Ultrastructure of exine of gymnospermous pollen grains from Jurassic and basal Cretaceous deposits in Northwest Europe and implications for botanical relationships

Abstract

Ultrathin sectioning and examination of gymnosperm pollen grains from various Northwest European, mid-Mesozoic successions has confirmed or added weight to previous suggestions of botanical relationships for some taxa, and indicated possible affinities for others. The sections of Araucariacites australis, Callialasporites dampieri, C. trilobatus and C. turbatus support a derivation from the coniferous family Araucariaceae. Although differing in gross morphology from pollen of both extant Ephedra and Welwitschia of the order Gnetales, Eucommiidites troedssonii has ultrastructural characters in common with these, and with Triassic Equisetosporites. Cerebropollenites macroverrucosus is widely accepted as not being closely comparable to pollen of modern Tsuga, but ultrastructural similarities suggest the possibility of an ancestral connection. The structure of the exine of Clavatipollenites hughesii sensu Schulz from Jurassic deposits is fundamentally different from that of Cretaceous grains referred to the same species, confirming observations made previously on the basis of analysis under the light microscope, and suggesting a possible derivation from cycadalean rather than angiospermous plants. Species of Chasmatosporites may also be linked to the Cycadales, or perhaps to the Ginkgoales. Pilasporites couperi is shown to be a gymnosperm pollen grain rather than a pteridophyte spore, but more precise (family) links have not been established. The protosaccate structure of Quadraeculina anellaeformis is clearly demonstrated but its botanical affinity remains enigmatic. Previously suggested relationships between Spheripollenites and members of the Araucariaceae, Cupressaceae and Taxaceae are not supported. They do not seem to lie with any modern gymnosperm family, nor with ancient angiosperms.