Stratigraphy and Fossils of the Mesoproterozoic Appekunny and Greyson Formations, Montana, USA: Insight into Early Eukaryotic Ecology and Taphonomy

Session: New Voices in Geobiology

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Abstract:

Fossil-bearing strata from the Mesoproterozoic Era are rare with macroscopic fossils being even rarer; the ~1.45 Ga Belt Supergroup stands out as containing both diverse microfossils and macrofossils.  Notably, the Appekunny and Greyson Formations, Montana, USA contain the world’s third oldest macroscopic fossils—Horodyskia moniliformis and Grypania spiralis—and the Greyson Formation has a diverse assemblage of microfossils including Tappania plana, Dictyosphaera macroreticulata, Satka favosa, and Valeria lophostriata.  The extraordinary paleobiological utility of the Appekunny and Greyson Formations has raised questions about their correlation and depositional settings; existing correlations are mainly pinned to their stratigraphic position below red-beds.  We systematically probed the stratigraphy and paleobiology of the Appekunny and Greyson Formations to assess the temporal and environmental correlations between these units and how the stratigraphic context illuminates the body fossils found within them.  Patterns in the presence or absence of fossils throughout a stratigraphic succession can be attributed to temporal (i.e., evolutionary), environmental, ecological, or taphonomic factors or a combination thereof; untangling these is challenging, but a systematic study has allowed us to test whether signs of life are stratigraphically controlled and then to assess the importance of environment vs. taphonomy. Magnetostratigraphy suggests that the members bearing Horodyskia moniliformis are not time-correlative, and rock magnetism and lithostratigraphy highlight that the Appekunny Formation was deposited at shallower depths than the Greyson Formation.  We interpret that this putative fossil’s appearance is due to a mixture of taphonomy and environment.  However, lithostratigraphy highlights that Grypania spiralis fossils are limited to a very specific stratigraphic height/lithology; we interpret this to be a true environmental feature.  Organic-walled microfossils have been challenging to find in these units, but finds have clear lithological controls and less clear linkages to stratigraphy; we suggest that taphonomy plays the most critical control on these organisms’ preservation.  Paleoredox analyses suggest waters were well-oxygenated (in opposition to earlier ferruginous results), but that sediments became anoxic and sulfidic.  Combined with paleobiological data from underlying units, this study tantalizingly suggests that oxygen does not appear to have been a first-order control on eukaryotes presence or their growth. Overall, the careful construction of basinal stratigraphic context for key paleobiological finds can shed light on eukaryotic evolution and ecology and should be more regularly applied to Precambrian successions.