The best headline about Dickinsonia, ever?

Dickinsonia is one of those pesky Ediacaran mysteries which persistently defies classification. Think of an animal group and someone has suggested that Dickinsonia belongs to it. Think of a group outside of Animalia and there’s a decent chance that Dickinsonia has been placed there too. In more recent years, there has been a tendency to see it as a potential placozoan – the simplest of animals – often in a very cautious manner, or at least that it is at the same grade of complexity (for an example of the case for a placozoan affinity, see here, for insight into just how difficult it has been to classify Dickinsonia, see here). A recent study into the development of Dickinsonia claims to provide strong evidence that it is indeed an animal, though without assigning it to any particular group. It effectively states that Dickinsonia is an animal, so let’s put aside the claims that it is not and focus on where it fits on the animal evolutionary tree. See for yourself, here.

This gained a lot of press coverage back in September, though one, in particular, stood out to me. The Week decided to go with the brilliant headline “550-million-year-old thingamajig determined to actually be an animal”. I honestly can’t express how much I love that Dickinsonia has been labelled a “thingamajig” as it is perfect. Dickinsonia is head-scratchingly confusing, it is rightly considered to be a Rorschach Test for palaeontologists, it is, quite simply, a baffling thingamajig. It’s just a shame that the term has no taxonomic value.

Own your own Ediacarans

If they were alive today they would probably have made terrible pets, but you may well have had some of them in a home aquarium. Like sponges in modern aquariums, the likes of Charnia and Charniodiscus might have featured in the odd fish tank, absorbing nutrients and sitting around looking pretty. Most Ediacarans depended on the slimy microbial mats in some way or another, whether they grazed on it or lived within, beneath or on it. And over all the Ediacarans were not a very mobile bunch. As fossils, however, they are hugely important in helping us to understand the evolutionary history of animals (and in helping us to admit to how little we know). Some of them are recognisable to any fossil-nerd worth their salt. Last year I invested in casts of some familiar Ediacaran beasties from a company called GeoEd, which makes replicas of thousands of important specimens:

I opted for some of the most recognisable taxa, as you can see I bought Charniodiscus, Charnia, Spriggina, Dickinsonia and Parvancorina. I have not been disappointed and would thoroughly recommend buying some from there if you, like me, are interested in the Ediacaran. The prices are very good, but do note that they don’t include shipping costs – those come after you complete your purchase.

The only issue I have with them is that the information can be outdated. For starters, the Ediacaran period fossils are listed as Vendian, a term which you will find in a lot of older texts about the period. I recently bought a cast of Charnia as a gift for my friend Dean Lomax and noticed that the little informative label on the back listed it as a Pennatulacean (a sea pen) despite that this interpretation was rejected scientifically by Antcliffe and Brasier (2006) who noted that sea pens grew by adding polyps to the bottom of the frond, whilst Charnia grew by adding segments to the tip. (I also hadn’t noticed that the descriptions on the website mentioned whether they were in positive or negative relief – the pictures are misleading – so pay careful attention if this matters to you.) Here’s the one I bought for Dean, the image pinched from his Facebook, and as you can see it is a very well made cast:

Many key Ediacaran specimens can not be collected, particularly those from Charnwood Forest in Leicestershire, so this is your only way to get your hands on them. Casts are extremely useful for studying such unique specimens and on top of that can make excellent ornaments for the enthusiast.

If anyone knows of any other good sites offering Ediacaran casts please let me know in the comments, I’d love to get my hands on some more and can do a comparison and a bit of advertising.

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]

 

Further thoughts on Retallack’s terrestrial lichen hypothesis…

Last night I addressed the claims of Gregory Retallack from a recent publication in Nature, see 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.)

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:

Dickinsonia? Is that you?

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 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.]

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 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.