Ginkgoales

Ginkgoales are a gymnosperm order containing only one extant species: ''Ginkgo biloba'', the ginkgo tree. 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 conifers.

Evolution

Ginkgophyta and Cycadophyta 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 plants, 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''

References

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Gymnosperm orders
Jurassic plants
Extant Permian first appearances