Nonbiomineralized carapaces in Cambrian seafloor landscapes (Sirius Passet, Greenland): Opening a new window into early Phanerozoic benthic ecology

Widespread microbial mats and the absence of significant bioturbation resulted in a poorly developed mixed layer and extensive cohesive substrates during the Ediacaran-Cambrian transition. Large nonbiomineralized arthropod carapaces overprinted with trails, interconnected burrow systems, narrow-caliber structures with dendritic terminations, and annulated burrows are abundant in the Early Cambrian Sirius Basset Lagerstatte (Greenland). Taphonomic controls were partially responsible for the pronounced association of these structures and carapaces, but ecologic conditions are envisioned as playing a significant role. Although some trace fossils (e.g., annulated structures) may be related to scavenging and/or deposit feeding, morphologic evidence suggests that this is not always the case. Interconnected burrow systems provide evidence for re-use of structures, suggesting grazing on or farming of bacteria. Sulfur bacteria were probably abundant in Cambrian sediments, requiring a source of sulfur or sulfide and limited quantities of oxygen. Anoxic sediment provided a source of hydrogen sulfide, whereas burrow outlets connected to the oxygenated sediment-water interface. Carcasses and molds of nonbiomineralized organisms may have acted as attractors, supporting a community of small invertebrates. Carapaces created a sharp boundary, further decreasing oxygen diffusion into the sediment. This steep oxygen gradient at the organic surface of the carapace promoted the growth of sulfur bacteria that could have served as the primary food for small macrofauna and meiofauna.