Gyrolithes from the Ediacaran-Cambrian boundary section in Fortune Head, Newfoundland, Canada: Exploring the onset of complex burrowing

摘要

The beginning of the Cambrian explosion is characterized by the onset of infaunalization and the appearance of systematic patterns of burrowing. The trace fossil Gyrolithes is common in the Ediacaran-Cambrian Global Stratotype Section and Point, where it shows a higher diversity and burrow depth than previously reported from any Cambrian spiral vertical burrows. Two ichnospecies are present: G. scintillus isp. nov. exhibits a small burrow radius to whorl radius ratio, whereas G. gyratus (Hofmann) exhibits an exceptionally large ratio that formerly led to its identification as an aberrant ichnospecies of Skolithos. The helical morphology of these Cambrian Gyrolithes is interpreted as having served two purposes: as an anchor in a relatively high-energy environment, and as an optimal shape for maximizing surface area for bacterial gardening. These shallow-marine gardening burrows share a similar feeding strategy as graphoglyptids, suggesting that shallow early Cambrian sediments may have been relatively poor in organics and thus further supporting the model for onshore-offshore migration of gardening burrows.