THE WINNER TAKES IT ALL: INTRASPECIFIC COMPETITION ON THE EDIACARAN SEAFLOORS

Intraspecific spatial relationships of fossil organisms at the meter-scale are typically impossible to analyze due to time averaging. Unique preservation at Nilpena Ediacara National Park, South Australia (NENP) allows for entire bedding planes of the Ediacara Member of the Rawnsley Quartzite to be excavated revealing over 300 m² in situ communities of Ediacaran organisms. This enables the spatial distribution of these early animal communities to be examined, revealing information about their biology and ecology. Aspidella is a form taxon interpreted as a holdfast that is found in abundance at NENP. Three bedding planes —STC-AB, TC-MM2, and WS-MAB— are dominated by Aspidella, preserving at least 100 individuals on each bed. Here we examine these populations using spatial point patterns analysis and examine how different size cohorts in the population are distributed. Results show that Aspidella were spatially thinning as they grew, with smaller sized populations being best-fit to aggregation and larger individuals being either random or heterogeneous in their distributions. This implies that as small Aspidella grew they competed for ecospace resulting in the mortality of certain individuals, an ecological dynamic that occurs commonly in modern ecosystems. Thinning additionally explains the common occurrence of Aspidella margins touching, which would result from multiple Aspidella growing into each other. This work provides some of the earliest evidence of spatial thinning in the fossil record and reveals that Ediacaran ecosystems had certain parallels to those observed in the modern.