Drivers of Ecological Dynamics in Ediacaran Avalonian Communities (~574-560ma).

The Avalon Biota (~580 — 560 Ma) records the oldest, deep-water communities of early animals. The preservation of these Ediacaran communities is exceptional, with thousands of these immobile animals killed then preserved in-situ beneath under-water volcanic ash flows, capturing a census of life at this time. These snapshots of Ediacaran life enable us to use a toolkit of quantitative ecological methodologies, normally only applicable for extant taxa, to enable rigorous hypothesis-testing to investigate the ecological dynamics of these early animals. We have mapped out 34 bedding plane surfaces to a 50 micron resolution from Charnwood Forest, UK, Mistaken Point, Spanaird’s Bay and Discovery Geopark, Newfoundland, Canada spanning 574 Ma – 560 Ma, using a combination of laser-line probe, LiDAR and photogrammetry. This data covers out 1076.86 m2 of fossilerfersous bedding planes with 53 taxa found within 20,242 specimens. The sized of the bedding planes was highly variable, from 0.8 m2 to 144 m2 so we used Species Area Curves to determine the extent of sampling, finding only 13 reached saturation in terms of captured diversity. However, our sensitivity analyses demonstrated that while taxonomic richness within a given community can’t be routinely captured, the Shannon and Simpsons diversity indices are robust. We found no biogeographical, environmental or stratigraphic associations with these diversity metrics. To investigate the mechanisms of Ediacaran eco-evolutionary dynamics we quantified vertical competition using distinct vertical stratification (i.e., tiering) and horiztonal competition using spatial point process analyses. We found that the presence and strength of intra-specific competition was significantly reduced by stoloniferous reproduction, which our mechanistic models showed subdued selection pressure and diversity within the Avalon Biota. Inter-specific competition between taxa pairs was structured through a trade-off between horizontal and vertical dimensions, and was not shaped by phylogenetic relatednessor morphology. There was a significant negative association between horiztonal (segregation intensity) and vertical (tiering) competition, which varied across communities, taxon pairs and taxon height gradients. These changing competition patterns indicate that inter-specific competitive relationships were not fixed properties of particular taxa but instead operated as an active ecological process.