Corystospermales
| Corystospermales | ||||||||||||
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| Triassic to Chalk | ||||||||||||
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predominantly Gondwana |
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Corystospermales | ||||||||||||
The Corystospermales are a Mesozoic order of the extinct plant group of seed ferns .
features
Many of the individual organs that are assigned to the Corystospermales were not found in organic connection with one another. Common to all well-preserved finds is the presence of specific secretory ducts in all organs that are lined by a unicellular epithelium.
leaves
The form genus Dicroidium , which belongs to the Corystospermales, is the most common leaf type of the Triassic in Gondwana . The leaves are characteristically bifurcated. The variability within the genus is very large, there are simple to double (possibly also triple) pinnate leaves, the leaflets themselves are in turn entire, pinnate or needle-shaped. The Rhachis anatomy resembles that of the more recent cycads . Further Gondwana leaf genera are Johnstonia , Xylopteris , Diplasiphyllum and Zuberia , which, however , are all assigned to Dicroidium by some authors . The unbranched, double to triple pinnate leaves of Pachypteris and Thinnfeldia are known from the Jura of Europe . Pachypteris is also known from the Jura and the Gondwana Cretaceous.
Tribes and habitus
There are several genera considered to be the tribes of the Corystospermales. Rhexoxylon has an unusual anatomy, the trunk consists of several rings of conductive tissue. Depending on the species, they consist of centrifugally or centripetally formed secondary xylem . The individual wedges of xylem are separated by parenchyma , which is attached to the medulla . This anatomy was interpreted as a sign of a climbing habit. However, some species have large diameters, such as Rhexoxylon brasiliensis 46 × 37 cm, and Rhexoxylon brunoi 71 × 58 cm. It is possible that some species were climbers, while others grew upright. According to reconstructions by Bruno Petriella , the Dicroidium plant had a rhexoxylon trunk , was a medium-sized, unbranched tree with a habit similar to recent tree ferns .
Another strain that belongs to the Corystospermales is Kykloxylon Fremdouwensis , small leafy stems. They have a ring of primary vascular bundles around a central medullary canal. The secondary xylem is dense and has distinct growth rings. Axillary branching occurs here, the habitus resembles the recent Nacktsamern except for the leaves . Jeffersonioxylon has a diameter of up to 61 cm, the height is estimated to be up to 31 m.
The Antarctic Dicroidium plants are reconstructed as 20 to 30 m high forest trees with dense, pyknoxylem wood and axillary branching that results in a bushy crown. There is anatomical evidence ( periderm under the leaf bases) that the leaves were shed seasonally. The plants grew at 70 to 75 degrees south latitude and were exposed to complete darkness in the polar nights . The reconstructed Umkomasia uniramia-Dicroidium odontopteroides -plants had short and long shoots similar to the recent ginkgo .
A trunk with Pachypteris papillosa leaves is known from the northern hemisphere , which is around 50 mm in diameter, is slightly fleshy and the surface of which is covered with leaf bases. The species is reconstructed as a shrub similar to the recent genus Rhus .
Pollen organs
In contrast to the vegetative organs of the Corystospermales, the morphology of the reproductive organs is quite uniform. The most common pollen organ is pteruchus , which was found all over Gondwana. It is a branch on which microsporophylls sit helically . The sporophylls are leaf-like with a basal stalk and a flattened end. They carry 20 to over 200 pollen sacs on their abaxial side . Pteruchus specimens from Antarctica are up to 3 cm long with at least 10 microsporophylls, the oval blades of which are up to 7.5 mm long and 7.3 mm wide. They carry at least 80 pollen sacs of around 2.5 mm in length, which are almost in rows. Yorkshire Jurassic Pteroma is likely the pollen organ of Pachypteris papillosa .
The pollen is bisaccat (has two air cushions) with a distal germ groove (sulcus). The pollen finds are placed in the genera Alisporites , Pteruchisporites and Falcisporites .
Sperm-forming organs
The seed-forming organs are quite similar to the pollen organs throughout Gondwana. On the basis of the branching and the cupula morphology, three genera can be distinguished: Spermatocodon , Pilophorosperma and Umkomasia , the best known genus. On a main axis sit helically arranged side branches, on each of which one or more pairs of cupules sit. The cupula is helmet-shaped, bent back and contains one or two ovules . Except for the protruding micropyle, this is hidden in the cupula. Umkomasia is known from all Gondwana continents with the exception of India, and was also found in the Jura of Germany. In the late Antarctic Triassic, Umkomasia uniramia cupulae were found on axes with Dicroidium leaves, the first direct evidence that these organs belong together . The cupula of specimens found in isolation is usually torn lengthways, which is likely to have been the natural opening mechanism. The mature, isolated cupules are up to 7.5 cm long with a diameter of up to 1.3 cm.
Temporal and geographical distribution
The first representatives appear at the beginning of the Triassic in Gondwana. The main distribution is in the Triassic Gondwanas. Finds from the Jura and the Cretaceous both from Gondwana and from northern sites are rarer.
Significant sites are the Ischigualasto Formation from the Upper Triassic in Argentina, as well as in the Antarctic.
Systematic position
The position of the Corystospermales within the seed plants must be regarded as unexplained.
A possible ancestor of Corystospermales is the carbon- - perm -temporal Samenfarn Botrychiopsis .
supporting documents
- Thomas N. Taylor, Edith L. Taylor: The Biology and Evolution of Fossil Plants . Prentice Hall, Englewood Cliffs 1993, pp. 579-585. ISBN 0-13-651589-4
- EL Taylor, TN Taylor, H. Kerp, EJ Hermsen: Mesozoic seed ferns: Old paradigms, new discoveries . Journal of the Torrey Botanical Society, Volume 133, 2006, pp. 62-82
- EL Taylor: Enigmatic gymnosperms? Structurally preserved Permian and Triassic seed ferns from Antarctica . Review of Palaeobotany and Palynology, Volume 90, 1996, pp. 303-318.
Individual evidence
- ↑ a b c d e B.J. Axsmith, EL Taylor, TN Taylor, NR Cuneo: New perspectives on the Mesozoic seed fern or Corystospermales based on attached organs from the Triassic of Antarctica . American Journal of Botany, Volume 87, 2000, pp. 757-768.
- ^ EL Taylor, TN Taylor, H. Kerp, EJ Hermsen: Mesozoic seed ferns: Old paradigms, new discoveries . Journal of the Torrey Botanical Society, Volume 133, 2006, pp. 62-82