Sporfossiler nr.10 og 11 Ediacara

# 10-11: The Strange World of Ediacara

  Most Phanerozoic soft-bodied faunas come from shales that were deposited in low-oxygen environments without bottom life. In contrast, Ediacaran organisms are found in sandstones deposited in well-aerated, shallow marine habitats (wave ripples!). In our interpretation, most of them lived motionsless - like plants - and were related to the unicellular amoeba and foraminifers rather than to multicellular animals or plants. Examples of these strange life forms (Vendobionta) are shown on this and the two followings.


The sole face  on top (# 11) displays a large vendobiont (Charniodiscus). Because of its shape and the holdfast disk at the base of the stem it was also at first compared to modern seapens; but instead of the open filter required for the feeding of a polyp colony, the "feather" of Charniodiscus is a coherent foil. More likely, this organism was related to other quilted vendobionts, though it stood upright like a plant.

                      The bottom slab (# 10) shows the base of a storm sand. It was collected by Jim Gehling (Adelaide) and shows two different types of fossils, both with the quilted construction typical for vendobionts The ovate Dickinsonia was (and by some colleagues still is) believed to be a segmented worm. It even moved around as documented by resting traces (in opposite relief) next to the "death mask" of the Dickinsonia that made them. But there are no scratches that would have resulted from mechanical excavation. Probably the creature moved around on microscopic pseudopodia and then stopped to digest the biomat from above by the same organelles. The other form, Phyllozoon (see also # 13), shows a similar bilateral segmentation, but because of its elongation it rather resembles seapens. Such a relationship is discredited by the fact that its long axis may split, and by the occurrence of twinned specimens, in which several rows are connected side-by-side. Comparing the two forms, we can see the complementary preservation noted by Gehling in many specimens: only one species is preserved in sharp detail -- never both. In our specimen, Dickinsonia appears crystal-clear, while Phyllozoon (in the lower right corner) is barely visible. On other bedding planes it is just the opposite. Furthermore, quiltings form grooves in Dickinsonia, while they are ridges in the phantom. This phenomenon can best be explained by a microbial "veil" that separated both species in life. What we see in the present slab is probably the relief on the upper side of the separating biomat, in contrast to # 13.

                     

            This slab has a story. The impression on the left was found by Richard Jenkins (Adelaide) in the 70's, and described from a cast. The original remained in the field, which in the meantime had become a nature reserve. In 1992 the slab was offered for sale in Japan; but  as it had been figured in many textbooks, it was identified and repatriated by Neville Pledge to the Adelaide Museum in a spectacular police action. During the illicit excavation, however, the top part of a second specimen came to light. Unlike in slab #12, the two fronds point in opposite directions. This suggests felling by the oscillating currents of a storm rather than by a unidirectional current. To test this hypothesis, more of this spectacular bedding plane would have to be exposed and cast -- even though this might be against the policies of a National Park.         


     

                                                                                                                                            
Ediacara
        



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