Tag Archives: Precambrian

Intelligent Design’s problem with the Cambrian explosion

Last night I made a grave error. I made the mistake of attempting to peruse the Intelligent Design movement’s main website, Evolution News and Viewsto see what they have said about recent developments on the Cambrian explosion. I personally don’t think that researchers should give them much thought, but as I am currently just a blogger I will occasionally address them. It is my intention to get my hands on Stephen Meyer’s book Darwin’s Doubt and review it though that may be a long time coming, especially considering that they have made a follow-up to address critics. Until I have read their books, I don’t plan on getting into the details of their arguments. Right now, however, I am more concerned with their approach to the supposed debate.

Firstly, they sound desperate to appear original and as though scientists researching the Cambrian explosion are slowly coming round to their way of thinking. They often state things in a way which suggests that Meyer got there first, for example, Graham Budd was apparently recently “confirming Meyer’s denial” about Precambrian organisms, when the reality is that the interpretations Meyer favours were first offered by evolution-accepting palaeontologists. Cambrian and Ediacaran experts are constantly described as admitting to something which the ID crowd believes; their loaded language is meant to give the impression that they are way ahead of the experts.

The ID presentation of the Cambrian explosion appears to be that is that it was relatively short (10 million years appears to be a figure they will accept), that bilaterian phyla appeared very suddenly in the Cambrian, that there were no precursors found amongst Ediacaran organisms, and that there is no satisfactory evolutionary explanation for it. You will find all of these views, even the last one, preceding Meyer’s publications and from experts in relative fields. The problem for them here is that they are not trying to solve any problems – they already think that they have the answer, yet it really offers no explanation.

Secondly, they consistently complain that they are being ignored. Nobody name-drops Meyer, nobody cites his book, nobody addresses his main thesis. They even do this when discussing papers which focus on specific phenomena, as though every single paper relating to the Cambrian explosion must address their pet theory (I use that term loosely). They are like the guy in a bar who seems to want everyone to fight him (perhaps better left ignored). Scientists are quite happily dissecting every aspect of the Cambrian diversification, looking at the genetic changes involved, the divergence times, the environmental changes, taphonomic changes, identifying fossils and working out how they fit in, looking at the timings of the events and so on. The debate is ongoing, there are many, many voices clamouring to be heard, trying desperately to tie together an overwhelmingly large, yet incomplete, dataset which befuddles even the most astute mind. Teasing out cause and effect in deep time is difficult and frustrating, people come at it from different angles, new evidence and new ideas can cause major shifts in thought. Meyer and his crew are desperate to be the most heard voice, they want their issue addressed and until someone addresses it they will assume that they are being ignored (which is tantamount to admitting defeat, by the looks of it).

Their third issue is that their main focus isn’t actually at the heart of the Cambrian explosion, despite their best wishes. The diversification can be perceived in many ways, with current thought often favouring its interpretation as an ecological explosion. It has often been perceived as an explosion in disparate body plans, which is not exactly the wrong way to look at it, but can be seen as the result of the ecological driving forces. The ID proponents take this a step further; it isn’t simply about body plans – it’s about the new information behind those body plans. With their rapid appearance narrative of the Cambrian explosion, this perspective on the diversification seems like a major issue, a sudden, unprecedented influx of biological information. Understandably, when addressing some of the ecological forces at play we don’t necessarily need to address the genetic changes, but they can’t always accept that. The genetic changes are important, but it does seem to be the case that the genetic toolkit necessary was already largely in place well before the Cambrian explosion (sponges, for example, appear to have some functioning genes which are used in more complex organisms in the development of the nervous system). So some Precambrian organisms may have had the capacity for evolving some of the body plans we see in the Cambrian, but nothing to cause them to do so – having a football pitch, a ball and 22 people does not ensure that a football match will take place.

This pushes the issue back, which ID theorists would like to present as a retreat, as hiding from the problem. The reality is that when you are concentrating on the Cambrian explosion you look at the stage which has been set and then analyse the changes. The environment is part of the stage, the organisms which preceded the radiation are part of it too, and the genetic toolkit is part as well. This is not to say that no new genes were necessary for the Cambrian explosion, but that it is not a major issue. The evolution of regulatory networks and of new genes is a separate question, which I get the impression that they know as it allows them to paint this picture of retreating evolutionary biologists. They can keep pushing back and back because ultimately they know that the origin of information goes back to the origin of life and that is where they truly set up camp, not the Cambrian explosion.

In summation, one tactic of the ID proponents is to try to sound original, when the reality is that the majority of their views on the Cambrian explosion are taken from actual researchers who accept evolution. They also complain repeatedly that they are being ignored, often because their personal favoured views are not being addressed. Finally, their issue is not really with the Cambrian explosion, but with the origins of information at life’s beginnings. The Cambrian explosion isn’t what they think it is, but as long as they continue to present it their way they will always feel ignored and as though experts are conceding to them. They will just persist in offering only criticisms and complaints.

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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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Filed under Cambrian, Cambrian Explosion, Ediacarans, Evolution, Palaeontology

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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The Cambrian Explosion – The old fossils vs animals debate

In a world without fossils, we might never have known about the event commonly known as the Cambrian explosion. This perplexing episode in evolutionary history has been known since early geologists and palaeontologists first started piecing together the history of life on our planet; what they found was that the oldest rocks contained no fossils, overlain by rocks containing complex fossils. A lack of precursors for complex fossils, such as trilobites, did not fit the evolutionary narrative gaining traction at the time. Charles Darwin’s explanation for this puzzle was that the fossil record was incomplete; these ancestral forms had existed, but had either not been preserved or not been found. Since then, more fossils have been found, including a large variety in Precambrian rocks, and the timescale has been constantly refined, allowing for a better understanding of the timing of key events. The questions raised by Darwin’s ideas have not quite gone away: is the Cambrian fossil record a true representation of the emergence of major animal groups? or is it an illusion caused by the paucity of the fossil record?

Many palaeontologists would like to read the fossil record literally and support the idea that the Cambrian explosion was a real event, however, there is a story in the genes which is somewhat different. Geneticists can estimate the divergence times between lineages using molecular clocks, a technique which uses mutation rates to count back to the time when two species or higher groups split. When applied to the origins of major animal groups, these estimates have varied widely and have been constantly refined. One consistent theme is that these dates are always substantially before the Cambrian, including the origins of phylum-level groups. It is always to be expected that a group or species would have evolved earlier than they appear in the fossil record, but not to the degree shown by molecular clocks for the Cambrian. If the story of the genes is read literally, then there is a lot of cryptic evolution in the fossil record, making the Cambrian explosion either an illusion caused by an absence of fossils, or by an increase in preservation potential due to the origin of hard parts and/or increased size across multiple groups. The conclusions of a recent molecular clock study appear to support the idea that the Cambrian explosion is a preservation event, not the rapid appearance of major animal groups.

In a paper titled Uncertainty in the Timing of Origin of Animals and the Limits of Precision in Molecular Timescales, dos Reis et al. present some robust new data and provide an excellent overview of the limits of molecular clock studies. Their estimates vary widely depending on certain assumptions, which are addressed in detail, but all of them do support an early divergence of major animal clades. They conclude:

This is the long-standing conundrum of the Cambrian—whether the first animal fossils faithfully reflect an explosion in animal biodiversity or merely an explosion of fossils. The results of our study—which integrates fossil and molecular evidence to establish an evolutionary timescale—suggest that the Cambrian explosion is a phenomenon of fossilization, while biological diversity was established in the Neoproterozoic. Integrating all of the sources of uncertainty that we explore (Figure 6, Table 1) allows us to conclude that crown Metazoa originated 833–650 Ma, fully within the Cryogenian, while the component clades of crown Eumetazoa (746–626 Ma), crown Bilateria (688–596) Ma, crown Deuterostomia (662–587 Ma), and crown Protostomia (653–578 Ma) all diverged within a Cryogenian to early- or mid-Ediacaran interval.

The results of our analyses leads us to reject the hypothesis that metazoans, eumetazoans, bilaterians, protostomes, deuterostomes, ecdysozoans, lophotrochozoans, or, for that matter, any of the component phylum-level total groups, originated in the Cambrian.

Figure 6 from dos Reis et. al (2015)

The conclusion that the fossil record is not accurate for the Cambrian may seem dismissive, just as many palaeontologists dismiss molecular clocks as inaccurate when they ostensibly contradict fossil data. This incongruence between the two datasets might itself be an illusion. Might there be reasons to think that both are accurate, but that we are mistakenly expecting them to answer the same questions?

It is relatively easy to picture a number of scenarios which could create the illusion of a surprising increase in complexity when really we just do not have the fossils. The Cambrian fossil record clearly does show that there was a dramatic increase in fossilisable hard parts, as we see a handful of shells at the end of the Ediacaran, through the increasing diversity and complexity in the Small Shelly Fossils (SSFs); soft tissues do not preserve well, so it might be a case that hard parts evolved in animals which had been milling around for millions of years prior. Or could it be the case that they simply were not big enough to be preserved? An impression of a soft-bodied animal in sand cannot be smaller than the individual sand grains. The meiofauna live in those sorts of surroundings, in between the grains of loose sediment, where oxygen levels are often low; animals too small to see without a microscope, complex yet only found fossilised in exceptional circumstances. And maybe it is the case that we have fossils of them but are looking with the wrong expectations? In these scenarios, the genes are right and it is the fossils which do not give enough of the true picture.

The Ediacaran period is famous for its menageries of soft-bodied organisms, preserved as impressions in sand, or by volcanic ash smothering their soft forms; any organism large enough would surely have been preserved. Within this period of time, we also find the Doushantuo fossils, visible only through the use of high-powered microscopes. Preserved in phosphate, they are largely an assortment of little ball-shaped clusters, thought by some to be embryos, yet by others to be single-celled organisms in the process of division. During the Cambrian and later, phosphate deposits such as these contain the elusive meiofauna, yet they are absent from the Ediacaran. Instead, we see a finer degree of fossilisation. Preservation in the Precambrian is arguably better than the Cambrian and later – preservation has evolved, but not towards higher fidelity.

Geneticists can also shed light on this question of preservation potential. Animals have a number of microRNAs, which are regulatory RNA genes that can be used to track the evolution of complexity, put simply because more complex animals have more microRNAs than simpler organisms. When some of the simpler animals are analysed, such as acoel flatworms and rotifers, it is found that they have lost microRNAs and become less complex, so they cannot be used to work out what a simple bilaterian ancestor might have looked like. Instead, they seem to indicate that the common ancestor of bilaterians was relatively large and complex, and at the very least a persistent puzzle.

It is possible that we already have fossils of bilaterians from the Precambrian, but that we are not identifying them correctly. The Ediacaran form Kimberella is often considered to be a possible mollusc, whilst its contemporaries Spriggina and Parvancorina are sometimes linked to arthropods, yet all of these are questionable. Recently, the shelly Ediacaran organism Namacalathus has been identified as a lophophorate, placing it with animals like brachiopods and bryozoans, but this is undeniably controversial.

Bilaterians don’t often sit still. They wriggle around on the sea floor, they churn up the sediment in search of food and shelter, they swim around looking for food. Burrows and trails are abundant in the fossil record, often found alongside body fossils, but also in sediments where body fossils could not be preserved. They give us insight into the nature of the sediment, the life habits of animals, and the evolution of complexity. Trace fossils tell a similar story to the body fossils. The sediments in the Precambrian were quite hard and largely devoid of oxygen – not conducive to the existence of meiofauna –  and lack any clear evidence of bilaterian activity, then animals began to wriggle about in the sediment in the Cambrian, allowing oxygen to reach greater depths. The first stage of the Cambrian shows a large increase in the ways animals interacted with the sediment, but it was not until the second stage of the Cambrian when they started to burrow up to a metre deep and ecosystems took on a new level of complexity. The trace fossil record suggests that the Cambrian explosion was a very real event, but that body-plan diversification happened before changes in ecological structure.

The data from trace fossils hints at how the record of body fossils and the information in the genes could both be telling the same story. It is possible that molecular clocks are right about major animal groups diversifying much earlier than the Cambrian, but the conclusion that the Cambrian explosion is an illusion caused by a poor fossil record is misguided. The fossil record does indeed appear to record an actual evolutionary event, but it might not be the origins of these major animal groups – it may instead be their ecological expansion, not least the widespread evolution of hard parts. Under this scenario, we are still lacking in key fossils, but that lack is not the explanation for the incredible diversification through the start of the Cambrian period. The Cambrian explosion would therefore not be a case of “animals or fossils” but an ecological restructuring which could not have been known without the fossil record.

References

dos Reis, M., Thawornwattana, Y., Angelis, K., Telford, M.J., Donoghue, P.C.J, and Yang, Z. 2015. Uncertainty in the timing of origin of animals and the limits of precision in molecular timescales. Current Biology. [Link]

Erwin, D.H., Laflamme, M., Tweedt, S.M., Sperling, E.A., Pisani, D., Peterson, K. 2011. The Cambrian conundrum: Early divergence and later ecological success in the early history of animals. Science 334: 1091-97. [Link]

Mangano M.G., Buatois L.A. 2014. Decoupling of body-plan diversification and ecological structuring during the Ediacaran – Cambrian transition: evolutionary and geobiological feedbacks. Proc. R. Soc. B 281: 2014003. [Link]

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Filed under Cambrian, Cambrian Explosion, Ediacarans, Evolution, Palaeontology

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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Filed under Ediacarans, Palaeontology