Dinoflagellates are a diverse group of flagellated eukaryotes, the majority of which are marine plankton, and are one of the most important groups of primary producers in the world’s oceans. The dinoflagellate fossil record is based on their zygotic resting cysts; this record indicates that the Bajocian of the Mid Jurassic (~170–168 Ma) represents a critical interval in dinoflagellate evolutionary history, marked by a rapid increase in the diversity of cysts from the family Gonyaulacaceae. From the Bajocian onwards, the Gonyaulacaceae have remained one of the most diverse and abundant groups of dinoflagellate cysts in the fossil record. Even so, Bajocian dinoflagellate cysts themselves have received relatively little study, leaving the patterns of this radiation unresolved. In this thesis, I examine the Bajocian diversification of gonyaulacacean dinoflagellate cysts in northwest Europe via quantitative palynological analysis, and relate this into a broader stratigraphic and palaeoenvironmental context. The dinoflagellate cyst record of the three key study areas demonstrates an increase in gonyaulacacean dinoflagellate cyst diversity through the Bajocian, irrespective of differing palaeoenvironmental settings. However, palynological and sedimentological data record systematic changes in lithostratigraphic composition and/or depositional environment which reflect changes in sea level. The integration of these data with biostratigraphic records indicates that the pattern of the radiation in Europe was strongly influenced by sea level, with the increase in gonyaulacacean diversity mirroring a major second-order transgression. On a finer scale, the main pulses of first appearances correlate with third-order transgressive episodes. A rise in sea level, coupled with changes in the tectonic configuration of ocean gateways, appears to have controlled the pattern of diversification in Europe. These palaeoceanographic changes may have enhanced water-mass transfer between Europe, the northwest Tethys Ocean, and the Hispanic Corridor, which promoted the floral interchange of dinoflagellates. Comparison to global data demonstrates that gonyaulacacean dinoflagellate cysts increased in diversity on a global scale through the Mid Jurassic. Whilst sea level rise and associated large-scale palaeoenvironmental shifts appear to have controlled the pattern of dinoflagellate cyst appearances in several regions (e.g. eastern Canada, Israel), there is no direct correlation between dinoflagellate cyst diversity and sea level rise on a global scale. Within dinoflagellate cyst assemblages from Europe, the spatial and temporal distribution of key taxa can be related to differing palaeoenvironmental settings of the basins studied; but certain patterns may reflect wider palaeoenvironmental drivers. Carbon isotope records generated for this thesis, and their correlation to other European sections, demonstrate that the Early Bajocian in Europe was marked by a positive shift in δ13C. Previous work has linked this carbon cycle perturbation to a phase of enhanced continental weathering and associated run-off. In several European basins, the Early Bajocian was marked by an acme of the genus Dissiliodinium; this genus may have bloomed in response to elevated nutrient levels. Intriguingly, a similar pattern is seen within dinoflagellate cyst assemblages from Australia, indicating this interval of palaeoenvironmental change may have had a global extent.