Extensive bioturbation in a middle Cambrian Burgess Shale-type fossil Lagerstatte in northwestern Canada

Mudstones hosting Burgess Shale-type preservation of soft-bodied fossils are commonly held to be characterized by little to no bioturbation. This has been taken as evidence for bottom-water dysoxia or anoxia, along with anaerobic conditions in the sediment, which favored preservation of soft tissues by hindering decay. Although invisible on fresh and weathered surfaces, laminated claystone comprising the middle Cambrian (Drumian) Ravens Throat River Lagerstatte in the Rockslide Formation of the Mackenzie Mountains, northwestern Canada, is revealed by preparation of surfaces sawn parallel to bedding to exhibit extensive burrowing. Four types of burrows are distinguished: (1) rare large forms averaging 15 mm in diameter; (2) backfilled vertical burrows 3-6 mm wide; (3) oblique to horizontal burrows 2-4 mm wide and typically with meniscate backfilling; and (4) tiny, short, mostly vertical burrows 0.5-1 mm in diameter. The third group is the most common, locally completely bioturbating laminae and penetrating worm carcasses; it conforms to Planolites. A variety of ethologies is indicated, with the large type seemingly serving as a dwelling burrow and the smaller ones from deposit-feeding. Although dysoxic bottom conditions probably developed occasionally, the widespread burrowing argues for predominantly oxic conditions, and it indicates that restriction of bioturbation was probably not the most important factor leading to soft-tissue preservation. Bioturbation might be more common in other Cambrian Lagerstatten than is currently believed, and it is possible that low-oxygen conditions at the seafloor were not fundamentally necessary for Burgess Shale-type preservation.