Tuesday, November 4, 2008

An introduction to the Ediacaran fauna

The Ediacaran fauna (pronounced edi-ak-ran) is a Precambrian (Neoproterozoic) assemblage, which existed from about 600 million years ago to approx 545 million years ago.

The fauna has now been found on all continents except Antarctica. However, the most important sites are: Namibia; Newfoundland & MacKenzie Mountains’ Canada, the White Sea Coast Russia; and the Flinders Ranges, South Australia.

One of the best localities and the place where the significance of the fauna was first recognised is here in South Australia. The name Ediacara comes from the site where the fossils were first recognised as Precambrian. Fossils were found in Namibia about 25 years earlier, but due to a mistake over the age and the fact that the finds were published in foreign language journals (German) their significance was not realized at the time.

Whilst the fauna has a world-wide distribution, it is important to note that there are significant differences in the make-up of the fauna at different localities. This is due, in the main, to environmental conditions. The Australian and Russian forms are similar and the rocks are indicative of a shallow water ecosystem. The Newfoundland forms contain decidedly different elements and the rocks are indicative of a deep water setting.

The fossils occur as depressions up into or extensions down from the bottom of thin quartzite beds. The fossils were formed by the covering of the muddy, biofilm covered, shallow sea floor, and the organisms on it, by mantling, thin sand bodies. Those organisms which where able to support the sand created depressions up into the overlying sand body. Those organisms which were either lying in or were forced into the mud by the sand, allowed sand to fill in the void left as they decayed (to produce downward extensions on the bottom of the sand body).

The majority of fossils are of rounded forms, reminiscent of jellyfish and in fact these were classified as jellyfish for a long time eg. Cyclomedusa (above), Mawsonites (left). (up to 35 cm across) Other forms included occasional 'sea-pen-like organisms (Pennatulacean colonial octocorals) which appear very similar to forms extant today (up to half a metre tall). A couple of possible annelids such as the large sheet-like form Dickinsonia (top) which looks like a flat pancake with segmentation, a gut and a definate head end (up to 75 cm long); and Spriggina which looks like a cross between a bristle worm and a trilobite (5 cm). A possible arthropod is also present, Parvancorina a recent specimen of which shows gills and possibly legs (3 cm).

A unique form is Tribrachidium (right), which as its name suggests is based on a tripartite body plan, but may well be some form of lophophore (similar to brachiopods and bryozoa)(3 cm).

It was thought that the fauna was dominated by the motile, free-swimming medusoids, which created a problem of preservation since medusoids do not, as a general rule spend a lot of time on the sea bed in the adult form. However it has shown that, whilst medusoid forms are represented, the vast majority of rounded forms are the anchors of sea-pens.

Modern sea-pens have a round, bulbous structure near the base of the organism which is highly muscled (seen in the middle of the disc on Charnodisucs at left). The organism uses this 'organ' to burrow into the soft muddy sediment and then as an anchor to hold the organism in place. During burial by the mantling sands, the stem of the 'sea-pen’ breaks, and the body of the 'sea-pen' collapses. Since the 'sea-pen is held up by hydrostatic pressure, the rip deflates the 'blade', the 'blade' becomes mixed with the sand, thus diminishing its preservation potential. The bulb, on the other hand, is already buried. In life the bulb is filled with water, so when the stem breaks away, the bulb fills with sand.

Since the underlying mud is approx. 80% water, as it dries out the thickness of the bed diminished to only a few cms, resulting in a flattened, rounded outline to the fossil. The various classifications on the 'medusoids' was due to surface ornamentation (ribs, concentric circles etc.), these are now thought to be the manifestation of muscle bands due to different degrees of decay before final 'molding'.

Thus the fauna has a decidedly benthonic bias, rather than being made up of free swiming forms as previously thought.

There are two main theories as to the affinities of the Ediacara fauna. One, originally put forward by Martin Glaessner is that most of the forms are related to modern forms, if not direct precursers. The other, proposed by Dolf Seilacher is that the Ediacaran fauna represents a unique bodyplan which arose early in metazoan evolution and became extinct before the Cambrian and thus all the forms within the fauna are members of a now extinct, separate phylum - the Vendozoa, with no connection to modern forms- or even Cambrian forms.

However, close examination of the fossils shows that many of the forms do indeed have a striking resemblance to Cambrian if not modern forms. Finds of 'sea-pen'-like organisms in the Burgess Shale (Thaumaptilon, which are very similar to Ediacaran forms appears to extend the range of such forms well into the Cambrian. Also, the form Kimberella can be probably placed within the Mollusca on structural and trace fossil grounds.

Thus several groups within the Ediacaran fauna exist today, and so the whole fauna did not become extinct. This is not to say that there are not some unique forms, there are, but the idea that they are all unique is overstepping things. My own opinion is that several groups of extant organisma can be traced back to the Ediacaran fauna. However, the origin of the metazoans is another matter. The Ediacaran fauna appears as a fully intergrated ecosystem with some quite advanced forms (eg. the colonial octocoral 'sea-pens'), so the question of origins has to be pushed back even farther, maybe as a consequence the late Proterozoic glaciation that ended approx 650 million years. Body fossil evidence will probably never be found, since they occur in meiofauna - too small to leave anything but chemical traces.

The various elements of the Ediacara fauna are united by one common character, none have any hard parts. There is no evidence of mineralisation in any fossil so far found. Thus the preservation of essentially 'soft bodied' organisms presented something of a quandary, especially as they are preserved in what is now quartzite.

It was thought that fossilization was due to a unique sedimentological facies, namely the ripple-topped sands mantling muds and that the fossils were constrained by the occurrence of this facies. However, the Ediacara Member in the Flinders Ranges contains 5 separate facies, ranging from thinly laminated silts to high energy, coarse sandstones, each of these facies if fossiliferous to one degree or another. Thus fossilization is not facies controlled, but occurs due to the interplay of a number of factors.

Amongst the factors which allowed the preservation of the Ediacaran fauna are (in no particular order): collagen, the lack of bioturbation , and the lack of predation.

The ability to produce collagen is important because collagen is relatively inert, strong and flexible. A collagen outer layer helped hold the organism together. It also allowed the organism to retain its shape when covered by the mantling sand to produce the fossils. Also, since collagen was a relatively new compound, the micro-community took a while to realize that collagen was a food source.

The reliance on passive adsorption of oxygen over the whole body meant that tissues had to be close to the surface in order to obtain oxygen by simple diffusion. This meant thin bodies. There is very little constraint to the size such organisms could reach, provided they stayed thin, hence half metre long 'flat' worms. However, flat, thin bodies are very bad at burrowing, which meant that vertical burrowing was virtually unknown during this period.

This meant that any organism that was buried was not disturbed, disrupted and ultimately destroyed by bioturbation (as it common today).

Since there were no hard parts about, predation was well nigh impossible, except possibly by disgorging some sort of dissolving fluid and sucking up the resultant gastronomic soup. But, definitely no chewing! Therefore, once the organism shuffled off to join the ranks of the choir immortal, it's mortal remains did just that - remained. They hung around on the surface, undisturbed for a considerable period of time, waiting for the mantling blanket of sand.

As was mentioned before, collagen was probably still a relatively novel compound at this time, so it was resistant to decay.

However, these conditions did not last. An interrelated series of events which included, increasing oxygen levels, the acquisition of mineralization capabilities, the rise of predation and the ability to produce a round cross-sectional body plan (by confining oxygen scavenging to certain parts of the body and using fluids to carry oxygen through the body) conducive to burrowing, soon demolished what was a pristine preservational environment. The so-called extinction of the Ediacaran fauna is IMHO largely illusionary for several reasons:
  • Of 7 cnidarian groups represented in the Ediacaran fauna, 4 appear to be ancestral to living taxa.
  • There is no close time control in respect of the supposed episode of extinction.
  • The disappearance of the fauna is largely due to the closure of a taphonomic or preservational 'window'.
  • The uppermost facies of this period throughout the world indicate a shallowing upward cycle, resulting in environments likely not conducive to preservation.


12 comments:

  1. Very nice post, thanks. How lucky for Canadians to have the Burgess Shale (as the archetype of Cambrian fossils) and the Newfoundland & MacKenzie Mountains (as one archetype of Ediacaran fossils)!

    I trust you're read Knoll's 'Life on a Young Planet"? It's a great book.

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  2. @ Chris

    Very good article. I find this period of time fascinating! Interesting ideas about the preservation.

    @changcho

    I hadn't read Knoll's book but checked it out and ordered it and a couple of others in that area. Thanks!

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  3. Ripper post Chris. I'll be watching for more Ediacaran episodes.

    PS. Have you read "Rockstar" the biography of Raymond Sprigg, yet (it spends far too little time on the Ediacaran fossils, sadly).

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  4. Very nice. I really only got into this stuff recently, I knew the general info, but not the more detailed stuff. Anyone have any good links for the hypothesis that changes in ocean chemistry played the major role in the initial development of hard parts?

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  5. Thanks for the great post. Very informative, partly because it's so candid. I've been reading various sources and there's still a few things I don't understand. Perhaps they could be addressed in a post-intro post. First, what's with the gigantism? Can this be a simple mutation in the DNA? I read another article of yours about absorbing oxygen through the "skin"... which seems a good way to produce large, flat bodies but I still wonder... is it
    that when the various biochem pathways are determined then change can happen suddenly? Are we talking about going from microscopic things to Ediacarans in less than a million years (which, of course would be many, many generations)?
    Also, the ability to be stationary and resist motion (e.g. ocean currents), to have holdfasts, etc., seems quite complex to me... are there scenarios for how this could come about? Thanks! (my background is astronomy,so I'm still coming up to speed here...)
    Daniel Hudon, Boston University

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  6. Daniel,
    I'll do a piece on giantism shortly

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  7. Facts are really clear and informative.. thank you very much for posting it.. it really helped me a lot in teaching my subject Earth Science..My students were very interested and amazed on this regard..

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  8. Regarding what you said here: "Body fossil evidence will probably never be found, since they occur in meiofauna - too small to leave anything but chemical traces.

    The various elements of the Ediacara fauna are united by one common character, none have any hard parts. There is no evidence of mineralisation in any fossil so far found. Thus the preservation of essentially 'soft bodied' organisms presented something of a quandary, especially as they are preserved in what is now quartzite.", I do have something to say.

    Please see the following Nature Precedings web page. http://precedings.nature.com/documents/4423/version/1

    Macro Ediacaran body fossils were found by me in central YunNan province of China. The taphonomy of these fossils is totally different from all other known places. I nick-named it as "salad bowl" where big chunks/fragments of multiple organisms were buried together, rather than the cast/imprint fossils found before. Initial chemical and optical works were done. Under electronic microscope, cellular structures were found.

    As I commented in one of your other post, I'm looking for international scholars to join me in this very interesting project.

    So, please join me.

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  9. I visited Mistaken Point to see the Newfoundland Ediacaran fossils last summer [trip report - http://trackinglifethroughtime.wordpress.com/] - the most enchanting place I have ever been. I just wish I had read this informative post first. And many thanks to Timothy Huang for his comment, leading to his article on the Yunnan discoveries. Here I am eyeballing Ediacaran fossils with a point and shoot while he is using an electron microscope! But both techniques have their strengths. I'd love to hear about the latest from Yunnan...

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  10. "Thus several groups within the Ediacaran fauna exist today, and so the whole fauna did not become extinct. This is not to say that there are not some unique forms, there are, but the idea that they are all unique is overstepping things."

    - I don't think that's the case. You make it sound as if it's a few scattered oddballs, and mostly things that are representative of the modern fauna. But in fact, the vast majority of Ediacaran taxa have not been successfully linked to modern taxa. Kimberella is really the odd one out, and one of the rarest and least representative members of the fauna. The dominant forms are all "oddballs".

    There are also no sea pens; sea pens have gaps between their branches, and are considered very derived cnidarians. Their earliest fossil record (leaving Thaumaptilon aside) is the Devonian. There are no other cnidarians in the Ediacaran fauna (and certainly not earlier) - the best candidate is the very late Ediacaran Namapoikia - a possible coral. I have issues with the connection between Thaumaptilon and sea-pens, and also with the connection between Thaumaptilon and Charnia.

    But certainly, the Ediacaran fronds don't bear any resemblance to sea-pens beyond the superficial outline - no sign of gaps between the branches, no sign of the subdivision of branches that is seen in sea-pens. Indeed, Ediacaran fronds - rangeomorphs, charniids, and Pteridinium - each have their own unique anatomies; it wouldn't even be correct to throw them into the same group as each other, let alone sea-pens.

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