Caytoniales
| Caytoniales | ||||||||||||
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Fossil of Sagenopteris phillipsi |
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| Temporal occurrence | ||||||||||||
| Triassic to Chalk | ||||||||||||
| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Caytoniales | ||||||||||||
| HHThomas |
The Caytoniales are a Mesozoic order of the extinct plant group of seed ferns . Their seeds are enclosed in structures called cupula, so that they are systematically often placed in the vicinity of the covering plants.
features
Caytonia must have been a small tree.
Frequent fossils are the hand-shaped leaves of the form genus Sagenopteris, composed of three to six leaflets . In Sagenopteris serrata , the leaflets are up to seven centimeters long and lanceolate. They have a distinct midrib , the side veins are anastomosing and form a reticulate vein structure. In Sagenopteris phillipsii , the cuticle is thick and the cell walls of the epidermal cells are straight. Another leaf genus is Ruflorinia from Argentina.
Caytonanthus is the pollen- producing organ. It consists of a slender axis on which there are flattened, feathery side branches. There are one to three elongated synangia on each branch . Each of them is about one centimeter long, narrowed at the distal end and consists of three or four pollen sacs , which are arranged around a central tissue. The synangia are radially symmetrical and open towards the central part. The epidermal cells are delicate and spindle-shaped. Thick-walled fiber cells are likely to be found below them.
The pollen grains are assigned to the genus Vitreisporites . They are small and have two air sacs (bisaccat). In Caytonanthus arberi , the grains are around 25 micrometers long and have reticulated lines inside the air sacs. In the ultrastructure of the sexine vesicles can be seen (alveolate). The surface is characterized by a distal, conspicuous germinal groove (sulcus). In Caytonanthus kochii , the pollen grains are slightly larger at 30 micrometers.
The structures that carry the ovules are placed in the genus Caytonia . They consist of an approximately five centimeter long axis on which almost opposite cup-shaped cupules stand. Scars on some axes suggest that the cupules were shed. Each cupula is round and has a diameter of up to 4.5 millimeters. The cupules are bent back to the axis, the opening points to the main axis. The opening is lip-shaped. A cupula contains eight to 30 ovules, depending on the species. The ovules stand in a row along the midrib of the cupula. Each ovule stands on a delicate stalk, is orthotropically aligned, radially symmetrical and about 2 millimeters in size. The integument of the ovule consists of an outer, single-row epidermis, underneath a row of radially arranged, thick-walled cells. From Caytonia sewardii an interior is cuticle known. The outer part of the integument may have developed into a fleshy berry. The integument was probably traversed by vascular bundles . Fallen seeds are sometimes placed in the amphorispermum genus . Pollination did not take place on the lip of the cupula, but on the micropyle of the ovule. It is possible that the pollen was transported into the interior of the cupula and to the micropyle by means of pollination drops. It is not known whether fertilization was carried out by swimming gametes or by means of a pollen tube .
Caytonia does not produce angiosperm fruit as originally believed after its discovery. The cupula structure, however, is a method of enclosing the seeds for protection and is therefore similar to the carpel of angiosperms.
The cupules of Ktalenia from Argentina contain only one or two ovules.
distribution
The genus Sagenopteris is widespread in the northern hemisphere and was found in the USA, Greenland, England, Canada and Japan, as well as in the southern hemisphere. However, reproductive organs are much rarer, and Sagenopteris is likely to belong to taxa other than the Caytoniales. Reproductive organs are known from the Jurassic of Europe and Australia, the Cretaceous of Siberia, Argentina and the Antarctic Peninsula.
The representatives are likely to have occurred mainly in swampy locations.
Systematic position
As with all Mesozoic seed ferns, their relationship to the other seed ferns is very questionable. In many cases the Caytoniales were also placed close to the Bedecktsamern.
The systematic position of the Caytoniales also depends on the interpretation of the cupula. Some authors interpret it as being homologous to the external integument of the angiosperm ovule, the structure of the angiosperms would have been created by reducing the ovules in the cupula to one. Accordingly, the Caytoniales would be the sister group of the anthophytes . Other cladistic studies saw the Caytoniales as a sister group of the Glossopteridales .
Hilton and Bateman propose to provisionally summarize the Caytoniales together with the Bennettitales and Pentoxylales as the development stage "higher seed ferns", which are directly opposite the Bedecktsamern .
Botanical history
The group of Caytoniales were first described in 1925 by HHThomas from the middle Jurassic of the coast of Cayton Bay in Yorkshire . He initially suspected that the Caytoniales were early representatives of the Bedecktsamer. The leaves of the genus Sagenopteris were known for a long time.
supporting documents
- Thomas N. Taylor, Edith L. Taylor: The Biology and Evolution of Fossil Plants . Prentice Hall, Englewood Cliffs 1993, pp. 575-579. ISBN 0-13-651589-4
- EL Taylor, TN Taylor, H. Kerp, EJ Hermann: Mesozoic seed ferns: Old paradigms, new discoveries . Journal of the Torrey Botanical Society, Volume 133, 2006, pp. 62-82
Individual evidence
- ^ J. Hilton, RM Bateman: Pteridosperms are the backbone of seed-plant phylogeny . Journal of the Torrey Botanical Society, Volume 133, 2006, pp. 119-168.
- ↑ in: HH Thomas: The Caytoniales, a New Group of Angiospermous Plants from the Jurassic Rocks of Yorkshire . Philosophical Transactions of the Royal Society of London. Series B, Vol. 213, 1925, pp. 299-363.
