EARLIEST EVIDENCE FOR OVERGROWTH AMONG EDIACARAN FRONDS AT NILPENA EDIACARA NATIONAL PARK, SOUTH AUSTRALIA

Abstract Biotic interactions are a cornerstone of modern marine ecosystems, shaping community diversity and persistence through time. For most of the fossil record, however, such interactions are difficult to detect 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 unique opportunity to test whether biotic interactions were already common 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 investigate whether the recurring pattern of “kissing”, where the margins of adjacent Aspidella make contact, reflects a reproductive process (e.g., budding or fission) or a competitive process (overgrowth). Our results show that 75% of kissing Aspidella exhibit boundaries deformed by their neighbors, that body size and density are strongly negatively correlated, and larger individuals tend to be more spatially isolated. These results 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.