THE STIRLING BIOTA – A PALEO/MESOPROTEROZOIC CONUNDRUM

The Stirling Biota of the Stirling Range Formation in Western Australia is represented by trace fossils and discoidal fossils in low-grade metamorphic sandstones. The discoidal forms were identified as Ediacaran taxa a decade ago and used to assign an Ediacaran age to the deposits, although Rb–Sr whole-rock dating had suggested a minimum age of 1.1 Ga. Our radiometric and paleontological work on the fossil-bearing sequence indicates that its depositional age is in fact 2.0–1.2 Ga (i.e. 2–3 times older than the Ediacaran), that the discoidal fossils are of uncertain nature, but that the associated trace fossils nevertheless indicate the presence of animal-like organisms. The traces are preserved in convex hyporelief on the sole of a thick bed of fine-grained sandstone. They consist of fine ridges, about 0.5–1.0 mm wide and high, forming parallel-sided pairs, 1.5–2.5 mm wide and up to more than 2 cm long. The ridge-pairs may be straight, but usually curve more or less irregularly. A recurring morphology is characterized by the ridges at one end coming together in a U-shape and at the other end flaring to about 3.5 mm width before terminating. There is no evidence of deeper penetration into the underlying sediment. The ridges are interpreted as natural moulds of mucus-reinforced sediment strings formed by the surface movements of a vermiform organism. The organism had well-developed mucus-producing capacity and probably a hydrostatic skeleton and musculature to allow it to change shape. Whereas in today’s biota this would be a description of an animal, it is possible that the traces were made by extinct multicellular or syncytial organism outside the crown-group metazoans. Whichever type of organism made the traces, the Stirling biota offers a glimpse of a Mesoproterozoic or even Paleoproterozoic biosphere which was more complex than the singularly microbial–algal world that is usually assumed.