Ginkgoales are a gymnosperm order containing only one Neontology species: Ginkgo biloba, the ginkgo tree.[Christenhusz, M.J.M., J.L. Reveal, A.Farjon, M.F. Gardner, R.R. Mill, and M.W. Chase (2011). A new classification and linear sequence of extant gymnosperms. Phytotaxa 19:55–70. http://www.mapress.com/phytotaxa/content/2011/f/pt00019p070.pdf] The order has a long fossil record extending back to the Early Permian around 300 million years ago from fossils found worldwide. The order was a common component of Permian and Triassic flora before the super dominance of Conifer.
Evolution
Ginkgophyta and
have a very ancient divergence dating to the early
Carboniferous.
The earliest representative of the group in the fossil record is probably
Trichopitys from the
Asselian (299-293 million years ago) of France. The earliest representatives of
Ginkgo, represented by reproductive organs similar to the living species, first appear in the
Middle Jurassic, alongside other, related forms such as
Yimaia and
Karkenia, which have differently arranged reproductive structures and seeds associated with
Ginkgo-like leaves.
The diversity of Ginkgoales declined during the Late
Cretaceous and
Cenozoic, coincident with the rise of
Flowering plant, with all Ginkgophytes aside from
Ginkgo being extinct by the end of the Cretaceous.
The only remaining Ginkgophyte was
Ginkgo adiantoides – a polymorphic species.
Modern
Ginkgo trees are native to China.
Reproduction
Ginkgo trees produce ovulate and pollen-bearing structures. These structures are
dioecious, in that male and female structures come from different
Ginkgo plants.
The pollen organs are very similar to angiospermous
catkins. They come from the
axils of the bud scales, and the leaves from the
Ginkgo tree spur shoots. Pollen is contained in sacs of two to four at the tips of sporophylls on the strobiloid. Ovules of
Ginkgo trees come from stalks from leaf axils on the short shoots, each containing two ovules. The ovule is fertilized by the flagellated male gametes, which can move about freely. This fertilization process begins on the tree itself in the spring. The swollen fruit-like
ovules, about 2–3 cm in diameter, fall from the tree in the fall, and fertilization continues into the winter/spring.
This ovule contains a single large seed, similar to that of a
cycad.
Morphology
Ginkgophyte wood
Fossils that appear
Ginkgo-like are filed under a
morphogenus called
Ginkgoxylon, Ginkgomyeloxylon, or
Protoginkgoxylon. Fossilized ginkgophyte wood is not commonly found in the record, possibly because it degrades easily, and possibly because it is difficult to tell apart from the much more pervasive
conifer samples.
Like conifer wood, it has secondary thin-walled xylem and a primary vascular system composed of eustele and bifacial vascular
cambium. The tracheids in the secondary
xylem rays have pitting that occurs only on the walls and is circularly bordered.
Ginkgophyte foliage
Ginkgophyte foliage has stayed largely consistent since the
Mesozoic. Its historically wide territory makes it an important leaf morphology, and its unique
stomata and isotopic profile give it a key role in recreations of the Mesozoic and
Cenozoic. Leaf fossils that resemble the Ginkgophytes are known as
Ginkgoites. There are similar, now extinct, morphogens, such as
Sphenobaiera, which describes fan-shaped, deeply divided leaves without clear petioles.
The distinctive shape of the modern Ginkgo biloba gives the impression of a very narrow leaf morphology, but the group is varied and diverse. The genus Ginkgo by itself contains a range of morphologies. Ginkgo digitata, from the Jurassic, has long, wedge-shaped laminae with the intercostal regions covered in stomata and resin bodies, while G. pluripartita has at most 2 cm-long leaves and is intercostally hypostomatic.
Fossil gallery
File:Ginkgo biloba leaf 01.jpg|A 6.7 cm tall Ginkgo biloba leaf, with insect herbivory. Klondike Mountain Formation, Republic, Ferry County, Washington, USA, Eocene, Ypresian, 49 million years old
File:Ginkgo biloba 01 SR 87-36-02 A.jpg| A 70 mm-wide Ginkgo biloba leaf. Klondike Mountain Formation, Republic, Ferry County, Washington, USA, Eocene, Ypresian, 49 million years old
File:Ginkgo biloba MacAbee BC.jpg| Ginkgo biloba Eocene fossil leaf from the Tranquille Shale of MacAbee, British Columbia, Canada
File:Ginkgo huttoni.jpg|Fossil of Ginkgo huttoni. Photo taken at Naturalis Museum in Leiden, The Netherlands.
File:Ginkgo huttoni 1.jpg|Fossil of Ginkgoites huttoni
