Squishy bodies, strange seas: the mystery of Burgess Shale-type fossilization explained?

The Proceedings of the National Academy of Sciences recently published a landmark, yet profoundly simple, report offering direct geochemical evidence of a viable mechanism for Burgess Shale-type preservation (Gaines et al., 2012). One example of such well preserved fauna (called Marella) is shown below:

Results of the international team’s research, summarized here, suggest that soft-bodied fossils were preserved in the Middle Cambrian by a combination of favorable ocean chemistry and sedimentary ‘sealing’. In short, oxygen-poor basins promoted the preservation of organic matter while scant sulfate concentrations inhibited microbial scavengers that thrive on bacterial sulfate reduction. The researchers also discovered thin laminae of lime cement (calcium carbonate) overlying the fossils. These laminae effectively would have ‘sealed’ the substrate in which the fossils were forming, thereby allowing organic tissues to decay at rates sufficiently low to be imprinted on the muddy layers of deep ocean sediments.

Mystery solved? Or perhaps, in more scientific terms: ‘Mystery framed within a plausible hypothesis with replicated empirical support!’

Or so those pesky paleontologists and paleoceanographers would have us believe, according to Brian Thomas of the Institute for Creation Research. Not surprisingly, Mr. Thomas is skeptical of the described mechanism and reaffirms his belief that Burgess Shale-type fossils are instead evidence of “the Genesis Flood [and an] unprecedented scale of violent geologic activity” (read the full article here).

Violent geologic activity? These words don’t seem to fit with the fine-grained canvases upon which the soft-bodied aliens of the Cambrian ocean were stamped. Therefore, I should attempt to explain his thought process. Let’s begin by asking: 1) Why does Mr. Thomas reject the mechanism offered by Gaines et al.? and 2) What alternate explanation can he offer for these taphonomic anomalies (i.e. cases of exceptional preservation) and associated geochemistry? The answers are as deceptively simple as his interlocutors are profound.

1) Because he can, and thusly he misleads his readers. Mr. Thomas offers no substantive critique.
2) He does not, though he still pretends to.

I do not mean to be crass, but I am often discouraged by the ease with which Mr. Thomas and Young-Earth Creationist authors dismiss the latest research by misrepresenting rather basic geological observations and principles (see below). The tactic is not only dishonest, but leaves millions of readers—who place their trust in these authors—ill-informed and pursuing a false hope. Let’s consider how Mr. Thomas approaches the work by Gaines et al. (2012):

“Marine biologists…never [find] fossils in the process of forming…because when a sea creature dies, its carcass is totally recycled within weeks. So, if a creature’s soft parts are going to fossilize, it has to happen extremely fast.”

Preservation of soft parts requires something wholly different than rapid fossilization. As Gaines et al. noted in an earlier publication (2008, Geology), the chemistry of Burgess Shale-type (BST) fossils indicates that fossilization did not occur rapidly (i.e. through early permineralization, or a replacement of soft tissues with other minerals). Instead, the process of organic degradation had to be slowed significantly. The most recent paper described independent evidence at multiple sites around the globe for this mechanism, confirming the original hypothesis.

Mr. Thomas continues by sarcastically recounting the authors’ description of the mechanism by which these fossils were preserved. For example, he speaks of ‘the assumed “global ocean” of the time’, adding quotes unnecessarily to cast unwarranted doubt. He then asks:

“But did all that really happen? Probably not, since most of those events invoke unexplained phenomena, such as strange ancient sea chemistry and a puzzling absence of bioturbation. Plus, if sealing buried creatures under limestone explains Burgess Shale-type fossils, then similarly preserved fossils should be found beneath the continent-covering limestones all over the globe, and not just in isolated pockets.”

The chemistry of the Cambrian ocean is far from unexplained, and I highly doubt Mr. Thomas is familiar with the plethora of research that currently describes our understanding of it. For example, the oxygen content of the Cambrian ocean is fairly well constrained through proxy evidence (chemistry of preserved minerals and organic matter) as well as geochemical modeling. I know partly because I have presented on the topic at geological conferences and contributed some data. Also well documented are evidences for low sulfate concentrations (less than 1/10th of modern values) during the Middle Cambrian. These explanations are neither novel nor unique to the conclusions by Gaines et al. (2012). However, they were not previously associated with BST fossilization.

Oxygen and sulfate concentrations may also be lower in deeper ocean basins restricted from rejuvenating circulation. Why is this important? Because it inhibits organisms from burrowing in the oceanic sediments! Where no oxygen is available, multicellular eukaryotes cannot survive, so their absence here is far from puzzling! Unless, of course, you posit that a global flood was responsible for the deposition of these rocks, in which case it is puzzling that burrowing animals are absent from layers where other animals are abundant.

Lastly, Mr. Thomas strains at credulity when reasoning that BST fossils should be found across the globe under limestone everywhere. As Gaines et al. point out, it is not the simple presence of limestone overlying mudstone that is significant, but the sudden deposition of calcium carbonate mud to seal off microbial scavengers in environments that also contain the aforementioned geochemical conditions. This only describes a tiny percentage of ocean basins in history. But how does the limey cement arrive in the first place?

“The study authors cited evidence of “rapid entombment” by “bottom-flowing density currents.” And whereas standard geology has no explanation for such widespread catastrophic undersea density flows, creation scientists can cite the Genesis Flood in describing the unprecedented scale of violent geologic activity implied by these and other fossils.”

I am shocked to read that Mr. Thomas believes geology offers no explanation for density currents (which actually occur on a daily basis around the globe, even in terrestrial lake systems!). A simple exercise, which elaborates my surprise, requires nothing more than GoogleMaps. My example (there are thousands more, which you ought to explore) comes from near Lompoc, California:

What looks like a pitchfork, connecting the shallow continental shelf to the deeper oceanic basin, is a network of submarine channels that formed by turbidity currents. Density flows form as a result of subtle differences in temperature, salinity, or suspended sediment load between adjacent water masses (see a fun video demonstration here). These bottom-water currents are capable of transporting calcium carbonate mud from shallow water to deeper basins. And—as Gaines et al. noted—they also provide a convenient trap to preserve soft-bodied fossils under favorable geochemical conditions.

To conclude, I urge you not to be misled by Mr. Thomas’ swift, unwarranted dismissal of the latest research in Cambrian fossil preservation. Thusly, a great disservice is done to these fine examples of early life, preserved by an incredibly delicate process. Mr. Thomas’ claim that BST fossil beds imply violent geologic activity is flatly wrong. Rapid burial alone cannot explain their preservation, and swift currents—along with the intense weight of overlying sediments—are not likely to leave such squishy bodies intact. Furthermore, Mr. Thomas offers no alternative explanation for the geochemical data reported by Gaines et al. (2012), generally interpreted to reflect Cambrian ocean chemistry and microbial activity.

His credibility is totally lost by saying simply ‘No, this can’t be right, because then I might be wrong.’

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