TRACE FOSSILS AND TRACE FOSSIL-LIKE BODY FOSSILS OF THE EDIACARA MEMBER, RAWNSLEY QUARTZITE, SOUTH AUSTRALIA

The Ediacara Member of the Rawnsley Quartzite is exposed west of the Flinders Ranges in South Australia. Preserved here are the fossils of the diverse Ediacara biota, which includes the trace fossil Helminthoidichnites, and a previously undescribed fossil informally referred to as “stitch and groove” (SNG), of which superficially resembles a trace fossil. However, SNG does not fulfill basic criteria for having a trace fossil origin. By comparing SNG with the unequivocal trace fossil Helminthoidichnites, the trace fossil-likelihood of SNG can be assessed.

The Ediacaran trace fossil record is the best way to constrain the first appearance of motile bilaterian animals, so distinguishing between an Ediacaran trace fossil, body fossil, or other sedimentary structure is important. This can be difficult, however, given the prevalence of Ediacaran tubular (and thus trace fossil-like) fossils.

Helminthoidichnites is a levee-lined, millimeter-scale furrow, made within biomat coated substrates. Several lines of evidence make Helminthoidichnites an unquestioned trace fossil: 1) the trace exhibits consistent evidence for sedimentary displacement in the form of raised levees, 2) diameters are consistent along any given specimen’s length, 3) the trace cuts through body fossils of the Ediacara biota, 4) the trace is confined to the soles of very thin (≤ 10 mm thick) beds, and 5) Helminthoidichnites occurs in both positive and negative hyporelief and epirelief on the soles and tops of beds, meaning the original trace was prone to collapse depending on its position within biomats.

SNG is an elongate groove that can reach ~82 cm in length and up to ~1 cm in width (most grooves are typically ~1–4 mm wide). Unlike Helminthoidichnites furrows, SNG grooves widen and contract along a straight path, resulting in a superficial stitching pattern (e.g. one groove can range from 1–3.5 mm in width); a trace should only widen at a turn. Furthermore, SNG only occurs in negative hyporelief on bed soles, and in positive epirelief on bed tops; this is consistent with an organism that, upon burial, was more resistant than a trace to structural failure. Also, instead of being limited by bed thickness like Helminthoidichnites, SNG is preserved on the soles of beds with thicknesses of 0.1 mm to 15.5 cm.