PRESERVATION, DISTRIBUTION AND BEHAVIORAL IMPLICATIONS OF THE EDIACARAN TRACE FOSSIL HELMINTHOIDICHNITES: EDIACARA MEMBER, UPPER RAWNSLEY QUARTZITE, SOUTH AUSTRALIA

The Ediacara Member of the Upper Rawnsley Quartzite is exposed west of the Flinders Ranges in South Australia, Australia. Preserved here are the fossils of the diverse Ediacara biota, which includes the trace fossil Helminthoidichnites. Originally described by Glaessner as “Form B,” Helminthoidichnites occurs globally and provides unequivocal evidence for the presence of motile bilaterian animals. The form, distribution and preservation of Helminthoidichnites were assessed in order to better understand the paleoecology and paleoethology of its constructor.

The levee-lined, millimeter-scale furrows of Helminthoidichnites have a guided meander morphology and are preserved primarily on the bases of very thin beds in both positive and negative hyporelief. On excavated beds, Helminthoidichnites occurs in dense clusters on beds < 1 cm in thickness and is extremely rare on beds > 1 and < 2 cm in thickness. There is no evidence for Helminthoidichnites occurring on the bases of beds with thicknesses greater than 2 cm. Indeed, within an individual bed of varying thickness, the trace fossil is restricted to the thinner portions. These data are consistent with an undermat mining lifestyle and imply that the original organism was limited to mining under only very thin layers of sand, very close to the sediment-water interface; with microbial mats and low levels of bioturbation preventing deep-tiered oxygenation of sediments during the Ediacaran, such a behavioral pattern is not surprising.

The furrow diameters of Helminthoidichnites are consistent within individual traces, but exhibit a range of values between separate traces. This suggests varying sizes of the trace constructor, and is important for calibrating early bilaterian ontogenies. Furthermore, Helminthoidichnites appears to branch. However, because individual furrows never convincingly connect, these ‘branches’ are in fact two separate traces with one trace ending as it intersects the other. These terminations are interpreted as directional indicators of an organism that was avoiding a previously constructed trace, perhaps to forage more optimally for food.