The evolution of organism sediment interactions in the early benthos

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The evolution of the Ediacara biota and its transition into the metazoan biotas of the Phanerozoic remains as a significant challenge to modern paleontology. One of the defining characters of the classical Ediacaran biota is that, with few rare exceptions, they were dominantly non-motile epi-benthos. This immobility is likely to have posed a significant challenge to the organisms themselves since hydrogen sulfide tends to build up under any object at the sediment-water interface that cuts off the supply of oxygenated seawater. We have recently proposed that reclining Ediacaran taxa with high surface areas were adapted to not only cope with hydrogen sulphide, but may have actually used that hydrogen sulphide to fuel a symbiosis with either ecosymbiotic or endosymbiotic thiotrophic bacteria. That inference has a number of implications that my students, colleagues and I intend to explore in the coming years.The proposed program of research aims to address the lack of understanding of a number of inter-related aspects of the early evolution of the benthos and their relationship to the evolution of seafloor ecosystems through the Ediacaran-Cambrian transition via the exploration of several lines of investigation:1) The rangeomorph-dominated soft-bodied assemblages of the Ediacaran in Avalonia need to be reassessed in the light of the revelation that the Ediacaran biota might have had chemosymbionts, in order to determine which elements of the Ediacaran biota might be sediment recliners (part of the microbe-sediment system), and which organisms were erect in the water column as many authors have assumed. This understanding is key importance for realistically assessing the palaeobiology, palaeoecology and evolution of the Ediacaran biota, which usually has most Ediacaran taxa erect in the water column instead of growing along the sediment-water interface.2) With our recent recognition of late Ediacaran meiofaunal burrows (and their potential impact on ecosystems) there is a need for an extensive search for meiofaunal bioturbation. In this respect, new discoveries of organic foraminiferal linings and agglutinated tests from 565Ma in beds with meiofaunal bioturbation preserved below matgrounds must be explored to better understand which animals were the first burrowers, and therefore how burrowing evolved.3) The post Ediacaran (i.e. Cambrian) trace-fossil rich faunas of the Burin Peninsula of Newfoundland represent the earliest evidence for burrowing below the sediment water interface, but have been little studied at the scale of individual burrow through serial grinding, and detailed petrographic and geochemical analysis. This bottom-up approach is needed to ground-truth assertions that have largely been based on bulk-sediment proxies for global oceanic chemistry. These approaches will allow us to better understand the earliest animal burrows and their impact on the early seafloor.