Abstract:

Aims: Fossil leaves of the genus Quercushave 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 Quercusis 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. delavayiwas 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 based 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. simulataand 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.

Key words: Quercus, cuticle, evergreen broad-leaved fores, Pliocene, Yunnan