The Winner Takes It All: Competitve Overgrowth Among Ediacaran Fronds at Nilpena Edicara National Park, South Australia

Biotic interactions are a fundamental component of modern marine ecosystems, influencing community diversity and persistence through time. Across much of the fossil record, however, evidence for such interactions is difficult to identify due to limited in situ preservation. At Nilpena Ediacara National Park (NENP), South Australia, the exceptional preservation of tens of thousands of fossils from the Ediacara Member of the Rawnsley Quartzite provides a rare opportunity to evaluate whether biotic interactions were already prevalent in some of Earth’s earliest ecosystems. Here, we analyze three populations of the widespread form-genus Aspidella, interpreted as the holdfast of the frond Arborea, to test whether the recurring pattern of “kissing”, where the margins of adjacent Aspidella come into contact, reflects a reproductive process (e.g., budding or fission) or a competitive interaction (e.g., overgrowth). Our results show that 75% of kissing Aspidella display boundaries deformed by neighboring individuals, that body size and density are strongly negatively correlated, and that larger individuals tend to be more spatially isolated. Together, these findings indicate that kissing Aspidella at NENP is best explained by competitive interactions rather than reproductive processes. Because overgrowth is widespread among modern marine invertebrates, and kissing Aspidella occur globally, multiple frond taxa may have employed this strategy, suggesting that competition for seafloor space was already a pervasive feature of Ediacaran ecosystems.