Explaining the exceptional preservation of Ediacaran rangeomorphs from Spaniard's Bay, Newfoundland: A hydraulic model

Exceptional 3-D preservation of Ediacaran rangeomorph fossils is found on a single bedding plane at Spaniard's Bay, Newfoundland. This high-quality preservation has previously been explained by entrainment of organisms within the T_d-e mudstone division of a distal turbidite, followed by encasement within concretions. Our sedimentological and taphonomic analysis reveals a clear association between these fossils and evidence for erosive unidirectional flows, including scours marks, tool marks, ridge-and-groove marks, parting lineations and current crescents. We suggest an alternative sequence of events that runs broadly as follows: (i) rangeomorph discs were anchored to the seafloor during deposition of planar laminated silts (our unit 2, <10mm thick; T_d), now bearing pyrite framboids and pyritized organic matter; (ii) rangeomorph fronds were then felled and entrained by high velocity unidirectional currents, to lie within their own erosional scours at the top of unit 2, or to form tool marks; (iii) this topography was then draped and cast by soft-weathering sand (unit 3, T_c) associated with the growth of early diagenetic pyrite around sand grains. Pyrite grains also appear to have replaced clumps of organic matter. Fossil impressions have since been exposed by differential weathering of the ferruginous sands with respect to the silts. This new context now provides a parsimonious explanation for a range of hitherto paradoxical structures. Features previously regarded as microbial mats ('bubble trains') that formed in the lee of sinuous ripples on the top of unit 2 may be explained as load-casts, or by localised gas escape within areas of lowered hydraulic pressure. Rangeomorph fronds remarkably preserved in positive (rather than the more usual negative) epirelief are explained by means of sediment-casting of branches that became ruptured in the high velocity current. Paradoxical structures previously thought to be enclosing biological 'sheaths' around rangeomorph fronds are reinterpreted as scour marks, whereas imbricate overlaps of first order branches in Beothukis, Trepassia and Avalofractus are explained by hydraulic shear, driven by overlying currents across ruptured and deflated fronds. We find that rangeomorph bodies could be deflated, imbricated, folded over, inverted, and infilled with fine sediment. Our hydraulic model provides a null hypothesis against which future observations of rangeomorph fronds can now be tested. It removes some significant anomalies in our understanding of rangeomorph architecture, and provides a better understanding of the physical properties of their body tissues, permitting the possibility of a reasoned consideration of their puzzling biological affinities.