Plant Science Portal 
The Ginkgoales: A case study

The Ginkgoales were once a large group of gymnosperms, now reduced to a single species; Ginkgo biloba.

Ginkgos are sometimes referred to as "maidenhair trees" for their superficial foliar resemblance to the maidenhair fern (Adiantum spp.). They are also sometimes referred to as "living fossils" for the close resemblance of Ginkgo biloba to fossilised extinct species.

Both fossils and extant plants are easily recognised for the distinctively shaped leaves which feature an open dichotomous venation pattern.

Ginkgo biloba is deciduous, and it is thought that the now extinct ginkgos were also deciduous because of the large number of fossilised leaves which have been found.

Ginkgo biloba is dioecious, which means that there are separate 'male' and 'female' trees and this appears to be the case with the ancient species also, although there was much variation in reproductive structures between species and genera.
Ginkgo biloba
Ginkgo biloba.
During the Permian (~280 Ma) the ginkgos appeared in the fossil record, though they were more prominent in the Jurassic (~190 Ma). The oldest fossils come from a single location in what is now the Asiatic part of Russia. During the middle Jurassic, there was a great increase in ginkgo fossils created throughout the northern parts of Laurasia.

During their peak, it is thought that there were as many as 16 genera of ginkgos which formed a major component of the world's flora. Maximum diversity was reached during the Cretaceous in the Northern Hemisphere. Ginkgo was also present in Gondwana, and in Australia, Ginkgo was widespread along the eastern coast about 125-220 Ma.

By the Palaeocene Ginkgo adiantoides was the only remaining species of Ginkgo, and showed a close similarity to Ginkgo biloba in leaf shape. This species was mainly distributed in warmer northern parts of the world where the climate was tropical, however as the climate cooled during the Oligocene, it established a more southerly distribution. During the Pliocene, G. adiantoides was particularly common in Europe, but it later declined and was extinct on that continent by the end of the Pliocene (2.5 Ma).

The evolutionary origins of the Gingoales is not clear as the living and extinct species share many vegetative characteristics with the conifers and cordiates.
Ginkgo fossil
A fossil of a Ginkgo species dating from the Permian.
However the reproductive structures are more similar to cycads. Unfortunately molecular research has not shed a lot of light on this problem. Part of the difficulty has related to the abundance of foliar fossils, but a relatively limited number of fossils of the reproductive organs. It has been suggested that the Carboniferous species Dicranthophyllum moorei may be a possible ancestor to the ginkgos based on its slender forking leaves which look ginkgoalean, but the evidence for this is not strong and open to much debate.

Other possible ginkgo ancestors include Polyspermophyllum and Trichopitys.

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