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Studies in Neoproterozoic Paleontology

  • Author(s): Riedman, Leigh Anne Smith
  • Advisor(s): Porter, Susannah M
  • et al.

The Neoproterozoic Era was one of major biotic change against a background marked by fluctuations in oceanic and atmospheric chemistry, formation and rifting of the supercontinent Rodinia, and at least two global glaciations (Snowball Earth events). Presented here are three studies of differing aspects of those biological changes.

The first is a systematic study of the diverse and well-preserved, organic-walled microfossil assemblage of the Alinya Formation of eastern Officer Basin, Australia. The use of scanning electron microscopy (SEM) revealed an unexpected level of morphological detail not visible in transmitted light microscopy and led to the recognition of new species and emendation of existing species as well as establishment of degradational sequences. In total, thirty-three taxa are described here including nineteen previously named forms, five newly described species and two new combinations.

The second study describes the organic-walled microfossil assemblages from five successions that span the first (Sturtian) glaciation (~717 Ma) and interglacial interval (>635 Ma), and integrates those data with a critical evaluation of primary paleontological literature of units deposited from ~850 to 650 Ma. The described successions from Australia and Svalbard record low species richness throughout this interval and when placed in context of all available body fossil data from the mid-Neoproterozoic, indicate global species richness may have decreased much earlier than previously realized. This finding of temporal decoupling between loss of richness and glacial onset suggests the extinctions previously associated with the Snowball Earth glacial events may not have been glacially driven.

The last of these three studies provides a broader view of the early to middle Neoproterozoic biosphere (1 Ga to 635 Ma) and describes application of the CONOP correlation and seriation algorithm to a new database of paleontological, geochemical and radiometric data. Paleobiological (first and last species appearances), geochemical and age events were placed into an ordinal sequence and calibrated to the geological time-scale to reveal a high-resolution species richness record for the first 80% of the Neoproterozoic Era. Major features of this record include an increase in species richness ~805 Ma, sustained high richness levels until a decrease ~770 Ma and a short-lived increase ~760 Ma before a steep decline ~750 Ma. The findings of the two studies described above can be placed within the context of this broader synthesis: the diverse assemblage of the Alinya Formation is representative of the richness peak between ~805 and 775 Ma and the successions recording the Sturtian glacial and interglacial assemblages in Australia and Svalbard are indicative of the extended nadir that began ~750 Ma with a recovery in species richness delayed until after the termination of the second (Marinoan) glacial event.

These three studies, together, describe a broad view of the early to middle Neoproterozoic Era and detail important vignettes within that story. From a more detailed and temporally constrained record of the Neoproterozoic biosphere, relationships between biotic and abiotic events during this transformative time can become better understood.

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