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BIOSTRATIGRAPHY, PROVINCIALISM AND EVOLUTION OF EUROPEAN EARLY JURASSIC (PLIENSBACHIAN TO EARLY TOARCIAN) DINOFLAGELLATE CYSTS

Department of Earth Sciences, University of Perugia, 06100 Perugia, Italy, e-mail: Rbucefa{at}tin.it

JAMES B. RIDING

British Geological Survey, Keyworth, Nottingham NG12 5GG, United Kingdom, e-mail: jbri{at}bgs.ac.uk

New and previously published dinoflagellate cyst data from the Pliensbachian and early Toarcian of northern and southern Europe have been compared in order to define successions of Boreal and Tethyan bioevents respectively. Significant stratigraphical differences between these two regions indicates that strong provincialism affected dinoflagellates in Europe during the Early Jurassic. This therefore precludes the erection of a pan-European palynostratigraphy for the Pliensbachian to early Toarcian interval. Early Jurassic dinoflagellate cyst provincialism has been quantitatively assessed using the Koch Index of biotal dispersity and the Simpson Coefficient of biotal similarity. These methods conclusively demonstrate that marked provincialism occurred at this time, and that the Boreal and Tethyan realms represent two distinct phytoprovinces, based on their respective dinoflagellate cyst assemblages. The Boreal Realm was characterised by relatively high diversity and the dominance of Luehndea spinosa, Mancodinium semitabulatum and Nannoceratopsis spp. Valvaeodinium may also be abundant and diverse. The Parvocysta suite first appears in the Bifrons Zone and becomes relatively diverse. During the Pliensbachian and early Toarcian, the Tethyan Realm was characterised by low diversity dinoflagellate cyst floras. Mancodinium semitabulatum and Mendicodinium spp. typically dominate the assemblages. Valvaeodinium spp. are present in the early Toarcian (Tenuicostatum Zone) and the Parvocysta suite is extremely rare, represented only by Susadinium scrofoides. The transition between the two realms was diffuse. The intermediate area, between palaeolatitudes 25° and 30°, was characterised by mixed Boreal and Tethyan biotas. Dinoflagellate cyst distributions appear to have been strongly controlled by palaeolatitude, indicating that sea water temperature was a major controlling factor. This phenomenon is best exemplified by the distribution of Valvaeodinium. Dinoflagellate cyst distributions were also strongly sensitive to both coastal/oceanic settings and palaeosalinities. The relationship between Early Jurassic dinoflagellate cyst evolution and global palaeoceanographical changes have been investigated via the evaluation of diversity and the rates of speciation, extinction, and turnover. The differing patterns of evolutionary rates in the Boreal and Tethyan realms are explained as interplay between the two dinoflagellate cyst provinces and palaeogeographical changes.

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