Sporfossil nr. 12-13                           # 12-13: Ediacaran Snapshot  <>
                      Another important locality with Ediacaran fossils was discovered in southeast Newfoundland (Canada) in 1968. In the Mistaken Point Ecological Reserve, large inclined bedding surfaces are exposed on cliffs washed by the Atlantic Ocean. Waiting there for the rare sunny day and the slanting sunlight between 5 and 6 p.m is rewarded by the view of thousands of fossils. They are always located at the base of light-colored turbidites (see legend # 16) of volcanic ash. As the coarser-grained ash blanket weathers more readily than the underlying indurated shale, the fossils in this case appear as impressions at the top of the originally plastic shale bed. Also, Mistaken Point fossils have become laterally squeezed by the collision between the Avalon plate and Proto-North America (Laurentia). In order to restore their original shapes and orientations one has to retrodeform the images with the computer, using the now elliptical holdfasts as a standard..

                      Like their Australian relatives, the vendobionts of Mistaken Point are built like air mattresses. But instead of the serial pattern resulting from the sequential addition of new "segments" (like in annelid worms), their quilting pattern resembles the venation of plant leaves: once established, the inter-"vein" areas expand and become successively subdivided. In the terminology of the mathematician Benoit Mandelbrot such growth leads to "fractal" patterns (see plexiglass graphic by Gene Carrozza, New Haven in the introductory panel). The fractal mode serves the same purpose as serial subdivision: keeping the absolute size of the quiltings steady during growth of the organism. The presence of similar allometric compartmentalization in oversized unicellular organisms (e.g. large foraminifera) suggests that the living content of the vendobionts was syncytial, i.e. they had multiple nuclei, but were not truly multicellular.

                      Important information about the life style of the Newfoundland vendobionts can once again be derived from preservational circumstances. In the top slab (# 12), all stalked forms are oriented alike. We know the direction of the smothering turbidity current (arrow) from cross-bedding observed by Friedrich Pflüger in the overlying ash bed. Thus the alignment is equivalent to tree felling patterns after a storm: like trees, the organisms were attached to the bottom and held the fronds up into the water column, rather than being suspended from a buoyant "life saver". The erect life style, however, poses another problem. Firstly, the holdfast disks (circular when retrodeformed) never became uprooted. Aso, disks would be rather ill designed for attachment to loose sediment. This contradiction is solved by assuming that the sediment surface was sealed by a leathery microbial mat. On such a substrate, holdfast disks could function just as they do on a rock.

                      Other vendobiont species on the same bedding plane must have also been adhesively attached to the mat, because their long axes remained randomly oriented in spite of the turbidity current, in which any loose objects would have been transported and aligned before they became buried. A few of these carpet organisms became partly flipped over. They prove that both sides were identical, like those of a quilted mattress.

                                           


12
Rebecca Bendick (senior thesis, Yale University) has done statistics of this rare "fossil snapshot" on a much larger surface. She found out that the vendobionts tried to keep a certain distance from one another, but preferred to be in the neighborhood of their own species. Also, the age structure of the community suggests that survival was not yet controlled by predation -- another indication of a peaceful Garden of Ediacara. The lower cast (# 13, shown only as a drawing) shows a similar snapshot from Ediacara itself. On the sole face we see 16 equal-sized individuals of the Vendobiont Phyllozoon in clear preservation, while the phantoms of Dickinsonia can hard be recognized. As in # 10 we deal with a sole face, but in this case we view no the upper, but the lower surface on the biomat, which is also characterized by "elephant-skin structures. This means that Phyllozoon lived below the mat and Dickinsonia on top, where it could move around on its pseudopodia (see # 10). The associated sand sausages (known under the informal name Aulozoon, are probably backfilled burrows of a flatworm grazing the biomat from below    



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