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On Sat, 22 Jul 1995, Michael Kirby wrote: > are, however, terrestrial lichens living on soft substrates. Another big > problem with relating Ediacaran organisms to living marine lichens is that > it appears that marine lichens have only recently invaded the oceans, > probably long after the Precambrian. Kappen (1973, p. 364) noted that "The > very pronounced zonation of lichens in and around aquatic environments and > the specialization of only a few taxa show that aquatic habitats represent > extreme environments" for lichens. Though Retallack's (1994) lichen > hypothesis may at first glance seem plausible, the closer you look at the > available information on lichens today, the less plausible his hypothesis > becomes. There are just too many inconsistencies that must be overlooked > in order to accept Ediacaran organisms as lichens (as we know them today). Since "lichens" don't form a clade, but represent a way of organizing symbionts, it's always possible that the Ediacaran could have been crawling (metaphorically) with lichens that had no relationship to any living lichens -- maybe they weren't algal-fungal, but fungal- cyanobacterial, or actinomycete-cyanobacterial, or hyphochytridiomycete-thraustochytridial-glaucophyte-rhodospirillid. But I have no idea how you would turn this into a testable hypothesis. Also, a photosynthetic organism lying underwater on soft muds or sands would tend to get buried quickly, unless it had some way of digging itself out or moving sediment off itself. Some Ediacara fossils were flat, lay flat on substrates, and reached nearly a meter in diameter; I don't see how this could be done unless they had some way of clearing sediment from themselves. Might that be why marine lichens aren't found on soft substrates underwater -- because they'd get fouled and buried before growing to any size? > > On the flip-side, I think that we should reexamine Ediacaran localities to > see if any of them could be non-marine. If a marine environmental > interpretation is upheld for these localities, then based on what we know > of marine lichens today, I think that we can safely reject the hypothesis > that the Ediacaran organisms were lichens. I would not rule out, however, > the possibility that some (though not all) Ediacaran organisms may > represent some other kind of autotroph or plant-like organisms which went > extinct right before the Cambrian explosion and therefore left no living > descendents for us to examine today. > The Estonian geologist Enn Pirrus has examined Eastern European Vendian sections, looking at sedimentology and clay geochemistry (I can get refs if anyone wants them -- they're in English). It seems that the lower Vendian, the Redkino Stage, was marine, and is characterized by Ediacara-type faunas. During the late Vendian, apparently the E. European basin was isolated from the ocean and became fresh water (glacial melt, perhaps?) -- and at this time, the Kotlin Stage, Ediacara fossils become extremely rare, but "carbon films" become common. So if this interpretation of the facies is correct, then the Ediacara fauna is definitely not non-marine -- at least not in north Russia, and the fauna there is very similar to the Australian fauna. Retallack's paper hypothesized that the Australian forms were living on soil subaerially. Maybe. Maybe not. None of my field observations in Russia suggest that any of the fossils there lived on land. Can anyone add to this? Part of Retallack's evidence comes from supposed paleosols in the Ediacara Hills, and I don't know paleosols at all. . . In fact a few localities with Ediacara fossils (North Carolina, Newfoundland, Siberia) are supposed to represent deep-water facies, which means that if those fossils are autochthonous, they`re not likely to be photosynthetic. Maybe they were non-photosynthetic lichens. <tongue in cheek> I've got it! They must have been non-fungal lichens that lacked algal symbionts! </tongue in cheek> My two renminbis` worth for the day. Ben Waggoner UCMP Berkeley, CA 94720
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