Northward expansion of the southern-temperate podocarp forest during the Early Eocene Climatic Optimum: Palynological evidence from the NE Tibetan Plateau (China)

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).

始新世早期（距今53.3至41.2百万年，即Ma）常被视为温室地球的典型模式。位于青藏高原东北部（North-eastern Tibetan Plateau，以下简称NETP）的西宁盆地中，该时段被一套分布广泛、定年精确的蒸发岩与红色泥岩沉积序列所记录。本研究聚焦于早始新世气候最适宜期（Early Eocene Climatic Optimum，以下简称EECO；53.3至49.1 Ma）的孢粉记录，探讨该套沉积中裸子植物化石花粉的组成特征。此外，我们还研究了这些花粉属的现存近缘类群（nearest living relatives，以下简称NLR）的植物亲缘关系，以及它们在青藏高原东北部始新世地层中出现的古生物地理意义。

为达成研究目标，我们将透射光显微镜技术与激光扫描、电子显微镜技术相结合，获取高分辨率图像以展示化石与现生裸子植物花粉的形态变异。此外，还开展了形态计量学分析，以探究这些类群及其近缘类群的亚属及属内变异。

为明确数据的古生物地理背景，我们基于全球化石花粉数据库的资料，分别为Phyllocladidites以及其他罗汉松科（Podocarpaceae）类群绘制古生物地理分布图，并将其与全球生物多样性信息设施（Global Biodiversity Information Facility，以下简称GBIF）的现代记录进行对比。我们还评估了现存近缘类群的气候适配范围。

分析结果证实，西宁盆地EECO沉积中存在Phyllocladidites（现存近缘类群为Phyllocladus，罗汉松科）与Podocarpidites（现存近缘类群为Podocarpus，罗汉松科）两类花粉。此外，基于文献的比较研究表明，Parcisporites极有可能是Phyllocladidites的晚出同物异名。本研究的发现进一步显示，Phyllocladidites标本所属的支系多样性远超此前认知，且在EECO时期的生物地理分布范围远广于现今。基于现存近缘类群的气候适配范围，我们推测西宁盆地在EECO时期的古气候条件更为温暖湿润。我们得出结论：典型南温带罗汉松松林的叶枝型针叶树在EECO时期发生了北扩，随后又经历了局地灭绝。

正文图件的配套支撑数据详见：https://doi.org/10.1016/j.revpalbo.2023.104914

附表S1：Podocarpus与Phyllocladus的清理后GBIF出现数据（对应图2）
附表S2：PEX样品的孢粉形态计数数据，年代跨度为53.02（52.81~53.52）至52.36（52.11~52.57）Ma（Meyer等，2022）（对应图4）
附表S3：本研究所用全部现生及化石裸子植物花粉的测量数据（对应图5）
附表S4：用于绘制古地理图的Phyllocladidites与Parcisporites化石花粉数据集（对应图6）
附表S5：用于制作地层范围图的、按生物地理分区整理的Phyllocladidites与Parcisporites化石花粉数据集（对应图7）
附表S6：基于网格化清理后GBIF出现数据的Podocarpus与Phyllocladus气候数据（对应图8）