Revisiting the old haunt: Paleodictyon and Protopaleodictyon from shallow-marine deposits of the middle–upper Aeronian (Silurian) of the Yangtze Platform, South China

Abstract

Graphoglyptids such as Paleodictyon and Protopaleodictyon are typical members of the Nereites ichnofacies, but occasionally they also occur in shallow waters beyond the deep sea. Here, we report Paleodictyon arvense, P. goetzingeri and Protopaleodictyon isp., alongside other trace fossils of the proximal Cruziana ichnofacies from the middle–upper Aeronian, Yangtze Platform, South China. The trace fossils were recognized within intervals deposited in the transitional zone between the lower shoreface and the inner shelf settings, as well as the delta front environment. Compiling examples of shallow-water Paleodictyon and Protopaleodictyon suggest that these trace fossils occurred in a variety of shallow-marine settings, including nearshore, shelf, and slope areas, as well as fluvial and lacustrine environments. Graphoglyptids originated in shallow shelves before migrating into deeper facies during the Early Ordovician. The return of Paleodictyon and Protopaleodictyon in shallow-marine facies after that time generally reflects high sedimentation rates, low bioturbation intensity, low benthic-food delivery and occasional event deposition in relatively quiet local circumstances. The onset of prevailing suboxic, stressed conditions during the Aeronian may have presented a colonizing window for the tracemakers of Paleodictyon and Protopaleodictyon to expand their life habitats in shallow- and marginal-marine settings, prior to higher stages of biotic recovery during the early Silurian.