云南宜良上新世栎属植物研究及其古环境指示意义

Fossil plants of Quercus from the Pliocene of Yiliang,Yunnan Province and their paleoenvironmental implications

栎属(Quercus)植物叶片化石在新生代地层中分布普遍,对亚热带常绿阔叶林具有重要的指示意义。本文对采自云南省宜良县上新统茨营组中的栎属植物叶片化石进行角质层结构分析,结合叶形态特征,系统描述了4种栎属植物:阔叶栎(Quercus latifolia)、线叶栎(Q.scottii)、楔基栎(Q.simulata)和黄毛青冈相似种(Q.cf.delavayi)。通过对比化石和现生植物的叶形态和角质层特征,本文认为叶片形状、叶基和叶尖特征、二级脉数量、表皮细胞和垂周壁特征、气孔大小和形状、气孔密度、毛基类型、形态和分布情况可以作为区分不同种类的特征,但是不能只考虑其中的一个因素,应该综合这些特征作为判断依据。结合宜良植物群中其他化石资料认为,包括壳斗科柯属(Lithocarpus)、栲属(Castanopsis)以及樟科、榆科、木兰科等在内的植物,代表了以栎属青冈组为优势树种的半湿润常绿阔叶林。宜良植物群与同时期相近纬度的植物群相比,植被类型相似,但是落叶成分较少。宜良植物群中的线叶栎、楔基栎和阔叶栎同晚始新世的线叶栎以及早中新世的楔基栎和阔叶栎相比,形态并未发生大的变化,与现生植物也非常相似,进一步说明云南现代常绿阔叶林的主要成分来自于古老植物的承袭,没有发生较大的改变。

Aims:Fossil leaves of the genus Quercus have been widely reported from the Neogene.However,many fossil species were previously named without epidermal features.The present specimens from the Pliocene Ciying Formation in Yiliang,Yunnan Province provide detailed information about cuticular features,which can be useful for understanding the morphological variation at the genus and species level.Moreover,we explored the evolution of the evergreen broad-leaved forest in Yunnan Province,of which the genus Quercus is still the dominant element.Methods:We observed the cuticular characteristics of the specimens using standard maceration and mounted it in glycerol on glass slides.By comparing leaf physiognomy and cuticular characteristics with previously reported fossil species and extant leaves,the present specimens were identified at the species level,containing three extinct and one existing species.Combined with else fossil plants collected from the Pliocene Ciying Formation,the vegetation form of Yiliang flora was recognized and compared with other floras of the same age in Yunnan Province.Results:We systematically described leaf physiognomy and cuticular characteristics of four species of the genus Quercus:Q.scottii,Q.simulata,Q.latifolia,and Q.cf.delavayi.The present Q.cf.delavayi was similar to Q.praedelavayi,Q.tenuipilosa and living species Q.delavayi in leaf morphology and epidermal structures except for the trichome base densities.The compound trichome bases densely occurring in the advanced veins may be related to a defense function.The nearest living relative of the studied fossils were assigned base d on morphological similarity.The fossil components indicated a semi-humid evergreen broad-leaved forest existed in the Pliocene of Yiliang area,which was dominated by Quercus sect.Cyclobalanopsis.The vegetation form was similar to the floras distributed in low-middle latitudes,rather than in middle-high latitude areas.The morphological characteristics of the present fossils Q.scottii,Q.simulata and Q.latifolia were similar to the Q.scottii of the late Eocene,Q.simulata and Q.latifolia of the Miocene,respectively,and all of them matched the nearest living relative species well.This implies that the dominant elements of the evergreen broad-leaved forest were inherited from the distant ages without great variation.Conclusion:By analyzing leaf physiognomy and epidermal features,we determined that the present specimens belong to four species of the genus Quercus(Q.scottii,Q.simulata,Q.latifolia,and Q.cf.delavayi)and the most useful features for distinguishing among specimens are leaf shape,leaf base and apex shape,number of secondary veins,epidermal cells and anticlinal walls,stomatal size and frequency,trichome base type and density.However,of these characteristics cannot be used alone.The distribution and main components of evergreen broad-leaved forests in Yunnan have not changed greatly since the Cenozoic,which could be related to the complex topography and diversified environment of Yunnan Province.