Leaf polymorphism of beeches from the Jura (France) in the late Pliocene (Piacenzian): contribution to taxonomy in palaeobotany

Abstract

Fossilized beech leaves were extracted from nodules in the Bletterans plateau (Jura, France) dating back to the Late Pliocene (Piacenzian, -2.7 Ma). During this period, seasonality was much less pronounced than it is today. A warm climate allowed the development of deciduous tree vegetation, comprising a mixture of tropical, subtropical, and temperate forests without modern analogues, including the presence of Fagus and Quercus. Various qualitative and quantitative morphological traits were studied, as well as comparisons with leaves from fossil species such as F. menzelii and F. kraeuselii,and leaves from present-day species like F. grandifolia and F. sylvatica. The most discriminative features include the basal angle, the angle between primary vein and secondary veins, the angle between primary vein and tertiary veins, and the density of tertiary veins. Three distinct groups stand out, referred to as morphotypes indicating environmental variations rather than genetic groups. Leaves of F. sylvatica separate into two distinct morphotypes, whereas the American species F. grandifolia, and the European species F. kraeuselii and F. menzelii, form a third morphotype. The Bletterans samples are close to these three morphotypes. These morphological correspondences do not allow associating a species to the jurassian samples, but they support the existence of leaf morphotypes acclimated to fluctuating environmental conditions.