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