Wall ultrastructure of the Permian pollen grain Lueckisporites virkkiae Potonié et Klaus 1954 emend. Clarke : Evidence for botanical affinity

Abstract

Permian spore–pollen assemblages are dominated by striate bisaccate pollen grains. The botanical affinity of many of these is unknown, which limits their value in ecological reconstruction and biostratigraphy. Lueckisporites virkkiae Potonié et Klaus 1954 emend. Clarke 1965 is a bitaeniate bisaccate pollen grain of uncertain botanical affinity that dominates late Permian Euramerican assemblages, but also bridges phytogeographical provinces. It is classified into three “variants,” which capture a spectrum of morphological variability, but further complicates matters by questioning whether it comprises a natural taxon. Here we report on a TEM analysis of the gross structure and wall ultrastructure of L. virkkiae variants from the upper Permian (Lopingian) of Kimberley, Nottinghamshire, UK. The variants have the same gross structure and wall ultrastructure. The exine consists of a three-layered ektexine composed of a thin tectum, an alveolate infratectum and a thin foot layer, subtended by a solid endexine. Alveolae are either small and rounded, columellate-like, or irregularly shaped, depending on whether the exine is viewed in transverse or longitudinal section. Sacci are protosaccate, with narrow and irregular, or large and voluminous, endoreticulations. Similarities in gross structure and wall ultrastructure suggest that the current taxonomic grouping is correct with the variants representing the range of variation within a single species. We suggest that the differences in morphology are subtle and attributable to harmomegarthy. Comparisons with fossil and modern conifer pollen grains suggest an affinity with conifers. Co-occurrence with the Zechstein conifer macrofossil Pseudovoltzia liebeana (Geinitz) Florin 1880 (Majonicaceae) indicates this as a possible source.