Palaeobiological significance ofPlagiogmus arcuatusfrom the lower Cambrian of central Australia

The Cambrian trace fossil Plagiogmus is the floor of a backfilled burrow which is common in the deltaic sediments of the Arumbera Sandstone in central Australia. The Plagiogmus forming organism/s was/were connected to the surface by a siphon that formed a longitudinal furrow. Collapse associated with the movement of the siphon through the sediment may have produced a variety of trace fossil morphologies similar to the ichnogenera Gordia, Cochlichnus and Taphrhelminthopsis. Other parts of the Plagiogmus burrow may be compared with Olivellites, Aulichnites, Laminites, Climactichnites and Psammichnites ispp. The distinctive transverse bars of the Plagiogmus structure are interpreted as being produced by a posterior sucker used by the animal during locomotion. It is interpreted that the burrow fill was composed solely of feeding wastes and not the spoils of tunnelling. The laminae within the backfill are not related to the transverse bars. Bilobed burrows overlying the Plagiogmus component are related to the presence of a siphon that we interpret to have been used in surface deposit feeding. The biological affinities of the Plagiogmus-forming animal remain obscure, but it was probably vermiform and shared characters with the Mollusca, Annelida, Hirundinea and/or some echinuran worms.