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At 09:30 AM 08/10/2002 -0700, you (Matthew E Clapham) wrote: >Dr. Savage, > ............snip...................... > >It is possible that there are non-photosynthetic vascular plants in the >fossil record, but I don't think there were any at Mistaken Point - simply >because vascular plants did not evolve until ~150 million years after the >Mistaken Point biota (in the Silurian/Devonian). I don't know if there >are any instances of algae that can live non-photosynthetically (if there >are I would be interested...) but in any case I feel it would be >unnecessarily complicated to suggest that Mistaken Point is the lone >example from the fossil record of a diverse, abundant community of >non-photosynthetic algae. Because of their unusual morphology, it is >always easy to argue that the Ediacara biota (and particularly the >Mistaken Point biota) was something other than animals (a separate >kingdom, lichen, fungi) but recent results on their community ecology has >emphasized the striking similarities to modern epifaunal slope animal >communities. Matthew, I certainly was not suggesting that the fossils at Mistaken Point might be vascular plants! I was merely using a highly complex non-photosynthetic flowering vascular plant as an example of how versatile nature can be. Sometimes it surprises us. In biological classification today (which still tends to be arbitrary and recently has undergone immense change due to increasing insight into DNA affinities), I doubt very much that 'algae' not able to photosynthesize, whether single or multi-celled, would be considered as algae. Indeed, even some photosynthetic algae are considered to be more closely related to protozoa than they are to other algae. Water molds, slime molds and chytrids are plant-like organisms typically lacking of photosynthetic pigments. .........snip.................... >Holdfasts alone (without stems and fronds) are preserved on some >ash-covered bedding planes, but it is much more common for the entire >frond impression to be preserved. Part of the reason that no fossils have >been discovered within the tuff beds is that: 1) the later tectonic >deformation created a much stronger cleavage in the ash beds than in the >turbiditic strata, and 2) the ashfall tuff is only the (relatively) thin >bottom portion of the entire ash bed - the bulk of the thick (30-50 cm) >ash beds appear to be remobilized ash turbidites. Holdfast discs from >other localities have been found with stems projecting up into overlying >beds. It is also true that non-stalked species like spindles were >abundant on some surfaces (the D and E surfaces especially), but other >surfaces (e.g., Lower Mistaken Point) are nearly exclusively populated by >stalked frondose species. These community differences are actually >meaningful (more on this at GSA and in an upcoming paper). We feel that >the Mistaken Point bedding surfaces represent census populations of the >benthic communities - no planktonic/pelagic organisms are included. The >main line of evidence for this is taphonomic: that the organisms (even if >they had a density of 1.6 g/cm3) wouldn't sink faster than the crystal >tuff (which had a density of ~2.5-2.7 g/cm3). In addition, the fact that >the fossils with holdfasts have strong preferred orientations on all of >the Mistaken Point surfaces (related to current directions) whereas the >spindles are randomly oriented with no current accumulations (the >equivalent to log-jams). Although transport cannot be conclusively ruled >out, the taphonomic evidence against it is quite robust. This rate-of-sinking argument seems reasonable. It also seems plausible to me that it might take some time, following shock-wave killing of sea-surface organisms, before initially upward ejected ashes would begin to settle on the sea surface. There must be measurements on this in relation to recent eruptions?? Anyway, I'm sure it could be determined experimentally. The preferred orientation of the fossils could equally be a result of the direction of current flow over the bedding surface induced in response to the volcanic eruption. The oriented fossils are all essentially fusiform in shape and, I think, their long axes would tend to become aligned parallel to the flow direction even if they were settling from above. I did note one frond-like fossil with an orientation at least 20 degrees different from its neighbours. ............snip........... >The circular structure has been extensively interpreted as a holdfast >because the frondose taxa are felled downcurrent from it, with an >extremely strong preferred orientation, on every surface. If these >fossils were oriented after or during settling from a planktonic lifestyle >then other organisms (like spindles) should also be similarly >oriented. This is not observed in any situations. You mention that >plant morphology can be quite plastic - the morphology of organisms at >Mistaken Point was anything but plastic. "Segmentation" is quite rigidly >adopted in many taxa, and the morphology of those taxa is quite rigidly >developed. It is possible that the holdfast was (also) designed for >nutrient transport in some kind of chemosynthetic/chemosymbiotic >lifestyle, but the development in these communities of a complex tiering >structure that is nearly identical to that of Phanerozoic >suspension-feeding animal communities suggests that most if not all >organisms at Mistaken Point were actually suspension-feeders. There would >not be as great of an impetus to tier even if they were deriving dissolved >carbon or other nutrients from the water column. I disagree here. The frond-like organisms with putative stalk and holdfast would tend to drag, the bulbous holdfast serving as an anchor. The spindles, tapering from midpoint to both ends, would tend to tumble and become variously oriented, as you note they are. Regarding morphology, I "heard" descriptions of bilateral symmetry on the field trip, but I looked carefully and rarely saw any really convincing evidence for that. Instead, I saw abundant evidence for non bilateral development (I have photos!). I also saw what I believe are different stages in development of a single species, although as I understand it they are being interpreted as different species. Whether plant or animal, it is normal as an organism grows for its shape/appearance to change, sometimes profoundly. The morphology of Mistaken Point taxa may be rigidly developed, but the actual specimens show considerable variation. I think this question needs a lot of very careful study. >Hopefully this has satisfactorily addressed your points. Best wishes, > >Matthew E Clapham I really appreciate your response. Rod Savidge
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