Talk:Cambrian explosion/Archive 2

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Contents

Pictures and refs

As per to do list - I have some stuff I can put here when I have a moment!

Grahbudd 07:00, 25 January 2007 (UTC)

Timeline / Nature review

I've replaced Graham's timeline with a wikified one that's easier to read on the page (and a bit more colourful). I've also amended a couple of the dates where new data has allowed their refinement. Verisimilus 18:03, 10 March 2007 (UTC)

Nice, Verisimilus. But there some minor mistakes. Since you have the source file, please correct the timeline figure: there should be Chengjiang, not "Chengjang" (add missing "i"), and correct the "Cambrian Era" to "Cambrian Period" or "Pal(a)eozoic Era".

Done. For future reference, the timeline is editable at Cambrian_explosion/Timeline.Verisimilus 10:02, 14 March 2007 (UTC)

Dickinsonia ridges

Body ridges are a common feature in organism fossils 
of the period, suggesting possible external food intake.

This doesn't sit too comfortably with me... there are many other possible reasons for ridges (e.g. gas exchange, structural support, artefacts of segmentation, hydrodynamics?) that don't fit this hypothesis; indeed other animals (trilobites, earthworms) are 'ridged' whilst not having this mode of life. I'm not convinced this is factual enough to be considered Encyclopædic content... whilst there is one prevailing view that they did have this mode of life I don't think a ridged appearance is necessarily evidence for it. Verisimilus 21:09, 15 March 2007 (UTC)

Viral hypothesis

Alternatively, some new form of microbial or viral 
infection agent may have developed which could 
transfer genes between species, speeding up
evolution by "sharing" useful features. This is 
somewhat comparable to the Recombinant DNA
process in sexual reproduction, but on a cross-species
level.

After a quick search I can't find anything in the literature (or even in the link supplied) that suggests this hypothesis is anything more than an interesting but entirely unsupported and loosely defined conjecture... Hence I don't feel it should be included here as 'encyclopaedic content' unless a reliable source is found.

Verisimilus 13:21, 5 April 2007 (UTC)


Structure of the article

The structure of the article could be improved considerably. The "Dating the Cambrian" section serves little purpose; that topic is covered throughout the article, and what is not could be merged into the "Significance of the explosion" section. I also suggest putting the history section immediately after the intro, before the Significance section. Also, "Timing of the Cambrian Explosion" seems to be more or less dealing with the same issues as "Causes of the Cambrian Explosion". I would suggest removing the causes section from "Significance of the data" and putting it in it's own top-level section, and using a trimmed-down version of the timing section as an intro to the causes section.--ragesoss 23:55, 19 April 2007 (UTC)

Please write in plain English

I mean, write for the intelligent lay person. Please write where someone can actually learn something.

Wikipedia is still in the process of becoming. It might make it, it might not, it needs some help! For example, bluebirding--writing just to string together blue words that refer me elsewhere, rather than talking to me when I’m right here. I’m here and now, and I’m interested, please teach me. Use Carl Sagan or any other good writer as a model (maybe James Trefil, the guy who wrote “A Scientist in the City”), rather than using some artificial, formalistic style.

Maybe at the beginning of the article, talk just like you would if an intelligent, interested 10th grader were right in front of you.

Later on, as if I was an intelligent college student.

Later on, as if I was an intelligent younger colleague.

And later on, toward the end of the article, talk to me about the cutting edge of research going on right now as if I am your fellow colleague.

This will give the article a sense of development, and please do not do this mechanically. Surprisingly, if you allow the writing to stay a little unfinished, it’s often better. The article does not need to be perfectly consistent in any regard. It just needs to be good. Good is better than perfect (in this context, and in many other contexts as well!). Write just a little bit more formally than how you would simply talk to me. That’s what would help me to learn the subject matter and really get a lot out of it.

Okay, the part on the Ediacaran fossils: “In addition, in the Ediacaran Period immediately preceding the Cambrian, apart from the trace fossils and tubes previously mentioned, the record contains the highly enigmatic “Ediacaran” biota, which despite decades of study and a flurry of recent intense interest, remains very hard to place in the context of animal evolution.”

(That’s vague, that’s hedging your bets. That is “safe” writing. At issue is whether the Ediacaran creatures died off or not. Go ahead and say it.)


“Some taxa such as Kimberella are thought by some to represent bilaterians . . .”

(And I’m not sure whether these are part of Ediacarans or not.)

(next paragraph) “Perhaps the most promising area for study is the Doushantuo Formation of China, spectacular fossils from which are probably around 580 million years old or younger.”

(So I realize now it’s probably other early fossils (or fossil traces), but . . . more bluebirding! So, the purpose of a wikipedia article is just to bluebird, it’s not actually to teach anything.)

I think about two whole paragraphs on the Ediacaran fossils would be a lot better. It’s important enough. It’s a relatively known precursor. And for me personally, writing that builds on something I already know often makes for very satisfying reading. And afterall, the Cambrian Explosion is perhaps the greatest mystery in the history of life on Earth, and it’s a major issue in SETI and astrobiology and how likely life elsewhere in the universe might be. It’s worth spending some time on the precursors. And long is good. If it’s going to be a real teachful article, it’s probably going to have to be long. So a couple of paragraphs on Ediacarans, with the single bluebird if I want to learn even more. Now, discrete footnotes are fine, and are in fact good as long as they don’t interfere with the flow of the writing. For example, here are two websites on Ediacaran fossils (and good websites at that, at least to my untutored eye!) [1] [2] .

If someone says something like ‘appropriate for an encyclopedia,’ I’m going to say you’re letting a formalistic standard stand in the way of communication.

(I realize this may not be the most good-natured piece I’ve ever written. But it's frustrating when you look something up and you don’t learn anything.) FriendlyRiverOtter 20:45, 20 April 2007 (UTC)

Thanks for your comments

It's quite a challenging set of demands, which I mainly agree with in principle. The balance I think that needs to be struck is one between an article that is concise enough to be readable without the attention wandering, and yet in depth enough to capture the imagination. One could argue that to discuss the Ediacaran biota at length would be to stray unnecessarily from the subject - and one could argue that the interested will follow the 'bluebird' whilst the uninterested will not be distracted from the main flow of article. Regarding the levels of 'in depthness' - I agree with this in principle, but it would be quite difficult to fit into the current article, which has several distinct sections. I feel that to a degree the article already does mention points in brief before expanding on them; but there could well be scope for improvement. I'd wholeheartedly encourage you to have a dabble with the article, if you feel able! | VerisimilusT 09:54, 21 April 2007 (UTC)

More Judicious Use of Technical Terms

First off, VerisimilusT, thank you for your very gracious response. You took it better than I hoped (and I was frustrated when I wrote it), this was a post I was a little bit worried about, so thank you for your thoughtful response. However, I cannot take you up on your generous offer. I do not feel at this time I have the kind of readable writing style that will be acceptable to the majority of wiki users (people will be editing out what I think are my best parts and it will just drive me crazy), and besides that, I don’t feel I know enough about evolution! What I do hope is that I will be appreciated as someone on the other side, because it sure seems like everyone and their brother wants more formality in the writing. More formality? I ask, why can’t we explain it in plain English and introduce the occasional technical term that sincerely advances understanding? I think we can.

Okay, as an example, take Thomas Kuhn’s THE STRUCTURE OF SCIENTIFIC REVOLUTIONS. We could give a good meaty description in four sentences. And not a baby description, but a good description for a college audience. Four sentences. And I think that good description would have zero or maybe one technical terms. The technical terms are not what’s paramount. It’s explaining it to someone just as smart as you are (you’ve got to give your reader that respect), but just someone who doesn’t happen to know this particular area.

Okay, the Ediacaran animals, and the picture looks kind of like a sand dollar, but with lines radiating out from near center. Alright. Well, to an order of approproximation, all species are extinct. Appropriately 99% of all species that have ever lived are extinct. And evolution is bushy. Looking backwards, you can see a main line. But at any particular time, it’s bushy and going in all kinds of different directions and you have no way of knowing what’s going to end up being the future main line. So, it would be no great surprise if the Ediacarans had gone extinct. FriendlyRiverOtter 09:39, 27 April 2007 (UTC)

A 'main line' in evolution?

I'd have to disagree heartily on your 'main line' point. Evolution is indeed 'bushy', and just like a bush, there is no 'main line' - just twigs and twigs. It's popular - and easy - for humans to think that all evolution was directed towards them, but that's not how it works - things 'devolve' as fast as they 'evolve', and 'progress' can only be defined and measured by us, it's not an inherent property of the system. Evolution is change, and it takes us to decide when to call it 'progress'...

Re. the 'not unlikely' extinction of the Ediacarans: Species could be thought of as being a little like needles on a pine tree. It's not a perfect analogy, but an extinction of an entire clade would be like removing every single pine needle from an entire bough - if just one remains, the clade remains extant (i.e. non-extinct). Other, better documented mass extinctions have hit the pine tree like a strong wind - needles have fallen largely at random, and whilst sometimes one branch is decimated more than another, a major bough being completely stripped is incredibly rare. Twigs may be stripped (and go extinct), but larger boughs would always keep a few needles...

I suppose the counter-argument would go that the branches were twiggier back in those early days. Most people expect the 'Ediacarans' to be a major bough on the tree, but maybe at the time this was no bigger a branch than the trilobites', which went extinct at the end of the Permian... | Verisimilus T 12:26, 27 April 2007 (UTC)

To us of course! (our perspective)

And if bears had developed as the dominant intelligence, the “main line” would have gone to them. If it had been small dinosaurs, a la Harry Turtledove’s very imaginative colonization series, they would have been the “main line,” and mammals would have been just a little curious offshoot.

And if we were polar bears, seals would occupy a very central part of our mental lives. If we were antelopes, we would be disproportionately interested in the health of grasses in fields, and so on, and so forth. (And if we were insects, well, I don’t know if I can even imagine that, what kind of consciousness would we have?)

All well and fine, as long as we keep it in balance, that our main issues are not necessarily the only things of significance, nor of value (and that is a whole other discussion! we’re only recently deciding/discovering that whether animals think (like us) may be only one issue in the question of how we should treat them).

The Cambrian explosion was a step forward in complexity, although not necessarily in robustness, biomass, etc. In fact, Stephan Jay Gould said, “There are as many ways to adapt to local environments by becoming less complex is by getting more complex . . .” [3] .

Now, whether intelligence/language is a similar major step forward in complexity, that is a fascinating question of both philosophy and science, and I’m not sure the answers we might possibly get can be more than simply tentative (although we can get into emergent properties and lots of other cool stuff).

And regarding the Ediacarans, is there a good estimate of how many species, and how many separate families? 206.106.1.64 22:37, 30 April 2007 (UTC)

There's a list of Ediacaran genera which covers most of what's been discovered, but due to the lack of fossils for so many of the organisms that must have existed, it will always be difficult to tell the true number of species.
Of course, an artefact of the period is that as the organisms were so much closer to the trunk of the evolutionary tree, that then closely related organisms are classified today into as the parents (stem groups) of large-scale groupings - at the time, they would have been twigs that an Ediacaran palaeontologist would call different species or genera, but those species have gone on to give rise to the mighty boughs that we call different phyla today.
If you've got access to the journal Palaeontology, there were two particularly good articles in the January 2007 edition - Butterfield's 'Macroevolution and Macrocology through deep time' and Bambach's 'Autecology and the filling of Ecospace' - which address the issue of Ediacaran diversity. (Brasier's article's worth a scan too).
The concensus seems to be that diversity during the Ediacaran was undergoing a period of flux, driven by an initial peak of oxygenation allowing the diversification of benthic (bottom-dwelling) organisms, and a stabilisation of the oxygen levels which allowed the diversification of large motile metazoans (animals). Before the Ediacaran started, a stable but very un-diverse assemblage existed; during the Ediacaran, stability dropped as niches were created and diversity rose; after the Cambrian explosion, diversity was high - Butterfield posits an almost stable level - and stability low but (Butterfield would have it) gradually rising.
Verisimilus T 11:41, 1 May 2007 (UTC)

Life most bushy during middle stages of new niches?

At the beginning of the Ediacaran, the sideshoots are going to be awfully close to the trunk as you say, and it’s going to be awfully hard to tell between them and the trunk itself. Some of the sideshoots will develop into big bushy branches, but still, isn’t the default for any particular early species is that it’s more likely to be a sideshoot than an ancestor of a later species, isn’t evolution is that bushy?

As I understand it, species are coming and going all the time, and the average lifespan for a new species, within an order of magnitude, is approximately 10 million years?

Thank you for the part about oxygen levels. That’s exactly the kind of thing I need to start learning (oxygen first building up to a higher level, then stablizing at a more medium level, along with, I’m sure, a lot of bumps along the way). I know the feedback mechanisms of ecology can be very complex, and with a lot of different feedback mechanisms going on at the same time. So, I embrace the complexity (I mean, what else can I do!).

Now, I understand that sometimes (certainly not always) life is most bushy during the middle stages of colonizing a new ecological niche, and becomes less so as one or several species becomes dominant. An example of this might be the evolution of horses. And another example might be the evolution of us humans. At the time of Australopithecus and as one species was evolving toward (through intermediary first?) Homo habilis, there may have been half a dozen species living at the same time (Australopithecus africanus . . . afarensis . . . boisei . . . robustus . . . and probably several other cool ones as well). And considerably later on, we and the neaderthals were two branches. We did not have the relationship of ancestor-descendant, but rather of cousins. We both evolved from Homo erectus. (Arguably, one of the saddest things parts of evolution is that our cousins the neanderthals are no longer here.)

And to keep all this in time perspective, this is the last (approximately) 3 million years of human evolution compared to the 542 million years since the Cambrian Explosion. FriendlyRiverOtter 02:45, 8 May 2007 (UTC)

Possible additional examples of bushiness.

The best visible example might be beetles. Beetles have radiated in many and sundry directions, many, many thousands of species, running into the hundreds of thousands. And yet, it’s not really a question of “progressing.” The different species of beetles are merely different.

And on the microscopic scale, bacteria do the same thing to an even greater extent.

And as far as familiar animals, okay, how about Darwin’s finches? Most probably, the branch started with a single species from the South American mainland, and from there bushed out with different species on different islands and in different niches [4] .

Or, how about the carnivorous mammal that was the ancestor of whales? And from this one species, life bushed out to a number of species, with two main branches, the toothed whales and the baleen whales.

Of course, with the exception of bacteria, the Cambrian period was before all of this. The animals of the Cambrian are less familiar, so we’re going to need to take more time in description and be more generous with photographs. FriendlyRiverOtter 00:29, 21 May 2007 (UTC)

Burgess Shale some 30-odd million years AFTER start of Cambrian

From the timeline at the beginning of our article, the Burgess Shale fossilizations happened approximately 505 million years ago, which is thirty-seven million years after the start of the Cambrian. That’s something that needs to be pointed out, because sometimes the Burgess Shale is so emphasized that it feels like it happened at the beginning. And it didn’t.

And, regarding our topic of life being bushy, I have long heard that a number of creatures found in the Burgess Shale were unique, never to be seen again, including having bodily architecture different from all other animals that have ever walked, creeped, crawled, or swum about the Earth. Now, frankly, looking at the photos of the fossils, that’s not immediately obvious to me. But I am willing to listen to arguments and perhaps become convinced. For example, there’s a little creature that looks like a worm with spines. Okay, it’s interesting. If I saw a creature like that swimming in a pond today it would certainly get my attention, but I’m not entirely convinced that it has a unique bodily architecture.

What I do understand is that one species of worm—with the three bodily layers of ectoderm, mesoderm, and endoderm—became the blueprint for many, many later animals (including all the familiar ones!). FriendlyRiverOtter 05:28, 29 May 2007 (UTC)

How long did it take?

This article appears to lack a clear statement of how long the Cambrian explosion took to occur. Was it 100,000 years, 1 million years, or 10 million years? TimVickers 02:31, 14 May 2007 (UTC)

Who knows?! I think most people would be reluctant to put even an order of magnitude to it, for after all it depends on exactly what is meant by the explosion in the first place - and whether there even was a genuine explosion! Verisimilus T 10:21, 14 May 2007 (UTC)

If we don't know we should say we don't know and give the outer limits of the estimates. An uneducated reader might think it happened in a year or less, since "explosion" is usually kept for very rapid events indeed! TimVickers 20:21, 14 May 2007 (UTC) How about (using numbers from top of my head): Estimates of the length of time over which these changes occurred vary and the Cambrian explosion may have only taken 10,000 years or been as slow as 10 million years.

Whilst 10 million years is a suitable maximum, I'm not sure what to propose as a sensible lower boundary. Again, I suppose the question is which changes you're including within the explosion. The appearance of skeletons by definition did happen overnight, although of course this is not a very informative statement! Verisimilus T 20:33, 14 May 2007 (UTC)
Okay. The literature consensus appears to be of a length of about 9-10Ma for the appearance in the skeletal fossil record of many of the modern phyla. See Valentine, J.W.; Jablonski, D.; Erwin, D.H. (1999-03-01). "Fossils, molecules and embryos: new perspectives on the Cambrian explosion". Development 126: 851-859. Verisimilus T 15:12, 17 May 2007 (UTC)

Sounds great, "estimated to have taken about ten million years" would be suitably cautious. I'd suggest putting this in the lead somewhere, to give the casual reader a clear idea of what "rapid" means in geological terms! TimVickers 12:49, 19 May 2007 (UTC)

Why do we necessarily want something "suitably cautious"??? I'm in favor of laying it on the table! Just identify it as such. For example, "an aggressively short estimate would be _____" and include a footnote or two. But please, talk to me as openly and as casually as you would if I were your colleague.
And by the way, isn't this the main debate between Richard Dawkins and the late Stephen Jay Gould, the question of how much of a role 'punctuated equilibrium' plays? FriendlyRiverOtter 23:33, 19 May 2007 (UTC)
Now, Tim, you’re being a little bit tongue-in-cheek, a little bit ironic, right? It's as if you’re kind of joking about a corporate rule that we all have to follow even though we all know it’s a little goofy and over-restrained, right?
Only in this case, as I’m going to advocate in the next section, we don’t necessarily need to follow the goofy rules. We can engage instead, as it were, in Writers’ Liberation!
And on the scientific question, although not real common, aren’t there a fair number of examples of continuous rock record between the Precambrian and Cambrian? FriendlyRiverOtter 00:29, 21 May 2007 (UTC)
Yes, I'd imagine so - but the only way we can really answer the question is to have a continuous record of what life is present at all times during the Cambrian. Because fossilisation is so rare and biased, that's not something we can ever come close to, and whilst the resolution will steadily improve as we discover more localites, we'll never get close enough to a complete picture! Verisimilus T 11:23, 21 May 2007 (UTC)

I can accept that. That’s reasonable. The fossil record is incomplete, and the rock record perhaps even more so, especially rock layers from 542 million years ago (I mean, a lot of plate tectonics come and gone!). What I would ask, could we take three examples and explain them in some detail. For example, I’m going to assume the Burgess Shale is fine-grained. And I seem to recall reading that it happened in a slump, so that a bunch of little animals were trapped all at once. I would like to have that confirmed.

Again, our article has a lot of ‘bluebirding!’ We don’t really explain things, don't really teach things, don't really describe things. We mainly just mention things. FriendlyRiverOtter 05:28, 29 May 2007 (UTC)

That's because the topic is so huge! To cover everything in the detail it deserves in the one article would take a lot of space!
There are two problems with using the Burgess shale in this context. The first is that at 510 million years old, it documents the fauna long after the explosion had reached its climax. The other, that it represents exceptional preservation. Whilst it's true that this is what makes it so interesting, it also means that it's unlikely to be representative of the biota or ecosystems dominant at the time, and its interpretation on a bigger scale is difficult.
I'm afraid that this article has drifted to the bottom of my list of priorities at the moment, but it will receive some more attention sometime in the next month or two!
Verisimilus T 12:17, 29 May 2007 (UTC)
Everything you have contributed is a gift. And I know, I’m a voice in the wilderness, I am very much a minority opinion. All I’m saying is, if you feel the formal style is a good communicative style, if you can write it kind of fast, if you enjoy writing it, then by all means, continue. But, if you’ve sometimes wondered whether this is really the best way (or only way) to write and communicate, then please just take what I’m saying as encouragement from one reader to try something new. FriendlyRiverOtter 22:21, 15 June 2007 (UTC)

All writing has positive transfer, including formal writing

Does learning on a Dvorak keyboard slow you down and get in your way when you are later trying to learn on a regular Qwerty keyboard, or vice versa? No, studies have been done and it’s just the opposite. Learning on either keyboard helps you if you later move to the other. There is a positive transfer of learning.

I figure it’s probably the same with writing. So, if one has spent time on the formal style most common on wikipedia, there will be positive transfer as one makes the transition to a more communicative style. And I urge you all in fact to make the transition to a more communicative style!

Take a look at Richard Dawkins. Or look at COSMOS (the book) by Carl Sagan, and compare it to any introductory astronomy textbook for that time period, or even for the following ten years. I would suggest that it’s so much better in large part because Carl took the risk of being criticized. He wrote in his own voice. So if you do a good job of communicating with the interested lay person (and I hope that’s one of your prime audiences), you will probably be criticized as sounding too much like an essay or sounding too much like a magazine article. Please take those as compliments!

Learn from good writers like Dawkins, but mainly find and develop your own voice. For example, Richard uses the method of rephrasing something in different words. This can be useful in that it kind of puts up an intellectual signpost and makes the terrain more understandable, but then he kind of overdoes it. So, if you keep working on your writing and allowing your own unique voice to continue to develop, you might even become a better writer than Richard Dawkins. How about that! Please do not let any artificial constraints hold you back. FriendlyRiverOtter 00:29, 21 May 2007 (UTC)

Timeline

I can't get the Ediacaran, as presented here, to match the Ediacaran biota tineline. which is right? Adam Cuerden talk 19:07, 28 June 2007 (UTC)

Not quite sure what you mean. Both appear to end at 542Ma to me... I hope there's not a compatability problem with the template! Verisimilus T 19:39, 28 June 2007 (UTC)
I mean some of the events in the Ediacaran. ~575mya seems far too late for the first Ediacarans, likewise, the "last global glaciation" seems excessively late. Adam Cuerden talk 21:05, 28 June 2007 (UTC)
Ah. The "Global glaciations" on the Cambrian timeline refer to the Marinoan, which are poorly dated - between 570 and 590 million years ago. And the first "Ediacaran fossils" refers to the first bona fide communities (rather than, e.g., the much-debated Twitya discs) - i.e. the Avalon assemblage. It's all very hazy down there - I think that this timeline has the right level of detail for this article, whilst the EB article needs to have more dubious finds mentioned. Although perhaps some clarification would be useful... Verisimilus T 21:31, 28 June 2007 (UTC)
Ah, right. Fair enough. Just slightly confusing, as the Marinoan glaciations are linked to Snowball Earth (which talks solely about the Cryogenian) and so on. Adam Cuerden talk 23:24, 28 June 2007 (UTC)


Need to rephrase and slightly re-structure?

Some private correspondence gave me the initial impression that this article needed major improvements in content, but after reading it twice I think the content is good (of course there's room for improvement in anything, and it will have to be updated to take account of new discoveries and analyses).

By far the largest difficulty is the article's academic vocabulary and style, which will put off non-specialist readers, who should be Wikipedia's target audience. So it needs lot of re-phrasing.

I also think minor re-structuring would help, for example:

  • Intro as is. I disagree with one peer reviewer's suggestion that the intro should contain the difficulties the CE presents for Darwin's theory of evolution. The main reason for these difficulties is that Darwin's theory is gradualist, and it's probably necessary to explain "gradualist", which would make this material too long for the intro.
  • Theoretical significance as at present, plus explanation that Darwin's theory is gradualist.
  • Difficulty of dating as at present but in much less academic language. I'd also check to see if the situation has improved recently.
  • Difficulties of analysing fossils (and quote the Anomalocaris comedy of errors).
  • Types of evidence - trace fossils; hard-part fossils; soft-part fossils from lagerstatten; molecular phylogeny. All in simple terms, with explanations of what they can tell us.
  • Evidence from the Cambrian. I'd present this separately from and before pre-Cambrian content, because the Cambrian fossils explain the term "Cambrian explosion".
    • Lack of pre-Cambrian metazoan fossils when the term "Cambrian explosion" was coined.
    • Tommotian small shelly fauna.
    • Sub-sections as appropriate on fossils from later stage of Cambrian. I'd check for all lagerstatten and especially to see what's come from China recently. Also check for evidence relating to organisms other than multi-celled animals (the intro currently mentions phytoplankton). Add images to illustrate some "wierd wonders" as preparation for the discussion of "riot of diversity" vs "acclerated normal evolution" (Hallucinigenia and 5-eyed Opabibia would be good).
  • Pre-Cambrian evidence of multi-celled organisms. All discovered after the term "Cambrian explosion" became common in scientific usage.
    • Body fossils
    • Trace fossils
    • Molecular phylogeny
On second thoughts I may prefer:
  • Body fossils - pre-Cambrian and Cambrian. This gives the reader a more continuous chronology. It should appear before other types of evidence because: most paleontologists still treat body fossils as the primary source; it was body fossils that raised the apparent problem of the CE.
  • Trace fossils - pre-Cambrian and Cambrian.
  • Geochemical
  • Molecular phylogeny.
  • Phylogentic reconstruction (last because most speculative).Philcha 17:26, 19 July 2007 (UTC)
  • Was the CE real? E.g. is the pre-Cambrian evidence of multi-celled organisms enough to make the term "Cambrian explosion" out-dated? Tentative conclusion: evolution was apparently very fast in the early Cambrian.
  • Interpretations of the CE:
    • "Riot of diversity" (Gould).
    • "Normal evolution on rapid timescale" (e.g. Conway Morris). This may require explanation of the clades lophotrochozoa (includes wiwaxiids, halkeriids, annelids, mollusks, brachiopods) and ecdysozoa (includes lobopods, arthropods, onychophorans).
  • Theories about causes. At present I think the following section on "Timing" should be incorporated into "Theories about causes".

I have a suspicion that a cladogram would be helpful, but wonder:

  • where's the best place for one?
  • should include all multi-celled animals or just bilaterians?
  • how far should it extend towards modern lineages?

Other points to be resolved:

  • How much should the article say about the correlation between the "riot of diversity" interpretation and the theory of punctuated equilibrium, and between both of these and Gould's ideological biases?Philcha 14:19, 15 July 2007 (UTC)
  • How much to say about how Creationists, supporters of Intelligent Design and some Islamists quote the CE as a refutation of Darwinian evolution? Pro: it's a hot topic and some readers will be disappointed if they don't see coverage. Cons: hard to do briefly in what is a long article anyway; hard to avoid charges of partisanship (either way) unless the discussion is long.Philcha 10:34, 21 July 2007 (UTC)

Philcha's notes

Related WP articles:

A couple of comments

I think that since I originally re-wrote this article, it has (inevitably) become more fractured in its viewpoints - ie it has steadily got less coherent. This is not a trouble as long as the article still holds together - otherwise it will never make FA status - something that seems very distant right now.

When I re-wrote the article, I certainly and deliberately downplayed the "sensational" aspects of the CE, as typified by Gould's book, and still (apaprently) the majority view of popular understandings of the subject. But this viewpoint only ever had limited purchase in academic study of the CE, and it is certainly completely obselete now. Specifically: i) Things do not suddenly appear "explosively" except under the influence of taphonomic windows (ie the reason everything appears around Chengjiang time is preservational); but trace fossils and other information inform us pretty clearly that crown group bilaterians did not evolve until late Ediacaran time; ii) similarly, trace fossils do not "explosively appear" at the base of the Cambrian, but rather, diversify rapidly from latest Ediacaran time onwards ; iii) the whole debate between molecular clocks and the fossil record has almost collapsed as MC's have been adjusted to give dates close to that of the fossil record.

As fas as "bluebirding" goes: I am not sure exactly what this means, but I think that one of the things that needs to be corrected about the Cambrian Explosion is that we know everything about it. In fact, almost every aspect of it is the subject of ongoing research; and it is right to reflect these uncertainties rather than present a misleadingly simplistic "solution" to whatever the problem is meant to be. We do not know what the Ediacarians are: indeed, I would say that we have little clue at all. We do not know if oxygen was important. We do not know the phylogeny of most of the major groups involved, so this makes interpreting their fossil record problematic. And so on.

A minor point: the dating of the lagerstätten in the text is inconsistent with the dating in the diagram. The dating of the Sirius Passet is uncertain given that it is based on a Holmiid-like trilobite which is pretty dissimilar to the other holmiids: but if this is broadly right, then SP is probably end Atdabanian; and almost certainly considerably older than the age given in the diagram. It could actually be considerably older than this, ie basal Atdabanian. Chengjiang is, as far as I understand based on the brachiopods, probably early Botoman.


Dating the "speed" of the "explosion": this is highly constrained by the exceptional preservations. All we know is that by Sirius Passet/Chengjiang time (c. 516 Ma or so), loads of stuff was around, and that in Doushantuo time (perhaps around 580-560??), nothing was. In between we have increased diversity of small shellies and trace fossils, plus things like trilobites and echinoderms coming in at around 520 Ma. I don't think one can pull out an arbitrary chunk of 10 million years from this interval and say that THAT was the "explosion"...
Grahbudd 18:19, 19 July 2007 (UTC)

Re-write

This has been my "next project" on Wikipedia for a long time, and it's about time I got stuck in! I'd be very grateful if you'd keep a critical eye on my efforts and expose any errors that creep in!

Graham - "Bluebirding" (as far as I can gather) is the practice of using a link to another wikipedia page in place of a full definition. Ideally the naïve reader should be able to follow the whole article without clicking elsewhere - which is probably an ambitious target in an article such as this!

I'll try and reflect the "Cambrian effervescence" viewpoint more strongly - I've been wary of avoiding "Point of View" discussions, for as you say there still seems to be a large portion of scientists that haven't come round to your point of view yet. I suspect that any eventual Featured Article reviewers would pick up on this strongly, since Gould's views probably represent most people's familiarity with the subject.

Philca - I hope you don't mind if I add to your proposed article structure above!

Verisimilus T 21:04, 20 July 2007 (UTC)

Free free to add to my structure outline - I've already added some second thoughts.Philcha 09:17, 21 July 2007 (UTC)

I'm not sure that the Cambrian Explosion was purely an animal phenomenon, and had carefully worded the introduction to avoid giving that impression. The planktonic realm also underwent a rapid diversification - see reference in lede. Certainly Butterfield's theory of trophic cascades as an amplifying factor requires there to have been one.[1] It may be worth adding a small section discussing this - but I'm not sure that it needs dwelling on. Verisimilus T 12:39, 19 August 2007 (UTC)

Everything I've seen on the CE focuses on animals, but I admit that could represent "animal chauvinism" rather than the truth. I'll check out Butterfield. You said, "The planktonic realm also underwent a rapid diversification - see reference in lede," but the string "lede" does not appear in the article or this Talk page (except for your last post and this one) - please give more details.
Sorry: the "lede" is the introductory section of the article, before the table of contents. Verisimilus T 10:23, 20 August 2007 (UTC)
Whatever the outcome, I think we need to clarify the scope of the subject.
BTW you'll notice that I've commented out some of the previous content. I intend to review it later to see how it fits into my planned structure - some will, but some is excessive detail for this (already long) article, e.g. too much about the Burgess Shales per se rather than the evolutionary implications of the fossils found there.Philcha 15:05, 19 August 2007 (UTC)
In the intro it is said Prior to around 580 million years ago, organisms were on the whole simple, comprised of individual cells occasionally organised into colonies this contradicts the statement made on the Ediacaran biota page which ststes ancient lifeforms, of the Ediacaran Period, that represent the earliest known complex multicellular organisms. I think a consideration of past and present life from this cambrian point must always be taken into account when the article is rewritten. Enlil Ninlil 18:56, 19 August 2007 (UTC)
Good point - I'll change the intro to "Prior to around 610 million years ago, ...". Philcha 21:00, 19 August 2007 (UTC)
But complex Ediacarans are unknown from before ~580 - I don't think the Twitya disks (Aspidella) can really be considered to count on the basis of current knowledge. Verisimilus T 10:20, 9 September 2007 (UTC)
re. the article's scope: I feel that it should be as broad as possible, to cover the full magnitude of the event occurring: anything relevant (e.g. plankton) ought certainly to be mentioned. As you say, however, the article length is a possible concern... Verisimilus T 10:23, 20 August 2007 (UTC)

Comments on edits

Phlya

Just scanning through your additions, your "Why is it such a big issue" section, whilst a great addition to the article, may not stand up to close scientific scrutiny.

For starters, the analogy with the mammals - whilst interesting, and possibly helpful - is not without caveats. For example, it's recently become apparent that the mammals diverged long before the end of the Cretaceous, and the extent of the diversification may not be as large as it has been thought. (A little like the Cambrian, I suppose...)

That's the point. A general reader will not know about the molecular phylogeny analyses, but needs to be told that rapid evolutionary radiations are fairly common. People with a strong interest in paleontology. like you and me, will know about the molecular phylogeny analyses and see the parallel.Philcha 10:21, 21 August 2007 (UTC)

Also, I'm a little wary of the picture you paint of the phyla - it perhaps paints a misleading picture. Yes, interest in the subject arose because of the "appearance of phyla", but the concept of stem groups (see Budd 2000, in the article references, for a good explanation) is more useful in geological terms than the Linnean hierarchy. Perhaps a tighter or more thorough discussion has a place in the article, as this is an interesting area that can spark some confusion about the explosion? Certainly Gould put across the "phyla appearing at once" argument, and it may be worth dismantling that.

Once again, it's about the general reader and about the popular stereotypes with which general readers start. Yes, I will contest the "phyla appearing at once" view, which I never found convincing (it's too much like Saltation (biology), which is not credible in terms of morphological viability or population genetics). The real problem is how much one has to know in order to understand any analysis of the Cambrian explosion, which means the article has to start (over-)simple and increase the pace and intellectual intensity in a controlled manner.Philcha 10:21, 21 August 2007 (UTC)

Duration

Estimates of the Cambrian explosion's duration range from about 10M years (from 530 to 520M years ago) to about 80M years (starting about 580M years ago, in the Ediacaran age).

I've already struggled with putting a length on the explosion, and decided against it, as per Grahbudd's comment above. I'm not sure that it's helpful to pigeonhole time into "before and after the explosion". It was a smooth curve, and different definitions will cover different amounts of it...

I understand the difficulties and objections, and I'll cover the different views and definitions. But the duration is the central point - did the 10 or so modern phyla which contribute most of the fossil record evolve from "blobs" in a time similar to or less than what it took for e.g. mammals to evolve from therapsids? If we already had fossil evidence that evolution from "blobs" (similar to porifera and / or cnidaria) to modern coelomate phyla took a few hundred M years, there would be no "explosion".
I got the figures from reading around, and will have to add the refs.Philcha 10:39, 21 August 2007 (UTC)

Scarcity of body fossils

All but an insignificant amount of fossils are marine, as terrestrial settings only fossilise in exceptional circumstances. It's not cover by more recent rocks that is an issue, but the total sedimentary rock volume preserved; this follows an exponential curve into the past, as rocks are eroded or metamorphosed over time.

Must add refs for the ravages of subduction of marine sediments. But (general reader again), it's easier and useful to start with the simple point that older rocks are more likely to be covered (which is still relevant, e.g. the Burgess Shale is halfway up a mountain, with more recent rocks above, and other potential lagerstatte may have been preserved but remain buried).Philcha 11:05, 21 August 2007 (UTC)

The paucity of body fossils is probably a combination of taphonomic factors and a lack of hard parts.

That's what I was saying, but simplified and expanded for the general reader.Philcha 11:05, 21 August 2007 (UTC)

Worth clarifying too that it's only the MIDDLE Cambrian that contains lagerstatte - after the explosion had died down.

I'll check the dates of lagerstatte as I proceed, but everything I've read treats the Botomian age as "early Cambrian", so the Chenjian and Sirius Passet lagerstatten are about 50% to 75% of the way through the early Cambrian.
I agree that the lagerstatte do not cover the crucial first half of the early Cambrian, and have edited the article to mention that.Philcha 11:05, 21 August 2007 (UTC)

And of course lagerstatte don't display first or last occurrence dates - but the scarcity of fossilisation means that the usual record doesn't usually contain such dates either.

Agreed - I've edited to make this point.Philcha 11:05, 21 August 2007 (UTC)

Trace fossils

A trace fossil is any trace of an organism other than its body itself - and not restricted to burrows, as your introduction seems to suggest.

I'm over simplifying for the general reader again. Note the word "mainly". I'll edit this if the survey of the evidence includes other relevant types of trace fossil.Philcha 11:25, 21 August 2007 (UTC)

Do you have a citation for the necessity of Earthworm complexity? I suspect you're probably right but would like to be sure.

I've read it and will hunt it down and include it, but probably as one of the finishing touches. My priority is to produce a coherent article for the general reader then attend to satisfying purists later (not a dig at you, I read and agree with your comments on the hassles of the FA process).Philcha 11:25, 21 August 2007 (UTC)

What does one mean by "comparable complexity to an Earthworm"? it's perhaps worth clarifying this.

Yes it needs clarification - later. It's another example of how much one needs to know in order to understand the Cambrian explosion. Possibly it's best to explain a bit more about the mechanisms of the earthworm (wide range of specialized tissues, segmented coelom which acts as a hydrostatic skeleton giving a platform for the peristaltic action of muscles, etc.).
I've just checked coelom and found that it is far too anthropocentric, and will rewrite that some time to support evolution-related articles (found the same problem with suture and implemented a temporary fix; the full solution is to make suture a pure disambig page and rename the current content of suture as "Suture (surgical)" - which will upset the medical / veterinary people).Philcha 11:25, 21 August 2007 (UTC)

Molecular clocks

You're trying to summarise a huge amount of information in a short section.

Isn't that what Wikipedia does :-)Philcha 11:34, 21 August 2007 (UTC)

The pitfalls of the molecular method can probably be covered with a link, and a statement that "Molecular estimates have dated the explosion at dates ranging from 1Ga to 500Ma ago, and should thus be treated with caution".

This is a general section describing types of evidence and their uses and pitfalls. I intend to summarise (!?) the body fossil, trace fossil and geochemical evidence in chrono order of the rocks, and then summarise mol phylo analyses in a separate section (separate because not based on rocks).

Given that one of their assumptions is often that of a constant rate of evolution, their application may be more in spotting something abnormal happening in the Cambrian, such as Butterfield's 10-fold increase due to trophic cascades, than to providing a date for the explosion, which we know anyway to have happened long before the divergence of lineages.

I'm not sure I completely understand that comment. but I've edited to point out more clearly that mol phylo can show family tree relationships.
I totally don't understand "... the explosion, which we know anyway to have happened long before the divergence of lineages" - can you please explain.Philcha 11:34, 21 August 2007 (UTC)

-

I hope not to cast a negative view - there are many improvements in terms of style and some content - but there are also many small but significant errors. I'm wary of straying too far from Budd's version of the article without good reason, as it's very easy for a non-expert author to add small comments which turn out to portray an incorrect or misleading impression on the reader.Verisimilus T 10:59, 20 August 2007 (UTC)

No risk of my taking offence - I've found your comments helpful. I'll welcome more - this is the most difficult paleo subject I've tackled because: it strains my paleo knowledge to (?beyond) the limit; making it intelligble to the general reader is very difficult, because one needs to know so much (e.g. taphonomy, non-fossil evidence-gathering techniques such as mol phylo and geochemistry, basic biology including phyla and grades, etc. etc.).
Re "small but significant errors", I'm always willng to listen but may not always agree - it all depends what what each of us has read and how we interpreted it. And in the early stages of the article I deliberately over-simplify for the sake of the general reader - specialists don't read Wikipedia articles on their specialism.
I started editing the article because I wasn't happy with the structure and style for a whole host of reasons. So I'm not wary of straying too far from Budd's version, because I think there are plenty of good reasons for doing so.Philcha 11:48, 21 August 2007 (UTC)

More comments

I do not wish to sound argumentative, but. The original article was one reviewed by Nature; and was found to contain a substantial number of errors. As this was the case, and as the structure was a complete hotch-potch, I totally rewrote it. I agree that the version I produced was somewhat stodgy and academic; but it was, I modestly suggest, both comprehensive and accurate. I have watched this article evolve away from what I wrote with a mixture of some dismay and amusement. Now it contains about as many mistakes as it did before. For example:

i) "The Cambrian explosion describes a profound and very rapid diversification of life[1] on Earth that occurred more than half a billion years ago."

First, how can "the Cambrian explosion" "describe" anything? And now the opening paragraph makes no reference to what the Cambrian explosion is about, ie the rapid expansion of the _fossil record_; but rather talks about several layers of inference from it, including the idea that "evolution would accelerate by an order of magnitude" - Butterfield sourced and respectable, but what does it *mean*?

I see what you mean. Apart from the grammatical aspect (easily solved by wrapping the term in quotes), the observed phenomenon is that (until the discoveries of the last few decades) the fossil record apparently showed a sudden appearance of diverse metazoa - and the intro should stick to that, although it should also mention the concern that it caused for Darwin.
To get it really right, we need to resolve a discussion (above) with Verisimilus who argues on the basis of Butterfield's work that diversification of plankton should be included in the definition of the Cambrian explosion; while I think that Butterfield follows the common view that the Cambrian explosion was about animals, and he offers diversification of plankton and its consequences (enrichment of food chains; increase in marine O2 concentration as heavier zooplanktonic feces sank and sequestered carbon on the seabed) as a possible driver for the evolution of larger, more diverse metazoa (he also seems to support the idea of an "arms race" between predators and prey). What is the consensus definition of the CE?Philcha 23:30, 29 August 2007 (UTC)
As I'm writing this, a statement in the intro is "In the following 70 million to 80 million years, the rate of evolution accelerated by an order of magnitude..." and this seems to not make sense to me, as stated. Is "the rate of evolution" referring to actual evolutionary mechanisms causing evolutionary change ten times faster than before, or is it referring to an acceleration in the diversification of species? These are two different things, and it seems to me that what is being referred should be stated much more clearly, because as it stands it is ambiguous. [[[User:Greeneto|Greeneto]] (talk) 17:34, 22 May 2008 (UTC)]

ii) History of the concept: Darwin's difficulty was increased by the fact that in his time no sedimentary rocks much older than 500M years had been reliably identified, and Darwin regarded the Silurian as the earliest known geological period.[5]

But this is a misleading statement. Darwin was well aware of Sedgwick's work on the Cambrian (indeed, he accompanied him on his fieldwork to North Wales). It is true that in the 1850's the high-water mark of Murchison's Silurian concept was reached; but this paragraph makes it sound as if rocks as old as the Cambrian were not known to Darwin: they were.

I should have said something like "in Darwin's time rocks of Cambrian age were regarded as belonging to the Silurian period" (referring to Darwin's supposition that life must have been around for at least as long before the start of the "Silurian" as since then).Philcha 20:02, 29 August 2007 (UTC)
I've deleted the Silurian bit - it's an unnecessary complication for readers who are not interested in the history of Victorian stratigraphy.Philcha 10:40, 30 August 2007 (UTC)

iii) "Why is it such a big issue?" My original effort was stuffy, but this is too far too far in the opposite direction. And what follows is both misleading and a more or less complete non sequitor. " So at first sight a wide range of complex animals evolved from "blobs" in under 30M years". Why reference Cohen's out of date text book here, when there are zillions of more up to date works? And who says they were blobs anyway? And the misleading discussion of phyla is out of place.

We have to remember that that users of Wikipedia are not professional specialists like you or even armchair enthusiasts like me. The CE is a difficult subject for a general encyclopedia because one needs to know so may facts and concepts just to begin to understand it. If readers are familiar with the term "Cambrian explosion" at all, it is probably from articles or web pages for or against Creationism, or popularisations of "Wonderful Life". So I phrased this introductory section accordingly. The Creationist attempt to use the CE as a refutation of Darwinism is largely based on an essentialist conception of "phylum", so I quoted Cowen (whose book is AFAIK a fairly widely used one) to correct this conception.
PS: "zillions"????Philcha 23:30, 29 August 2007 (UTC)

iv) Duration of the explosion. I've already commented on how misleading this is. Any case, the whole idea of "Even if the process took 80M years, it would still be exceptionally fast - for comparison, the much less radical evolution of mammals from therapsids took about 70M years. " begs several questions. Says who? If one had a time machine and could watch what was going on, I expect it would not seem bizarre at all.

The 80M years is at the long end of the range of estimates I've seen, and dates from the earliest known fossils of the probable mollusc Kimberella. Do you know of earlier good evidence for coelomates?Philcha 23:30, 29 August 2007 (UTC)
The main problem about the CE is its apparent speed. Another example would be evolution of birds (about 50M years from Staurikosaurus to Archaeopteryx - if Archie was a bird). Evolution of mammals and birds are modest examples compared with how long it took for eucaryotes to evolve from from procaryotes (2B to 2.5B years) or metazoans to evolve from single-celled eucaryotes (500M to 1.2B years, depending mainly on when eucaryotes appeared). As a professional palaeobiologist you may consider these comparisons misleading - but then it is necessary to explain why, for the benefit of the general reader and even more so for armchair enthusiasts like me (perhaps a little knowledge is a dangerous thing, but the only cure is more knowledge).Philcha 10:40, 30 August 2007 (UTC)

v). "Body fossils Body fossils preserve all or significant parts of organisms and are therefore the most informative type of evidence. Unfortunately they are increasingly rare as one looks further back in time: The rocks in which they are buried are usually covered by more recent rocks. All life in the early Cambrian was marine, and subduction destroys marine sediments within 200M years of their formation, unless they are raised above sea-level by other tectonic processes such as orogeny (mountain-building)."

It is misleading to state that fossils get rarer as rocks get older. Number of fossils is controlled by outcrop area of fossiliferous rocks; and this is controlled in turn by tectonic cycles: rocks get buried and then exhumed over a few hundred million years. Even if one accepts that _on average_ older rocks are less likely to be preserved, this does not mean necessarily that the quality of the fossil record gets worse (see eg Benton et al. Nature 403, 534-537 (3 February 2000)).

I'll check the Benton article.Philcha 22:39, 29 August 2007 (UTC)
Can you please provide the exact title of the Benton article so I can search for it online. My previous comment assumed it was in the refs for Cambrian explosion, but a text search of that page for "Benton" got nothing.Philcha 13:06, 31 August 2007 (UTC)
Hi Philcha. I think the reference is:
Benton, M.J., Wills, M.A. and Hitchin, R. (2000). Quality of the fossil record through time. Nature 403, 534-537.
Cheers, --Plumbago 13:13, 31 August 2007 (UTC)
Thanks!Philcha 22:53, 3 September 2007 (UTC)

vi) Oceanic floor is destroyed by subduction; but the continental shelves where diversity is highest are continental, and thus do not get subducted in general.

I agree, I got that wrong.Philcha 22:39, 29 August 2007 (UTC)

lagerstätten: one needs to qualify this term to show that you mean conservation rather than concentration deposits.

I wrote "lagerstätten ..., which preserve soft tissue in very fine detail", meaning conservation lagerstätten without the additional technical term, out of consideration for gerenal readers.Philcha 22:39, 29 August 2007 (UTC)

vii) "Trace fossils. Trace fossils consist mainly of tracks and burrows on and a few centimeters under what was then the seabed. Animals under a centimeter long are unlikely to leave tracks or burrows on the surface; and burrows under the surface suggest the presence of animals which were almost as complex as earthworms..."

Trace fossils can in general go down a metre or more (Cambrian ones are much less tiered). Plenty of organisms smaller than a few cm can leave trace fossils; and you do not need to be particularly complex to burrow under the surface per se (e.g. the cnidarian "sea pansies" do it).

I'll have to check out sea pansies, and the depth and complexity of Ediacaran, early and later Cambrian burrows.Philcha 23:38, 29 August 2007 (UTC)

viii)"Molecular phylogenetics can provide interesting suggestions about the phylogeny ("family trees") of organisms provided one can avoid the difficulties created by saturation. But many paleontologists believe that fossils are still the best form of evidence and should take precedence if there is any conflict between the results of fossil-based and molecular analyses."

Name names! I think this sets up a false dichotomy.

The Evolution of mammals presents 2 such conflicts: the debate about when modern orders of mammals diverged and the conflict between anatomically-based classifications of modern mammals and the Afrotheria model.Philcha 23:30, 29 August 2007 (UTC)

ix) Triploblastic. Trichoplax is an example of a taxon that is not sponge or cnidarian but is not triploblastic; and ctenophores are not usually counted as triploblastic either.

Fair point. How about "Most types of living animal are triploblastic - the best-known exceptions are Porifera and Cnidaria."Philcha 22:39, 29 August 2007 (UTC)

x) Diploblastic. It is increasingly being recognised that anthozoan cnidarians are essentially bilaterian in many ways; but are not triploblasts.

Can you please more info / refs. Are you referring to planula larvae of cnidarians? If so, the Wikipedia planula article seems to imply that medusozoans also have a bilaterian life-cycle stage. Is that article correct?Philcha 22:39, 29 August 2007 (UTC)

xi) Coelomates. Not all Cambrian fossils are coelomates. E.g. the cycloneuralians.

I looked up cycloneuralians and found that they are now classified (along with arthropods) as Ecdysozoa, and that cycloneuralians include priapulids, which occur in the lagerstätten. In that case the CE is not all about coelomates. I've edited that.
The classification Ecdysozoa: cycloneuralians + arthropods also implies that they evolved from non-coelomate ancestors, so the schizocoels are paraphyletic. Have I understood this correctly?Philcha 10:40, 30 August 2007 (UTC)

xii) Trace fossils 1000M years ago?

Even the principal author of this report (Dolf Seilacher) has abandoned the idea that they are biogenic.

Please provide a ref for Seilacher's change of view - I've seen passing mentions of it but that's all.Philcha 23:30, 29 August 2007 (UTC)

xiii) Ediacaran organisms.

This section is highly biased. E.g. Spriggina is "probably a trilobite" - I would just flatly disagree with this and many other people would too. Citing MacMenamin on this topic does not, alas, fill me with confidence.

I cited MacMenamin in good faith based on what I saw at the time, but have seen reviews of his book which have given me second thoughts. I'll look for more material on Spriggina.Philcha 23:30, 29 August 2007 (UTC)

xiv) "If traces such as Cruziana and Rusophycus were produced by arthropods, that would indicate that arthropods or their immediate predecessors had developed exoskeletons, although not necessarily as hard as they became in the Cambrian.[22]"

Both these _are_ Cambrian fossils, as this section states itself.

Should have said "... later in the Cambrian."Philcha 22:39, 29 August 2007 (UTC)

xv) "The earliest Cambrian trilobite fossils are about 530M years old, but even then they were quite diverse and world-wide, which suggests that these arthropods had been around for quite some time.[32]"

Most people would date the oldest trilobites to be beginning of the Atdabanian, ie around 520 million years ago or so. Look at http://www.palaeos.com/Timescale/timescale.html - the timescale given here is, I note, not in accord with this. Incidentally, the Burgess Shale is middle Cambrian, but sites in the Lower Cambrian in the time scale!

What's the most widely-accepted timescale for significant events, fossils, etc. of the Cambrian? I admit I was getting uncomfortable about this issue, because every article I read uses different dates. Do we have to give up the idea of absolute dates and use relative ones, e.g. with the start of the Cambrian as zero?Philcha 22:39, 29 August 2007 (UTC)


xvi) Sirius Passet fauna.

Again, I dispute the age, it is probably 10 million years younger than this.

See my previous response - I really would appreciate some guidance on how to present the timing of events, because it's a key issue.Philcha 22:39, 29 August 2007 (UTC)

xvii) "The strangest-looking animals from Sirius Passet are Pambdelurion and Kerygmachela. They are generally regarded as anomalocarids because they have long, soft, bodies with a series of broad fin-like flaps along each side. The fossils found so far show no trace of gills or other breathing apparatus and no trace of segmentation. Both were apparently blind, as the fossils show no trace of eyes. Pambdelurion had a large mouth on the front of its head, flanked by a pair of thick, segmented appendages slightly longer than the swimming flaps and equipped with a flexible spine on each segment. It may have fed on plankton. Kerygmachela had a pair of long, slender trailing appendages at the rear end, and a small conical mouth flanked by robust, unsegmented appendages which had short spines on the front edge and were tipped with longer spines. The spiny front limbs suggest that it may have been a predator, but its small mouth suggests it would have been restricted to very small prey."

Kerygmachela was described with gills: indeed the title of the first paper to describe it was "a Cambrian gilled lobopod from Greenland". I should know, as I wrote it. In the fuller description it is described as possibly possessing eyes too. It is highly misleading to say that neither show any traces of segmentation; au contraire, they are both clearly segmented. ii) Pambdelurion's mouth is not terminal, but ventral. And the idea that is was a planktivore is fanciful (whether or not it is actually true). (eh, I described this fossil too).

Looks like I was naive to trust the Wikipedia articles on Kerygmachela and Pambdelurion and some other secondary sources. Can you please provide refs? Did you find evidence of eyes in Pambdelurion? Were the gills below the flaps (similar to biramous arthropods) or above (as in Opabinia)? Does the ventral mouth of Pambdelurion argue strongly that the animal was not a planktivore?Philcha 22:39, 29 August 2007 (UTC)
And can you suggest where to get some decent drawings of these critters, instead of the lurid cartoon?Philcha 22:29, 3 September 2007 (UTC)


xviii) Chengjiang.

e.g. "Chenjian contains other animals which have puzzled paleontologists but are now regarded as lobopods and fairly closely related to arthropods. Anomalocaris was a mainly soft-bodied swimming predator which was gigantic for its time (up to 70cm = 2.1 feet long; some later species were 3 times as long); the body had a series of broad fin-like flaps along each side, and at the rear a pair of "fans" arranged in a "V" shape. The fossils found so far show no trace of gills or other breathing apparatus and no trace of segmentation. ".

Again, gills were described by Whittington and Briggs )185) and the animals are clearly segmented. Not even I have claimed that Anomalocaris in the strict sense is a lobopod: it is clearly not, in fact!

Do you mean "Whittington and Briggs (1985)"? (I find typing in these wretched textareas difficult too, I've probably missed more of my own typos than I've fixed in this discussion). So what are the current ideas about classification and phylogeny for anomalocarids?
Somewhat off-topic, I'm not convinced by suggestions that Laggania was a planktivore, as the dorsal eyes on short stalks and the long, fairly coarse spines on the appendages suggest a bottom-feeder to me. Is there any support for its being a bottom-feeder?Philcha 22:39, 29 August 2007 (UTC)

xix) Misszhouia is not a "soft-bodied trilobite".

I'd read that it was closely related to Naroia and Gould ("Wonderful Life") describes Naroia as a trilobite, based on Whittington's analysis of the segments from which the various head appendages grew. Are there any decent refs for Misszhouia?Philcha 22:39, 29 August 2007 (UTC)

xx) "analyses of both of these animals were hilariously wrong, but they are now regarded as lobopods, and Anomalocaris is very similar to Opabinia in most respects (except the eyes and feeding mechanisms) - see above."

Unfair: both accounts were excellent given the material at hand. And Anomalocaris is not a lobopod. And it is not really fair to call Odontogriphus a mystery any more.

I probably let off too much steam there, and have removed the offending comments about Anomalocaris and Hallucigenia. And calling Odontogriphus a mystery was an exaggeration, as the current theories appear to be that it was either a mollusc or a more basal lophotrochozoan - I've edited it.Philcha 22:39, 29 August 2007 (UTC)

Again, I do not want to be argumentative: but I think that the current version of the article now contains even more mistakes than before the rewrite. Further, it puts far too much emphasis on the various exceptionally preserved faunas, which after all all date from after the period of the most important evolution events. Thus, the overview of the whole thing is now missing, but rather the article is evolving into a list of dubious facts about the exceptionally preserved fauna. Finally, it is too much in thrall to the Gould view which is not really held to be true by most palaeontologists.

I am not in thrall to the Gould view, which sounds too much like saltationism to me. If Kimberella is a mollusc, as far as I can see that would refute Gould's idea of a riot of disparity within a few million years of the Cambrian; and the time-range of "halwaxiids" (from Halkieria scales in the small shelly fauna to Wiwaxia and Orthrozanclus in the Burgess Shale) also argues against the view that coelomates evolved and diversified in a very short time. Unfortunately Gould's view is the one most likely to be known to general readers (mostly second-hand or worse), so I don't think the article can ignore it. The real task is to: present a summary of what is known, in terms the general reader can understand; refute well-publicised misconceptions, otherwise they gain adherents by default; then present viable theories about what happened and why.
I think the "weird wonders" from the lagerstätten are important because a lot of Gould's argument is based on them (and Gould's view is the one most likely to be known to general readers); while if they turn out to be stem-group relatives of well-known phyla then the CE looks more like normal evolution on a rather fast timescale.
Gould made a big deal about a wide evolutionary radiation which was then pruned (by chance and / or natural selection). I think the article should point out that this is a normal pattern, for example Triassic archosaurs (phytosaurs, aetosaurs, rauisuchians) and Mesozoic near-mammals (especially multituberculates).
Please be as argumentative as you like - I'm just an armchair enthusiast who is painfully aware both of his own ignorance and of how easy it is to take prior knowledge for granted when writing about the Cambrian explosion.Philcha 22:39, 29 August 2007 (UTC)


cheers

Graham Grahbudd 14:12, 26 August 2007 (UTC)

Recent Edit

"Today, creationists with a poor grasp of science maintain public interest in subject, highlighting the Victorians' concerns about its incompatibility with evolution."

I have to agree with removing "with a poor grasp of science." It seems unverifiable and pov. I would have taken this out myself, but it seems that has already been done and undone. Momo Hemo 09:48, 22 September 2007 (UTC)

Creationists with a good grasp of science don't highlight the Victorians' concerns about its incompatibility with evolution, because they realise that the concerns are outdated and have been addressed by recent (i.e. the last hundred years) science. I feel that it's important to make this point in case anyone mistakenly thinks that the creationists' arguments have a scientific foundation. Verisimilus T 10:09, 16 November 2007 (UTC)

Quick note on Ediacarans

Just to point out that Knoll's paper post-dates Conway Morris's. No-one really believes that Thaumaptilion is an Ediacaran any more (it's probably not an Octocoral either) and a mass extinction is almost universally accepted at the base of the Cambrian. Verisimilus T 14:43, 22 December 2007 (UTC)

Welcome back! I googled again and found more refs to claimed Ediacarans in the Cambrian, and have added one. Looks to me as if at present it's a rather grey area, and "...have been reported ..." in the Cambrian explosion article is suitably non-committal - unless you know of a knockout against the idea of Ediacaran survivors.
Re Ediacaran-Cambrian mass extinction, I'm struck by the lack of enthusiam for this idea in Marshall (2006) and by other comments, e.g. at [23]. In fact I've just looked back at my notes (much higher in this Talk page) and there seem to be rather a lot of explanations offerred for the carbon isotope anomaly, some of them not at all "the usual suspects", e.g A METHANE FUSE FOR THE CAMBRIAN EXPLOSION: CARBON CYCLES AND TRUE POLAR WANDER. I'll have to work this one in - even if it's one of the less popular hypotheses, it will convey an impression of how different the world has been in the past (despite being governed by the same rules of physics); and I should check out other hypotheses.
Still on my to-do list: molecular phylogeny; most of the analysis / explanations (I started the analysis prematurely, then realised I hadn't eaten my molecular greens).
More generally, how do you think it's shaping up so far? Philcha (talk) 17:20, 22 December 2007 (UTC)
Glancing at that second reference, the "Ediacaran taxa" described are simply microbial structures (See Grazhdankin), not "Ediacarans". I have to admit I've not come across Marshall before, and it's usually wise to take the views of one person in a small journal with a pinch of caution... there really are no Cloudina, Namapoikia, Namacalathus, Ediacaran fronds, Dickinsoniids, etc after the Cambrian, despite a growing body of seemingly suitable preservational settings. Of course, proving their absence is always going to be impossible!
And isotopic anomalies are interesting, but can usually be explained in a plethora of ways. Methane causes of rapid warming (which if I remember correctly is what that article was getting at) are not unique to the Cambrian, either - they've been touted as a cause of the end-Permian (and, I think, an amplifier of the K-T) extinctions too. There's not even agreement on whether temperature should be positively or negatively correlated to diversity! Verisimilus T 18:27, 22 December 2007 (UTC)
Oops, I put empty "name" params in the "ref" tags for that para, and that confused Wikimedia into pointing to the wrong citations. Fixed it now, points to [24], which says "Ediacaran fauna". Is that better, or are there still doubts about Ediacaran survivors?
Fair comment re Marshall, but it'll get the analysis section off to a good start. In an article of this size I find it helps to block it out using a few refs, then refine as I read through more.
I'll check for refs to Ed-Cm mass extinction to see if that's the current consensus. I'm aware of the methane-extinction link - I wrote the relevant part of Permian–Triassic extinction event (which I intend to revisit when I can re-find some refs re tolerance for hypoxia and hypercapnia - more fool me for losing them). Re temperature and diversity, I found something that said the Ed-Cm negative C-13 anomaly was greater than the P-Tr's and another re Ed-Cm anoxia (have to re-find those, too !!@@##) The trouble is in an article of this size I think one has to get a decent draft up then refine (hopefully with help), otherwise it stays 90% complete for ever. Philcha (talk) 19:31, 22 December 2007 (UTC)
I did get confused by the citations, but checked the edit history to see which citation you meant! As I said, the "Ediacaran fauna" mentioned are all microbial colonies - though I did find reference to a Cambrian Swartpuntia elsewhere which I wasn't sure what to make of (which, of course, I've now lost...) Verisimilus T 22:41, 22 December 2007 (UTC)
Is the Swartpuntia ref EARLY CAMBRIAN EDIACARAN-TYPE FOSSILS FROM CALIFORNIA Hagadorn et al. Journal of Paleontology.2000?
Re Cyclomedusa, Nimbia and Tirasiana, Grazhdankin has re-classified these as colonies of bacteria - how much support for that is there in the literature? Philcha (talk) 23:47, 22 December 2007 (UTC)
I think that was the paper - well found. I've not seen Grazhdankin's work dismissed anywhere in the literature (I know Guy has his own ideas about them, but his alternatives are not as widely accepted) - and my sense is that the microbial hypothesis has the backing of most Ediacaran palæontologists... Verisimilus T 14:01, 23 December 2007 (UTC)
What about something like: "More recently Ediacaran organisms have been reported from the Cambrian itself,{ref to Conway Morris 1993} but most of these are now generally regarded as microbial colonies, {ref to Grazhdankin} although it still appears that one genus of "classic Ediacaran" animals survived into the Cambrian.{ref to Hagadorn} Nevertheless many Ediacaran organisms ....{ref to Amthor et al.}" And what is the citation for the relevant Grazhdankin paper?
  • See refs 32-3 on Ediacaran biota. And remember that most of them are possibly/probably not animals. Verisimilus T 18:29, 23 December 2007 (UTC)
What am I missing? These refs are about the the Ediacaran period; and 33 (abstract) says, "At least some Ediacaran discoids can be compared with extant concentric ring-shaped microbial colonies that grow in hypersaline microbial mats. Insofar as most of the remaining record of Ediacaran discoids can be attributed to the holdfast structures of non-radiate modular organisms, there is no support from the fossil record for identifying a radiate ancestry for the Metazoa." To me this implies that some discoids are holdfasts and therefore that Cambrina discoids may be holdfasts. And Shu, Conway Morris et al. report another Cambrian "Vendobiont" [25]. Looks to me like an on-going debate and the safest position for now is non-committal. Philcha (talk) 19:54, 23 December 2007 (UTC)
  • Reference 33, Ediacaran microbial colonies, makes the strong case. Re. Conway Morris's "Vendobiont", this theory was blown out of the water by Jon Ancliffe (2007) - very neatly, I think - in "Charnia and sea pens are poles apart", where he shows that the growth mode of the rangeomorphs excludes the sea pen hypothesis which S.C-M. pushes. So these fossils could be early sea-pens (although FWIW Ancliffe doesn't believe this in the slightest - the octocoral fossil record otherwise begins in the mid-Tertiary) - but show no synapomorphies with the Ediacaran fronds: SCM's argument is based on superficial resemblance, no more, and I don't think there's any support for it. Verisimilus T 10:12, 24 December 2007 (UTC)
Thanks for your patience in making this clear. What's the Ancliff (2007) ref (it's not in Ediacaran biota)?
I'd better summarise how I understand our discussion, so that I can edit the relevant section: S.C-M. et al. have reported 2 finds that they claim are Cm surviving "Vendobionts"; Grazhdankin et al and Ancliffe (2007) argue convincingly that these are not "Vendobionts" (Grazhdankin identifies some as microbial colonies; what exactly does Ancliffe argue for?); but Swartpuntia (Hagadorn et al.) may be a genuine Cm "Vendobiont". Have I got this right? Philcha (talk) 10:53, 24 December 2007 (UTC)
  • It's reference 47 - you might have had a chance of finding it if I'd spelt Antcliffe correctly! JA doesn't argue for anything - in his opinion, they're just more "weird things from the Cambrian". He simply points out that they can't be Ediacaran. Your summary is spot on! Swartpuntia is a genuine "Vendobiont", the question is whether Hagadorn's fossil is really Swartpuntia. Verisimilus T 12:49, 24 December 2007 (UTC)
Thanks! I omitted one ref from the ones we've discussed (Crimes & McIlroy, http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=4813), but that names genera that Grazhdankin discussed, so I've treated it as a failed claim.
Re spelling of author's names, I usually select the text in my browser (double click if 1 word, drag mouse if several), copy by CTRL-C and paste by CTRL-V - saves a lot of typos (to which I'm prone, alas). This method works fine for any Windows version; I'd expect equivalents for Linux or Mac if you use one of these. Note: in Win, CTRL-X = "cut" Philcha (talk) 14:02, 24 December 2007 (UTC)

The Cambrian explosion is obviously a very important subject, but the page as it stands is forbiddingly long. Would it be possible to summarize and redirect to subpages to make this page more inviting to the paleontology non-specialist? — vivacissamamente (talk) 16:30, 30 December 2007 (UTC)

That's one of the things I intend to consider seriously once I'm happy that the content contains no major errors and does not omit anything important. Philcha (talk) 11:12, 31 December 2007 (UTC)

"Draft one": Suggestions for improvement

Hi,

My initial thoughts after a first read through is that the length, unfortunately, needs reducing a lot. While there may be a case for explaining every scientific concept within the article, there are many sidetracks that are given undue weight - for example, far too much time is spent describing individual organisms, which are interesting - but stray from the central point of the article. In a shorter article, the descriptions would make interesting asides, but I think that it is important to keep as close to the subject as possible. The first paragraph of "How fast did the main metazoan groups evolve?" is the meat of the story, and the previous dozen sections could almost be omitted in favour of a modest expansion of this.

Also, there's a lot of repetition - much of the previous version appears to have barely been touched, where incorporation may have been more useful.

I think there's a difficult balance to strike between engaging the non specialist by explaining unfamiliar concepts, and keeping the article short enough to be readable within the short attention span of the average reader. As it stands, it perhaps lies a little on the former side, and a more concise style of writing may help balance the scales.

The other thing that perhaps gets lost in the size of the article is a central line of argument - the article doesn't seem to "flow" comfortably, and the reader isn't really led from one section to the next.

I hope these comments are helpful, sorry if they're a little vague at this stage! After a significant trimming, this article should well be on the right tracks...

All the best, Verisimilus T 18:55, 30 December 2007 (UTC)

A few more points:

  • Body fossils preserve all or significant parts of organisms
misleading. "All" an organism is never preserved. Significant in what way?
  • among other reasons because the rocks in which they are buried are usually covered by more recent rocks
This reason doesn't quite back up the preceding statement; erosion is probably a more important factor. Do we need one single reason quoting?
I've inserted erosion as another factor.
  • But there is evidence that some types of animals or parts of animals are relatively likely to be preserved as fossils in some environments and times, and extremely unlikely to be preserved in other environments and times
Very vague.
This summarises 2-3 rather technical pages in one of Butterfield's articles, and gives a ref. I don't see how to be more precise without a big increase in words and technicality (e.g Doushantuo-style vs Burgess-style preservation) - please edit if you see a way!
  • these changes were most significant before the start of the Cambrian
How are you measuring significance? The change in chemistry around extinction events was probably more significant than that before complex life existed, for example.
This also summarises a long, technical discussion in the Butterfield articles. I'm referring to background changes, not driven by any exceptional events. Again, please edit if you can see a way to clarify without increasing the length or getting very technical.
  • for example any increase in the marine biomass would reduce the concentration of carbon
Doesn't sound true, though I may be wrong
Based on Butterfield again. Increase in phytoplankton sucks CO2 out of the water, and resulting increase in zooplankton preying on phytoplankton, scavenging, etc. reduces the amount of C that re-enters solution, and that leaves more free O2.
  • shells or exoskeletons.
Shells ARE exoskeletons.
Technically perhaps yes, but AFAIK in popular usage the word implies "jointed" - see for example a lot of science fiction, and Exoskeleton has a similar implication.
  • Note: “hard parts” in paleontology means “mineralized” –
why not just say "The mineralised hard parts" in the first place, to remove the need for this sentence?
"The mineralised hard parts" could mean "... as opposed to non-mineralized hard parts", e.g. a heavy layer of chitin. And at this stage I think the article should explain rather than assume conventions of paleo-speak.
  • an unusually high number of lagerstätten
Is it really unusually high?
Apparently - see Lagerstätte.
  • German for storage place
Is this the best translation? I prefer "Mother lode" or similar; I've not heard storage place before.
It's the literal translation.
  • lagerstätten are much better than most fossil beds,
better?
  • they probably do not preserve the majority of types of soft-bodied animal;
Has this been quantified? Be careful of using lagerstatten to mean "konservat-lagerstatten" - remember the term also applies to konzentrat-lagerstatten.
"... do not preserve the majority ..." is based on Butterfield. Aty present Cambrian explosion does not mention any konzentrat-lagerstatten, so I thought I'd spare the general reader that distinction.
  • known lagerstätten cover neither the crucial first half of the early Cambrian nor the period immediately before that.
There are Ediacaran lagerstatten; most of them don't show much; some Ediacarans are preserved as carbonaceous films.
The Ediacaran lagerstatten listed in Lagerstätte are well before the boundary.
  • Trace fossils consist mainly of tracks and burrows on and under what was then the seabed.
Geochemical traces and stromatolites are also considered trace fossils.
I wasn't aware of that. If you think it's important I'll change occurrences of "trace fossils" to "tracks or burrows".
  • moderately complex animals
I'm still not quite sure I like this term.
It's the same old struggle to keep it brief and simple. Would you prefer "triplobastic"?
  • lighter isotopes such as 12C respond to these more quickly than heavier ones such as 12C
a 13 is needed. And I'm not sure I buy the superseding argument - I don't know how much a mass extinction affects the biomass (rather than diversity)
From what I've read, it's generally accepted that for the number of taxa to be reduced there must be a greater reduction in the number of individuals, even in the surviving taxa. Also it now seems generally accepted that recovery from mass extinctions is slow (5-10MY), so the reduction of abundance automatically caused by extermination of taxa is not made up quickly.
  • and has most often applied to evidence from comparative anatomy
been?
Originally. Cladistics was invented by Hennig in 1966, AFAIK mol phylo only got going in the mid-1970s and then on proteins rather than DNA (e.g. the work of Allan Wilson and co.).
  • timing is the central issue in the Cambrian explosion
Not sure I agree
My understanding of the entire debate is that, if there were clear evidence of rise of metazoa and especially triploblasts over the course of hundreds of millions of years, there would be no debate. And the debate is littered with words like "abrupt" and "rapid".
  • [Stromatolites] peaking
What exactly do you mean by "Peaking"?
The next phrase says "declined in abundance and diversity". And IIRC the refs are not much more precise.
  • stromatolites declined again when the abundance and diversity of marine animals increased in the Ordovician Radiation;
My first reaction to this is that "Correlation does not imply causality"...
True as a matter of general principle, for example they could both be effects of some other cause. But the authors who comment on the decline of stromatolites after 1250MYA and those who comment on the inverse relationship across two mass extinctions interpret the data as causal linkage.
  • Their increasingly spiny forms in the last 1 billion years is probably due to the need for defense against predators,
defence / not sure I believe this
In Wikipedia articles I use American spelling, which seems to be the norm (likewise "...ized", "...or"). The interpretation is from the ref.
  • Trace fossils 1 billion years ago?
Seems misleading to include this considering that even the author of the paper no longer believes this.
Readers may have seen older articles that use this interpretation, so I think it's useful to knock it on the head. I'd like to provide a ref for Seilacher's change of view, do you know of one?
  • Cryogenian glaciations
... so what? What was the impact of these glaciations, were they global, why should they affect anything?
Fair questions, to which I don't yet have answers - except that the Gaskiers glaciation is later than the earliest Doushantuo fossils.
  • possibly an echinoderm, although it lacked a feature present in later echinoderms
Then by definition it was a stem group Echinoderm.
I don't think that follows, e.g. it could have secondarily lost stereom. I don't know of any evidence of its position in a cladogram.
  • like a rather loose, wobbly stack of cones
The ones I've seen are nothing like loose (although maybe C. germsi is?)
Play fair! I wrote "looks like ...", to point out that they're not as tightly nested as e.g. empty icecream cones.
  • (the organisms that looked most different from any of to-day’s animals
This seems like a very loose definition of the group!
If there's a better definition of "classic Ediacaran" that's not too long and technical, please edit it in.
  • Change in carbon isotope ratios at Ediacaran-Cambrian boundary
That's three times it's been mentioned now, in varying degrees of detail... unwarranted repetition?
You're probably right. But IMO pruning the article is one of the final stages, after we're happy with the accuracy, completeness and flow. Philcha (talk) 12:37, 9 January 2008 (UTC)

Verisimilus T 18:01, 6 January 2008 (UTC)

Hi, I've had a few more looks through this article, and I think that while there are a lot of sections I would have taken a different approach to, it does cover all the major areas without incorporating any major errors. I would suggest that the next thing to consider is its flow, and of course its length. There are a few bits of repetition that need to be addressed (e.g. the δ13C spike), and there are places where it reads more like a list of facts and explanations than a coherent article. Further, I've commented out the last bit of the original article - there doesn't seem to be much point in having both sections included. If you've got all the information you need from it, you may want to delete it entirely to accelerate page load times a bit. All the best, Verisimilus T 09:36, 10 February 2008 (UTC)
P.S. I suspect you may have attached too much weight to McNamara's comment on stromatolite decay (doi:10.1126/science.274.5295.1993f). I couldn't find any papers directly discussing his conclusions, but there are other things that can cause changes in stromatolite abundance. I'm not nearly knowledgeable enough to make an intelligent comment on the viability of these, but the article seems to be saying "if Awramik is right, the rise isn't at 700, but 1250Ma".
Awramik's paper* doesn't really do much more than say "Stromatolites start going extinct at about the same time that we have metazoan fossils" (i.e. the Ediacaran biota, according to the 1971 interpretation). It lists the other factors that can affect stromatolite diversity, but doesn't consider whether they could have any effect.
Verisimilus T 09:57, 10 February 2008 (UTC)
Thanks for "it does cover all the major areas without incorporating any major errors" and for the vote of confidence implied by "I've commented out the last bit of the original article". I'll start looking at how to make it shorter and tighter. Philcha (talk) 11:44, 21 February 2008 (UTC)
  • *Stanley M. Awramik (19 November 1971). "Precambrian Columnar Stromatolite Diversity: Reflection of Metazoan Appearance". Science 174: 825--827. doi:10.1126/science.174.4011.825. 

Infobar on Right

The info bar appears corrupted, there seems to be code overtop of it.207.6.125.46 (talk) 22:59, 27 January 2008 (UTC)

{{for loop}} is awaiting updating to work with the new preprocessor. Someone is bound to fix it soon. Verisimilus T 00:12, 28 January 2008 (UTC)

Viral cause

I've just removed a new section suggesting that "Although no direct evidence exists, a new kind of viral or microbial infection process is one possible route that genetic material may have taken in exchanges between early species." As far as I am aware this idea doesn't have any evidence in its favour, nor support from the scientific community; I feel this article should restrict itself to scientifically supported ideas, as there are thousands of intriguing but un-testable ideas out there, and we can't reasonably expect to cover them all. Comments in its defence are welcome!

Verisimilus T 09:20, 10 February 2008 (UTC)

Making the article shorter

After reading through the article, at present I'm willing to remove:

  • Section "Cryogenian glaciations"
  • The reference to the Gaskiers glaciation in section "Doushantuo Formation".
  • The reference to [[Namapoikia] in sections "Ediacaran organisms". But it's also mentioned in section "Doushantuo Formation", and the cited article refers (indirectly) to [[Namapoikia].
  • Section "Change in carbon isotope ratios at Ediacaran-Cambrian boundary" - but some of the content would have to moved into section "Carbon isotope fluctuations" (within "Changes in the environment", within "Possible causes of the explosion").

After that I find it difficult to identify content that can be removed, since: the pre-Cambrian content shows that complex metazoans did not originate in the early Cambrian and their emergence was probably not sudden; the descriptions of early Cambrian animals illustrate the exceptionally high level of disparity.

Of course I'm not best placed to be objective about this because I wrote most of it. Constructive comments and suggestions would be very welcome! Philcha (talk) 12:40, 21 February 2008 (UTC)

Re-arrange to place the sizzle before the steak?

I notice no-one has so far suggested ways of making this long article significantly shorter without sacrificing important content. I've had a crazy idea that might help general readers while keepng the details that armchair paleontologists like me will want to see: place the analysis (starting with "How real was the explosion?" before the potted history of life from 1BYA to 500MYA; if necessary clarify the issue first by defining the 3 main views that have been historically important (that there were no pre-Cambrian metazoans; that coelomates and possibly triploblasts evolved and diversified extremely fast in the early Cambrian, as Gould and Whittington argued; and that triploblasts and probalby coelomates were present a lot earlier, although there was a surge of diparity in the early Cambrian). So the order of the main sections would be: Intro; history of the debate; definitions of terms used commonly in the analysis (e.g. triploblast); the analysis ("How real was the explosion?", etc.); definitions of terms used in surveying the evidence; types of evidence; potted history of life from 1BYA to 500MYA? Would that work as a method of presentation? And would it be intellectually satisfactory? Philcha (talk) 19:54, 14 March 2008 (UTC)

That sounds like a good idea. At the moment, I like the "How real was the explosion" section; I like the "possible causes" section - these are what the focus of the article should be. But I'm still not sold on the necessity of the in-depth discussion of each and every fauna. In my opinion, this article would benefit greatly if the vast "Evidence in rocks" section were reduced to a similar length to the former two. You sum up much of the section very concisely in "How fast did the main metazoan groups evolve?" and I would not be surprised if you could fit in a section of similar length to sum up the non-metazoan activity. At the moment you have presented a very thorough coverage of what the diversity comprises, but it's not really quantified - and you stray somewhat from the section title.
I would consider re-grouping the section by "taphonomic modes", something along the lines of:
  • Trace fossils
    • Stromatolites
    • Trace fossils
  • Body fossils
    • "Palynomorphs" (i.e. organic-walled acritarchs)
    • Phosphatisation - with example of Doushantuo
    • Ediacara-type preservation
    • Burgess-shale type preservation - with one case study explored, and a mention that others include an array of different organisms from different times
    • Small shellies
    • Orsten
    • "Conventional" fossil record.
  • Other evidence
    • Isotopes (δ13C and perhaps strontium?)
Maybe a subsequent section entitled "diversity and disparity" would be interesting, discussing methods of quantifying the two, and detailing how they both changed between the Ediacaran and Ordovician.
If you must, you could have a section entitled "Controversies about fossil interpretation" to cover your 'molluscs, annelids or brachiopods' section - again, this is interesting and fun science, but is of little relevance to the Cambrian explosion. (It's no surprise that there were some stem groups around.)
Much of the other text currently in the section would be a great addition to the individual sub-pages, which are in many cases upsettingly sparse - and I'd encourage you to move much of the content there.
How does that sound? Verisimilus T 11:34, 15 March 2008 (UTC)
Hi, Verisimilus. I understand the attractions of your proposals, as the length of the article worries me a little too. But I think the devil's in the details:
  • There's a guideline somewhere that articles should be as free standing as possible. "How real was the explosion?" relies on a lot of information and concepts presented in the "potted history", for example (just 2 of many): the inverse relationship between stromatolites and metazoans; the idea of “aunts” or “great-aunts” of modern groups (that's the real purpose of the stuff about anomalocarids and halwaxiids; but just saying they were “aunts” or “great-aunts” of modern groups would not help the non-specialist reader without explanation of the relevant features of the animals).
  • Farming out some of the supporting information to other articles would also be tricky, as some supporting articles might not exist yet (so it would be necessary to research enough material about these subjects to make worthwhile articles), and others may have structures that do not easily accommodate the information that would need to be incorporated; in the worst case it might be necessary to re-structure whole packages of articles, and I know how difficult that is (I'm part way through such a project, History of IBM mainframe operating systems, see its Talk page for why; there's been no movement for a few months because I'm waiting for input / responses from others whose knowledge I value). At the back of all this, I don't feel qualified to intervene on a large scale in articles about such early stages in the evolution of life on earth (I'm only an armchair paleontologist, and the pros generally stick to 1 field / time interval, except on theoretical issues).
  • Arranging a lot of the material chronologically makes the most parsimonious assumptions about the reader's prior knowledge. That was my main reason for editing this article: the previous version would have only make sense to at least a long-time armchair paleontologist who already knew most of the potted history and the concepts (to be more precise, it read like an extended abstract).
  • Some readers may have read about the CE elsewhere (? most probably Wonderful Life ?); some of these will expect to see this article deal with the same issues as other publications; some need to be informed that the ideas they found in these publications are not necessarily the final or only answer (John Locke: "... it is ambition enough to be employed as an under-labourer in clearing the ground a little, and removing some of the rubbish that lies in the way to knowledge ...").
I also have a few questions about your comments:
  • You suggest "a section of similar length to sum up the non-metazoan activity". Sorry if I'm being thick, but what non-metazoan activity? The stromatolite and acritarchs are only mentioned as indicators of metazoan activity. I think the article deals with the Ediacaran fauna in the same way (if these don't count as metazoans). That leaves only the plankton (Butterfield), but you're the last person I'd expect would want to see his analysis downplayed :-)
  • You said, " ... very thorough coverage of what the diversity comprises, but it's not really quantified." What srt of quantification did you have in mind? And can it be explained satisfactorily to the non-specialist reader (starting with me!).

Philcha (talk) 19:33, 28 March 2008 (UTC)

An Avalon (Ediacaran) explosion?

I've reinstated this section. The idea is quite new (Jan 2008), and may get its own Wikipedia article in time. Meanwhile mentioning it briefly in this article will: (a) remind editors to check up on whether it gets enough scientific support and additional info to merit its own Wikipedia article; make the CE seem less surprising because there's evidence of another explosion. If the idea of an Avalon explosion survives scientific investigation, eventually someone will use the 2 explosions as the basis of some principle in evolutionary theory, then we can produce an article about that too (unless they are simply taken as examples of Valentine's "empty niches" theory and its consequence that explosions at each grade happen only once). Philcha (talk) 18:45, 28 March 2008 (UTC)

Definitions section

I'm sure sure if this has been discussed previously, but the Definitions section seems a little misguided. The beauty of a hyperlinked encyclopedia is that a reader can instantly go to the article of an unfamiliar word. If the reader is unfamiliar with a word, such as phylum, they can go to the relevant article, fill in any gaps in their knowledge and then return. Ashmoo (talk) 13:26, 7 April 2008 (UTC)

There's a Wikipedia guideline Wikipedia:Summary style that articles should be as self-sufficient as possible; it also makes the point that that diffferent readers want different levels of detail. The summary here aims to present all and only what readers need to know in order to understand the CE and why it's an important subject. The Coelom article is too techical and detailed for this purpose, and Triploblasty is a stub. The intro to the Phylum article is closer to what the CE article needs, but is still a bit detailed and does not make the point that the Linnean classicification has its problems, especially with long-extinct animals. Philcha (talk) 15:54, 8 April 2008 (UTC)

Quantum Evolution

Someone has recently added a section "Quantum Evolution":

Quantum evolution is the theory that the emergence of primitive consciousness accelerated the course of evolution. Major evolutionary transitions are hypothesized to arrise when small populations—isolated and limited from gene flow—would fixate upon unusual gene combinations.

I can see a few things wrong with this:

  • The article Quantum evolution explains that the idea is based on based mainly on genetic drift, and does not mention "the emergence of primitive consciousness".
  • Quantum evolution's originator, George Gaylord Simpson considered the idea "controversial and hypothetical".
  • The section is not supported by any citations that provide evidence or arguments that Quantum Evolution is relevant to the Cambrian Explosion.

I will remove the section in about a week if those objections are not met. Philcha (talk) 18:07, 9 May 2008 (UTC)

I was thinking of removing it myself. I'd take it out for now, and insist on a reliable source being provided before it was reinserted (at a minimum). Smith609 Talk 20:16, 9 May 2008 (UTC)
I suspect that the user intended to refer to Quantum evolution (alternative), which indeed seems to include a bit of the mysticism mentioned. Anyhow, (s)he has not responded to the note fairly put on the user talk page; and I think a bit more argument will be needed to convince at least me that this indeed is a theory held by some scientists within the field. Hence, I fully support the removal. JoergenB (talk) 20:09, 28 May 2008 (UTC)

Removed "Conventional record" and "Exceptional preservation"

I've removed these sections plus the stump of some old content about molecular phylogeny; this material had caused confusion because it was commented out. To make matters worse, the stupid doibot inserts HTML comments but is not smart enough to leave commented-out content alone, so it creates nested HTML comments and therefore a real dog's doo-doo. The last version containing the whole of the commented out material is [26] in case anyone wants to mine the old stuff for additional info. Philcha (talk) 12:08, 11 May 2008 (UTC)

Increase in abundance and spininess of acritarchs

A 2nd para has recently been added to "Increase in abundance and spininess of acritarchs":

Further evidence that predation, or at least herbivory, on plankton first appeared around this time comes from a consideration of taxon longevity. The abundance of planktonic organimsms that evolved between 1,700 and 1,400 million years ago were limited by nutrient availability - a situation which limits the origination of new species because the existing organisms are so specialised to their niches, and no other niches are available for occupation. Around about 1,000 million years ago, species longevity fell sharply, suggesting that predation pressure by protist (and possibly metazoan?) herbivores became an important factor. Predation would have kept populations in check, meaning that some nutrients were left unused, and new niches were available for new species to occupy.[2]

The way I read the cited paper (Stanley (2008). "Predation defeats competition on the seafloor" (extract). Paleobiology 34: 1. doi:10.1666/07026.1. ), it attributes the drop in species durations for Proterozoic Phytoplankton to "protistan herbivory", and the next sentence, "Not surprisingly, the average duration of acritarch species declined further when metazoans diversified, being less than 10 Myr for species that arose in the Early Cambrian," does not commit itself about when metazoan herbivory / predation became significant. Philcha (talk) 19:20, 13 May 2008 (UTC)

True, I think it was useful to raise the point that metazoans were not necessarily the sole perpetrators, as was implied by my reading of the previous paragraph; it also emphasises the important role of predation in evolutionary diversification. I've struck the reference to metazoa entirely. Smith609 Talk 07:00, 14 May 2008 (UTC)
The way it reads now looks to me as if it's saying that there was a significant non-metazoan pressure on acritarch populations. I think that there are 2 ways of looking at it in the context of the Cambrian explosion:
  1. It could be taken as weakening the suggestion that metazoans created a new selective pressure in acritarchs and other plankton beginning 1 BYA.
  2. Alternatively one could say it's irrelevant to the argument in the 1st para of the section, because protists' method of attack is chemical (engulfing and ingesting), while increased size and spininess would have defended against mechanical attack (piercing, ripping). But that's open to the objection that increase and size and spininess would have defended against envelopment. The question then would be whether acritarchs' size and spininess increased beyond what was necessary to defend against envelopment by protists; if they did, presumably metazoans provided the rest of the selection pressure. This line of argument is ridiculously WP:OR, as Stanley nowhere mentions the size and spininess of acritarchs, nor predators' methods of attack.
That leaves interpretation 1 as a competing explanation of changes in acritarchs after 1BYA, i.e. it weakens the inference of metazoan predation. Is that what you intended? If so , I don't think it would be WP:OR to say it has also been noted that acritarch species longevity declined after 1 BYA and this has been interpreted as the result of predation by protists, i.e. relatively large single-celled organisms. Philcha (talk) 09:37, 14 May 2008 (UTC)


I thought I'd better read the reference supporting paragraph one, and thought I'd "return fire"... I think this article is being over-interpreted too, without mention of its caveats.

“ Their increasingly spiny forms in the last 1 billion years probably result from the need for defense against predators, especially predators large enough to swallow them or tear them apart. ”

—Current article - italics don't appear to have support from article
“ Processes may be of different kinds and of different functional significance (for example, they may also be selected for as a means to increase water friction or adhesiveness).

The presence of process-bearing acritarchs is therefore not a definite indication of the presence of predators/grazers.

 ”

—Bengtson (emphasis mine)

This article probably isn't the place for a full-blown discussion, but I think that the presentation of facts as it stands is a little skewed. I would be tempted to move it to a new section at Acritarch, and to take this opportunity to slim the C-ex article slightly.

How about combining the stromatolite and acritarch sections into a new, brief "Proterozoic microorganisms" subsection, and using a sentence along the lines of:

“ Marine organisms (acritarchs and stromatolites) show signs of increased predation pressure around 1,000 million years ago, which may reflect predation by protistan microorganisms. They suffer a sharp decline in abundance at the start of the Cambrian period, which has been linked to the raditaion of metazoan grazers. ”

—Just a rough idea

This tells the casual reader everything they need to know in a concise form (concision being my main concern for this article at the moment, as you know!); the current, more detailed discussion can then be moved to Acritarch and Stromatolite for the interested - a "further information" link could be provided to emphasise this point. Moving the discussion to the acritarch page will allow us to expand the discussion of the papers a little more in the article, to give the readers a fairer reflection of what is and isn't being asserted, without scaring off the casual reader of this article. Smith609 Talk 10:16, 14 May 2008 (UTC)

Thanks for making me read Bengston's paper again, he has a great knack of having fun while presenting serious palaeo analysis - my favourite, out of many contenders, is "... the Cambrian explosion was not just a calcareous dress-up party ..."; second place may go to his diagram of stromatolite and acritrach diversity, which has snowflakes to mark glacial periods and a dagger to mark an apparent acritarch mass extinction. Points about stromatolites that are relevant to the CE:
  • "Stromatolite diversity" is mainly morphological and is not the same as taxonomic diversity. But it does seem to be a good measure of their abundance, and it's necessary to use it that was as it would be very difficult to produce a true abundance measure (kg of stromatolite fossils per tonne of sediment deposited in shallow seas).
  • Modern miocrobial mats are not disturbed by burrowing animals less than a few mm in size. If these had been around from 1 BYA, one would expect to find trace fossils, but there are no convincing finds. (A little bit of WP:OR - perhaps the arms race has made modern mats tougher).
  • Conclusion re stromatolites: "the grazing hypothesis may be incomplete, but it seems to explain more of the demise and the present distribution pattern than do alternative or complementary hypotheses, such as geochemical trends, competition from eukaryotes, or taxonomic artifacts. At present, however, the pattern of stromatolite decline can only be taken as suggestive of widespread and abundant grazing organisms."
Now for acritarchs:
  • A few weakly spiny forms before 2 BYA; but process-bearing forms very rare before 1 BYA. Diversities rose after 1 BYA and peaked just before the Neoproterozoic glaciations 700-600 MYA.
  • The inverse correlation between stromatolite and protist abundance after 1 BYA does not indicate an antasgonistic relationship, as protists are common in modern microbial mats and help to stabilise the sediment.
  • Other Neoproterozoic show signs of anti-predator defences, e.g. melanocyrillids. What are these and from when? Googles was no help.
  • "The evolution of diverse and complex acritarchs during the Neoproterozoic suggests activities by planktic and/or benthic predators." A few lines later: "Secondly, the spiny processes are not unquestionably antipredatory aptations. Thirdly, the Proterozoic predators need not be animals — they could be protists having no direct phylogenetic connection with the Metazoa." I'm not sure about B's 3rd point - size and spines would be good defences against engulfing but not against chemical attack; they would be better defences against mechanical attack (piercing, ripping, grinding) - by (later in the paper, discussing chancelloriids) predators larger than the gaps between spines.
Finally: "The combined indications from the decline of stromatolites and the diversification of acritarchs suggest that it may have begun around 1 Ga."

My thoughts on how this affects Cambrian explosion:

  • I understand your desire to make Cambrian explosion more concise - I'm still puzzling over how to do this without leaving non-specialist readers bewildered. I'm reluctant to eliminate chunks of the story of Proterozoic animal evolution, because I'd hate to see the article start becoming as abstract as Graham Budd's version was in mid 2007. As I've said before (ad nauseam), IMO Wikipedia's main audience is non-specialists, and I think we have to assume zero relevant prior knowledge. The Cambrian explosion is a much more difficult subject than e.g. any dinosaur article, because a dinosaur article can rely on most people's familiarity with dinosaurs' nearest living relatives, birds and crocs. For the CE we have to explain basic invertebrate zoology and ecology, trace fossils, changes in the environment, taphonomic factors, geochemistry, etc., etc. I've been an armchair paleontologist for decades, but all I knew of Precambrian life before starting work on Cambrian explosion was: Gould's account of Ediacaran organisms; Gould's account of the Burgess Shale's "wierd wonders" and Conway Morris' opinion that Opabinia and anomalocarids were "aunts" of arthropods; Pikaia may have been a cephalochordate; the "superphyla" Ecdysozoa and Lophotrochozoa; protostome / deuterostome and schizocoel / enterocoel; Williamson's heretical but rather plausible ideas in Larvae and Evolution; Snowball Earth; stromatolites; Margulis' symbiotic hypothesis of the origin of eucaryotes; a hazy memory of pop science articles about Shopf's research on the earliest life. That's a tiny fraction of what I learned while researching for Cambrian explosion, but probably exceeds by as many orders of magnitude the average reader's prior knowledge.
  • No change to the section on stroatolites, but add Bengston to refs following "The most widely-supported explanation is that stromatolite-building organisms were the victims of grazing animals, which would imply that sufficiently complex animals were common over 1 billion years ago".
  • Tone down the acritarchs section to "Their increasingly spiny forms in the last 1 billion years may have resulted from the need for defense against predators." Philcha (talk) 14:05, 14 May 2008 (UTC)

Making the article shorter

For the reasons described in a previous post (Thread "Increase in abundance and spininess of acritarchs", post by Philcha at 14:05, 14 May 2008 (UTC)) I'm very reluctant to drop significant chunks of the potted history of life in the last billion years. But in re-reading the article I think there parts than can be dropped or shortened without serious harm:

  • Reduce last sentence of "Body fossils" to "Part of this is due to changes in the chemistry of the oceans, which were partly caused by the on-going evolution of life, and these changes were most significant before the start of the Cambrian – for example any increase in the marine biomass would reduce the concentration of carbon, and the appearance of sponges reduced the concentration of silicon.[17]
  • In "Body fossils", shorten the first 2 sentence so of the para about lagerstätten to "The Cambrian fossil record includes an unusually high number of lagerstätten which preserved the fossils' soft tissues in extremely fine detail, allowing a very informative study of animals that normally would not have left fossils. The fine detail of the deposits This has allowed paleontologists to examine the internal workings of animals which in other sediments are only represented by shells, spines, claws, etc.
  • In section "Increase in abundance and spininess of acritarchs", add a brief explanation of "Snowbal Earth".
  • Delete the section "Cryogenian glaciations".
  • In section "Doushantuo Formation" delete the para about Gaskiers glaciation; and delete from the final sentence "such as Namapoikia (see below)", so we can delete mention of 'Namapoikia from section "Ediacaran organisms".
  • In section "Ediacaran organisms" delete mention of 'Namapoikia.
  • Possibly move the stuff about 3 assemblages from section "Ediacaran organisms" to section "Ediacaran and Early Cambrian diversification of trace fossils", which is the only place where it's used.
  • Delete the para about Namapoikia from section "Ediacaran organisms".
  • Delete from section "Ediacaran organisms" the para about extinction of Ediacaran biota.
  • In section "Sirius Passet fauna", delete the short para about Pauloterminus - it's not used elsewhere in the article and the "bivalve" carapace was a fairly common feature of early Cambrina arthropods but implies nothing about a relationship with molluscs.
  • In section "Sirius Passet fauna", reduce the para about Pambdelurion and Kerygmachela to:
Reconstruction of Kerygmachela from Sirius Passet, viewed from the top, with the head to the right. The shaded areas on the lobes (flaps on the sides) are thought to have functioned as gills.
Reconstruction of Kerygmachela from Sirius Passet, viewed from the top, with the head to the right. The shaded areas on the lobes (flaps on the sides) are thought to have functioned as gills.
"The strangest-looking animals from Sirius Passet are Pambdelurion and Kerygmachela. They are generally regarded as anomalocarids because they have long, soft, segmented bodies with a pair of broad fin-like flaps on most segments and a pair of segmented appendages at the rear. The outer parts of the top surfaces of the flaps have grooved areas which are thought to have acted as gills. Under each flap there is a short, fleshy leg. This arrangement suggests the animals are related to biramous arthropods. Both were apparently blind, as the fossils show no trace of eyes. Kerygmachela had a small conical mouth flanked by robust, unsegmented appendages which had short spines on the front edge and were tipped with longer spines. The spiny front limbs suggest that it may have been a predator, but its small mouth suggests it would have been restricted to very small prey. Pambdelurion lacked trailing appendages but had a more typically anomalocarid-style mouth, a relatively large ring of crushing plates under the front of its head. Its mouth was flanked by a pair of thick, segmented appendages slightly longer than the swimming flaps and equipped with a flexible spine on each segment.[69]"
  • In section "Chengjiang fauna", delete the para about Vetulicola.

Comments welcome! Philcha (talk) 18:08, 14 May 2008 (UTC)

You can probably guess that I thoroughly approve!
In other news, I remember some time ago searching the literature for a written definition of the Cambrian explosion. I recently came across:
“ the Early Cambrian adaptive radiation of several eukaryotic clades, including the Metazoa, documented by the truly rapid expansion in the fossil record of characteristics such as large size, morphological complexity, skeletalization, and infaunal activity. ”

—Source
It's not immediately obvious how best to include it in the article but thought I'd throw it out there for your consideration. Smith609 Talk 19:31, 19 May 2008 (UTC)
I've done the edits in the list above (with regret, as we've had some fun discussing the topics that have been sacrificed). I'm now really struggling to see how to make the article any shorter without assuming too much prior knowledge in readers. Suggestions welcome!
Re the quote "the Early Cambrian adaptive radiation of several eukaryotic clades, including the Metazoa, documented by the truly rapid expansion in the fossil record of characteristics such as large size, morphological complexity, skeletalization, and infaunal activity" that Smith609 posted:
  • I think the intro already says that, but in simpler language.
  • Although it's accurate, it might mislead non-specialist readers into thinking it says that complex metazoa started in the Early Cambrian - it had me confused for a second, until I interpreted the word "expansion" really precisely (implies they were there before, but there was an inflation in the Early Cambrian).
A couple of things bug me:
  • Smith609's 2nd para in section "Increase in size and spininess of acritarchs", beginning "Further evidence that predation, or at least herbivory, on plankton first appeared around this time comes from a consideration of taxon longevity". I'm not sure what it's supposed to tell the non-specialist reader, in fact I'm not sure myself what it implies. Stanley's paper discusses only planktonic acritarchs, while Bengston's discusses both planktonic and benthic acritarchs. Does the para mean, "OK, there may have been predation, but protists are as likely as metazoanas to have been the culprits"?
It's supporting the assertion that herbivory existed. I would object to the article implying that this meant the predators were metazoans, which I don't think Stanley even mentions. If I recall correctly, I inserted the rider partly to counter this "metazoan predators" allegation made in the article at the time. In fairness, another major reason for putting it there was that it was Really Quite Interesting. Maybe it doesn't fit into the vision of an "accessible to all" article. See below for thoughts on how to overcome this (I suspect a new section may be necessary...) Smith609 Talk 17:52, 20 May 2008 (UTC)
  • The only distinctive thing that section "Chengjiang fauna" brings to the party is the first genuine chordates. Otherwise all the same and more is in "Burgess Shale" (Second verse, same as the first, But a whole lot louder and a whole lot worse!). From a purely logical point of view I'm tempted to merge the accounts of the Chengjiang and Burgess fauna, but it just doesn't feel right. I'm also tempted to drop the last para of "Chengjiang fauna" because that just lists "normal-looking" fossils that contribute little or nothing to the debate. Can anyone help me make up my mind(s)? Philcha (talk) 22:45, 19 May 2008 (UTC)
I was tempted to combine the two into "Burgess-shale type lagerstatte" or similar. Sounds like a good plan! Smith609 Talk 17:52, 20 May 2008 (UTC)