Exine ultrastructure of in situ pollen from the cycadalean cone Androstrobus prisma Thomas et Harris 1960 from the Jurassic of England

Pollen grains extracted from the cycad pollen cone Androstrobus prisma Thomas et Harris 1960 from the Bajocian of Yorkshire were studied by means of LM, SEM and TEM. The species is characterized by rounded-oval, inaperturate pollen with an indistinctly verrucate surface and predominantly homogeneous exine with occasional alveolate areas. Present and earlier published data on the fine morphology of pollen grains of fossil cycads are discussed in the light of the differentiation between Mesozoic non-saccate and presumably monosulcate pollen produced by a number of gymnosperm groups. Although the exine pattern in pollen grains of fossil cycads is usually indistinct in transmitted light, SEM reveals minute, but various sculpturing that can be used for taxonomic purposes, particularly to differentiate between species. Elongated ectexinal alveolae, situated mostly in one row and covered by a thin tectum, is an unequivocal cycadalean character. Sufficiently well preserved fossil cycad pollen clearly shows this type of exine ultrastructure. Poorly preserved cycad pollen grains commonly show an alternation of alveolate and homogeneous regions in the exine with predominance of homogeneous regions. This peculiar mode of preservation can be used as a hint to reveal cycadalean affinity. Inaperturate pollen grains are unknown in bennettites or ginkgophytes. Therefore, if a dispersed, non-saccate and boat-shaped pollen grain is proved by means of electron microscopy to lack an aperture, this would indicate a possible cycadalean affinity.