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Mesozoic |
Department of Geological Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK
The Late Cenomanian bio-event is accepted as a globally synchronous extinction event that is characterized by significant biological changes which are coincident with isotopic and geochemical anomalies. The extinction event is characterized by changes on the macro-, meio-, micro- and nanno-scale and, in many examples, it has been suggested that these are ‘step-wise’. In the United Kingdom successions the steps are emphasized by depositional non-sequences, but in all cases the biological changes are in the same order. In microfaunal terms this involves a marked reduction in the benthonic foraminifera followed closely by a restriction of the planktonic foraminifera to surface-dwelling morphotypes. The event is also characterized by short-lived floods of Heterohelix sp. (small, biserial planktonic foraminiferids), Bulimina sp. (small, benthonic foraminiferids often characteristic of low-oxygen environments, calcispheres (?calcified dinoflagellates) and radiolaria. Following the extinction events, in the very latest Cenomanian, the recovery phase begins. Almost immediately the calcareous nannoplankton flora is restored but the dinoflagellates do not recover until much later in the Turonian. The benthonic foraminifera recover slowly with the fauna of the Early Turonian being of low diversity, with long-ranging taxa — including a large (?deep water), internally complex, agglutinated genus (Labyrinthidoma). The planktonic foraminifera recovered quickly with the Praeglobotruncana, Dicarinella, and Marginotruncana faunas appearing in succession within 100 000–200 000 years of the end of the extinction event. Using a model of the normal food chain it is possible to identify the order in which the basic building blocks needed for ecosystem recovery are put in place following the extinction event.
