Opportunistic behaviour after the Toarcian Oceanic Anoxic Event: The trace fossil Halimedides

We present an analysis of the first well-documented record of the trace fossil Halimedides in Lower Jurassic (Toarcian) sediments, from the West Iberian Palaeomargin (Lusitanian Basin, Portugal). The abundant and well-preserved specimens allow a detailed ichnological characterization, supporting interpretation of the associated palaeoenvironmental conditions in terms of substrate consistency (firmgrounds), food availability and ethology of tracemaker (agrichnial/sequestrichnia behaviour). The appearance of this enigmatic trace is just after the Toarcian Oceanic Anoxic Event (T-OAE), in agreement with previous records associated with oceanic anoxic events as the OAE-1 and OAE-2. However, in the case study, oxygenation at the seafloor can be discarded as the major factor inducing colonization by Halimedides tracemakers. The rapid appearance of this trace after the T-OAE is related to the opportunistic behaviour of the tracemaker. Agrichnial/sequestrichnia behaviour — capturing and storing food particles— together with efficient microbial gardening favoured rapid colonization of the comparatively oligotrophic seafloor in a transgressive context. The record of Halimedides could be appraised as a tool to evidence significant palaeoenvironmental changes such as those associated with bio-events.