A modern analogue for the trace fossil Gyrolithes : burrows of the thalassinidean shrimp Axianassa australis

The tidal flats at Praia do Araça, Brazil have muddy siliciclastic sediments on the surface and a layer of heavily packed shells down to 30–40 cm depth. The most obvious element of the infauna is the thalassinidean shrimp Axianassa australis. Several animals were captured with a yabby pump. Burrow openings were characterized by a low mound (1-2 cm high and 10–30 cm in diameter at the base) with one or two simple holes nearby (20-70 cm away). Counts along two transects showed a mean density of Axianassa burrow openings of 1.4 m−2 (range: 0–7), mounds ranged in density from 0 to 3 m−2 (mean 1.25). Three nearly complete (and several incomplete) resin casts showed a unique burrow shape, with spiral shafts leading to wide horizontal galleries from which several evenly proportioned corkscrew-shaped spirals branched off, leading to further horizontal galleries at greater sediment depths. Burrows had up to 15 such spirals and a total length of over 8 m. The total burrow depth was between 106 and 130 cm. The role of the spirals and the similarity of Axianassa burrows to the trace fossil Gyrolithes are discussed.