Parrotia (Hamamelidaceae) pollen morphology and a glimpse into the fossil record and historical biogeography

Abstract

The affiliation of fossil pollen grains of Hamamelidaceae with extant genera is still a challenge for palynologists, probably due to the scarcity of pollen morphological documentation and comparison of Hamamelidaceae genera and species and their fossil counterparts. Parrotia once had a much wider geographical range during geological time according to fossil records, including macrofossils (e.g., plant leaves, and flowers) and microfossils (e.g., pollen). However, assigning fossil pollen of Parrotia is yet difficult regardless of some previous work available on pollen morphology.In this study, we present new results after intensive re-examination on the pollen morphology of Parrotia using light microscope (LM) and scanning electron microscope (SEM), aiming to assess pollen diagnostic features as a guide for fossil pollen recognition. We also compared the reticulum sculptures and nanoscale ornamentations on the muri around lumina for six genera in the tribes Fothergilleae with the representative genera from the neighboring tribes Hamamelideae and Eustigmatea. Hamamelidaceae fossil pollen types from Miocene sediments in southeastern China were examined and identified. We found that 1) pollen grains of Parrotia persica and P. subaequalis have significantly different dimensions of the length of lumen and density of lumina (numbers/10 μm2) under SEM, and the lengths of polar axis, equatorial axis, colpus, bottom side of polar view and height of polar view under LM, respectively; 2) the peculiarities of reticulum, microechinae on the muri and arrangement pattern under SEM are useful diagnostic features to differentiate pollen types of Fothergilleae groups down to genus level from two neighboring tribes; 3) two sister species of Parrotia show identifiable characteristics of reticulum and microechinae based on high resolution imaging with SEM, however, it should be noted that assigning fossil pollen to Fothergilleae still need caution, because of some similar looking close genera under LM; 4) inter- and intraspecific pollen variations need to be carefully considered, when identifying dispersed or fossil pollen grains of Parrotia under low resolution of LM imaging; 5) nanoscale structures based on high resolution imaging for pollen surface sculptures are important keys to distinguishing different fossil pollen types of Hamamelidaceae, including Parrotia.Most of fossil sites of Parrotia are concentrated in Europe-Eurasia and Japan in East Asia, but there are relatively fewer in China and America. To have an insight into the historical biogeography of Parrotia, the global fossil records of Parrotia were comprehensively reviewed and visualized in three large regions (northern American, Europe-Eurasia and East Asia). The palaeophytogeography of Parrotia in light of fossil pollen in East Asia is also generally discussed. Further investigations of the fossil record of Parrotia are expected prior to fully picturing its past phytogeography and species diversity through time, particularly some missing links during Pliocene and Quaternary in China.