An overview of fossil Ginkgoales

Neither direct fossil evidence nor consensus exists on the origin of the Ginkgoales and their phylogenetic relationships with other seed plants. The bases for assigning most Palaeozoic leaf fossils to Ginkgoales are shaky. There are eight morphogenera considered more or less well defined and useful for classifying Mesozoic leaf and shoot compressions/impressions, and only two or three morphotaxa of anatomically preserved wood fossils have generally been used. About nine genera of ovulate organs, however, have been reported in the Mesozoic. Whole plant reconstructions suggested for a number of well-preserved ginkgoalean plants are enumerated. Their associated (or connected) organs, and their occurrences and distributions are cited in detail. There are three or four major evolutionary lineages so far recognized among Mesozoic Ginkgoales: the Ginkgo-Grenana-Nehvizdyella lineage, the Karkenia lineage, the Yimaia-Toretzia/Umaltolepis lineage and perhaps the Schmeissneria lineage. Ginkgoales may be classified into five to six families, with a number of accessory morphotaxa and unclassified taxa. The general evolutionary trend among ginkgoaleans is reduction of both vegetative and reproductive organs. The reduction trend is seen clearly in the genus Ginkgo and roughly recapitulated in the developmental sequences of the living species. A similar reduction sequence runs in parallel in other lineages of Ginkgoales. Ginkgoales flourished during Jurassic and Early Cretaceous, but a significant radiation of the group had occurred already in Late Triassic when Ginkgoales were present in high taxonomic diversity and showed considerable morphological innovation. Geographically, Ginkgoales are mainly distributed in Laurasia and probably originated there. The earliest records are from Laurasia as is the relict living fossil. Ginkgoales may have lived in various climates and diverse habitats, although most flourished in mesic and temperate climates, and the Late Cretaceous and Cenozoic ginkgos were largely confined to riparian environments. es in micro- and ultrastructure studies and chemical investigations on the cuticle and megaspore membrane of ginkgoalean fossils are also summarized. Further studies in these fields may provide useful information on the ecology and palaeoclimatology of Ginkgoales as well as their taxonomy.