The largest complex burrow system from the early Cambrian of Sweden, probably made by priapulids in response to tidal currents

An extensive burrow system is reported for the first time from the early Cambrian (Series 2, Stage 4) Lingulid Sandstone of the File Haidar Formation in south-central Sweden. It occurs in a sandstone bed of nearshore origin with tidal influence and is preserved by four aligned rows of subvertical burrows, rows separated from each other by 30–35 cm. The rows are up to 140 cm long and contain up to 31 individual burrow shafts, although their unpreserved number probably was higher. The shafts have a thick concentric, spiral or eccentric lamination and are stained with iron hydroxide minerals. Analogue burrow systems from the same outcrop area indicate that the shafts were arcuate and inclined, probably merging basally. These features are characteristic for ‘Fucoides’ circinatus as originally described from this region. Lateral burrow adjustments resulted in the development of a weak protrusive spreite progressing in a north-westerly direction, roughly normal to the palaeoshoreline. Compared with similar ichnotaxa and modern analogues, priapulids are the likely producers of this complex burrow system, although polychaetes must also be considered. These worms were feeding on suspension or selectively on surface deposits, probably combined with a predatory lifestyle of a dwelling worm waiting for epibenthic prey. The alignment of the burrows in rows might be a response to acting tidal currents (e.g. ebb and flood) and gradually shifting substrate in such a tidal-dune environment. These findings represent the largest burrow system known from the Cambrian and document ecosystem engineering at the dawn of the Phanerozoic in response to hydrodynamic processes.