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Paper No. 15
Presentation Time: 8:00 AM-6:00 PM

SIZE DISTRIBUTIONS, MORPHOLOGY AND TAPHONOMY OF THE FORM GENUS ASPIDELLA: EDIACARA MEMBER, SOUTH AUSTRALIA


DZAUGIS, Matthew P., School of Marine Science, University of Texas Marine Science Institute, 750 Channel View Drive, Port Aransas, TX 78373, GEHLING, James, South Australian Museum, Adelaide, 5000, Australia, DROSER, Mary L., Department of Earth Sciences, University of California, Riverside, 900 University Ave, Riverside, CA 92521, TARHAN, Lidya G., Department of Geology and Geophysics, Yale University, P.O. Box 208109, New Haven, CT 06520, DZAUGIS, Mary, Department of Earth and Environmental Sciences, University of Rochester, Rochester, NY 14627 and RICE, Dennis, South Australian Museum, North Terrace, Adelaide, 5000, Australia, matthew.dzaugis@maine.edu

The form genus Aspidella occurs prolifically on the bases of fossiliferous beds of the Ediacara Member, Rawnsley Quartzite of South Australia. Systematic excavation of 20 beds totaling over 300 sq. meters has yielded over a thousand specimens of Aspidella characterized by variable yet distinctive morphology. Aspidella is here interpreted as the fluid-filled holdfast of a variety of similarly fluid-filled frondose organisms.

Within individual beds, Aspidella occupies a wide range of sizes. It dominates three beds occurring densely packed in some areas. Diameters range from < 1 cm to 14 cm. Specimens on all three beds demonstrate a right-skewed population consistent with those reported from Newfoundland and the White Sea.

Specimens of Aspidella occur as a minor component on all of the other beds and occupy the same size range with one notable exception - an enormous specimen of Aspidella with a diameter of 50 cm occurs on a bed dominated by Dickinsonia and Funisia with only one other small specimen of Aspidella.

Aspidella is characterized by variable morphology within a bed. In rare cases, Aspidella with stalks are preserved on some beds. In other cases a ghost drag-mark of Aspidella is preserved where the organism was pulled out of the substrate. Common morphological characters include radiating ridges, concentric circles and central bosses. Importantly, the presence of these characters is not related to size and is interpreted to be a function of the taphonomic history of these discs.

The organism would likely have lived with its holdfast secured in or under the microbial mat, its stalk and frond protruding above the mat and into the water column. Upon severance of the stalk, the body-fluid would drain out and the hollow holdfast filled in with sand. Subsequently, following burial and lithification, the internal mold created by this infilling would be preserved as a convex circular structure adhering to the base of the overlying bed. In these cases, the sand-filled structure can be observed in cross-section. In other cases Aspidella was plucked away from the substrate and left a pedal hollow that was cast by the overlying bed. More commonly, however, Aspidella holdfasts occur as composite, collapsed casts preserved in continuity with the overlying bed and formed through burial and casting (smothering) of the epimat surface.

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