonstrating that palaeosols are present, but from what I can tell he does not achieve this. [Edited addition: he notes that the red beds are often underneath beds of different colouration and considers this to be evidence that they were red when deposited. Rocks weather at different rates dependent on their composition, so this is not a surprise.]
In addition to addressing the sedimentology of the Ediacara Hills, there are other localities in which these fossils can be found, and Retallack’s explanations have to account for those too (particularly the White Sea deposits in Russia). Charniodiscus is amongst those labelled as lichens in his study, yet they are also found in the Mistaken Point deposits, which are deep marine, way below the photic zone which lichens require and notably not on land (Retallack thinks that these need re-evaluating).
When it comes to a period of time so mysterious as the Ediacaran it is good to have some unusual ideas, especially if they can potentially give insight into the Cambrian Explosion (I haven’t properly discussed that here, as it is irrelevant if Retallack’s classifications are wrong). If you are going to make such an unusual claim then you need some compelling evidence, but it seems that Retallack has not managed to provide that, so his publication in a top journal is bizarre to say the least. His claim for lichen affinities in Ediacaran biota has been assessed and found wanting since he first proposed them. His new claim, that the environment of deposition was terrestrial, appears to be based on flimsy interpretations, presenting no data which refute a marine environment. It has been described as ambiguous, and I concur. It seems to me that Retallack has gotten too ahead of himself, as I am sure many of us would if we thought we had cracked two of the biggest mysteries of the fossil record. We’ll continue to scratch our heads over the biological affinities of Ediacaran organisms, we’ll continue to be puzzled by potential causes of the Cambrian diversification, as Retallack has not given us anything to truly connect the dots.
So imagine that you have been transported back around 550 million years. It isn’t as cold, but there are brisk winds as you are near the shore. You look around and see no signs of life, not even lichen or fungi on the floor beneath you. You take off your shoes and go for a paddle in the water, able to feel the ripple marks under your feet, noticing how slimy it all feels. Fortunately you brought a snorkel with you, so you wade out a bit further and dip under the water for a better look. The water is so unusually clear and you notice lots of unfamiliar organisms in the microbial mats below. Some are sticking out of it, extending upwards much like a plant would do, some are embedded in the mats, and some simply appear to be resting on top of them. You don’t notice any movement, except those caused by waves, as you would have to be watching for quite some time to see any motility. You do notice that there are marks on the mats, some scratchess where something has scraped the mats away, some oval shaped marks which look as though something has sucked the life out of the mat below, before moving on for another meal. You could even identify the culprits if you stayed long enough, watching this peaceful underwater world. This isn’t Retallack’s Ediacara, but it appears to be much closer to the right one.
Resources and References
Naturally you might want to read the paper in question if you can access it, which can be followed up with one of the responses, again, if you can access Nature. The journal also gave an article by Brian Switek which gives a decent overview. The press releases contain a decent amount of information, and you can check out ScienceNOW’s article, ScienceDaily, ABC Science, NPR (where you can listen to people with funny voices talking about it too) and there is the University of Oregon page. Those are just the articles I used and there will be many more out there. When more of the experts start responding I will make sure to share those too.
I also began this blog post using references, but as I started writing this late at night and I have been rather ill the last few days, I gave up being thorough. The ones I did cite are as follows.
Brasier, M.D. and Antcliffe, J.B. 2008. Dickinsonia from Ediacara: A new look at morphology and body construction. Palaeogeography, Palaeoclimatology, Palaeoecology. 270, 311-323.
Retallack, G.J. 1994. Were the Ediacaran fossils lichens? Paleobiology. 20, 523-544.
Retallack, G.J. 2007. Growth, decay and burial compaction of Dickinsonia, an iconic Ediacaran fossil. Alcheringa: An Australian Journal of Palaeontology. 31(3), 215-240.
Sperling, E.A. and Vinther, J. 2010. A placozoan affinity for Dickinsonia and the evolution of late Proterozoic metazoan feeding modes. Evolution & Development. 12(2), 201-209.
Waggoner, B.M. 1995. Ediacaran Lichens: A Critique. Paleobiology. 21(3). 393-397.