Neogene Picea leaves from the Baoshan Basin of Yunnan and their implication for paleoaltimetry
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Abstract:
Picea A. Dietrich. (Pinaceae) has a rich fossil pollen record. Macrofossils of the genus known to date are mainly represented by wood and seed cones, while leaves are scarce. Moreover, reported fossil leaves lack microscopic examinations, limiting their taxonomic resolution. In this study, we report a large number of three-dimensional preserved leaves and leaf fragments from the Neogene (latest Miocene to early Pliocene) Yangyi Formation of the Baoshan Basin, western Yunnan Province. These fossil leaves are compressed and flattened. The leaf apex is acuminate and its base is truncated with no petiole. The midvein is obviously raised on both the adaxial and abaxial surfaces. On the adaxial surface, a stomatal band is present on both sides of the midvein, each comprising five or six, mostly continuous stomatal files. The epidermal cells are rectangular to elongate with undulate anticlinal walls. On the abaxial surface, stomata are absent, and the epidermal cells are rectangular to elongate with undulate anticlinal walls. Based on comparisons of gross leaf morphology and micromorphology with extant species of Picea, we found that the fossil leaves are most similar to Picea brachytyla var. complanata (Mast.) W. C. Cheng ex Rehder. Due to the absence of other associated plant organs, it is uncertain if these leaves truly represent this living species. In consequence, we tentatively assign these fossil leaves to Picea brachytyla var. complanata (Mast.) W. C. Cheng ex Rehder. Based on modern altitudinal range (2000–4000 m) of Picea in Yunnan along the southeastern margin of the Tibetan Plateau, the Baoshan Basin might have reached a considerably high elevation, at least higher than the modern level (1650 m), during the latest Miocene and the early Pliocene. Given the warmer climate of that period, Picea may have occupied even higher elevations than their modern counterparts. This finding enriches our knowledge on the elevation history of the southeastern margin of the Tibetan Plateau.
