Remarkable fossils on unremarkable bedding planes? The role of true substrates in the fossil record of the Ediacaran macrobiota

Strata from the late Ediacaran Period (∼579–539 Ma) host fossils of Earth's earliest assemblages of diverse macroscopic organisms: the Ediacaran macrobiota. Many Ediacaran macrobiota taxa are preserved as cast-and-mould impressions of wholly soft-bodied organisms on the tops or bases of beds (termed here Ediacaran impression fossils, or EIFs). Occurrences of EIFs are frequently interpreted to preserve in situ palaeocommunities representative of original ecosystems. Observations of the bed-scale sedimentology associated with EIFs in the Mistaken Point Formation of Newfoundland (Avalon-type biota), Rawnsley Quartzite of South Australia (White Sea-type biota) and Kuibis/Schwarzrand subgroups of Namibia (Nama-type biota) suggest in situ burial was favoured in scenarios of high sediment supply and concentrations, in conjunction with elevated aggradation rates, which together suppressed local turbulence and limited erosional scour. The resulting fossiliferous surfaces can be regarded as true substrates – surfaces preserving the sediment–water interface in instances where intervals of sedimentary stasis were succeeded by depositional events without associated erosion. We here examine key factors that underpin true substrate construction against the record of in situ EIFs in space and time and discuss the implications of considering EIFs within a true substrate framework. Our assessments highlight several points: (1) EIF preservation is facilitated in the first instance by routine compound sedimentation states where the state immediately preceding the burying bed was one of stasis, with any post-deposition modification (e.g. following decomposition of the entombed organic matter) only minimally impacting the final record; (2) Burial may occur at any stage of community colonization, growth, shifts in morphology in response to environmental triggers, functional collapse, or individual decay and degradation; (3) True substrates hold variable amounts of stratigraphic time, complicating comparison of community ecology between different Ediacaran biotic assemblages; and (4) The availability and accessibility of certain sedimentary facies more amenable to true substrate preservation will impact the apparent distribution of the Ediacaran macrobiota. Future research on Ediacaran impression fossils requires consideration of sedimentation regimes and bed construction to decouple biases inherent to these processes from genuine biological signals.