Is Ediacara really stranded?

Heh, when I wrote a confused post about a paper by Greg Retallack that argues that classic Ediacaran fossils like Dickinsonia come from a terrestrial rather than an underwater environment, I said there’s sure to be responses. And I completely managed to miss the responses in the very same issue of Nature, apparently published online on the same day. *shameface* (I don’t think I got the commentary piece by RSS???)

One of them was actually quite nice to Retallack. L. Paul Knauth’s name doesn’t ring a bell, I suspect he’s the “geologist” out of the “palaeontologist and a geologist” the intro mentions. Of Retallack’s analysis itself, all he has to say is that Precambrian sediments can be very difficult to interpret, and one will need genuine expertise in fossilised soils ‘n’ stuff to evaluate Retallack’s claims. However, Knauth rejoices over the mere fact that there are unorthodox opinions like Retallack’s out in the open. In which he is certainly right – science wouldn’t go anywhere without disagreements.

The other commenter, Shuhai Xiao, is not so kind. (Him I’ve actually heard of; he’s published some seriously interesting stuff about Ediacaran fossils.) His commentary is kind of a polite way of saying “what a load of nonsense”. Like Knauth, he considers the evidence for the terrestrial origin of these rocks ambiguous, but he also emphasises features of the rocks that fairly unambiguously point to a marine environment. Funnily enough, he brings up geology that isn’t totally impenetrable to me as evidence, like a neat photo of Dickinsonia specimens on a slab of rock covered in nice symmetrical-looking ripples (the kind that forms under quiet waves). There’s also the fact that I forgot about when I wrote the other post: Dickinsonia itself is sometimes associated with crawling traces. Whatever that thing was and wherever it lived, it ain’t no lichen.

That’s reassuring in terms of not standing my worldview on its head, but I really wish Xiao had been less vague about some of his points. For instance, “the isotope signatures of carbonate nodules in the Ediacara Member can be accounted for by post-depositional alterations that do not involve pedogenic processes,” he says, with no further explanation and no citations. I’m thus far on Xiao’s side, but that doesn’t turn the above into a good argument…

Oh well. Let the debate rage on 🙂

(As of yet, no citations of Retallack’s paper on Google Scholar. We’ll definitely check back later. If I remember…)

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4 thoughts on “Is Ediacara really stranded?

  1. Greg Retallack January 24, 2013 / 05:20

    Xiao’s argument that Dickinsonia has to be marine because found on ripple marks makes no sense to me. These kinds of ripples are common in floodplains, below, in and above alluvial soils. Deformation of the ripple crests by microbial crust and Dickinsonia is evidence that these ripples formed well before the growth of crust and Dickinsonia. And yes they are symmetrical, thus oscillation ripples formed under only a few centimetres of water and certainly not evidence of current as indicated by Xiao. His poitn here seemed even vaguer than in other comments. Am I missing something here!

    • Naraoia January 24, 2013 / 15:23

      First of all, thanks for the comment. I didn’t expect to get this kind of attention!

      Of the two of us, I’m far more likely to be missing something 🙂 I make no secret of my rudimentary geological education. Out of that whole commentary piece, “symmetrical ripples = waves” was basically the only geological concept I had any familiarity with. (I don’t think Xiao said that those ripples were evidence of current. They seemed to be different but related points to me.)

      Random question: has any experimental taphonomy been done on mushrooms and lichens and other candidates for Ediacarans’ identities? I vaguely recall that such experiments with jellyfish convinced people that Ediacaran “medusoids” weren’t jellies, and I really liked how rotting lampreys and lancelets informed the interpretation of Cambrian chordate-like fossils… It sounds like a potentially useful approach to other weird fossils as well.

      (I wish Shuhai Xiao also showed up here. I’d very much enjoy a little informal back and forth!)

  2. Greg Retallack March 28, 2013 / 20:04

    Google alerts make possible a remarkable ability to trace news and publications on any breaking topic, and this one has been a doozy. If it is any comfort, I do not comment on the crazy-time blogs.

    Experiments in preservation of mushrooms, lichens and cyanobacterial mats have been done recently by Mihail Tomescu, and they show that such organisms are preserved in much the same way as fossil plants. This tallies with their fossil record, preserved as both compressions (like Ediacaran fossil) and as permineralizations (like the Doushantou Ediacaran lichen). Simpson and colleagues (cited in my Nature paper) have also found described Cretaceous fungal-lichen biological soil crusts preserved like Ediacaran fossils.

    • Naraoia March 28, 2013 / 21:23

      Thanks!

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