Tag Archives: Ediacarans

Ediacaran ecosystem engineers – the Savannah hypothesis and our Skynet-type origins

Image from Wikipedia.

Out on the savannah, it is easy to find certain resources as they tend to be concentrated in limited areas. Trees, termite mounds, changes in terrain, all contribute to this concentration of resources. It is this sort of environment which has been hypothesised as resulting in our own bipedality which enabled human ancestors to move efficiently between resource hotspots. In a recent review, Budd and Jensen have proposed a ‘savannah’ hypothesis as an explanation for the evolution of bilaterian animals and consequently the Cambrian explosion.

Bilaterian animals are widely thought to have driven the Cambrian explosion, particularly as they function as ecosystem engineers altering the environment around them. The burrowing activities of bilateral organisms altered ocean chemistry and the nature of the sediment, opening up more resources to be exploited and resulting in a cascade of diversification. But why did bilaterians evolve in the first place? Ecological causes for the Cambrian explosion tend to presuppose features they should be explaining, such as the ability to burrow or the presence of predation (both likely contributed enormously to the diversification, but were also caused by it). Environmental causes tend to suggest limiting factors such as a lack of oxygen, which may actually be mistaking cause for effect.

The savannah hypothesis suggests that the Ediacaran biota also functioned as ecosystem engineers, causing carbon hotspots in the sediment and water around the organisms which were exploited by bilaterian animals which went on to diversify, eventually displacing their Ediacaran providers. Dissolved organic carbon in Ediacaran seas would not likely have clumped together, instead being spread out through the water column – not an economical resource for active organisms. Burrowing is highly energetic and would require dissolved carbon to be concentrated; without the Ediacaran organisms it would have been too diluted and sequestered away by the abundant microbial mats. Just like trees on the savannah, the Ediacaran biota concentrated dissolved organic carbon, providing sufficient resources for active burrowing and the need for motility.

In their thorough review, Budd and Jensen challenge the view that Ediacaran organisms went extinct by the start of the Cambrian period having been outcompeted and devoured by bilateral organisms. Instead, they survey putative evidence that shows that bilaterians first appeared towards the end of the Ediacaran period and that Ediacaran-type organisms persisted well into the Cambrian (and perhaps longer). At first, bilaterians would have been dependent on the Ediacaran ecosystem engineering, but went on to evolve their own sessile forms, such as crown-group sponges, and predatory habits which made them a threat to the Ediacaran biota – comparable perhaps to humans in the Terminator franchise creating Skynet and setting up their own demise.

They also reviewed the phylogeny of basal animals and take the view that sponges form a single clade which is the sister group to all other animals. They coined the term “Apoikozoa” which encompasses all animals and their sister group the choanoflagellates. And they made a case for Ediacaran organisms being early animals, albeit hugely problematic, whilst being highly critical of some of the optimistic interpretations. It is a paper which has provided a lot to mull over.

References 

Budd, GE. and Jensen, S. 2015. The origin of the animals and a ‘Savannah’ hypothesis for early bilaterian evolution. Biological Reviews. doi: 10.1111/brv.12239

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Tribrachidium – three arms and a lot of mystery

If you were to have been wading out into the Ediacaran seas, waves lapping against your legs, the slime of the microbial mats squidging under your feet, you might have noticed groups of a unique little organism called Tribrachidium. About the size of a coin, Tribrachidium had three ‘arms’ spiralling gently away from its centre, resembling a Celtic pattern. It rested on the sea floor in a variety of settings, mostly shallow with wave action, not moving, just letting the waves wash over it. It’s one of those organisms which is difficult to classify, but it has been possible to make inferences about its ecology as a recent study has done.

The Ediacaran period has been perceived as being ecologically quite simple, followed by the Cambrian explosion which saw a sharp rise in the number of ecological habits of animals. Bush, Bambach, and Daley (2007) developed a classification of life modes based on the area of marine ecospace inhabited relative to the seafloor, the level of movement of the organism, and feeding mechanisms. Theoretically, there are 216 distinct combinations, 92 of which are found in the Phanerozoic, 98 are possibly not viable, and 26 are possible but have not evolved. When applied to the Ediacaran  (Bush, Bambach, and Erwin 2011), there are just two modes of life in the older Avalon assemblage which is dominated by frond-like organisms, there are ten in the younger White Sea assemblage, and five in the younger Nama assemblage which sees the first skeletal forms. Contrasted with this, the first half of the Cambrian saw a rise to around 30 different modes of life (still only a third of modern life-modes).

A new study by Rahman et al. (2015) looked at the possible feeding habits of Tribrachidium and suggested that it had a mode of life previously unknown from the Ediacaran, indicating that Ediacaran ecosystems were more complex than thought (add this to the optimistic approach to the Ediacaran period). They narrowed down the possible feeding habits of Tribrachidium to just two: osmotrophy and suspension feeding. Osmotrophy involves the passive absorption of organic matter dissolved in the water, a common approach in the Ediacaran due to increased amounts of organic matter in the water, found in organisms such as Charnia and Charniodiscus which had large surface areas to absorb nutrients. Suspension feeding involves the trapping of organic matter in specific parts of the organism and requires a method of directing the water towards those traps.

With the two possible modes of feeding in mind, the researchers used computational fluid dynamics (CFD) to observe how water would have flowed over the organism, a method commonly used in engineering. Osmotrophy requires that the water flow over as much of the organism as possible, as has been observed in some of the frondose Ediacarans. By contrast, suspension feeding requires that the flow would be directed and focussed. What their tests found was that the water was directed passively by the arms, funneling it towards three depressions called ‘apical pits’ where it slowed down so that food particles would fall out of suspension. This directed movement fits neatly with a rare ‘gravity settling’ mode of suspension feeding, rather than with osmotrophy. They explained it as follows:

In summary, our CFD analyses demonstrate that the external surface morphology of Tribrachidium altered ambient water flow to produce low-velocity circulation above extremely localized areas around the organism, which is consistent with the interpretation of Tribrachidium as a suspension feeder rather than as an osmotroph. Specifically, we find that the three primary branches act to slow water flow and direct it up toward the apex of the organism, where small-scale recirculation develops directly above apical pits. This recirculation occurs at a range of simulated current velocities regardless of the organism’s orientation to the principal direction of flow. We suggest that this low-velocity zone of recirculation allowed larger particles to fall out of suspension, whereupon they were collected in the apical pits and subsequently metabolized (suspension feeding via “gravitational settling”). This hypothesis suggests that Ediacaran organisms used a larger diversity of feeding strategies than is currently appreciated and that they may have played a role as rudimentary ecosystem engineers, albeit in a fashion that became rare in the Phanerozoic with the disappearance of microbial matgrounds.

Computer simulation of water flow around Tribrachidium.

If it is indeed accurate, this study sheds light on the nature of the taxonomically tricky Tribrachidium, whilst expanding our understanding of Ediacaran ecosystems as more complex than previously thought. However, it is worth noting that fossils of Tribrachidium found so far show no signs of a mouth or other feature for ingesting filtered particles, it may simply be part of a larger organism – the holdfast of a frond-like organism, for example.

References

Click here for an interview with one of the authors.

Bush, AM., Bambach, RK., and Daley, GM. 2007. Changes in theoretical ecospace utilization in marine fossil assemblages between the mid-Palaeozoic and Late Cenozoic. Paleobiology 33:76-97.

Bush, AM., Bambach, RK., and Erwin, DH. 2011. Ecospace utilization during the Ediacaran radiation and the Cambrian eco-explosion. In Quantifying the Evolution of Early Life, edited by Laflamme, M., Schiffbauer, JD., and Dornbos, SQ, 111-33. Dordecht, Netherlands: Springer.

Rahman, IA., Darroch, SAF., Racicot, RA., and Laflamme, M. 2015. Suspension feeding in the enigmatic Ediacaran organism Tribrachidium demonstrates complexity of Neoproterozoic ecosystems. Science Advances 1, 10.

Singer, A., Plotnik, R., and Laflamme, M. 2012. Experimental fluid dynamics of an Ediacaran frond. Palaeontologica Electronica 15:1-14.

 

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An optimistic approach to the Ediacaran biota?

There’s a bit of a problem with the Ediacaran fossil record – it’s not what was originally expected and the organisms we do find are problematic. Based on the complex and recognisable fossils of the Cambrian, it was anticipated that more primitive forms would be found in the Precambrian, and, in a sense, they were. When they were first recognised, Precambrian organisms appeared to fit what was predicted; amongst them, palaeontologists recognised possible sponges, jellyfish, assorted worm-like creatures, putative arthropods and echinoderms. The more they were studied, however, the more problems in classifying them arose.

At some point along the line, near enough every Ediacaran fossil which had been linked to modern phyla have been reassessed and their connections found wanting. There is a handful which can still tenuously be linked to modern groups, but there is an apparent dearth of expected animal fossils, especially when the molecular clock data is taken into account. There appears to be an evolutionary gulf between the Ediacaran biota and the Cambrian explosion fauna.

Part of the problem is preservation – comparing the fossils of the Ediacaran and the Cambrian is difficult considering that they are mostly preserved in very different ways; the fossils of the Ediacaran are soft-bodied organisms preserved mostly as moulds, the fossils of the early Cambrian are mostly tiny bits of shell and other hard parts, and then there are the exceptionally preserved organisms from deposits such as Chengjiang.

One approach we can take to link the Ediacaran and the Cambrian is to avoid trying to fit them into recognisable taxonomic groups, and instead focus on the attributes they share with modern animals, particularly their ecology. This was the process adopted by Mary Droser and Jim Gehling in a paper earlier this year, titled The advent of animals: The view from the Ediacaran. We can look at the Ediacaran period and see things which are usually associated with animals, even if we cannot properly classify the fossils in question.

Mobility

One thing which clearly sets animals apart is movement – worms wriggle through sediment, fish swim about, and, of course, us humans find as many different ways to move as possible. Many animals don’t move about for most of their lives, not least sponges and corals, both of which we might expect in the Ediacaran in some form, but movement on or in the sediment would potentially be evidence for bilateral animals milling around. Most of what we see from the Ediacaran are stationary organisms, attached to the sediment by a holdfast or resting on the surface. The earliest animal traces are from 565 Ma and are most similar to traces by the polyps of anemones, providing evidence of muscular contraction, evidence of which also comes in the form of the body fossil Haootia quadriformis which possessed bundles of muscle fibres and is a possible cnidarian. The most common trace fossils in later Ediacaran rocks are in the form of grooves and levees, called Helminthoidichnites, and are interpreted as being caused by an animal too small to be preserved and limited in size by the chemical conditions of the sediment. They appear to have been mining the microbial mats, also showing evidence of avoidance behaviour, and are likely to have been created by bilaterian animals.

A few Ediacaran body fossils are associated with traces as well, lending to their interpretation as bilaterian in nature. Kimberella is a box-shaped body fossil which is often associated with scratch marks (Kimberichnus) that has been commonly seen as bilateral and has even been considered to be a possible mollusc. The associated traces have been interpreted as evidence of mat grazing though there are differences between the grazing habits of Kimberella and those of molluscs. The likely related Dickinsonia and Yorgia have been found associated with faint casts of their bodies, which appear to be resting or feeding traces where they sat ingesting the microbial mat before moving on to another patch. They often also have possible muscular contraction marks, though this interpretation depends somewhat on their phylogenetic affinity. The advent of mobility is therefore not confined to the Cambrian period though it does see an increase in the number of modes of mobility, as it is a behavioural trait of bilateral animals found in the Ediacaran.

Reproduction

Sexual reproduction is another trait associated with modern animals found in the Ediacaran period. The puzzling organism Funisia is a collection of tube-like structures which were previously not even recognised as body fossils. They demonstrate branching patterns which are potential evidence of asexual budding, whilst their distribution appears to be due to the production of spats, a form of reproduction mostly found in sexual organisms. Though their phylogenetic affinity is puzzling, the likely sexual reproduction of Funisia highlights another metazoan trait found in the Ediacaran period.

Skeletonisation 

The Cambrian explosion was first recognised in the fossil record due to the geologically sudden appearance of skeletal parts. The evolution of hard parts appears to have been a key stage in the evolution of Metazoa though it is not restricted to the Cambrian. Droser and Gehling discussed the example of Coronacollina, a cone-shaped organism with long, straight spicules radiating outwards, interpreted as a sponge-grade organism which is important for being the oldest known multi-element organism. Other Ediacaran shelled organisms include Cloudina and Namapoikia which are possibly cnidarian-grade organisms but had their study been released more recently they would likely have included the latest interpretation of Namacalathus as a lophophore. Even if we cannot place them phylogenetically, the appearance of skeletal parts, particularly multi-element organisms, is a key step in metazoan evolution found in the Ediacaran.

Ecosystems 

Ediacaran fossils tend to have been preserved in the places they lived, as opposed to having been transported and dumped elsewhere. This allows them to be studied as communities and permits insight into their ecological nature. The Flinders Ranges of Australia contain a succession of beds which are characterised by a range of organisms in shallow marine settings. The same organisms tend to appear on each bed but with different abundances, suggesting a level of sophistication in communities similar to that in the Phanerozoic despite there being a lack of predation, organisms living in the sediment, and widespread skeletonisation.

Conclusions

Setting aside phylogenetic affinities, traits of modern animals are found in the Ediacaran period. An optimistic approach to the Ediacarans allows us to see signs of mobility and the presence of muscles, skeletonisation, sexual reproduction, and the beginning of complex ecosystems – all possible links to the animals found in the Cambrian, suggesting that poriferans, cnidarians and bilaterians were all found in the late Precambrian.

References

Droser, M.L. and Gehling, J.G. 2015. The advent of animals: The view from the Ediacaran. Proceedings of the National Academy of Sciences 112: 16. [Link]

 

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I Chuffing Love Correcting Science Headlines – Namacalathus

Science journalism has a bit of an issue. With science news sites all over the internet, many take on the clickbait approach, sometimes unintentionally. It seems that whoever decides the title has often not read the article itself, and certainly hasn’t read the research upon which it is based. This mismatch would not be as big a deal if people read past the title, but many seem not to bother. One of the repeat offenders for this is the website I Fucking Love Science. In case you were not in the know, it is effectively a site for perving on the sexier side of science, those bits that make the public say “wow” and then go about their day thinking about other things. They do, from time to time, have some genuinely interesting and informative content, but they still need to tighten things up a bit.

My example today is due to their coverage of a story I posted about myself very recently. The late Ediacaran shelly fossil, Namacalathus, has a new interpretation being offered, and this is fascinating for anyone interested in the Cambrian explosion. I took a more conservative approach, much as I have with the title of this post (I actually don’t mind swearing, I just didn’t feel the need), in part because I intend to look at many more putative Precambrian animals, but also because I like to remain sceptical with potentially big news.

The offending article can be viewed here and is actually one of their better offerings, but there are issues. Firstly, they went with the title Newly Discovered Fossil Suggests Complex Skeletons Evolved Earlier Than Thought. It isn’t a huge error, and thankfully the attractive part of the headline isn’t blatantly false, but Namacalathus was described fifteen years ago, which is hardly a new discovery. They don’t even go on to mention that Namacalathus was previously thought to be a possible cnidarian, nor do they mention that this previous interpretation was based partly on the nature of reproduction (asexual budding) which is now used to suggest that the organism within the shell was bilaterally symmetrical. They are dead on with their information about the shell formation (something I personally covered in little detail) but didn’t include this standout gem.

The part of the title which is meant to pique your interest is the fact that these complex skeletons evolved earlier than thought. Yet the quotation at the end of the article, from researcher Rachel Wood, quite clearly says that these complex animals were suspected, and perhaps it would have been worth mentioning the discrepancies between the fossil record and molecular clocks.

There are some other issues which crop up time and again with this subject. The first is that science journalists seem a bit baffled about how to present the Ediacaran biota, which is no surprise, as they are baffling, but the article puts it in a rather misleading way, saying, “Paleontologists still aren’t sure what kind of life they are, but they were likely plant forms, algae, microbial mats, fungi or very primitive life forms called protists.” Putting possible plant affinities first in the list could potentially mislead as this was stated in a sentence after mentioning the Avalon explosion, which involved the arrival of frond-like organisms such as Charnia, an organism which does resemble a plant despite it living too deep for photosynthesis to work. They also neglect to mention many of the attempts to classify Ediacaran organisms which have placed them close to the base of the Metazoa and even within it. They could simply have called them “possible primitive animals” and they would not have been mistaken.

The second issue is that they misrepresent the Cambrian explosion, describing it first as “the period of ancient time complex life appeared to suddenly and rapidly evolve in,” and later as a “sudden appearance of life.” This is especially odd, considering that they provide links along with each statement, the first of which describes most of the change happening in the second and third stages of the early Cambrian, “a period of about 13 million years,” which is hardly sudden. The second link mentions that apparent new evidence suggests that the Cambrian explosion may have involved more gradual change. It is misleading, though common, to state that the Cambrian explosion was sudden, especially without qualifying in its geological context, where “sudden” can mean several million years.

Overall, the article does not do a terrible job presenting Namacalathus in its new light, as it does manage to sound exciting to laymen and gives some good information on the shell structure (even if I did feel that it neglected the reproductive strategy), but it does commit the sins of having a misleading title, a confusing approach to the already confusing Ediacaran biota, and an exaggerated description of the Cambrian explosion. They could also have done with some dissenting or sceptical views from a leading Ediacaran palaeontologist, but that’s not always as easy.

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Further thoughts on Retallack’s terrestrial lichen hypothesis…

Last night I addressed the claims of Gregory Retallack from a recent publication in Naturesee here for my criticisms. As I was critiquing his claims I did not go into detail on some of the issues which I think it does raise. One is the nature of the Ediacaran biota, the other is the nature of scientific debate as perceived by the public.

On Ediacarans…

The Ediacaran biota really are the most mysterious in the fossil record. Whether you are looking at the frond-like Charnia, or the oval shaped Dickinsonia, or even Spriggina, which at first glance appears to have a head, you’re in for a lot of difficulty working out just what they were. There are so many questions which remain difficult to answer. Are they animals? Or are they “almost” animals? Are they more like fungi or lichens? Are they actually single celled organisms? Can they even fit into a known group or are they some unique evolutionary experiment? All of these have been suggested at some point. Do they all group together as more closely related to each other than to other groups? Or are they from many diverse groups, some of which are familiar to us? Again, the answers remain elusive.

Not only is working out relationships fraught with difficulty, but mode of life can be confusing too. Their ecosystem was very different to anything we have today. We cannot infer modes of life through phylogeny if we cannot discern their relationships. One palaeontologist will see an organism which might have been swimming or crawling around, whilst another sees it as sessile, absorbing nutrients passively. Spriggina is an excellent example, as many see it as some sort of proto-arthropod, yet its “head” has also been interpreted as a hold-fast as though it is a frond.

Spriggina fossil along with two reconstructions, one of which depicts it as a frond. Picture credit: Jack Unruh

Original ideas should definitely be welcomed, they can help us ask all sorts of questions which we might have overlooked, shedding more light on the nature of these fascinating organisms. Retallack did that when he first proposed that they might be lichens, back in 1994, but that is an explanation which has been assessed and found wanting. But this time he brings another novel idea: could the Ediacaran organisms have lived on land?

Terrestrial Ediacarans is an intriguing idea (except that they are not found in terrestrial deposits, contrary to Retallack’s claims). We very well could find something of the same age which is beyond microbial grade and inhabited the land. They could even be well known Ediacaran forms, for who are we to say that they could not have lived on land and in the sea? Modern organisms often tolerate a narrow range of environments, but we cannot claim the same for the past, not least because evolution often functions by an increase in generalists which later become specialists (this happens at all levels, from genes to species). We don’t know where Ediacarans fit onto the tree of life, so we cannot make a phylogenetic case against it. But without any evidence it is merely wild speculation; a nice idea, but not science unless you can back it. Retallack has tried. Retallack has failed.

In coming blog posts I will be exploring some of the weird and wonderful ideas regarding Ediacarans, of which Dickinsonia will be a focus as it seems to have been wedged into nearly every possible group at some point or another. Some of Retallack’s ideas will be presented, but they are not accepted and for good reason. (On a rather random note, check this out.)

Ciavatti 2008 apparently

Even though I think it is nonsense, I do like seeing reconstructions of Dickinsonia swimming and showing off its internal organs.

On Science…

There is, quite naturally, going to be a big response to Retallack. If he had published in a smaller journal as he has done in the past, then there would be less of a response (just the standard criticism), but he has published in what is meant to be one of the biggest journals and it is getting a lot of publicity, which sadly seems to happen often in palaeontology (Chatterjee’s bizarre views about large pterosaurs, for example, see here and here). Martin Brasier is reported to have said that he finds “Retallack’s observations dubious, and his arguments poor. That this was published by Nature is beyond my understanding.”

My biggest worry here is that people will mistakenly think that Retallack has a good case and that any resistance is because you mustn’t challenge scientific dogma. What it really shows is that if you are challenging a view which is supported by a lot of evidence, such as the marine environment of Ediacaran organisms, then you need to make a very compelling case. The response is because he has failed to do that. It also highlights that sometimes a good idea is wrong and that you need to accept it and move on (in rare cases sticking to your guns is a good thing, but not if you ignore contradictory evidence). Retallack’s lichen hypothesis never gained a following for good reasons; it was given a fair hearing and just did not stand up to scrutiny (on a related note, such ancient lichens are known and they do resemble lichens).

The public often sees debate as a bad thing for science. Many will no doubt see this as some form of bullying, as though Retallack is a heroic crusader, fighting a dragon called Dogma, guarded by those black knights of the scientific establishment. But really he is bashing a mop against the walls of a castle in an attempt to lay siege, even though the drawbridge has been lowered, the portcullis raised, and there is even a place at the table for him to eat.

Let’s end with some foliose lichen:

No.

Dickinsonia? Is that you?

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Lichen Controversy in the Land of Ediacara

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 NatureHe 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.]

Dickinsonia fossils, interpreted by Retallack as lichens.

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?

Image by Aleksey Nagovitsyn, pilfered from Wikipedia.

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).

In Conclusion

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 ScienceNPR (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.

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