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 wellpreserved 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 TOAE 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.