Abstract

A cladistic analysis of Mesozoic ginkgoaleans is made based on new evidence from the Middle Jurassic of Henan, China, together with previously known fossil records. Cordaites plus early conifers have been chosen as out-group for evaluating polarities of characters of ginkgoaleans. The analysis favours a monphyletic group of Ginkgoales including Trichopitys, Karkenia, Toretzia, Umaltolepis, Yimaia and Ginkgo. The most parsimonious cladogram shows that Ginkgo and Yimaia are phylogenetically closely related sister taxa and so are Toretzia and Umaltolepis, while Karkenia appears to represent a distinct evolutionary lineage of Ginkgoales in the Mesozoic. Based on the analysis, a provisional classification of Ginkgoales is proposed.

The ovulate shoots of Trichopitys may be regarded as a putative archetype or its analogue from which various ovule-bearing organs of ginkgoaleans originated. All Mesozoic members are characterized by having short shoots and laminated leaves. The main evolutionary trends of vegetative parts in Ginkgoales are considered to be reduction of shoots and webbing and fusion of leaves. Their fertile shoots also tend towards reduction. Ovules become sessibe and reduced in number but increase in size through geological time.