云南保山盆地新近纪云杉属针叶化石及其古海拔指示意义

Neogene Picea leaves from the Baoshan Basin of Yunnan and their implication for paleoaltimetry

松科(Pinaceae)云杉属(Picea A. Dietrich)花粉化石记录丰富,大化石则以木材和球果居多,而针叶化石报道较少且缺少微形态研究。云南保山盆地新近纪羊邑组产大量三维立体保存的云杉属针叶化石。这些针叶化石呈扁平条形,叶尖急尖,叶基截形,无叶柄;上表面中脉隆起,中脉两侧各有一个气孔带,每个气孔带有5–6条气孔线,气孔线多连续,少数不连续,气孔呈椭圆形至长椭圆形,非气孔带表皮细胞矩形至长矩形,细胞壁波浪状弯曲;下表面中脉亦隆起,无气孔,表皮细胞多为矩形至长矩形,细胞壁波浪状弯曲。通过与现生云杉属针叶的宏观形态和微形态对比发现,该化石与油麦吊云杉(Picea brachytyla var. complanata(Mast.) W. C. Chengex Rehder)在形态上最为相似。但由于缺少其他植物器官(如球果),难以判断化石是否真正代表了这个现生种,因此,将其暂定为似油麦吊云杉。基于云杉属在青藏高原东南缘的海拔分布范围(2000–4000 m),以及考虑到新近纪比现在更为温暖的气候环境,推测保山盆地在新近纪时期具有比现在(1650 m)更高的海拔。该推断进一步丰富了青藏高原东南缘的海拔演变历史。

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.