Ediacaran Discs: Elucidating the Form and Function of the Discoid Fossils at the Root of Metazoan Evolution
Recent suggestions that Ediacaran discs, the most common and ubiquitous fossils of the Ediacara biota, represent the holdfasts of fronds has led them to be relatively unstudied in recent years. This thesis utilizes biometric, sedimentologic, and morphologic data from Ediacaran sites in Avalonian Newfoundland and NW Canada to elucidate the biological and preservational nature of Ediacaran discoid fossils. A study on the Avalon and Bonavista peninsulas of Newfoundland comparing Conception-style preservation of fronds visibly attached to discoid holdfasts to Fermeuse-style preservation of isolated discoid fossils confirmed that isolated specimens of the discoid fossils Aspidella and Hiemalora represent holdfasts whose petalodia were not preserved. Taphonomic studies suggest that taxa such as Charniodiscus and Primocandelabrum had holdfasts at the sediment-water interface, whereas taxa such as Charnia had holdfasts that were buried in the substrate. Morphological and sedimentological studies of Aspidella, Eoporpita, and Hiemalora from the “June beds” in the Mackenzie Mountains of NW Canada showed considerably more internal complexity than has previously been described for Ediacaran discs. Aspidella appears to represent a membrane-bound exterior surface that surrounded a tiered series of laterally-adjacent, hollow, club-shaped lobes within the holdfast that, if found separately, would be referred to the discoid genus Eoporpita. Restudy of the enigmatic, Cryogenian-aged “Twitya discs” of Bluefish Creek in the Mackenzie Mountains of NW Canada reveals probable examples of Aspidella. This discovery extends the known time range of Aspidella by nearly 80 Ma and offers a glimpse of deep-water benthic organisms that presaged the Ediacara biota. The lack of a visible stem or petalodium on either specimen of Aspidella from Bluefish Creek may be taphonomic, in which case complex multicellularity may range considerably down into the Cryogenian. Alternatively, it may imply that the disc was the ancestral structure, and that the overlying frondose structure evolved later as an oxygen-collection structure under the low and fluctuating oxygenation conditions that characterized Ediacaran seas.