Leaf differentiation of extinct and remnant species of Zelkova in Western Eurasia

The genus Zelkova (Ulmaceae) has occurred in Europe and Southwest Asia since the Eocene represented mostly by the fossil-species Z. zelkovifolia. Currently, relict populations of Z. abelicea, Z. sicula and Z. carpinifolia can be found in Crete, Sicily and the Euxino-Hyrcanian province, respectively. To reveal relationships between extinct and extant Zelkova species, we compared fossil Miocene leaves of Z. zelkovifolia and leaves of extant species using morphological leaf characteristics and statistical methods (Tukey's test, discrimination analysis, principal component analysis, agglomeration). The fossil leaves of Z. zelkovifolia appeared more variable and generally intermediate between all three extant species. The lowest level of significant differences was found between Z. zelkovifolia and Z. abelicea in the leaves from fertile shoots and between Z. zelkovifolia and Z. sicula leaves from vegetative long shoots. The intermediate positions of fossil leaves of Z. zelkovifolia between leaves of Z. abelicea, Z. sicula and Z. carpinifolia could indicate that the extinct species was an ancestor of all three extant taxa. Consequently, this result suggests divergence between Z. abelicea, Z. sicula and Z. carpinifolia no earlier than the late Miocene and/or during the Pliocene.