Toarcian Oceanic Anoxic Event induced unusual behaviour and palaeobiological changes in Thalassinoides tracemakers

The Early Toarcian (Early Jurassic) global marine mass extinction is associated with one of the most important environmental perturbations of the Mesozoic, the Toarcian Oceanic Anoxic Event (T-OAE), usually interpreted as a global oceanic anoxic event. In the Lusitanian Basin (LB) — one of the reference areas for the study of the T-OAE — despite an elevated extinction rate among marine invertebrates, the anoxia hypothesis has been discarded based on geochemical and isotopic data. In the Fonte Coberta section the T-OAE is characterised by the extinction of the previously well-represented brachiopod taxa, a decreased abundance of ammonoids, and the establishment of a well-developed endobenthic community mainly composed by Thalassinoides tracemaker, which indicates favourable environmental parameters at least for this community. Thus, the palaeoenvironmental changes associated with the T-OAE event bear a minor incidence on the macrobenthic tracemaker community at the level of extinctions and changes in diversity or abundance; yet they exert a major effect on the behaviour and palaeobiology of tracemakers. The Thalassinoides specimens show an exclusively horizontal development and unusual architectural features during the extinction interval, tied to behavioural adaptations to nutrient availability and/or substrate firmness. Smaller burrow diameters are registered at the base of the T-OAE, and then an increase is observed during the T-OAE. This could be associated with a palaeobiological response (Lilliput effect) to palaeoecological changes at the beginning of the T-OAE, followed by a progressive change to pre-event environmental parameters. This study therefore sheds light on how environmental alterations can determine specific behavioural and palaeobiology changes of the producer, as reflected by variations in significant features of Thalassinoides architecture.