Phyllozoon and Aulozoon: key components of a novel Ediacaran death assemblage in Bathtub Gorge, Heysen Range, South Australia

Abstract

The recognition of fossiliferous horizons both below and above the classical Ediacara levels of the Flinders Ranges, South Australia, significantly expands the potential of this candidate World Heritage succession. Here we document a small window into the biology and taphonomy of the late Ediacaran seafloor within the new Nilpena Sandstone Member of the Rawnsley Quartzite in Bathtub Gorge, northern Heysen Range. A 1 m2 slab extracted from the gorge, now on permanent display at the South Australian Museum, has a death assemblage dominated by the erniettomorph Phyllozoon hanseni Jenkins and Gehling 1978 and a newly named macroscopic tubular body fossil – Aulozoon soliorum gen. et sp. nov. – on its fine sandstone bed sole. The orientations and juxtaposition of these taxa suggest overprinting of an in situ benthic Phyllozoon community by sand-filled tubes of Aulozoon carried in by a storm wave-base surge. Phyllozoon hanseni is a widespread species that is restricted to the Nilpena Sandstone Member of the Rawnsley Quartzite, whereas Dickinsonia costata ranges from the underlying Ediacara Sandstone Member into the Nilpena Sandstone Member. Fundamental differences in the ways these two vendobiont taxa are constructed and preserved may provide insights into their biology and phylogenetic affinities. In the Nilpena Sandstone Member, D. costata is joined by Dickinsonia rex Jenkins 1992, which appears to be confined to the member, and is here re-described to clarify its taxonomic status and stratigraphic distribution.

Following the logic behind biological interpretations of the Ediacaran biotas

“…Ernietta (Fig. 3) was a bag-shaped organism constructed from the same kind of modules as Pteridinium (Pflug, 1970(Pflug, , 1972bJenkins et al 1981;Dzik, 1999;Elliott et al 2016;Ivantsov et al 2016) and probably Phyllozoon (Gehling & Runnegar, 2021). Reconstructions by Jenkins (in Jenkins et al 1981) and Ivantsov et al (2019c) are remarkably similar and perhaps both incorrect in the same two interconnected ways.…”

“…A second specific example is the 'Muru pedotype' found beneath a 3-4 cm thick event bed in Bathtub Gorge, central Flinders Ranges, that buried and preserved a death assemblage of Phyllozoon fronds, Aulozoon tubes and Dickinsonia footprints (Gehling & Runnegar, 2021;Retallack, 2013, p. SI4). In this case, the underlying, equally thin sandstone, which is covered with counterpart casts of fossils found on the base of the overlying event bed, is well lithified and easily extracted from the outcrop (Fig.…”

“…Suggestions that Phyllozoon was a window lichen and Aulozoon a mycelial rhizome, with Dickinsonia as its mushroom (thallus; Retallack, 2007), are imaginative but unrealistic, given the fact that the putative rhizomes are in the event bed that buried the organisms, not in the underlying 'palaeosol' (Fig. 6; Gehling & Runnegar, 2021).…”

“…6. Fronds of Phyllozoon hanseni and tubes of Aulozoon soliorum on a piece of a 3-4 cm thick event bed from Bathtub Gorge, central Flinders Ranges (Gehling & Runnegar, 2021). The counterpart (a) is the top of the A horizon of a Muru palaeosol according to Retallack (2013).…”

“…3a) well separated by open spaces, rather different from the situation seen in the two reconstructions. This is an indication that the modules were independent units that merged during growth rather than subdivisions of the whole (Gehling & Runnegar, 2021). Seilacher (1992) had stipulated the opposite when characterizing the Vendobionta.…”