Caytoniales
Caytoniales Temporal range:
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Fig.1 A whole-plant reconstruction of Caytonia nathorstii[1] | |
Scientific classification | |
Kingdom: | Plantae |
Clade: | Tracheophytes |
Division: | †Pteridospermatophyta |
Order: | †Caytoniales Gothan 1932 |
Form genera | |
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The Caytoniales (Figs. 1-2) are an extinct order of
History
The first fossils identified in this order were discovered in the Middle Jurassic Gristhorpe bed of the Cloughton Formation in Cayton Bay, Yorkshire,[8] with the name of the bay giving the name to the group. They have since been found in Mesozoic rocks all over world.[9] It is likely that Caytoniales flourished in wetland areas, because they are often found with other moisture-loving plants such as horsetails in waterlogged paleosols.[1] The first fossil Caytoniales were preserved as compressions in shale with excellent preservation of cuticles allowing study of cellular histology.[5]
Description
The woody nature of associated stalks and preserved short shoots are evidence that Caytoniales were seasonally deciduous, shrubs and trees.[1] Caytoniales had fertile branches with seed-bearing cupules .[10] The ovules were located inside fleshy cupules with tough outer cuticle. Individual ovules had an apical tube called a micropylar canal, that allowed pollen to pass into the pollen chamber.[11] The outer layers of the cupules were fleshy and fruit-like; it is possible this was to aid in animal dispersal.[1] The cupules are 4-5mm in diameter and about 3 mm long[12] (Fig 1-2),[5] and resemble a blueberry. The extra protection of the reproductive organs gave rise to the idea that Caytoniales were predecessors to angiosperms, which have completely enclosed seeds.
The pollen grains were small, between 25 and 30 μm in diameter. The size of the pollen grains supports the idea that they were wind-pollinated, and their bisaccate wings may have enabled entry into the seed by a pollination drop mechanism.
The most common and widespread part found fossilized are leaves of Sagenopteris (Fig. 3).[5] These are compound leaves consisting of, usually, 4 leaflets arrayed in a palmate manner. The individual leaflets are up to 6 cm in length. The leaflets have anastomosing veins, like those of some ferns, but lacking orders of venation found in angiosperm leaves.[1]
Relationship to angiosperms
Caytonia was first described by Hamshaw Thomas in 1925. His close examination of the cupules led him to believe this was one of the earliest examples of angiosperms. He mistakenly thought the entire ovule was enclosed in the cupule, unlike typical
The enclosure of ovules in Caytoniales has nevertheless been considered an early stage in evolution of the angiosperm double integument, and the carpels formed from an elaboration of their stalk (Fig. 5[7]). Other theories for the origin of angiosperms derive them from Glossopteridales (Fig.5[13]), among other groups (see Evolutionary history of plants).
Gallery
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Fig. 2 A) leaf structure B) Venation C) Pollen sacs D) Pollen grain E) Seed structure F) Cupule G) Cupule from side H) Ovule
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Fig. 3 Fossil leaves of Sagenopteris phillipsii from the Gristhorpe Bed at Cayton Bay. Natural History Museum specimen photographed by G.J.Retallack
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Fig. 4 Fossil Caytonia nathorstii, the reproductive structure that encloses ovules. Natural History Museum specimen photographed by G.J. Retallack.
References
- ^ JSTOR 2399379.
- ISBN 978-0-226-58083-8.
- ISBN 978-0-309-07099-7.
- ^ ISBN 978-1-4067-1861-4.
- ^ .
- ^ .
- ^ .
- ^ Friis, Else (2011). Early Flowers and Angiosperm Evolution.
- ^ .
- ^ "Caytoniales † - Plant Evolution & Paleobotany". sites.google.com. Retrieved 2016-03-09.
- .
- ^ "Importance of the Caytoniales". www.hullgeolsoc.co.uk. Retrieved 2016-03-09.
- ^ .