|
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.
A
fossil of a Ginkgo species dating from the
Permian.
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.
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.
|
