Rare Fossils: Dead Animals Decay Rapidly

Why do some fossils leave soft tissue remains? It takes guts, some scientists propose.

Given that bacteria are the enemies of fossilization, could they actually play a role in preserving them? A new study thinks so. Science Magazine says,

The overwhelming majority of organisms will never fossilize. Preservation of an animal’s anatomy in rocks is a rare event requiring a strict set of geologic and chemical conditions. Fossilized soft tissues like skin or muscle are even rarer, as they decay very quickly beyond recognition before mineralization occurs. It would be tempting to assume that microbes—the great mediators of rot and recycling—would be a natural enemy to high-quality fossils, but [Philip] Donoghue’s time spent watching shrimp waste away seems to hint at exactly the opposite.

Donoghue’s team at University of Bristol, with others from Uppsala University, tested the rapidity of decay with brine shrimp. As expected, microbes quickly rendered them unrecognizable. If deprived of oxygen, though, the microbes could act as preservatives, the team thinks. PhysOrg explains:

In watching the process of decomposition, the team noted that bacteria in the gut set to work right away, multiplying massively as they engaged in eating the dead tissue around them, so much so that they completely filled the cavity and eventually caused it to burst, which gave them access to other internal organs. If the shrimp was in a low oxygen environment when it died, such as being buried in sand, then most of the decomposition occurred from the inside, and then stopped as the bacteria ran out of air. But, because gut bacteria carry a lot of calcium and/or phosphates and because they form biofilms, after they died, they left behind a mold of sorts that showed the form of the gut organs of their dead host.

This explanation predicts that exceptional fossils form in low-oxygen environments. It also predicts that the best preservation will be in the gut. For this reason, Donoghue’s team thinks that the evolution of a through-gut (mouth and anus) made exceptional preservation possible. Science Magazine explains:

The researchers also point out that animals with true “through-guts”—ones that contain both a mouth and an anus—are much more likely to leave behind high-quality fossils than animals like corals and jellyfish, which eat and excrete through the same hole and are home to far fewer bacteria. The evolution of the anus appears to have given rise to a more complex microbiome and, thus, that “definitely increases your chances” of leaving behind an exceptional fossil, Donoghue says.

And yet jellyfish fossils have been found in mass graveyards. Other soft-tissue remains, like the famous dinosaur red blood cells and osteocytes, were found in bone. The ink-sac of a squid still contained the carbon remains, enough to write words with it. Original material from feathers has been seen in Archaeopteryx fossils. These have nothing to do with gut bacteria. The explanation, therefore, seems inadequate:

For many years scientists have debated whether the “Cambrian Explosion” was the result of more species suddenly developing or whether it was just the result of more remains being fossilized and found. In this new effort, the researchers suggest it might have had to do with the development of the anus and a through-gut.

This cannot be true, since Ediacaran fauna have no guts, but are found around the world. These precede the Cambrian animals in the fossil record.

The original paper on the Proceedings of the Royal Society B is open access. Here’s its gutsy explanation for the Cambrian explosion:

The key role of gut-derived microbes in decay and, by inference, preservation means that the evolution of a through gut is likely to have important implications for preservation potential. Organisms that have blind guts, such as cnidarians, evert their guts such that they cannot maintain a gut flora. As a result, one might expect that such organisms would have little chance of preserving internal anatomy. Preservation must depend on the formation of favourable external biofilms that invade inwards, similar to the process observed in embryos, to stabilize their internal anatomical structure post-mortem, allowing a much longer window for internal autolytic processes to take effect and thus resulting in a much lower preservation potential for internal anatomy. This prediction is largely borne out by the fossil record. The overall quality of preservation is also often of a lower fidelity in described soft-bodied diploblast grade and blind-gut bearing organisms relative to groups possessing through-guts. For example, arthropods, annelids, priapulids and hyoliths can in many cases preserve aspects of gut, musculature and, in rare cases, neural tissues. On the other hand, diploblastic organisms, such as cnidarians, are typically found as impressions or outlines only (with the notable exception of very rare specimens of the probable cnidarian Olivooides [37,38]). This may go some way to explain the mismatch between phylogenetic and molecular clock expectations that diploblasts existed long before diploblast bilaterians, yet the fossil records of diploblast and triploblast eumetazoans is approximately coincident [39,40].

Under almost all circumstances, pseudomorphing of biological anatomy by biofilm-forming microbes [5,13] may be limited to small structures. This process can provide a good explanation for the preservation of microfossils such as fossilized embryos as well as internal microenvironments, such as guts, within larger fossils. However, it is only in the most exceptional examples of exceptional fossil preservation that microbes replicate and preserve internal anatomy more generally. Bacterial biofilm pseudomorphing of anatomical structure may not be an important mechanism in preserving macroscale animal remains, even though endogenous microbes are important vectors of the decay of visceral tissues that leaves cuticle articulated and intact in Burgess Shale-type preservation. Thus, endogenous microbes exert a fundamental control on the amount of soft tissue morphology, and therefore the amount of anatomical information, that is preserved in Konservat-Lagerstätten [exceptionally preserved fossils]. Hence, the evolution of a through gut is an important factor in both the ecology of metazoan diversification and its fossil record. This finding also suggests the bauplan of an animal may act as a strong control on the processes of subsequent taphonomic transformation into an exceptionally preserved fossil, when the basic conditions required for the genesis of Konservat-Lagerstätten are met.

The authors provide almost no reference to actual fossils when they state that “this prediction is largely borne out by the fossil record.” Brian Thomas at ICR has a list of 42 documented cases of soft tissue preservation (original tissue, not biofilms) found in fossils dating as far back as 360 million years in the evolutionary timescale. Most of them are not related to gut bacteria. They mention cnidarians (jellyfish) as unlikely to be preserved, but what about ctenophores (comb jellies) that are found in Cambrian strata? Soft tissue preservation is rampant in Burgess Shale fossils. Evolution News & Views reported fossils of modern-looking jellyfish dated by evolutionists at 580 million years old.

So either Donoghue’s team did not do a thorough literature search, or is ignoring this evidence. Having made a prediction, though, that animals with anuses are the most likely to preserve biofilms that create “pseudomorphs” of soft tissues in low-oxygen conditions, they have opened the door to falsification in future studies. On the face of it, their lab work on shrimp is unlikely to capture the variety of circumstances that preserve animal tissues.

Someone should call this the anal-retention theory of taphonomy.

Why is nobody asking hard questions of the Donoghue team? Aren’t they aware that many exceptional fossils have nothing to do with gut bacteria? Aren’t they concerned that soft-tissue fossilization raises serious questions about the dating of fossils into millions of years?

What is being ignored in this paper is far more important than what is being stated. Maybe they can’t stomach the notion that soft tissue challenges long ages. See Real Science Radio’s list of published papers on dinosaur soft tissue remains.