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…)