Morphology and ultrastructure of Tilioideae pollen: how to differentiate Craigia, Mortoniodendron, and Tilia.

Assigning fossil Tilioideae pollen grains to the extant genera Craigia, Mortoniodendron, and Tilia is difficult. Firstly, the pollen from these genera appears similar when studied only with light microscopy. Secondly, comparison data showing the morphological and ultrastructural range of Mortoniodendron pollen need to be better defined, and there is a lack of transmission electron micrographs of Craigia pollen. Cenozoic palynorecords from Europe, especially of Paleogene age, document several Tilioideae-type pollen of taxonomic uncertainty. It is unclear if they represent extant Tilioideae genera, extinct taxa within the subfamily, or other closely related fossil or living Malvaceae. To assist in future taxonomic assignments of fossil Tilioideae-type pollen, we investigated extant Craigia, Mortoniodendron, and Tilia pollen with light, scanning electron, and transmission electron microscopy. New pollen data are provided for all three genera, and previous research was compiled to provide the most up-to-date comprehensive morphological and ultrastructural summary of these genera. Our study shows that combining light- and electron microscopy makes it possible to distinguish among Craigia, Mortoniodendron, and Tilia pollen. This will help paleopalynologists to identify these genera in pollen assemblages or to exclude them and focus on other taxa. The correct assignment of Tilioideae pollen is important for unravelling the paleophytogeographic history of this subfamily and closely related Malvaceae. Also, Tilioideae genera are typical for particular climates and/or vegetation biomes. Therefore, assigning fossil pollen to any of the three extant genera will affect the paleovegetation reconstruction and paleoclimate evaluation.