Rheotaxis in the Ediacaran epibenthic organism Parvancorina from South Australia

Abstract

Diverse interpretations of Ediacaran organisms arise not only from their enigmatic body plans, but also from confusion surrounding the sedimentary environments they inhabited and the processes responsible for their preservation. Excavation of Ediacaran bedding surfaces of the Rawnsley Quartzite in South Australia has provided the opportunity to study the community structure of the Ediacara biota, as well as the autecology of individual organisms. Analysis of two bedding surfaces preserving large numbers of Parvancorina illustrates that individuals display a preferred, unidirectional orientation aligned with current, as indicated by the identified current proxies: tool marks, overfolded edges of Dickinsonia, felled fronds and drag structures generated by uprooted frond holdfasts. Taphonomic and morphological evidence suggests that the preferred orientations of Parvancorina individuals are not the result of passive current alignment, but represent a rheotactic response at some stage during their life cycle. These results illustrate a previously unrecognized life mode for an Ediacaran organism and arguably the oldest known example of rheotaxis in the fossil record. The morphology and previously suggested phylogenetic affinities of Parvancorina are also re-evaluated. Apart from possessing a bilaterally symmetrical body, there are no unequivocal morphological characters to support placement of Parvancorina within the Euarthropoda or even the Bilateria.