| Summary: | Quantitative analysis of the distribution of dispersed sporomorphs (spores and pollen) has been used to assess vegetation change through the Middle Jurassic Ravenscar Group (Aalenian–Bathonian) from North Yorkshire, UK. Aalenian, Bajocian and Bathonian strata possess relatively distinct sporomorph assemblages regarding taxon occurrences, abundances and diversities (richness), suggesting a dynamic history regarding the nature of parent vegetation. Aalenian palynofloras are composed of a heterogeneous mixture of conifers, ferns, simple monosulcate pollen producers, sphenophytes and Caytoniales; Bajocian palynofloras are co-dominated by conifers and ferns; and Bathonian palynofloras are highly rich and contain assemblages of abundant ferns, conifers, lycophytes, pteridosperms/conifers and Caytoniales. Temporal variations in assemblages are a result of long-term depositional environmental changes that influence depositional taphonomic processes and parent vegetation. Comparison of the dispersed sporomorph and plant megafossil record through the Hasty Bank plant bed demonstrates that both fossil assemblages represent different aspects of the palaeoflora due to a multitude of ecological and taphonomic biases. Specifically, conifers and ferns are underrepresented in plant megafossil assemblages, bryophytes and lycopsids are represented only in sporomorph assemblages, and sphenophytes, pteridosperms, Caytoniales, Cycadales, Ginkgoales and Bennettitales are comparatively underrepresented in sporomorph assemblages. Biases between assemblages include, variation in sporomorph production levels, depositional environment and differential sporomorph and parent plant durability. Megaspore occurrences from the Middle Jurassic of Yorkshire are reviewed and a new megaspore genus, named Reticuspinosporites whytei is formally described. Transmission electron microscope analysis of the spore wall ultrastructure suggests lycopsid and possibly selaginellalean affinities.
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