Fossil Friday: Ediacaran Animal Embryos Put to Test and Put to Rest
The Weng’an biota of the Doushantuo Formation in South China is a famous fossil Lagerstätte, which is of particular importance, because it is dated to an Early-Middle Ediacaran age (590-575 million years ago), right in the time when molecular clock estimates place the origin of crown group metazoan animal phyla. The absence of actual unequivocal fossil animals from this period has often been explained away as an artifact of an incomplete fossil record. However, the discovery of various Ediacaran localities of the Burgess-Shale-Type decisively refuted this artifact hypothesis (Bechly 2020b), because those localities would have easily preserved any early small and soft-bodied animals, but only yielded macroalgae and a few problematic forms of uncertain affinity.
The Last Straw
The phosphatized microfossils of the Doushantou Formation, which are three-dimensionally preserved down to the cellular level, represent the last straw to somehow align the molecular clock expectations with the actual fossil record. This explains the urge by paleontologists to readily interpret some of the Doushantuo fossils as metazoan embryos. To the great frustration of evolutionists, all these attempts proved to be highly contentious and ultimately failed to provide any convincing evidence for Ediacaran metazoans. I discussed the dubious nature of these alleged animal embryos in several previous articles (Bechly 2020a, 2020b, 2022a; also see Evolution News 2016), where you can also find all the references to the peer-reviewed literature.
Now a new study by Sun et al. (2024), published in the Proceedings of the Royal Society, looks at the developmental biology of the genus Spiralicellula that was previously proposed as a potential metazoan embryonic stage, a kind of planktonic larva. The authors found a very different developmental mode exhibited by Spiralicellula and all the other alleged metazoan embryos, lacking any cell differentiation, compared to any crown group metazoans. They therefore explicitly “reject a crown-metazoan affinity for Spiralicellula and all other components of the Weng’an biota, diminishing the probability of crown-metazoan diversification before the early Ediacaran.” The authors do not even consider Spiralicellula as a plausible stem-animal but conclude “that Spiralicellula is more likely affiliated with non-metazoan holozoans than with stem metazoans.” So the highly critical view that I elaborated in my previous articles is still well in line with the most recent mainstream research by the leading experts on these fossils.
A New Study
Therefore, it is well worth quoting at some length from the conclusions of this interesting and important new study:
While the embryo-like fossils from the Weng’an biota were once thought to represent the earliest metazoans, recent research has suggested that some species, including Caveasphaera [40], Helicoforamina [29], Ostiosphaera [47], Sporosphaera [33] and now Spiralicellula, have affinities that lie outside crown Metazoa. Interpretations of soma–germ cell differentiation in Megasphaera [21] have been used to support their interpretation as stem metazoans, at best. The primary evidence for a stem-metazoan affinity of Megasphaera derives from ‘matryoshka’ structures that are interpreted to reflect cell differentiation. Nevertheless, it remains unclear whether these structures are endogenous or exogenous in origin [7,48]….
The Weng’an biota, renowned for its exceptional preservation fidelity, is considered a distinctive taphonomic window that holds great potential for documenting the earliest metazoans. The absence of definitive evidence of crown metazoans in the biota is inconsistent with the expectations of the molecular clock estimates which posit a Tonian or Cryogenian origin for the clade [1,2]. It remains formally possible that the absence of crown-group animals from the Weng’an biota and earlier strata reflects the incompleteness of the fossil record, and the discovery of unequivocal metazoans from the Weng’an biota or older strata remains a viable possibility, not least given the discovery of crown metazoans, including cnidarians and bilaterians, within the later Ediacaran [50–54]. However, claims of crown metazoans from the Cryogenian [55,56] and Tonian [57,58] are all highly contested [59–62] and intense exploration of the Weng’an biota, the most exceptional of all sites of fossil preservation, has failed to yield the anticipated evidence of early crown metazoans, instead yielding only evidence of non-metazoan holozoans or possible stem metazoans. Alongside the Weng’an biota, the Doushantuo silicified Lagerstätte in South China serves as its lateral counterpart [28,63–65]. Despite differing preservational settings, this Lagerstätte remarkably preserves fossil structures down to a subcellular level. It contains a diverse array of microfossils, including cyanobacteria, acritarchs, multicellular algae, and embryo- like fossils [63,66,67], all found in the Weng’an biota. Notably absent, however, are fossils of crown-group metazoans. As such, the available fossil evidence suggests a relatively low probability of crown metazoans diversifying in the early Ediacaran, rather than ecological constraints within the Weng’an biota’s preservational setting. Such insights prompt a recalibration of molecular timescales in light of these discoveries.
In short: There are no fossil animals in the Ediacaran, when they should be found according to the gradualistic predictions of Darwinian evolution and according to molecular clock datings. The fossil record does not agree with either of these predictions, so that the theory fails the empirical test.
Time for a Better Theory!
In his New York Times bestseller book Darwin’s Doubt, Stephen Meyer (2013) considered that the Ediacaran Doushantuo fossils may indeed include actual animal embryos of sponges. As I have already indicated (Bechly 2020b, 2022a, 2022b), this concession may have been far too generous. Even mainstream evolutionist science more and more recognizes that there simply are no metazoan animal embryos in the Doushantuo Formation. So, where are all the postulated ancestors of the more than twenty different animal phyla appearing abruptly in the Cambrian Explosion? Even Richard Dawkins (2009) has admitted that the Cambrian shows us a substantial number of major animal phyla “already in an advanced state of evolution, the very first time they appear. It is as though they were just planted there, without any evolutionary history. Needless to say, this appearance of sudden planting has delighted creationists.” Well, maybe nature is telling you something.
References
Bechly G 2020a. The Myth of Precambrian Sponges. Evolution News May 12, 2020. https://evolutionnews.org/2020/05/the-myth-of-precambrian-sponges/
Bechly G 2020b. The Demise of the Artifact Hypothesis. Evolution News June 6, 2020. https://evolutionnews.org/2020/07/demise-of-the-artifact-hypothesis-aggravates-the-problem-of-the-cambrian-explosion/
Bechly G 2022a. “Lying on the Internet”? Debunking Dave Farina on Stephen Meyer. Evolution News December 1, 2022. https://evolutionnews.org/2022/12/lying-on-the-internet-debunking-dave-farina-on-stephen-meyer/
Bechly G 2022b. Let’s Help “Professor Dave” Understand the Precambrian. Evolution News December 2, 2022. https://evolutionnews.org/2022/12/lets-help-professor-dave-understand-the-precambrian/
Dawkins R 2009. The Greatest Show on Earth. Free Press, New York (NY), 470 pp.
Evolution News 2016. New Precambrian Embryos Are equivocal at Best. Evolution News August 18, 2016. https://evolutionnews.org/2016/08/new_precambrian_1/
Meyer SC 2013. Darwin’s Doubt: The Explosive Origin of Animal Life and the Case for Intelligent Design. HarperOne, New York (NY), viii+498 pp.
Sun W, Yin Z, Liu P, Zhu M & Donoghue P 2024. Developmental biology of Spiralicellula and the Ediacaran origin of crown metazoans. Proceedings of the Royal Society B 291: 20240101, 1–10. DOI: https://doi.org/10.1098/rspb.2024.0101
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