The taphonomy and evolutionary relationships of the Namibian erniettomorphs

Abstract

Farm Aar in Namibia contains a sequence of rocks from the late Ediacaran Nama Group. This site has produced a large number of fossils of Ernietta plateauensis and Pteridinium simplex. These two forms are incompletely characterised in the literature. Pteridinium was an elongate, three-vaned organism. It had a rubbery, semi-rigid consistency evidenced by the fact that it will bend, but does not fold or tear except at distinct points of weakness. It is tri-symmetric around the central axis. There is a distinctive offset symmetry along this axis where the vanes meet. The vanes are relatively thin and made up of parallel tubes, which are arranged in double-layers. This double-layer of tubes is also found in Ernietta. Ernietta was a sac-shaped organism with the same offset symmetry along a suture at the base of the sac, where sets of parallel tubes meet each other. Both of these organisms are found in Nama-style preservation – a taphonomy in which soft-bodied organisms are preserved three-dimensionally in sands and muds. Pteridinium is preserved within event beds, above scours where massive sand has been deposited and within waning (but still upper flow regime) flows depositing sand laminae. Ernietta is also preserved in transported deposits, but can be found on Aar in original life position, where fossils have been buried more gradually by mud. It is likely that the three-dimensional form of these organisms was preserved by the early lithification of surrounding sediment as a result of the precipitation of pyrite in association with bacterial metabolism. Both forms are also associated with membrane-like and bag-like structures. Farm Aar also bears domal structures – sedimentary structures that may represent a new form of biologically influenced soft-sediment deformation. The dual-layer of tubes making up the body walls of Ernietta and Pteridinium represents a synapomorphy unique to the Erniettomorpha. These two forms are the first-named members of this clade, which also includes the Namibian form Swartpuntia germsi and the South Australian form Phyllozoon hanseni. Swartpuntia represents a highly derived, late-appearing erniettomorph, while Phyllozoon may or may not represent a trace fossil form of Pteridinium. In either case, it is interesting in being preserved in a contrasting taphonomic setting – 'Flinders-style' preservation – to most other known erniettomorphs. The closest relatives to the erniettomorphs within the Ediacara biota are likely to be the rangeomorphs and dickinsoniomorphs. The key differences between the erniettomorphs and dickinsoniomorphs centre on the dual-layer of tubes, and the semi-rigid construction of erniettomorphs, which contrasts with the tendency of dickinsoniomorphs to fold and to be found draping small-scale sea-bed topography. Erniettomorphs were most likely osmotrophic organisms. Their anatomy results in body tissue being mostly structural, which may have placed a phyletic constraint against the development of surface area/volume ratios comparable to modern osmotrophs. They were second only to the rangeomorphs in terms of diversity within the Ediacara biota, producing global cosmopolitan taxa such as Pteridinium and novel body-plans such as Ernietta and Swartpuntia right to the end of the Ediacaran.