DOI: 10.17632/nm5cv7m56h.1
title: Northward expansion of the southern-temperate podocarp forest during the Early Eocene Climatic Optimum: Palynological evidence from the NE Tibetan Plateau (China)

description: The early Eocene (53.3 to 41.2 million years ago (Ma)) is often referred to as a model for a greenhouse Earth. In the Xining Basin, situated on the North-eastern Tibetan Plateau (NETP), this time interval is represented by an extensive and well-dated sedimentary sequence of evaporites and red mudstones. Here we focus on the palynological record of the Early Eocene Climatic Optimum (EECO; 53.3 to 49.1 Ma) and study the fossil gymnosperm pollen composition in these sediments. In addition, we also investigate the nearest living relatives (NLR) or botanical affinity of these genera and the paleobiogeographic implications of their occurrence in the Eocene of the NETP. To reach our objective, we complemented transmitted light microscopy with laser scanning- and electron microscopy techniques, to produce high-resolution images, and illustrate the morphological variation within fossil and extant gymnosperm pollen. Furthermore, a morphometric analysis was carried out to investigate the infra- and intrageneric variation of these and related taxa. To place the data in context we produced paleobiogeographic maps for Phyllocladidites and for other Podocarpaceae, based on data from a global fossil pollen data base, and compare these with modern records from GBIF. We also assessed the climatic envelope of the NLR. Our analyses confirm the presence of Phyllocladidites (NLR Phyllocladus, Podocarpaceae) and Podocarpidites (NLR Podocarpus, Podocarpaceae) in the EECO deposits in the Xining Basin. In addition, a comparative study based on literature suggests that Parcisporites is likely a younger synonym of Phyllocladidites. Our findings further suggest that the Phyllocladidites specimens are derived from a lineage that was much more diverse than previously thought, and which had a much larger biogeographical distribution during the EECO than at present. Based on the climatic envelope of the NLR, we suggest that the paleoclimatic conditions in the Xining Basin were warmer and more humid during the EECO. We conclude that phylloclade-type conifers typical of the southern-temperate podocarp forests, had a northward geographical expansion during the EECO, followed by extirpation.

Supporting data for figures in the main text [See: https://doi.org/10.1016/j.revpalbo.2023.104914]

Table S1. Cleaned GBIF occurrence data of Podocarpus and Phyllocladus (Fig. 2).
Table S2. Sporomorph counts for the PEX samples, ranging in age from 53.02 (52.81-53.52) to 52.36 (52.11-52.57) Ma (Meyer et al., 2022) (Fig. 4). 
Table S3. Measurements of all modern and fossil gymnosperm pollen that were used in this study (Fig. 5). 
Table S4. Fossil Pollen Dataset of Phyllocladidites and Parcisporites for constructing paleomaps (Fig. 6).
Table S5. Fossil Pollen Dataset of Phyllocladidites and Parcisporites  according to biogeographical realms for making stratigraphical range chart (Fig. 7).
Table S6. Climate data for grid-based cleaned-GBIF occurrence data of Podocarpus and Phyllocladus (Fig. 8).