Identifying Tilioideae pollen in the fossil record using electron microscopy
- Author(s)
- Christian Geier, Johannes Martin Bouchal, Silvia Ulrich, Fridgeir Grimsson
- Abstract
The Tilioideae (Malvacae) contains about 50 extant species assigned to the three genera, Craigia, Mortoniodendron, and Tilia. The pollen morphology and ultrastructure of Tilioideae is characteristic and pollen of this subfamily is relatively easy to recognize during routine light microscopy (LM) investigations. Tilioideae pollen is small to medium sized, circular to convex-triangular in outline, oblate, brevi(3)colporate with thickened nexine surrounding the endopori (costae), and has reticulate ornamentation that can range from perforate to reticulate. To assign fossil Tilioideae type pollen to extant genera, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) has to be employed. Mortoniodendron pollen grains are small (20-33 µm in LM) and with a coarse reticulum in SEM with psilate or crested muri. The lumina are filled with free-standing columellae which form an internal tectum in TEM. Craigia pollen is medium sized (29-37 µm in LM) and has nano- to microreticulate ornamentation with psilate or striate muri and psilate to perforated lumina. In TEM, Craigia pollen has an indistinct internal tectum. Tilia pollen is large (30–52 µm in LM) and has a microreticulate ornamentation and lumina that are irregularly polygonal and funnel shaped. An internal tectum is rare in Tilia. The costae at the apertures in Mortoniodendron and Craigia pollen are of similar thickness and width. The costae in pollen of Mortoniodendron usually form acute angles to the interapertural wall, but the costae in Craigia pollen consistently form right to obtuse angles to the interapertural wall. Tilia has the most massive and widest costae, and in pollen of some Tilia species the sexine bulges outwards, which adds to the thickening of the aperture. The importance of accurately affiliating fossil Tilioideae pollen to extant genera (or to exclude them) lies in their climate preferences. Mortoniodendron occurs in fully humid to monsoonal tropical rainforest climate and biomes. Craigia grows in subtropical winter dry forests with warm to hot summers. Tilia thrives in temperate broadleaf to mixed deciduous and conifer forests in subtropical, warm temperate, and boreal climates, it is cold resistant but prefers abundant humidity. Using combined LM, SEM, and especially TEM, to investigate fossil Tilioideae type pollen makes it possible to correctly identify Craigia, Mortoniodendron, and Tilia pollen, and to exclude similar pollen from extinct taxa of the subfamily as well as other closely related fossil or extant Malvaceae.
- Organisation(s)
- Department of Botany and Biodiversity Research
- External organisation(s)
- Österreichische Akademie der Wissenschaften (ÖAW)
- DOI
- https://doi.org/10.5194/egusphere-egu25-17212
- Publication date
- 2025
- Austrian Fields of Science 2012
- 105117 Palaeobotany, 106008 Botany
- Portal url
- https://ucrisportal.univie.ac.at/en/publications/6c82eccc-5423-4628-8a70-49b802ac1121