A NEW, BILATERALLY SYMMETRICAL, TUBULAR EDIACARAN ORGANISM, RAWNSLEY QUARTZITE, SOUTH AUSTRALIA
The P. ricei fossil consists of a sinuous to looping central tube up to ~80 cm long that is preserved as an irregular negative hyporelief groove on bed soles. The grooves typically have counterpart ridges on the tops of underlying beds. In some specimens, the groove (and underlying ridge) is continuous, but in others preservation is discontinuous. The width of P.ricei ranges both between separate lengths (<1mm–1cm) and within individual samples (<1mm–3mm). The widths of separate specimens can vary on the order of several millimeters (e.g., ~2mm–10mm). Well-preserved specimens reveal the presence of a segmented, outer tube that is preserved on the base of beds. The outer tube can be twice as wide as the central groove; segmentation continues across the central tube.
Among the Ediacaran organisms preserved at our field site, P. ricei is the only one that exhibits serial segmentation, and is the only one preserved on successive sedimentary beds.
The phylogenetic affiliation of P. ricei is unclear. While the groove may reflect the casting of ingested sedimentary detritus, thin sections of P. ricei ridges reveal no unconformity between the ridges and the rest of the sample. It is clear that P. ricei had an axial structure capable of withstanding initial burial without collapse. Rigid, lignified tissues characteristic of photosynthetic organisms (and which might explain the groove’s preservation) would not appear until the Paleozoic, making an algal origin for P.ricei unlikely. Further, bilaterians are characterized by through-going digestive tissue: a characteristic analogous to the through-going grooves and ridges of P. ricei. Lastly, the distribution of P. ricei on sequential sedimentary beds implies a preferred depositional environment above wave base.