Selective incidence of the toarcian oceanic anoxic event on macroinvertebrate marine communities: a case from the Lusitanian basin, Portugal

A macroinvertebrate assemblage has been studied from the Thin Nodular Limestones Member (TNL) of the Fonte Coberta section (Lusitanian Basin, Portugal), giving special attention to its trace fossils. The assemblage was studied to analyse the influence of the Toarcian Oceanic Anoxic Event (T-OAE) on the macroinvertebrate community. The TNL consists of thin alternations of marl/limestone and nodular limestone beds, showing clear differentiations from bottom to top. Integrative analysis of the macrofossil assemblage for the studied interval, conducted for the first time in the Lusitanian Basin, reveals three major features: (1) abundant trace fossils with dominance or near exclusiveness of Thalassinoides, and a local record of Spongeliomorpha; (2) scarcity, but not extinction, of ammonoids; and (3) the absence of previously abundant brachiopods. The findings reveal a selective incidence of the T-OAE on the macrobenthic community, with a major influence (extinction) of brachiopods, severe changes (diminution in abundance and diversity) on ammonoids and a minor effect on macrobenthic tracemakers. A complex relationship between different palaeoenvironmental conditions during the T-OAE can be interpreted, discarding anoxic conditions in the seafloor, although low oxygen conditions in the water column are a possibility. The increase in seawater temperature and availability of nutrients on the seafloor would have played an influential role.