Mopping up some Ediacaran Enigmatics

The most common form of preservation of Ediacaran fossils in the Flinders Ranges of South Australia is by sands covering objects on the sea floor and masking them. Much of the sea floor was covered with algal and bacterial mats, or films, that add an extra dimension to the structures preserved by the covering sands. We have to take this extra dimension into account if we are to successfully interpret what it is that has been preserved. Some organisms lived on the mats, some below, and some had elements below and elements above. Understanding the interactions between organism, mat and sand will allow us to better understand what was going on at the time and help separate out body fossils from trace fossils from non-biogenic traces.

A good example of this are "mops".

"Mops" are a series of abundant, distinct structures which look like the head of a mop - hence the name. "Mops" are always preserved in hyporelief, that is on the underside of the rock, unusually though, they can be either positive - standing up from the surface like a pimple, or negative - a depression in the surface like a dimple. Most Ediacaran fossils are usually either one or the other.

Tarhen et al. studied a series of "mops" which show a highly varied shape and structure, but found some consistent features.

Mop structures in hyporelief from the Ediacaran of South Australia.
Scale bar = 2cm. (Tarhan et al. 2010)

(1) "Mops" are always orientated within 10 degrees of the palaeocurrent as derived from a number lines of evidence.

(2) They have a distinct margin at one end - considered to be the distal or far end. This can be straight (arrowed in (A) above), curved (in (B)) or lumpy (in (C)).

(3) They have a series of roughly parallel lines running at around 90 degrees to the margin (arrowed in (B) above). These appear filament-like and can be either linear or wavy and tangled

These features can vary even within the same bed and between specimens in close proximity.

They do tend to appear in close association with the disc form Aspidella, which it though to represent the holdfast of a Charniodiscus-like organism or sea pen. But are they fossils or non-biogenic marks?

Well they appear biogenic. The association with Aspidella is too strong to be random. But are they body or trace fossil? Well, they are a little of both.

The association with Aspidella appears important. Aspidella is the holdfast of a sea pen-like organism, which had a large bulbous holdfast buried in the sediment, with a stalk and frond rising up into the water column. So the organism has some elements (the holdfast) below the bacterial mat, and some (the frond) above the mat.

In this configuration, the frond will be subject to water currents. What is though to have happened is that strong currents carrying the sand that will eventually overlay the organisms has hit the frond and basically dragged the whole organism including the holdfast, in the direction of the palaeocurrent. As the holdfast is under the mats, this dragging has uprooted the holdfast and dragged it through the microbially bound mat layer. The parallel lines represent torn-up bits of mat which were attached to the top of the holdfast. Lumpy margins of the 'mops' are probably caused by small lumps of sediment trapped next to the holdfast.

Formation of "mops". A: Normal conditions. B: Current drag. C: Current induced structures.
D: Sand deposition. E: Preservation. (Tarhan et al. 2010)

This explanation can explain a number of structures seen in Ediacaran rocks. One in particular is Pseudorhizostomites.

Pseudorhizostomites. Scale bar = 2 cm. (Tarhan et al. 2010).

Pseudorhizostomites has been interpreted as a rhizostomean medusa (a jellyfish without tentacles), and even a gas escape structure formed during the decay of an organism. But an interesting interpretation based on the 'mop' formation, is that they are Aspidella-like holdfasts that have been torn free by current action, but vertically rather than in a more horizontal direction. So they have been plucked out of the sediment rather than being dragged along.

Another structure that appears related is one associated with actual holdfasts.

Here the holdfast has remained intact and in place, but the stem has been flattened and the surface of the holdfast has been distorted by the sheer forces caused by the current pushing the frond and stem over.

Here is an annotated example that has been featured here before. The wrinkles (W) and the impression of the stem (S) are clearly visible and have been caused by the stem and frond being pushed over by a strong current which eventually deposited the overlying sand bed

The explanation of the "mop" structures is a neat use of the interplay between sedimentology and taphonomy, that ties together a number of structures.

Tarhan, L.G., Droser, M.L. and Gelhing, J.G (2010) Taphonomic Controls on Ediacaran Diversity: Uncovering the Holdfast Origin of Morphologically Variable Enigmatic Structures. Palaios V. 25, pp 823-830. DOI:10.2110/palo.2010.p10-074r

To Epirelief or Hyporelief, That is the Question . . .

My last post on the possible 770 million year old Ediacaran fossils from Kazakhstan prompted some comments regarding epi- and hyporelief (go read the post and comments).

Basically, Ediacaran fossils are usually found on the underside of the sandstone bed in either positive relief (sticking out from the surface) or negative (sunk into the surface) hyporelief (hypo- is sciencey for 'on the underside'). This is dictated by the mode of preservation (again see the original post), and means that we do not find them on the top surface of the sandstone bed, or in epirelief. (Actually we do find them in epirelief, but only at Mistaken Point in Newfoundland where they have been mantled by volcanic ash - but that appears to be a one off and so does not represent normal preservation).

As an example here is a slab I collected from the Flinders Ranges in South Australia. First the upper (epi) surface.


As you can see, its rippled - strongly rippled actually - which puts pay to the ideas that the Ediacaran faunas were deep water, but that's another story. However, what the top surface isn't, is fossiliferous. There are no fossils on it.

Now lets take a look at what's underneath.


Ta Da! This was taken in situ, about 5 minutes after I'd found it. The photo is a bit blurry 'cos my hands were stili shaking!

Notice the smooth lower surface. This is because it is mantling a mud which does not form ripples. Also notice that there are three large discs in positive hyporelief and a Dickinsonia in negative hyporelief. What do you mean no!?. Ok, here's an annotated version.


The question has been asked why the counterpart of a positive hyporelief isn't a negative epirelief on the bed below. In other words if the fossil is formed by sand flowing into a depression in the underlying mud to form a positive hyporelief, should there also be a corresponding negative epirelief on the top of the underlying bed?

The answer is yes, but we almost never find them. The reason is that the underlying bed is made of mud and so compresses during diagenesis which disrupts the fossil. However, the main reason is that the mudstone weathers much more readily than the sandstone and forms a very friable, crumbly rock that simply weathers away.

We do have some examples of underlying epirelief fossils, but they are very rare. To find them you have to excavate the sandstone with the underlying mudstone still in place. Even then, they are very fragile. Normally this never happens in nature as the mudstone erodes away. The fossiliferous sandstone then weathers out and falls downslope to be found by palaeontologists.

Below is a typical Ediacaran location. The fossils are eroding off the top of the hill and can just be seen in outcrop. but the fossils are found on the scree slope, as float - that is, material that has eroded out of the outcrop and has slid downslope


So the answer is, yes there are there epirelief fossils but we almost never find them as they have weathered away. But they would be in mudstones and not in sandstones.

2.1 Ga Multicellular Colonial Organisms - Update 2

In posts here and here I discussed why I thought the 2.1 Ga structures from Gabon, figured in the Nature paper, were actually microbial mats and not examples of multicellular colonial organisms.

I now have some further info which strengthens my view. I'd like to thank Dr Diana Cuadrado of the Instituto Argentino de Oceanografía who very kindly sent me some more images of microbial mats.

This mainly revolves around the claim in the paper that microbial mats . .

. . often leave characteristic in carbonate ad siliciclatic rocks. Such structures, however, including those formed in shales and mudstones, do not resemble the Gabon fossils". (p. 103)

As I've discussed previously, I disagree.

At the top of this post is an example of a modern microbial mat (top image), compared with a photo of the 2.1 Ga structures (bottom image). The black box in the top image represents the area of the lower image at the same scale (The bar scale is 10 cms in the top image and 1 cm in the bottom image). This reinforces just how small these 2.1 Ga structures are.

Here then, is more evidence that microbial mats can produce the structures seen in the 2.1 Ga specimens.


Here is a close up of a large gas bubble in a modern microbial mat (it was taken last week - you can't get much more modern than that!). As you can see by the comparison with one of the 2.1 Ga specimens at the same scale, gas bubbles in microbial mats are on the same scale as the 2.1 Ga specimens.

For me this is the clincher (if a clincher were needed)


The top photo is of a modern, ruptured gas bubble, the bottom is of a 2.1 Ga specimen. See how the modern microbial mat is flexible enough to fold and stay intact even when torn. But, see the folding to the top and right of the hole. It's almost an exact match for one of the 2.1 Ga specimens figured, to scale, below. No that's not quite right . . . it's an exact match!

Not only that but there is a small fringe beyond where the folds end in the modern example, similar to that seen in the 2.1 Ga specimen.

To say something like "Ta Da" at this point would be churlish, juvenile and unprofessional, . . . so

TA DA!

But wait, there's more.


This is a photo of a mature microbial mate with a couple of overturned pieces. Not the cracking in the larger piece around the margin. his may be the cause of the "radial fabric" of the 2.1 GA strucutures. The smaller piece has even more marked cracking and looks similar to this 2.1 Ga specimen below.


This latest evidence strengthens the argument that the 2.1 Ga structures are pyritised microbial mats and not multicellular colonial organisms.

2.1 Ga Multicellular Colonial Organisms - Umm, Not (update)

I previously discussed why I don't think the 2.1 Ga structures found in Gabon are multicellular colonial fossils.

Having looked at the Nature paper some more, another thing springs to mind.

Scale.

Here are some of the structures as figured in the paper.

Note the scale bars against each example. Those scale bars represent 5 mm! That's right millimetres!

These things are small. Especially the central area that contains the folding.

Now, compare that with the examples of bubbles in microbial mats

Photo from Cuadrado and Pazini (2007)

There is no scale, but some of the bubbles must be at least the size of the central folded area of the specimens figured in the Nature paper. This means we have structures documented today in microbial mats that equate in size to the central area of the structures from Gabon.

Now, what would happen to a gas or fluid filled bubble if it were covered with sediment and the gas/fluid escaped? It would deflate. Not deflate flat, as the microbial structure would be too thick. Maybe it would fold on itself just like the central structure in the specimen in row two above? Especially if there was an encouraging push from overlying sediment.

I'm even more convinced that the Gabon structures are microbial mats.

UPDATE 2

Diana G. Cuadrado and Natalia V. Pizani. (2007) Identification of microbially induced sedimentary structures over a tidal flat. Latin American Journal of Sedimentology and Basin Analysis. v.14 n.2 La Plata ago./dic. 2007

2.1 Ga Multicellular Colonial Organisms - Umm, Not

2.1 GA structures on black shale bedding surface. Scale bar 1 cm. (El Albani et al. 2010)

The latest edition of Nature has a paper claiming 2.1 billion year (Ga) old multicellular colonial organisms from Gabon. This is a remarkable claim, since the oldest definitive large multicellular organisms so far found in the fossil record are from less than 600 million years ago (It is still not certain if the 2 Ga Grypania spiralis is a eukaryote or large bacterial colony). So remarkable is the claim in fact, that it made the front cover of Nature.

I think they are wrong.

Lets be more specific here. There are two claims - that the structures are 2.1 Ga old, and that they represent large multicellular colonial organisms.

First the date. That appears well constrained. Chris Rowan has good coverage of the dating at Highly Allochthonous, so I have no argument against the age.

I don’t think they are multicellular colonial organisms. I think they are pyritised remains of microbial mats, or structures associated with microbial mats.

There’s been ample coverage of the paper, but I want to concentrate on the reasons why the authors think that the structures represent organised colonial organisms.

The structures are found in unmetamorphosed black shales. Over 250 specimens have been recovered. They are pyritised and embedded within the sediment. They range from 7-120mm in length, 5-70 mm in width, and 1-10 mm thick. They can occur in densities of up to 40 specimens per square metre, at random orientations, but all are horizontal to bedding. All are wrinkled to some degree, with some having large central pyrite nodules. Some show significant folding. All show radial cracks. The authors used micro-computed tomography (Micro-CT) to produce three-dimensional images of the structures. They also undertook carbon and sulphur isotope analyses of the host rock and the structures.

On the following lines of evidence the authors decided that the structures represented multicellular colonial organisms:

1. The structures shown under Mirco-CT are not the same as produced by microbial mats.


2. They are unaware of any inorganic process that could produce the folding patters seen in the structures.


3. The folding suggests an originally cohesive flexible sheet.


4. They are unaware of any inorganic process that could produce the radial fabric seen in the structures.


5. The radial fabric represents peripheral accretion of flexible organic matter.


6. The 12C/13C carbon isotope ratio in the host sediment is different to that in the structures suggesting the structures represent distinct organisms.


7. Steranes have been found in the shales, a compound associated with eukaryotes.


8. A lack of support for any other inorganic origin.

I think pyritisation of microbial mats is a better explanation. Here’s why.

I’d like to take 1, 2, and 3, together because they all relate to the physical properties of microbial mats.

First let's take a look at what we're talking about.

Micro-CT-based reconstructions and virtual sections of four specimens from the FB2 member of the Francevillian Group. First Column - original. Second column - volume rendering in semi-transparency. Column 3 - Transverse two-dimensional specimen. Column 4 - longitudinal section running close to estimated centre of specimen. Scale 5 mm. (El Albani et al. 2010)

The structures are pretty much all pyrite. The absence of pyrite is marked by the radial cracks. The bright central area in rows 3 and 4 represent a central pyrite nodule. This is not present in all specimens. This is important, as the large thickness values for specimens are all associated with this central nodule. Away from the nodule, or when a nodule is not present (e.g. row 2 above), the structure is a thin film of pyrite around 1-2 mm thick.

This is really important. All the folding is just that, folding. The apparent holes (black areas) in the images above do not represent complex structure within the specimen. The image is a slice through the specimen at a certain level. Where the pyrite film is folded below the level being imaged (downward folds), or completely above it (upward fold), you get a black space. The images appear to show complex structure, but it is simply a line drawn across an undulating surface. There is no internal structure here other than the radial fabric.

Further, and critically, the authors have concluded that the thin pyrite film and the central nodule represent different periods of pyrite formation. This is supported by sulphur isotope data, and I am happy to accept their conclusion.

This means that the large nodules have little to do with the original structure of the specimens, and, in fact they appear to occupy a central cavity in the specimen when they are present. An example of an actual nodule is below.

Section through a specimen showing central pyrite nodule surrounded on both sides by sheet material. Scale bar 1 mm. (El Albani et al. 2010)

In this specimen you can see that the nodule and the thin film on either side of it is almost all pyrite. The gaps in the center of the nodule are unexplained, but probably host sediment. Ignore the pretty colours. that refers to sulphur isotope analysis which I'm not questioning.

So, if we ignore the central nodules for the present, we are left with an original structure that is a thin film approx. 1-2 mm thick which has been thrown into folds in some specimens (e.g. row 2 above) but not in others (e.g. row 1 above).

The authors claim that the folding and the radial cracks cannot be accounted for by inorganic processes. I think they can.

Look! over there on the right! Is a 2.1 Ga colonial organism with a radial fabric? Is it a Proterozoic jellyfish? No, it's . . well, I'll explain later. But compare this example with rows 2, 3 and especially 4 of the Micro-CT images above. Notice that it has a central zone which is distinct from the periphery. Notice there is a faint dark band separating the central core from the periphery. Notice also the distinctive radial 'cracks' that spread out from the core Central mass? Check, radial cracks? Check. This would appear to be a good match to the 2.1 Ga structures.

But if you are not convinced, here's another variety of the same structure. this time we'll compare it directly with row 2 above as this has the best fold structure presented.




Can you see the fold pattern at the centre of the lower photos? It's quite similar to the fold pattern in the Micro-CT image. These images are of water escape structures in Ediacaran sandstones. The difference in colour you can see is due to the sandstone being deposited wet and dirty - i.e. waterlogged and with a significant amount of silt and clay (the red stuff). Escaping water carries the red silts and clays away with it, leaving the white sand behind.

Three things to note. They can produce radial structure, they can produce fold-like structures, and they are pretty similar in size even though they are from different sandstones.

Now, for the record, I don't think that the radial structures in the 2.1 Ga structures are water escape features (I've another idea for them), I'm just showing that such structures can be formed inorganically. Although I am suggesting that water escape could be an explanation for the folding we see in the row 2 specimen.

The authors discount water escape as an explanation because the underlying sediment shows no disruption such as that shown in my Ediacaran example viewed from the side. Plus, shales are not known for their permeability (ability to allow water to flow through it). But we are dealing here with microbial mats. Mats can act as a barrier to water or gas flowing below them. In such instances, rapid loading by waning storm sediments could cause water or gas trapped under the microbial mats to tear through the mat, creating some neat folding patterns and not disturb the underlying sediment too much.

Actually I'd better show you some mats so that you know what I'm talking about. Here's one.

A juicy, yummy microbial mat, full of cyanobacterial goodness from Yellowstone Park. Photo Carnegie Institution

(And yes I do consider Yellowstone to be a good analogue for the 2.1 Ga Proterozoic, because while the hydrothermal pools are aggressive to microbial mat grazers, so was the 2.1 Ga Proterozoic, because, well, there weren't any.)

Do you think the microbial mat above looks like what the authors have imaged? I do.

And just so we're clear on how flexible microbial mats can be, here's another photo.

Photo from Cuadrado and Pazini (2007)

Here are some more Microbial mats.

Photo from Cuadrado and Pazini (2007)

Photo A shows microbial mats draped (folded) over cracks. Photo B shows gas bubbles trapped under a microbial mat. What? you thought I just made that gas stuff up?!

But this is the clincher for me.

Microbial mats at West Chaplin Lake (Bowman and Sachs 2008)

There are a number of things to notice here. Firstly the mats are not one continuous sheet, but here are two discrete mats about the same dimensions as the 2.1 Ga structures. Also note the lower mat has been torn to produce a clean, sharp, high angled edge. This shows that these things can be torn without losing structure, so simple folding should be no problem. Speaking of folding, check out the left margin of the upper mat. See the fold? Remind you of anything? This maybe?


So microbial mats can fold and wrinkle without tearing. I agree with the authors that the structures represent originally cohesive flexible sheets, but microbial mats not colonial multicellular organisms.

There are other methods to fold or wrinkle microbial mats. Storms could rip up mats or partially dislodge them for example, and the authors interpret the environment as deltaic, influenced by storms. Elephant skin textures on bedding plains have been interpreted as being caused by wrinkling and folding microbial mats.

I hope this has shown that the structures seen in the 2.1 Ga specimens, the folding and wrinkling, can be produced by inorganic means on biological sheets without invoking a new class of organism.

OK, on to numbers 4 and 5, the radial fabric.

This is one of the main arguments for the structures being a colonial multicellular colonial organism and it certainly is striking. but, as I showed above, radial fabrics can be caused by other things. inorganic things.

This irregular radial fabric, along with the the fact that it is commonly deflected to meet the edge of the structures is taken as evidence that the structures grew by peripheral accretion of flexible organic matter.

However, there are a few specimens where this radial fabric appears to go right through the structure.

Scale bar 1 cm

In one example, the fabric appear to actually cut through the central nodule

Scale bars 5 mm

How could a growth-related fabric cut through a nodule that wasn't there until late in the diagenetic process?

I think it's down to water again.

Lets assume that the mats/colonial organisms have gone through initial diagenesis and have been converted to pyrite. As diagenesis proceeds and load on the sediment column increase, the mudstones start to compact. Muds can contain 60% water which means that they have a long way to compact. Problem. Pyrite sheets can't compact.

I think that as compaction progressed and the pressure increased, the thin outer margin split in a regular pattern. As the process continued, the cracks spread further into the body of the specimens and sediment would be forced into the cracks. As we have an example of crack through the central nodule, it may be that this cracking occurred late in the diagenetic process, when compaction was reaching it greatest extent.

So rather than be evidence of peripheral growth, the fabric may be diagenetic.

Number 6 and 7 are linked, 6 is the carbon isotope ratio, and 7 the presence of steranes.

The authors found a difference in the organic carbon isotopic composition between the host rock and the structures. This, they say, shows that the structures were distinct organisms.

Without going into too much detail, carbon is present in two main isotopes 12C and 13C with one extra neutron in the nucleus. Organisms preferentially take up 12C in different amounts depending on their metabolic pathways, causing a shift in the 12C/13C ratio. Measuring this difference or delta 13C can help identify the origin of the organic mater.

The host rock has a delta 13C value of -27 while the structures had a value of -32. Now -27 is roughly an average value for eukaryotes, and so would be expected to represent the fallout from the water column into the sediment of dead eukaryote plankton. A value of -32 is more like bacterial signature. This would tend to support my suggestion that these structures are pyritised microbial mat fragments.

Number 7 is the observation that steranes have been found in the shales (note the shales not the structures). Steranes are compounds associated with eukaryotes. But if the explanation for the delta 13C values is correct, the steranes would also be produced from the eukaryote plankton and can't be used to support the suggestion that the structures are eukaryotes.

Finally number 8, a lack of support for any other inorganic origin.

I think I've put up enough suggestions for inorganic input into the formation of these structures. but here is a rough explanation:

- Microbial mats grow on delta front muds - probably as individual round patches, possibly connected by thin connections to other mats.

- Storm activity rips up or distorts the mats into folds and waning storm currents deposit a layer of sediment over them.

- Pore fluids equilibriate and the mats are surrounded by sulphide dominated pore fluids.

- Pyrite replaces the mats.

- As the basin fills and the sediment column increases, the muds compact. The distortion and pressure casuse the pyrite structures to deform slightly and crack around the margins.

- As pressure and compaction increase the cracks propagate and second pyrite phase occurs in the central area of some structures. It may be that the cracks allow pore fluid and organic-rich sediment into the centre of the structure allowing the second pyrite phase to occur.

I'd like to commend the authors for taking a multidisciplined approach to this work. It's an approach which I believe should be done more often, and will be more widespread in the future.

I'd also like to commend them for their use of new techniques and I hope they will continue to use them to explore more of the geological record.

Sorry that this post has been a bit of a smorgasbord, but my aim is to present a number of possible scenarios for the formation of these structures that do not require that they be large, multicellular, colonial organisms. There are other, unfortunately more mundane, explanations.


UPDATE 1

UPDATE 2


Albani, A., Bengtson, S., Canfield, D., Bekker, A., Macchiarelli, R., Mazurier, A., Hammarlund, E., Boulvais, P., Dupuy, J., Fontaine, C., Fürsich, F., Gauthier-Lafaye, F., Janvier, P., Javaux, E., Ossa, F., Pierson-Wickmann, A., Riboulleau, A., Sardini, P., Vachard, D., Whitehouse, M., & Meunier, A. (2010). Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago Nature, 466 (7302), 100-104 DOI: 10.1038/nature09166

Diana G. Cuadrado and Natalia V. Pizani. (2007) Identification of microbially induced sedimentary structures over a tidal flat. Latin American Journal of Sedimentology and Basin Analysis. v.14 n.2 La Plata ago./dic. 2007

Jeff S Bowman and Julian P Sachs (2008) Chemical and physical properties of some saline lakes in Alberta and Saskatchewan. Saline Systems, 4:3 DOI: 10.1186/1746-1448-4-3