Dorvilleids and arabellids (Annelida) as indicators of dysaerobic events in well-laminated non-bioturbated deposits of the French Mesozoic
Abstract Oceanic anoxic environments are characterized by laminated deposits rich in organic carbon and sparse benthic organisms. In anaerobic environments the benthic boundary is free of fauna. By contrast, in dysaerobic environments, the benthos comprises echinoderms, bivalves, crabs, ostracods, foraminifera and polychaete annelids which are known from tubes and burrows. When extremely low-oxygen concentrations are present, but not anoxic conditions, the only survivors are known by their bioturbation. The palynological studies in process on French black shales of the Toarcian, Kimmeridgian and Cenomanian-Turonian boundary provide a good occasion to give an “identity” to these bioturbations. Worms involved in the disorganization of sediments are revealed by the remains of their jaw apparatuses which are called scolecodonts, and are assigned to the Dorvilleidae-Mochtyellidae complex and Arabellidae families. The inferred length of the annelids is very small; hence the bioturbation is no longer visible after burial diagenesis. In this case, laminated deposits, without benthic fauna and burrows, are not necessary anoxic deposits but could be dysaerobic ones.