Sunday, September 26, 2010

Palaeoporn 18

It's not size, but form that counts.

Look, I'm sorry about this. It's quite embarrassing, I know. But in my defense, I have blurred the naked Dickinsonia at the top right of the slab so as not to offend any sensibilities.

Anyhow, on to the real subject of this Palaeoporn, the structure with the highly descriptive and emotive name - "Form D". Yeah . . . "Form D" . . . no, honestly!

The Form D is the . . . um . . . rather flaccid structure above the compass. Form D was the name given to a group of thick, long, trace fossils of unknown origin. While they are not common, it doesn't take much poking about to expose them.

Given the lack of structure it was hard to attribute a source for the traces (they are not ribbed for example) thus making it difficult to erect a reasonable phylogeny and nomenclature. So they got stuck with the name "Form D" - as they were the fourth type of trace structures to be catagorised. Recently, they are thought to be, not trace fossils, but body parts. They appear to be association with the form Phyllozoon.

Phyllozoon Hanseni (yellow arrows) and Form D (blue arrows)
from the Ediacaran of Canada.

Phyllozoon is a more-or-less bisymmetrical leaf-shaped structure composed of a series of 'tubes' which meet at a central zigzag suture. There seems to have been only one layer of tubes forming the 'vanes' of the frond, with a stiff outer covering. Numerous individuals are commonly scattered over bedding planes, sometimes overlapping. The fronds have obvious polarity (top and bottom) and their common association with Form D is too frequent to be chance. It is possible that Form D is actually the stolon to which the Phyllozoon fronds were attached. They may well have looked something like this below, except Phyllozoon probably wasn't green.

The alga Caulerpa taxifolia doing a passable imitation of Phyllozoon. Photo credit.

I suppose we should be grateful that Phyllozoon wasn't called Phallozoon . . .

Sunday, September 19, 2010

Proterozoic Sponges Claim Doesn't Hold Water

The geologic account of ancient life is plagued with reports that do not withstand critical assessment. This is a special problem in the older rocks, where reports of spurious records continue to dilute the authentic record of evolution on the primitive Earth . . .

. . . It is left to the reader to draw conclusions about similar instances not here alluded to; the record of alleged pre-Phanerozoic life is full of them.
Preston Cloud (1973)

Harsh perhaps, but Preston Cloud's words should be engraved on the cover of every Proterozoic geologist's field notebook.

Frankly, the Proterozoic is weird. Most of what you think is organic, isn't. The vast majority of the rest is microbial mats. And the stuff you really, really think could be metazoan, is usually microbial mats playing silly buggers.

I don't want to become a party pooper (and no that's not a plug for Bora's pootopia). I don't, I really don't.

I want to be able to yell and scream about some new find that pushes our knowledge of the early evolution of metazoans back well into the Proterozoic. I do, I really do.

But I can't.

In Nature Geoscience, Maloof et. al discuss possible sponge-grade metazoans from the approx. 640 million year old Trezona Formation in the Flinders Ranges of South Australia.

The Trezona Formation just underlies the Marinoan glacial deposits (dated to approx. 635 million years ago) which mark the final throw of the of the Cryogenian dice, and are overlain by rocks of the Ediacaran Period. The Trezona marks the onset of the Marinoan glaciation and contains shallow water stromatolite flake breccia and bioclast packstones filling gaps between stromatolite heads:
. . . we identified a great diversity of bioclasts. Most packstones contain clasts of probable microbial origin, such as spalled flakes of adjacent stromatolite laminae and ripped-up and rolled-up sediments with cohesion enhanced by the presence of microbial mats. However, many bioclasts have anvil, wishbone, ring, and perforated slab morphologies that are difficult to assign to an abiotic roll-up or bacterial mat origin. in addition, the red colour and calcite composition of these distinctively shaped clasts are unique to the packstones (and even packstone clasts entrained in the overlying Elatina Fm diamictite as far as 65km from the nearest Trezona Fm stromatolite reef outcrop) and are not found in situ in the layers elsewhere in the Trezona Fm that could have been brecciated and transported. Therefore we suspect that the 1-cm-scale red bioclasts represent the remnants of a community of organisms endemic to the stromatolite-packstone environment. (Maloof 2010.)
This is what we are talking about

Outcrop photos from the Trezona formation showing the range of clasts
including "anvils" (d) and "perforated slabs" (g). (Maloof 2010.)

2D thin sections show the clasts are composed of a mixture of opaque clays, quartz grains and micro-crystalline calcite, finer than the surrounding matrix, with a sharp, continuous contact with micritic rims

Thin-section photomicrographs from the Trezona formation. (Maloof 2010.)

The authors then describe 3D shapes using serial sectioning (grid a bit off, photograph, grind a bit off, photograph). These 3D images show a three-dimensional network of 1mm diameter interconnected tubes. The tubes are also lined with micrite.

Three-dimensional reconstruction of the Trezona Formation
structures. (Maloof 2010.)

So we have stromatolites with infilling packstones contain a diverse set of bioclasts representing chips from associated stromatolites and ripped up and rolled sediments that have been glued together by microbial mats. However, the paper claims that some of these bioclasts have shapes, include anvil, wishbone, ring and perforated slabs, that cannot be caused by nonbiological actions for the following reasons:

1) The shapes cannot be formed by rock chips
2) The red colour and micrite composition is unique to the deposits - no source of the chips can be found.
3) The micrite coating in the tubes and around the clasts may represent weakly calcified cell layers.
4) The interconnected 3D tube structures and the 3D symmetry support a sponge hypothesis

1 and 2
OK, lets split these up. The first two deal with the lithology and composition of the clasts. The authors say that the clast shapes produced cannot be formed from mud chips or bits of stromatolite, and that is true. It is also true that there are no lithologies similar in colour or composition to the clasts anywhere else in the Trezona Formation. This means that the shapes cannot represent chips eroded off earlier formed mudstones because there are nor similar lithologies or rock types in the area, and even if there were, rock chips cannot create the shapes seen. But there are problems with using that as supporting evidence for a biological origin. There are other explanations. Other abiogenic explanations.

The local explanation for the clasts is that they are mud flakes not mud chips, that is, not mud chips eroded off pre-existing rocks, but mud flakes formed when muds are deposited in ephemeral or short-lived deposits, such as ponds after flooding surrounding areas, or overbank deposits.

Drying mud Gammon Ranges. Photo Credit.

These muds settle and dry out and can form all sorts of shapes.

Drying mud, Canyonlands National Park. Photo by Douglas.

In some instances the mud flakes can completely roll up

Drying mud from Death Valley. Photo by Don Gale.

Check out one of the rolled mud flakes pictured above compared with a couple of the clast shapes

Clearly some mud flakes can take on the shapes that mud chips cannot, and can form some of the shapes present in the Trezona Formation that are being used as evidence for a biologic origin.

But it gets worse. See the arrowed clasts in the image with the "anvil" shape (at right). These are clearly mud flakes. They show a common curled profile that can be seen in the examples of recent drying muds above. Nor do they contain the tubes that are supposed to be a feature of the "sponge" clasts.

Now, here's the interesting bit. Both the obvious mud flakes and the "sponge" clasts are the same colour AND have the same composition (opaque clays and microcrystalline calcite, with the occasional very fine quarts grain - see photo-micrographs above) in other words your typical mud overbank deposit. Especially since the Trezona is pretty much a lowstand deposit (lower sea level). Some of the limestones deposited prior to the Trezona Formation, when sea levels were higher, are now exposed, providing a source for carbonate and putting some distance between the Trezona depositional environment and any non-carbonate source material (meaning that only very fine grained material is likely to reach the depocentre).

I think it is clear that the red clasts are mud flakes from overbank deposits, rolled up and redeposited between the stromatolites. In other words, waters carrying red carbonate mud, overtop riverbanks and settle into ponds. The mud settle out and the ponds dry up. The mud then dries, with the carbonate providing a good cement. The mud cracks, curls and eventually are dispersed by winds rain or floods into the nearby sea. The mud could be sourced from further inland on the Gawler Craton, and the carbonate sourced from limestones deposited during the period before the current regressive cycle, of which the Trezona Formation represents a lowstand or low sea level phase. The ephemoral nature of these overbank deposits, and their lack of lateral extent means that they do not contribute to the rock record, but dry out, crack, curl and are dispersed by wind and/or water. This also explains the occasional presence of "silica blebs" within the clasts. These represent very fine sand grains incorporated into the muds as they are deposited.

Clearly the mud flakes and "sponge" clasts have the same source - which is not organic. The shapes and colour cannot be used as supporting evidence of an organic origin for the clasts.

The authors suggest that the red matrix could be a replacement:
Alternatively, the original organic skeletons could have been coated in a bacterial extracellular polymeric substance following their death. The chemical composition of the Fe, Na, K clays found in the Trezona Fm bioclasts is similar to that of augenic minerals precipitated by microbial biofilms during the replacement of soft tissues. The extracellular polymeric substance would have also formed a template for abiotic calcium precipitation. (Maloof 2010.)
Now if I'm reading that right, it looks like the authors are suggesting that the original skeleton of the "sponges" has been replaced by the red mud, since there is no evidence for spicules or any other type of a primary skeleton.

There are two problems with this explanation for the absence of a primary skeleton. The first is the fact that the red matrix composition is shared with clasts that are clearly mud flakes. The second problem goes to number three in their list of supporting evidence, that the micrite coating in the tubes and around the clasts may represent weakly calcified cell layers.

The authors comment that the "sponge" clasts are surrounded by a micrite rim (the dark outer rim around the clasts in the photo-micrograph at right. I don't know why they don't use the standard terminology - micrite envelope), which is also present lining the tubes. They suggest that:
the micrite of uniform thickness and texture coating both the exterior surface and interior canal walls could represent weakly clacified cell layers sandwiching the mesohyl of a sponge grade organism. (Maloof 2010.)
To their credit the authors then demolish this argument by pointing out that:
However, texturally similar (but less uniform thickness) micrite also coats peloids that do not seam to be part of the Trezona Fm organisms.
Yes, that's a bit of a problem. It's a bit difficult to suggest that the micrite envelopes are evidence of mesohyl when almost everything in the deposit has them.

Micrite envelopes are well known and are generally considered to be formed from filamentous organisms (bacterial, algal or fungal) who's filaments calicify and coalesce into a intertwined mesh around the clasts. In this case the areas between stromatolites are colonised by numerous filamentous organisms that grow around all the clasts present and eventually form the micritic envelopes. So having micritic envelopes around the "sponge" clasts and within the tubes cannot be used as evidence of sponge-grade tissues.

But there's another problem.

As stated above, the authors suggest that the mud matrix could be a replacement for the original skeleton structure around the tubes. The presence of the micrite envelopes is really problematic here. The authors comment that:
However, the contact between micrite coating and mixed clay-chert-calcite interiors is usually sharp, with no evidence of diffusive or porosity-following micritization. (Maloof 2010.)
This means that any replacement of the original skeleton must have occurred prior to the micrite envelope being laid down. However, the tubes must have been sealed during replacement as there is no replacement material in them. After replacement, the tubes must have opened again to allow the micrite envelope to form.


The evidence suggests that the red matrix is not a replacement, but a primary feature.

Which brings us to number 4, that the interconnected 3D tube structures and the 3D symmetry support a sponge hypothesis. I'll say up front that I do not know what the tubes are. They could be sponge tubes, but the evidence for that is equivocal at best.

A more likely explanation is that the "sponge" clasts are mud flakes that have been coated by filamentous organisms, rolled around and glued together. The tubes would then be where the filamentous outer coating of individual clasts, acting as a buffer and holding the clasts apart as a number of clasts were bound together.

It is interesting to note that an analysis of similar lithologies from rocks of the the equivalent Cryogenian interval south of Adelaide found;
. . . a light greyish microcrystalline limestone in which numerous flakes of grey calcareous mud are set. The flakes are typically intraformational and a gradation is noticed from intraformational breccia to edgewise conglomerate. Most of the flakes are flat, but there is a tendency to turn at the edges; length varies from several to 20 mm. in thin section or on polished faces there is a superficial resemblance to annelids (reference), but this is quite lost when the third dimension is considered. (Sprigg 1942)
In the Proterozoic, weirdness rules. This makes interpretation difficult. I don't think the evidence presented in the paper is enough for a sponge interpretation to hold water. However, we've been misinterpreting the Proterozoic for some time, and the authors are in good company.

Cloud, P. (1973) Pseudofossils: A Plea for Caution. Geology, v. 1, p. 123-127.

Maloof, A., Rose, C., Beach, R., Samuels, B., Calmet, C., Erwin, D., Poirier, G., Yao, N., & Simons, F. (2010). Possible animal-body fossils in pre-Marinoan limestones from South Australia Nature Geoscience, 3 (9), 653-659 DOI: 10.1038/NGEO934

Sprigg, R.C. (1942) The Geology of the Eden-Moana Fault Block. Transactions of the Royal Society of South Australia, 66(2), 184-214. Download

Thursday, September 9, 2010

Please Give to DonorsChoose

Be No. 1... Give to Public Schools in Need! - Go to

OK I'm back and finalising a couple of posts. In the meantime DonorsChoose is running again. is an online charity that makes it easy to help US school students in need. (Yes I'm Australian, but this is important.)

American public school teachers post classroom project requests on Requests are for arts, science, music, language, civics, sports, etc.

You can browse project requests and give any amount to the one that inspires you. Once a project reaches its funding goal, the materials are sent to the school.

Education is important. If you have a few dollars to spare please consider supporting teachers trying to give their students a good education.

A number of the projects are close to being fully funded, a few dollars could make all the difference.

Wednesday, September 1, 2010

We've Lost Cedric

Cedric the Tasmanian Devil has been put down.

Cedric was at the forefront of research into the deadly Devil Facial Tumor Disease, which at last estimate was infecting 60 per cent of the wild devil population in Tasmania.

He made waves in 2007 when he was injected with a strain of DFTD and survived. He even survived a second strain with only a couple of minor tumours, which were removed.

Cedric had shown an immune response to the disease, and was otherwise healthy. It was hoped that he would lead researchers to a treatment or vaccine for the disease, but an X-ray last week showed a number of tumours in the lungs, and it was decided to euthanase the 6-year-old Cedric.


Donations to help the Tassie Devils can be made here.