Biogeography of Bathonian to Oxfordian (Jurassic) dinoflagellates: Arctic, NW Europe and circum-Mediterranean regions

Abstract

Calculations of the Koch Index of biotal dispersity suggests that the range of dinoflagellate cyst diversity in the Arctic, NW Europe and circum-Mediterranean regions was relatively high in the studied Late Bathonian to earliest Callovian, Early to Middle Callovian, Late Callovian and Early Oxfordian time intervals. Although there are no great differences, the dispersity values increase from Late Bathonian to Early Oxfordian. The Simpson Coefficient of biotal similarity (Simpson, 1960), which has been applied to quantify the variations between the dinoflagellate floras within the different regions, suggests that the differences in composition of the dinoflagellate assemblages became less prominent during Late Callovian-Early Oxfordian time compared to the Late Bathonian-Middle Callovian. During the Late Bathonian to Early Oxfordian the true Boreal dinoflagellages assemblages (Sverdrup Basin-Svalbard/Franz Josef Land) show a southwardly decreasing similarity to the contemporaneousassemblages approaching the Tethyan region. Similarly, assemblages in the southernmost Tethyan region (Middle East-Iberian Penninsula) show decreasing similarity to the assemblages approaching the Boreal areas. The dinoflagellate assemblages in the Yorkshire/East Midlands area appear to be intermediate between the true Boreal and Tethyan marine microfloras, altjough they may show a slightly closer similarity to the Tethyan than the Boreal assemblages. This distribution patterns found among the dinoflagellate cysts appears similar to that observed for the ammonities, with distinct differences between Boreal (Arctic) and the Tethyan (Mediterranian) Provinces and with a mixing of elements within the Sub Boreal Province. From late Early Callovian times the previous faunal barriers were disrupted and it is possible to make correlations at the ammonite zonal level among the Arctic, NW central European and Sub-Mediterranean regions. The dinoflagellate assemblages are profoundly different between the Boreal assemblages (i.e. in the Sverdrup Basin, Svalbard and Franz Josef Land) and those of the Sub-Mediterranean province (southern Germany, France, Switzerland) and this continued through the Middle Callovian. The Late Callovian was a period of distinct diversification among the Boreal and Sub Boreal dinoflagellate cyst floras, as shown by an increase in number of species compared to the older Callovian assemblages observed within the Sverdrup Basin, Barents Sea Region, the North Sea area and the British Jurassic. Similar to Late Bathonian-Early Callovia time, the West European Sub Boreal Province in Late Callovian time also represented an area of mixing of Boreal and Tethyan marine microfloras. As evident within the Late Bathonian-Early Callovian interval, a transition between two distinctive dinoflagellate cyst provinces appears as a generally fluctuating line transversing the Britain-North Sea region. In overall character, the Early Oxfordian dinoflagellate cyst assemblages are very similar to those of the Late Callovian time. The Simpson Coefficients for the Early Oxfordian dinoflagellate assemblages display latitudinally decreasing similarity trends comparable to the older middle Jurassic time intervals. It is evident from the present study that the increase in the dispersity values and the decreased differences in composition among the dinoflagellate assemblages from Late Bathonian to Early Oxfordian times coincided with the disappearance of important land-barriers and the establishment of new open marine sea-ways between the Boreal and Tethyan basins. These changes in the provinciality of the dinoflagellates also coincide with a period when southward migration of boreal faunas into the Paris and Lusitania Basins and a migration of Mediterranean faunas into Eastern Greenland took place.