The stratigraphy, distribution and phylogeny of some Lower Cretaceous Circumpolles from southern England
Over 300 samples were examined from a wide range of Lower Cretaceous exposures. A total of 26 Circumpolles species were recognised, 7 previously published and 19 newly described in this thesis. Each species is described using optical (O.M.), scanning electron (S.E.M.), and, where possible, transmission electron microscopy (T.E.M.). Several new techniques for combined S.E.M., O.M. and T.E.M. microscopy are discussed. The stratigraphic distribution of these Circumpolles allows the recognition of 8 informal palynostratigraphic zones, 6 of which are recognised in both marine and non-marine basins of deposition. These are; the late Volgian, early Ryazanian, late Ryazanian - early Valanginian, late Valanginian, early Hauterivian, Hauterivian - Barremian, late Barremian - early Aptian and Aptian - Albian. Combined S.E.M. and T.E.M. analyses of the Circumpolles described highlights some important phylogenetic trends seen within the Circumpolles group of pollen grains and their parent plants the Cheirolepidiaceae. There is a progressive evolutionary change from an essentially gymnospermous, Late Triassic group to a more advanced Early Cretaceous group (exhibiting many morphological features similar to those found on more recent angiospermous pollen grains). This includes a change in the intexine from undifferentiated to reticulate and from granulate/columellate to alveolate. A change in the external microsculpture of the Circumpolles group is also evident from smooth, to roughened, to granulate, echinate and eventually microechinate. The range of variation shown by the intexine appears to have phylogenetic importance and useful for both generic and specific identification of Circumpolles. Microsculptural variation, however, may well be controlled by environmental factors such as climate and is more useful for specific and suprageneric classification. Of fundamental importance to the Circumpolles group is the change from distal germination in the Jurassic to rimulate germination in the Late Jurassic, Early Cretaceous and eventually to a colpate style of germination in the Aptian. This latter mode of germination was previously considered to be restricted to, and characteristic of, angiosperm pollen grains.