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Do New Ediacaran Fossils Muffle the Cambrian Explosion?

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Whenever the mainstream journals discuss the Cambrian explosion, you can expect three things:

  1. They will try to explain the Cambrian radiation using only unguided material causes like oxygen, temperature or chemistry. Mind and intelligence is forbidden.

  2. They will use anthropomorphic words or appeal to magic, saying that the Cambrian animals appeared, developed, innovated, arose, emerged, filled new ecological niches, or engaged in an evolutionary arms race.

  3. They will completely ignore the argument in Stephen Meyer’s book Darwin’s Doubt: that the sudden appearance of the Cambrian animals required vast amounts of information expressed hierarchically in new cell types, tissues, organs, and body plans. Since this is beyond the capabilities of neo-Darwinism, and the only cause we know that can produce this kind of information is intelligence, the Cambrian explosion provides powerful evidence for intelligent design.

The journal Geology just published new research dealing with the Ediacaran period — that lazy world of sessile marine animals that preceded the Cambrian. Let’s see if they live up to our expectations or have something new to say. Unless they find a gradual neo-Darwinian path from Precambrian microbes to trilobites, appealing to only material causes, without using magic words, we will have to call Strike Three.

Pour Sand, Add Cement, Mix

Derek E. G. Briggs, a Darwin proponent we called out last October, lends his name to a paper by lead author Lidya Tarhan and three others writing in Geology, “Exceptional preservation of soft-bodied Ediacara Biota promoted by silica-rich oceans.” Tarhan’s team took a closer look at the type sequence in Australia where the Ediacarans were first identified. Their goal was to understand how such soft creatures could be perfectly preserved as fossils in sandstone. In short, they propose that if you add the right silica cement to the sandy seafloor fast enough, you can get beautiful molds and casts of creatures before they decay.

Here we present evidence from the Ediacara Member of South Australia that Ediacara-style preservation was due to rapid, early-stage precipitation of silica cements, facilitated by the high silica saturation state of the oceans prior to the appearance of prolific silica biomineralizers. An early silicification model provides a coherent, mechanistic and empirically supported explanation for the widespread preservation of soft-bodied organisms of Ediacaran-early Paleozoic age as sandstone casts and molds. The prevalence of early silicification confirms that Ediacara-style fossil assemblages can provide an accurate window into life on the Ediacaran seafloor that can be used to reconstruct critical steps in the development and diversification of early animal ecosystems. [Emphasis added.]

You see right off the bat their focus is on the fossilization process, not on evolution. If you can explain the creatures’ preservation, they say, you can “reconstruct critical steps in the development and diversification of early animal ecosystems.” But that doesn’t follow. It’s only a half-truth. You might open a window on the conditions that existed when they were fossilized, but you can’t leap from there to a theory of how animals “developed” (euphemism for evolved) and “diversified” (another evolutionary euphemism).

In a nutshell, their premise is that a “taphonomic window” (opportunity for fossilization) opened in Precambrian days that permitted a unique kind of “Ediacaran-style preservation,” as can be seen in their beautifully detailed photos of Ediacarans, those mysterious frond-like and pillow-like multicellular colonies illustrated in Illustra’s film Darwin’s Dilemma. This taphonomic window persisted for “hundreds of millions of years” well into the Ordovician.

So why don’t we find Ediacarans after the Cambrian explosion? As the story goes, the new critters used up the cement. Modern oceans are undersaturated in silica, thanks to drawdown by diatoms, sponges, and radiolarians. That wouldn’t have been the case back in the Ediacaran. With more dissolved silica in the water, the Precambrian creatures could have been cemented in the sand, forming casts and molds before their tissues decayed away. But later, the silica-hungry newcomers broke the molds; that’s why the Ediacarans stopped being preserved. Most likely this reflected a global change in the chemistry of the oceans. What are the implications of their model?

Evidence for early silicification across a wide range of tissue types demonstrates the importance of a global and persistent environmental control on fossilization, i.e., high marine silica concentration. Resolving this long-standing taphonomic paradox allows genuine evolutionary signals (e.g., extinction events) to be distinguished from preservational artifacts. An early silicification taphonomic model indicates that the geologically abrupt appearance and subsequent disappearance of the Ediacara Biota is a valid evolutionary signal. It also provides the first empirical support for the contention that Ediacara-style fossil assemblages truly reflect the diversity, trophic complexity, and community-level ecology of Earth’s oldest fossil animal ecosystems.

That’s not helpful. For Darwinians, that is. They just said that the abrupt appearance and disappearance of the Ediacarans “is a valid evolutionary signal.” Meyer agrees that the Ediacarans appeared explosively (DD, pp. 86-88), and disappeared before the Cambrian explosion. He would only disagree that this is a valid evolutionary signal. Next.

Signals in the Desert

In a desolate area east of Death Valley, a team of scientists found “Two new exceptionally preserved body fossil assemblages from Mount Dunfee, Nevada, USA” (Geology). They were hopeful this site would shed light on the Cambrian explosion, because

Evaluation of hypotheses that relate environmental to evolutionary change across the Ediacaran-Cambrian transition has been hampered by a dearth of sections that preserve both the last appearance of Ediacaran body fossils and the first appearance of Treptichnus pedum [an index fossil for the start of the Cambrian] within carbonate-rich strata suitable for chemostratigraphic studies. Here, we report two new exceptionally preserved latest Ediacaran fossil assemblages from the Deep Spring Formation at Mount Dunfee, Nevada (USA). Further, we report these occurrences in a high-resolution carbon isotope chemostratigraphic framework, permitting correlation on a regional and global scale.

It’s a significant find. They’ve got the latest Ediacaran all the way to the first Cambrian. In between, they identified a “worm world” of ichnofossils (trails and burrows), some small shelly fossil traces, and — get this — the “carbon isotope excursion” that evolutionists believe indicates a global change in ocean chemistry and the carbon cycle. Their detailed geological cross-sections show this transition across 100 meters of strata. They can correlate some fossils with similar ones from China. It’s a perfect setup to refute Darwin’s Doubt.

The data presented here represent the tightest relationship documented to date between the negative δ13C excursion and biological turnover at the Ediacaran-Cambrian boundary, consistent with an environmental disturbance eliminating the last of the Ediacaran biota and paving the way for the Cambrian radiation.

Alas, things are not so simple. Once again, they agree that the Ediacarans went extinct before the Cambrian event. That means those weird sessile creatures didn’t have anything to do with the new Cambrian body plans. The Nevada site lacks the frond-like and mattress-like creatures for which the Ediacaran period is best known. As for Cloudina, (once considered a “small shelly fossil” but later reclassified), its classification as a metazoan living in the Ediacaran period seems questionable. It looks like a pile of cups, and has no organs or specialized tissues. It went extinct at the base of the Cambrian, so it cannot have been an ancestor to the new Cambrian phyla. T. pedum is hardly a better contender; it’s known by its burrows, and probably lacked hard parts.

The team’s geological chart shows lots of strata between the first and second fossil assemblages where transitional forms could be hiding, but they’re not there. You have Ediacarans, then blank space, then some tubular things, then the Cambrian index fossil T. pedum higher up. No evolution is obvious. No pre-trilobites. No pre-Anomalocaris. A Darwinian would be batting the air to consider this outcrop helpful for explaining the Cambrian Explosion.

As for that carbon isotope excursion, we’ve observed that its significance is questionable, since it doesn’t correlate tightly around the world. At best it is an effect of the explosion, not the cause of it. Similarly, T. pedum shows up in different places around the world, so it’s not exactly a precise marker either. Moreover, their correlations of the few worm-like body fossils with similar ones in China appear dubious. And embarrassingly, one fossil they call Conotobus used to be considered an ancestor to Cloudina, but here in Nevada it appears in strata above its descendent! So as noteworthy as the Nevada site is, it leaves the Cambrian explosion completely unexplained. Another strikeout.

Summing Up

The editors of Geology put the best spin on things that they can. James D. Schiffbauer, a geologist from the University of Missouri, reviews the papers in a piece titled, “The age of tubes: A window into biological transition at the Precambrian-Cambrian boundary.” He recalls his delight at reading Wonderful Life, Stephen Jay Gould’s marvelous description of the Burgess Shale. He remembers his astonishment at the newly discovered Ediacaran fauna in 1992, wondering what it meant for evolution. He says that the Ediacaran-Cambrian interval “has become one of the most intensively studied time slices in the geological and paleontological records.” Then he laments:

The past twenty years have brought significant new data to the table regarding the Ediacaran-Cambrian Earth system and biosphere, but a great many questions remain unanswered. Foremost of which, the pattern(s) of and mechanism(s) for biotic change during the Ediacaran-Cambrian transition are still largely unresolved.

Did these two papers offer new hope? Well, they helped rule out some unworkable ideas, like the “Cheshire Cat” theory that posited the Ediacarans disappeared gradually. No, Schiffbauer admits, “we observe that the fossil record of Ediacara organisms was truncated at this transition — whether by disappearance of their preservational regime, or by disappearance of the organisms themselves.”

As excited as Schiffbauer is about the new Nevada site with its tubular whatzits, he ends with more stories, more magical anthropomorphic talk, and more promissory notes. His last paragraph even discounts some of his own previous publications.

These tubular and other vermiform (worm-like) organisms have recently been implicated in marginalizing and competitively wedging out the classic Ediacaran forms, owing to such ecological novelties as ecosystem engineering and macropredation (Schiffbauer et al., 2016). However, as shown here in direct context with an isotopic record of environmental perturbation, perhaps the combined ecological and environmental stressors provided an insurmountable double whammy, forcing a coda for the classic Ediacarans. While the terminal Ediacaran of the Deep Spring Formation had been previously examined (e.g., Gevirtzman and Mount, 1986; Signor et al., 1987), Smith et al.’s new work has served to prop the window open for further refinement of the taxonomy of these tubular forms, as well as detailed investigation of their taphonomy and paleoecology. These “wormworld” organisms (Schiffbauer et al., 2016) inhabit an important transition, and their continued investigation may yield clarity into the patterns and mechanisms of biotic turnover at the Ediacaran-Cambrian boundary.

OK, when he finds said clarity, we’ll be glad to give him another chance at bat. Till then, we must call it as we see it. Strike three.

Photo: Golden spike indicating the base of the Ediacaran period, by Peter Neaum [CC BY-SA 3.0 or GFDL], via Wikimedia Commons.

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Evolution News & Science Today (EN) provides original reporting and analysis about evolution, neuroscience, bioethics, intelligent design and other science-related issues, including breaking news about scientific research. It also covers the impact of science on culture and conflicts over free speech and academic freedom in science. Finally, it fact-checks and critiques media coverage of scientific issues.

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