Abstract
Dinoflagellates underwent a major radiation during the Bajocian (Middle Jurassic, ~170–168Ma). The group originated in the Middle Triassic and dinoflagellate cysts were relatively low in diversity until the Bajocian, when over 100 species appeared. The Gonyaulacaceae expanded during this interval to become the dominant family of cyst-forming dinoflagellates, and has remained the principal dinoflagellate family throughout the Mesozoic and Cenozoic. However, Bajocian dinoflagellate cysts have received relatively little study. In order to unravel the pattern of the Bajocian dinoflagellate radiation, we have generated a high resolution, quantitative, palynological record through an expanded Middle Jurassic succession in Swabia, southwest Germany. Previous research has indicated a rapid, stepwise order of first appearances through the Bajocian. By contrast, we clearly demonstrate that there was a more gradual, continuous increase in diversity from the Late Aalenian to the Early Bathonian, although the number of first appearances was highest during the latest Early Bajocian to Late Bajocian. Major experimentation in excystment mode in the gonyaulacaceans occurred during the Late Aalenian and Early Bajocian, when multi-plate precingular, single-plate precingular and epicystal archaeopyle types first appeared. Through the integration of our data with published information from northwest Europe, we demonstrate that the appearance of dinoflagellate cyst taxa through the Late Aalenian–Early Bathonian appears to have been controlled by a major second-order transgression. Although the cause of this radiation is yet to be constrained, given the contemporaneous diversifications of other plankton groups, as well as ammonites, bivalves and fishes, it is likely that it formed part of the wider Mesozoic Marine Revolution. There is a prominent acme of Dissiliodinium giganteum in the Lower Bajocian which is a useful stratigraphical marker. Acanthaulax crispa is confirmed as a key index species for the Bajocian. One new species, Korystocysta aldridgeii sp. nov., is described.
Highlights
Dinoflagellates are a diverse group of flagellated freshwater and marine protists, the majority of which are photosynthetic or mixotrophic marine plankton
Dinoflagellate cysts were low in diversity throughout the Middle Triassic to earliest Middle Jurassic, until a major radiation occurred during the Bajocian (170–168 Ma), when ~ 100 new species appeared (Fig. 1; Woollam and Riding, 1983; MacRae et al, 1996; Riding et al, 2010)
As this work has examined the S. humphriesianum zone in unprecedented detail, we show that the lowest occurrence datums (LODs) of A. crispa is in the middle of the zone, we raise the highest occurrence datum (HOD) from the P. parkinsoni zone to the lowermost Bathonian, Z. zigzag zone, P. convergens subzone
Summary
Dinoflagellates are a diverse group of flagellated freshwater and marine protists, the majority of which are photosynthetic or mixotrophic marine plankton. Dinoflagellate cysts were low in diversity throughout the Middle Triassic to earliest Middle Jurassic, until a major radiation occurred during the Bajocian (170–168 Ma), when ~ 100 new species appeared (Fig. 1; Woollam and Riding, 1983; MacRae et al, 1996; Riding et al, 2010). The family Gonyaulacaceae expanded throughout the Bajocian and became the dominant group of cyst-forming dinoflagellates, which remains so to the Recent (Fig. 1; Fensome et al, 1996). Other groups of plankton radiated at this time, including coccolithophores and planktonic foraminifera (Hart et al, 2003; Suchéras-Marx et al, 2015), implying major evolutionary innovations occurred within plankton during the Bajocian
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