First evidence of epidermal structures of Ginkgo from the Mediterranean Tertiary

Abstract

Tertiary leaf compressions of a Ginkgo plant with cuticle displaying all taxonomically important epidermal features are reported for the first time from Greece, and for the Mediterranean area as a whole. The fossils are from Upper Miocene sediments of Vegora, north-western Greece, and assigned to Ginkgo adiantoides (Unger) Heer. The most conspicuous cuticle characteristics are prominent papillae on the subsidiary cells that sometimes completely cover the stomatal apertures. Such papillae also occur in leaves of G. adiantoides from the Pliocene of eastern Central Europe, but are absent in leaves from the Pliocene of Germany also assigned to this species. We observed a high variability in the degree of papillosity of subsidiary cells of stomata in leaves of living Ginkgo biloba that exhibited prominent papillae in mature sun leaves and a total lack of papillae in shade leaves of a potted plant. This may suggest that differences in epidermal characters between the fossil leaves from Greece and Germany are due to ecotypical/intraspecific variability and not to genetically fixed specific differences. It also suggests that a number of Tertiary Ginkgo species from Eurasia that were distinguished from G. adiantoides by having papillate epidermis cells and subsidiary cells of the abaxial cuticle should be included within G. adiantoides. Two lineages of Ginkgo can be distinguished for the Tertiary of the Northern Hemisphere. From the Palaeogene high latitudes leaves were reported that exhibit upper epidermis cells with prominent papillae similar to those of older Mesozoic Ginkgo, whereas G. adiantoides lacks the papillae of the upper epidermis, a feature which is shared with the living G. biloba. In general, the size of stomata is larger in extant Ginkgo leaves than in those of the fossils.