In the Ediacaran marine succession of the Cerro Negro Formation (Tandilia System, NE Argentina), abundant microbially induced sedimentary structures indicate general conditions of substrate biostabilization. Numerous discoidal structures in this succession were previously interpreted as moulds of soft-tissue holdfasts of sessile organisms, within the form genus Aspidella. In this study, we performed a detailed re-analysis of some of these features and discuss two alternative hypotheses to explain their genesis: (1) as the result of soft-sediment deformation and fluid injection structures; and (2) as structures of active animal–sediment interaction (i.e. trace fossils). We show that the dome-shaped discs are internally laminated, with a cylindrical to a funnel-shaped vertical tube at their central region. The presence of these downwards vertical extensions and other intricate internal arrangements cannot be explained under the taphonomic spectrum of discoidal fossils, but shows striking similarities to Intrites-like structures and other sand-volcano-like pseudofossils (e.g. Astropolithon). However, some structures are hard to distinguish from vertical dwelling burrows with funnel-shaped apertures and thick-lined walls, commonly produced by suspension- and detritus-feeding invertebrates (e.g. Skolithos isp., Monocraterion isp. and, less likely, Rosselia isp.). Since reliable age constraints are unavailable, and further investigation concerning other palaeobiological indicators is needed, the most parsimonious hypothesis is that of a structure derived from fluid-escape processes. Our study demonstrates the importance of detailed investigation on discoidal structures in either upper Ediacaran or lower Cambrian strata.

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The Ediacaran-Cambrian transition interval is described for the west part of the Gondwana Supercontinent. This key interval in Earth’s history is recorded in the upper and lower part of the Tagatiya Guazú and Cerro Curuzu formations, Itapucumi Group, Paraguay, encompassing a sedimentary succession deposited in a tidally influenced mixed carbonate-siliciclastic ramp. The remarkable presence of cosmopolitan Ediacaran shelly fossils and treptichnids, which are recorded in carbonate and siliciclastic deposits, respectively, suggests their differential preservation according to lithology. Their distribution is conditioned by substrate changes that are related to cyclic sedimentation. The associated positive steady trend of the δ13C values in the carbonate facies indicates that the Tagatiya Guazú succession is correlated to the late Ediacaran positive carbon isotope plateau. Sensitive high-resolution ion microprobe U-Pb ages of volcanic zircons from an ash bed ∼30 m above the fossil-bearing interval in the Cerro Curuzu Formation indicate an Early Cambrian (Fortunian) depositional age of 535.7 ± 5.2 Ma. As in other coeval sedimentary successions worldwide, the co-occurrence of typical Ediacaran skeletal taxa and relatively complex trace fossils in the studied strata highlights the global nature of key evolutionary innovations.