How could this fossil's soft tissue be preserved so well?

Question

I found this article for non biologists like myself. It describes, how astounding fossils were found in China. They belong to a crustacean like animal that lived more than 500 Ma ago. The exciting thing about them is, that the nervous system -soft tissue - is fossilised in its finest detail. The article says:

“Using fluorescence microscopy, we confirmed that the fibres were in fact individual nerves, fossilised as carbon films, offering an unprecedented level of detail. These fossils greatly improve our understanding of how the nervous system."

My question is: how can these soft tissues be so well preserved?

I managed to find the scientific publication and attempted to identify the answer myself, but it seems to me, the question is not touched upon. Perhaps, being a layman, I overlooked it, or it is obvious to biologists?

Answer 1

You're right that the authors don't comment on how they think the fossilization occurred. Since that process isn't the point of their article, my guess is that any discussion of it would have detracted from their research focus.

There are some ways that soft tissue fossilization can occur, but to specifically answer your question as it relates to this case, you'd have to know more about the geological conditions in which the fossils were found, which I don't see in the article. This article provides a nice review of soft-tissue preservation processes.

The main points to consider in soft-tissue fossilization, according to the linked article, are pressure, time, charge and chemistry.

Pressure: How much pressure was the fossil under? The higher the pressure, the less oxygen and fewer organisms can access it and degrade biological tissue.

Time: How quickly did the fossil become inaccessible to degrading organisms? The faster this occurs, the less likely that any soft tissue will rot away.

Charge: Does the sediment in which the fossil found carry a charge? Electrical charge acts both as an selective barrier to some organisms and chemical compounds that can hurry biological decay and provides a means for reaction with biochemical compounds, which brings us to the last point:

Chemistry: Can components of the biological soft tissue react with their surrounding minerals? If so, then the inorganic compounds present may be able to reduce lipids and other complex carbohydrates to molecular carbon and minerals in the form of the biological organism.

So say, for instance, that this organism was suddenly encased in a thick layer of clay. As the review states:

In clay-rich environments, the large surface area and charge of clay grains are thought to contribute to preservation in one of two ways. First, enzymes produced either through autolysis or by invading microbes are adsorbed to the surface of the clay grains and inactivated, slowing or preventing degradation (Butterfield 1990, Garwood et al. 1983). Second, molecules directly adsorbed to clay grains are resistant to degradation (Stotzky 1980), whereas protein-protein layers are readily degraded. This mechanism may explain the exquisite preservation of thin carbon films in impression fossils such as the feathered dinosaurs of the Jehol Biota (see discussion below). As tissues interact with surrounding clay grains, only the molecules in direct contact with clays are preserved as monolayers, whereas the rest are degraded normally. Thus, surface morphology is preserved, and original carbon might be preserved as well

Who knows if this process explains these particular fossils. ¯_(ツ)_/¯ As I said earlier, we'd need more information. The section quoted above, though, seems (to me) to provide a plausible hypothesis.

I hope this helps!

Answer 2

Three major factors.

1. the organism itself was small and thin enough to not be exposed as it decomposed, combined with the extremely fine grained nature of the matrix (rock) means the organism was held perfectly still even as all the moisture and active enzymes were literally leached or squeezed out of it.

2. Most of an organism is water, and that gets slowly removed leaving behind a stain almost as if you slowly dried out the organism while pressing it between two plates. Look at the old practice of pressing flowers under glass for ideas of how this can preserve things. Removing any space in the remains prevents the molecules from moving much.

3. It was buried quickly under anoxic conditions so little to no bacterial action was able to alter or disturb the remains as these processes reduced it to basically a stain of all the non-water components of the organism. What is left does not take up much space and microscopic mineral grains completely surround them, the molecules cannot move much becasue they have nowhere to go, that is why such fossils become very delicate once exposed the leftover material is then free to shift.

Sources Burgess shale examinationand experimentation in preservation

Answer 3

Preservation of individual nerve-fibres look quite extreme, but students of palaeo-sciences are quite familiar with seeing rock-slides containing fossilized tissues, where the contours of every cell is clearly visible.

wikipedia - image, cropped and rotated to save space. It represents a siliceous petrifaction mode of preservation, of a plausibly soft-bodied organism.

The most well-preservation of cells are found in petrifaction or cellular permineralization modes. (Though there are disputes about when to use these 2 terms (such as second one has a broader sense use), the 2 things are same.)

Though your mentioned page did not mentions anything about its plausible mode of preservation, it apparently looks like a compression fossil , which also contain preservation of harder structure such as plant-cuticle, but preservation of individual nerve fibers are really surprising. (However just speculating, may be it is actually a petrifaction fossil or something in-between of petrifaction and compression).

.........................

No one of us know, how exactly the fossilization processes take place, but things unanimously accepted are, fossilization requires very very special conditions/ situations, and which is very rare and chance dependent, and slow also.

It has been hypothesized that fossilization requires anoxic environment, source of sedimentary materials (preferably including finest-grain clays), soluble minerals, quiet or no-shake conditions.

For petrifaction-fossils, it has been hypothesized that a source of minerals that would deposit, is mandatory, such as the sulphides, silicates carbonates, sulphates etc of metals like iron, magnesium, calcium, etc. coming through volcanos, hot-springs etc. and also marine sources of salts may have played some role.

In case of silicified petrifaction, presence of soluble silicates are important. An increase in acidity (decrease of pH) can trigger the deposition of insoluble forms, thus may have generated silicified petrifaction.

.............

I've read about one example observed present day, that resembles (and may explain) permineralization-type fossilization, from Stewart- Rothwell's Palaeobotany.

"A park ranger reported finding the remains of a coyote in the outflow of a geyser. He left carcass undisturbed and found, when he returned some four years later, that the remains of coyote bones had already become silicified in the same way as the bones of dinosaurs".

Though this does not include very soft tissue, yet a present-day example of silicification.

....................

Reference:

All informations based on Paleobotany and the evolution of plants/ 2nd Edition, Wilson N. Stewart and Gar W. Rothwell/ Cambridge University Press.