The use of transmission electron microscopy when investigating fossil angiosperm pollen: A review and suggestions for future applications

It has been more than 65 years since transmission electron microscopy (TEM) was first used to study fossil angiosperm pollen. Since then, significant progress has been made in sample preparation protocols and TEM equipment, particularly in contrasting methods used to reveal detailed ultrastructural differences in pollen walls, as well as sensitive, high-resolution cameras needed for documentation. Early TEM studies on fossil angiosperm pollen focused on determining whether ultrastructure could be used to assign palynomorphs to spore-producing plants, gymnosperms, or angiosperms. Later research became more taxonomically focused, comparing fossil pollen walls to those of extant angiosperms. Most TEM research on fossil angiosperm pollen has until now been conducted in the USA, Western/Central Europe, and Russia, with limited contributions from other parts of the scientific community. There is a clear relation between the sample origin (country, locality) and author group, and between the type of pollen specimens (dispersed versus in situ) and the taxonomic resolution. In situ pollen from fossil flowers is usually assigned to families and genera, while most dispersed pollen is noted as Incertae sedis. Despite the obvious advantages of TEM analyses for determining the taxonomic placement and phylogenetic relations of fossil pollen, little effort has been put into this subject over the last decades. To increase the understanding of angiosperm evolution, TEM investigations of fossil pollen need to be routinely applied. New generations of palynologists need to pick up on this method, and an effort to investigate Cenozoic angiosperm pollen from a taxonomic point of view is overdue.