WHAT'S HIDING UNDER THE BED? BED-TO-BED HETEROGENEITY OF A NEWLY EXCAVATED SUCCESSION OF THE EDIACARA MEMBER, RAWNSLEY QUARTZITE, SOUTH AUSTRALIA

Previous excavations of a series of beds in the Ediacara Member of the Rawnsley Quartzite west of the Flinders Ranges, South Australia has revealed a high level of heterogeneity between beds in terms of both discrete identifiable fossils as well as diverse textured organic surfaces. This pattern is further supported by new excavations. A succession of beds was excavated for a surface area of nearly 30 sq. m including five beds ranging in thickness from 3-10 cm with numerous “shims” (thin beds less than 1 cm and typically less than 2 mm in thickness) occurring in between thicker beds. Of the five beds, three have abundant fossils. One bed is dominated by holdfasts with attached, but not well-preserved, fronds along with Wigwamia and Dickinsonia. A second bed is dominated by Dickinsonia and Parvancorina. Neither have well-developed textured organic surfaces (TOS). The discrete body fossil dominating the third bed is also Dickinsonia but the bed is otherwise covered with a “groovey” textured organic surface interpreted to be the result of abundant poorly preserved tubes. Thus, the apparent dominance of Dickinsonia is deceptive in terms of the overall paleoecology and species richness; though it is impossible to “count” the number tubes. Shims are dominated by Helminthoidichnites or by non-descript textured organic surfaces that are not attributable to tubes.

Importantly, surfaces produced by poorly preserved tubes are not uncommon in the fossiliferous beds of the Ediacara Member. Of 15 excavated beds, four have such surfaces but vary considerably in terms of the associated identifiable body fossils. However, the overall ecosystem structure represented by each of these four beds was likely far more similar than would be typically interpreted based on the body fossils alone. Excavation of Ediacara Member surfaces allows for a more complete picture of Ediacaran paleoecology. Data reveal a complex ecological structure with bed-to-bed heterogeneity reflecting environmental variability as well as taphonomic differences.