The nature of the Ediacaran macrobiota (c. 580-541 Ma) remains puzzling. These first assemblages of large, complex fossils may have included early animals; giant microbial forms; and organisms representing radically different body plans that went extinct. Discoidal impressions â some forming the base of Ediacaran fronds but most found as isolated discs â dominate the Ediacaran macrobiota. However round markings may also be formed in a variety of abiogenic ways. This study investigates the nature and biogenicity of discoidal impressions from two Ediacaran successions: the c. 560-Ma upper Burway Formation, Longmyndian Supergroup, Shropshire, UK; and several sites on the Bonavista and Avalon Peninsulas, Newfoundland, Canada, ranging in age from 565âc. 560 Ma. The investigation involved fieldwork, photography, serial grinding through cross-sections, and optical and scanning electron microscopy. It concludes that several Longmyndian discoidal forms are pseudofossils formed by sediment injection resulting from small-scale fluid escape inferred to be driven by microbial mat sealing. Turning to clearly biogenic impressions, comparison of the varied morphologies of holdfast discs associated with fronds preserved under ash and sand from several Newfoundland sites leads to a generic model of their architecture as consisting of enclosed chambers, a complex construction perhaps for strength or possibly symbiosis. Detailed observations of the rayed disc Hiemalora suggest that it may have had an amoeboid lifestyle. Finally, the key Ediacaran taxon Aspidella is separated from the discs Ediacaria and Spriggia, with which it has been synonymized, and interpreted as a possible polyp-like animal capable of limited movement. This thesis thus demonstrates that the earliest reported Ediacaran discoidal impressions are abiogenic, produced by mat-influenced processes particularly relevant to the Precambrian, and proposes models and interpretations for several key Ediacaran forms that have important implications for both the nature and diversity of the Ediacaran macrobiota, and early animal evolution.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:711877 |
| Date | January 2015 |
| Creators | Menon, Snehalatha Ramakrishna |
| Contributors | Robinson, Stuart ; McIlroy, Duncan |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://ora.ox.ac.uk/objects/uuid:ff1dc37f-711d-41f2-a3b8-733cd26cb571 |
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