What Happens Between Depositional Events, Stays Between Depositional Events: The Significance of Organic Mat Surfaces in the Capture of Ediacara Communities and the Sedimentary Rocks That Preserve Them

Abstract

In the absence of complex, bioturbating organisms, the seafloor during the Precambrian was covered in widespread organic matgrounds. The greatest diversity and complexity of organic mat textures occur in the Ediacaran fossil record as exemplified by the Ediacara Member of the Rawnsley Quartzite, which crops out in and around the Flinders Ranges, South Australia. This succession unambiguously demonstrates that heterogenous mats coexisted with and were central to the ecology and biology of the Ediacara Biota. Excavation of 33 fossiliferous beds with varying types and extents of organosedimentary surface textures provide the opportunity to utilize this record to develop criteria to evaluate the maturity or extent of growth of Ediacaran matgrounds and, using these characteristics, to examine the relationship between mat type, mat maturity and Ediacara Biota community structure. Based on the assumption that mat maturity represents an indicator of the duration of time between burial events, we can test predictions about the relationship between mat maturity and community development. We find that mat maturity, rather than the mat type itself, more strongly influenced the distribution of taxa and the development of Ediacara macroorganism communities. Using a ranked Mat Maturity Index, we find that although density of macroscopic body fossils and genus diversity correlate with mat maturity, evenness does not. We additionally find that the sessile taxa Obamus and Coronacollina are restricted to surfaces with mature mats whereas all other Ediacaran macrobiota show no connection to mat occurrence and maturity. However, we do observe that large Dickinsonia are more likely to occur on surfaces recording mature matgrounds. The exceptional record of mat surfaces preserved in the Flinders Ranges area demonstrates that, in addition to the apparent ecological role played by mat surfaces in Ediacaran communities, they were also likely a significant component of the Ediacara Member biomass and were integral to community function.

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“…Terminal segments are sub-triangular. These purported Dickinsonia specimens can be considered large specimens and as per recent understanding, the large Dickinsonia is found in the litho-facies with mature mat ground (Droser et al, 2022). However, there is no evidence of microbial mat-related structures on the bedding surface, tangential, and longitudinal sections of the exposed sandstone surfaces of the Auditorium Cave or on the nearby outcrops and thereby negating the possibility of occurrence of Dickinsonia in the depositional environment of the Maihar Sandstone.…”

Species of Dickinsonia Sprigg from the Ediacaran of South Australia

“…tenuis may be correlated with their presence on more mature mats under the assumption that these provided greater nutrients compared with less developed organic substrates (Droser et al . 2019, 2022). For example, the most abundant and largest D .…”

“…4I), respectively, which represent two of the most mature surfaces at NENP (Droser et al . 2022). However, large specimens of D .…”

Spatial distributions of Tribrachidium, Rugoconites, and Obamus from the Ediacara Member (Rawnsley Quartzite), South Australia

“…Although seven Ediacaran taxa have been cited as potentially mobile (Evans et al 2019), the majority of taxa from the Ediacara Member were sessile and lived atop or embedded in laterally extensive microbial mats (Droser et al 2019(Droser et al , 2022. These mats were abundant in the absence of widespread bioturbation and range in maturity between beds at NENP (Droser et al 2022).…”

“…Although seven Ediacaran taxa have been cited as potentially mobile (Evans et al 2019), the majority of taxa from the Ediacara Member were sessile and lived atop or embedded in laterally extensive microbial mats (Droser et al 2019(Droser et al , 2022. These mats were abundant in the absence of widespread bioturbation and range in maturity between beds at NENP (Droser et al 2022). Among the sessile organisms, the triradialomorph taxa Tribrachidium and Rugoconites occur in both South Australia and the White Sea of Russia and are abundant at NENP (Glaessner and Daily 1959;Glaessner and Wade 1966;Hall et al 2015Hall et al , 2018Boag et al 2016).…”