The taphonomy and affinities of the problematic fossil Myoscolex from the lower Cambrian Emu Bay shale of south Australia

Most of the specimens of Myoscolex ateles Glaessner, 1979, the most abundant soft-bodied taxon in the Big Gully fauna from the Lower Cambrian Emu Bay Shale of South Australia, preserve only the phosphatized trunk muscles, in striking contrast to the organic residues that characterize other Burgess-Shale-type biotas. This is the oldest phosphatized muscle tissue and the first thus far reported from the Cambrian. The extent of phosphatization implies a source in addition to the animal itself, and this is reflected in high levels of phosphate in the Big Gully sequence compared to other shales. The apparent anomaly posed by the extensive preservation of labile muscle tissue as opposed to the more decay resistant cuticle is explained by the role of bacterial processes in the preservation of soft tissues. New specimens of Myoscolex reveal a variable number of trunk somites with possible tergites, and flap-like appendages. There is evidence for at least three eyes on the head, and a proboscis may have been present. An annelid affinity is rejected and Myoscolex is reinterpreted as an Opabinia-like animal with possible affinities with the arachnomorph arthropods.