About 635 Ma ago fossils of large-bodied organisms first appeared in the geological record: some quilt like, others with a crude bilateral symmetry, more looking like ‘mud-filled bags’ and ribbed discs but none that can easily be distinguished as animals, plants or colonial microorganisms. First found abundantly in the Ediacara Hills of South Australia, hence their sack-name the Ediacaran biota, it now seems that they were distributed globally in the late Neoproterozoic Era. Interpreting their metabolism is risky enough – some are reckoned to be animals that absorbed nutrients through their skin, others said to be dependent on photosynthesis – but a controversy has raged for many years over the kind of environment in which they thrived. In a detailed 2012 study of sedimentary structures petrography in the South Australian sandstones from which they were first described, Gregory Retallack of the University of Oregon inferred that some lived on land and are now found in palaeosols: they include Spriggina, Dickinsonia and Charnia that are among the most favoured candidates for being animals or some kind. Others inhabited shallow water. Anticipating fiery disputes a Nature editorial appeared in same issue in which Retallack published his paper .
Retallack has now moved on to the even more fossil-rich Ediacaran sediments of Newfoundland (Retallack, G.J 2016. Ediacaran sedimentology and paleoecology of Newfoundland reconsidered. Sedimentary Geology, v. 333, p. 15-31). Eye-wateringly detailed sequence stratigraphy of the now famous Mistaken Point locality and others suggests that the ecosystem there was an intertidal salt marsh. In detail it contains evidence for shallow-water graded bedding, signs of regular storms and perhaps tsunamis together with interbedded palaeosols and subaerial volcanic crystal tuffs whose feldspars survive intact. The palaeosols can be subdivided into several pedogenic types akin to those used to classify modern soils. Unlike the arid setting of the South Australian Ediacaran sediments, whose palaeosols show signs of freezing, the Newfoundland package indicates humid, cool-temperature climes
As in Australia, the palaeosols are rich in Ediacaran fossils, including the best known; the leaf-like Charnia and its discoidal support structure that appears in Retallack’s reconstruction of the environment in an analogous way to salt-tolerant shrubs in modern tidal flats. They occur together with encrusting fossils that bear some resemblance to modern foliose fungi or lichens. Further chuntering in the palaeontological community seems inevitable, but the sedimentological observations alone knock one hypothesis on the head: it has been said that the graded bedding common to both major Ediacaran assemblages constitutes evidence for deep marine origins from turbidity currents. But there is further compost in which controversy may thrive, in that Retallack ascribes the repeated palaeosols to glacially controlled sea-level fluctuations: the Newfoundland sequence contains two diamictites interpreted as tillite, one dated at ~583 Ma the other undated but at the top of the sequence.
3 thoughts on “Further pounding for ideas on the Ediacaran fauna”
I’m sorry, but I’m afraid you have been duped. No matter how ‘eye-wateringly detailed’ the documentation of the stratigraphy, the fact is that the interpretation of that stratigraphy by Prof. Retallack is demonstrably incorrect. It is guided by his belief, published in 1994, that the Ediacaran biota are lichens (Retallack, 1994), which requires that the settings they are found in are terrestrial (Retallack, 2012).
His 2012 Nature paper, as well as several other recent papers he has published on the South Australian and Newfoundland sections, has received just criticism both at conferences and in the literature (see for example the publications listed below). I suggest that if anyone interested in this topic was to read these, they would come to a very different conclusion to the one presented in this article.
Liu, A. G., McIlroy, D., & Brasier, M. D. (2010). First evidence for locomotion in the Ediacara biota from the 565 Ma Mistaken Point Formation, Newfoundland: REPLY. Geology, 38(10), e224-e224.
Xiao, S., Droser, M., Gehling, J. G., Hughes, I. V., Wan, B., Chen, Z., & Yuan, X. (2013). Affirming life aquatic for the Ediacara biota in China and Australia. Geology, 41(10), 1095-1098.
Callow, R. H., Brasier, M. D., & McIlroy, D. (2013). Discussion:“Were the Ediacaran siliciclastics of South Australia coastal or deep marine?” by Retallack et al., Sedimentology, 59, 1208–1236. Sedimentology, 60(2), 624-627.
Thanks for the robust comment on Retallack’s paper. My remit on Earth-Pages is to report and summarise what I regard as tantalising developments across the geosciences. To be duped implies that someone has swallowed something, hook, line and sinker: I certainly gave no sign of having done that in this case, as regards Retallack’s inference that being found in a Neoproterozoic palaeosol implies a photosynthetic metabolism for any fossils. Mind you, I think his Figure 12 is a pretty transparent provocation to stoke up the furore he has sparked over the years. As regards his interpretation of the sequences as containing various types of palaeosol, consider this – in the absence of any fossils from the sequence would any sedimentologist demur at his drawing such a conclusion…? Sure, it would have been bolstered by the presence of halite pseudomorphs, but note he reported gypsum sand and cracks resulting from ice and desiccation in the 2012 Ediacara papers. Callow et al. (2013) do not really refute Retallack’s view of the red horizons in the Edicaran type locality, and I think his evidence for subaerial deposition in Newfoundland is just as defensible and awaiting refutation. Although I am neither a sedimentologist nor a palaeobiologist, his provocative views would interest most geoscientists. It is up to readers to draw their own conclusions, and you have been helpful in providing pointers to the contrarian camp!
I think it is fair to say that Retallack has form in seeing evidence for complex ecology on land far earlier than is generally accepted – having made similar claims for the presence of terrestrial arthropod trace fossils in the late Ordovician Juanita Formation (Retallack G and Feakes C, 1987 Trace fossil evidence for Late Ordovician Animals on Land, Science, Volume 235). It is, as you say Steve, up to readers to draw their own conclusions. Having read that paper I came to the view that the red colouration and caliche style weathering was more likely to be post sedimentary than syn. I shall read the latest paper with interest – thanks for drawing attention to it…