Filament microfacies at the onset of the Toarcian Oceanic Anoxic Event

The onset of the Toarcian Oceanic Anoxic Event (TOAE) in the NW Europe affected by extensive anoxia is characterized by a unique peak in abundance of the epibenthic bivalve Bositra radiata in the uppermost part of the Semicelatum Subzone of the Tenuicostatum Zone (in southern Germany or in the Cleveland Basin), typically occurring in laminated organic-rich beds just at the onset of the extinction interval (coinciding with the onset of the main negative carbon isotopic anomaly). New work on Toarcian successions in the Tethys Ocean (Eastern Alps) or in successions not affected by anoxia in the Beni Snassen Mountains (Ain Reggada) shows that this bivalve occurs in a distinct ~20-50 cm-thick (non-laminated) shell-rich interval that is also associated with the initial fast depletion in d13C at these sections. These bivalves are preserved as fragments formed by an outer prismatic layer in marls (with the initially aragonitic inner layer being dissolved) or are still formed by bi-layered remains in limestones, forming a distinct filament microfacies (stratigraphically preceding the onset of the classic filament microfacies formed by Bositra buchi since the bifrons Zone). Their valves tend to be oriented concordantly but are also frequently fragmented, vary in size and do not form any discrete pavements, indicating they represent within-habitat time averaged assemblages. These assemblages are either almost monospecific or B. radiata still co-occurs with micromorphic brachiopods (that went extinct during the the TOAE), and indicate that the unique seafloor colonization phase by this bivalve occurred over a relatively broad geographic extent of the western Tethys (both on its northern and southern margins) just at the onset of the main extinction phase. This research was supported by the Slovak Agency for Research and Development (APVV 22-0523).