Bennettitales
| Bennettitales | ||||||||||||
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Flower-like structure |
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| Temporal occurrence | ||||||||||||
| Triassic to Chalk | ||||||||||||
| 250 to 65 million years | ||||||||||||
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worldwide |
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Bennettitales | ||||||||||||
| Engler |
The Bennettitales are an extinct group of seed plants . They lived in the Mesozoic Ages and formed flower-like structures. In their characteristics, they resembled the one hand cycads , on the other hand the angiosperms and Gnetales . They are discussed as one of several possible precursors of the flowering plants.
features
Shoot axes
The shoot axes are often stocky and often pachycaul . The habitus of the plants, however, ranges from short and stocky to slim and heavily branched. The branching usually appears dichotomous .
The trunk is a Eustele with a pronounced pith . The bark is also strong. The wood is relatively narrow. The belt of leaf traces found in the otherwise similar cycads is missing in the trunk .
In the secondary xylem there are slender heads of tracheids , rarely spotted tracheids . The rays are uni- or biseriat. There are long resin channels in the base tissues .
leaves
The leaves of the Bennettitales stand at the ends of the trunk as a tuft of leaf fronds. The leaves are morphologically very similar to those of extant cycads . In contrast to these are the stomata formed syndetocheilisch. There are two equatorial side cells that are the same length as the guard cells. Both secondary and guard cells arise from a common mother cell, a form of formation that is rare and still occurs in Welwitschia , for example . In most plants, secondary and guard cells arise from different cells.
Young leaves are rolled up (circinate vernation ). The leaves are usually singly pinnate, rarely feathered or bipinnate. Feathered twice are about Banatozamites , Coreanophyllum and Nipponoptilophyllum . The nerve endings are usually open, less often closed. Several forms known only as leaf fossils are counted among the Bennettitales due to the anatomy of the stomata: the most common are Nilssoniopteris , Pseudocycas , Dictyozamites , Ptilophyllum , Pterophyllum , Otozamites and Zamites . Leaves are seldom preserved anatomically, an example is Otozamites mortonii , pinnate leaves whose vascular bundles in the rachis form an omega-shaped system in cross-section. Each vascular bundle is surrounded by a sclerenchymal sheath.
In addition to the foliage leaves, the Bennettitales have scale-like leaves, the cataphylls . Two generic names for these are Deltolepis and Cycadolepis .
Reproductive organs
The reproductive organs are in small to medium-sized, hermaphroditic (bisporangiaten) or unisexual (monosporangiaten) cone-like short shoots, which are often referred to as flowers .
The axis of the cone is short, has a determined (limited) growth and is known as the receptaculum. The receptaculum is cylindrical, conical or dome-shaped. Anatomically it is a eustele with a large marrow . From the Eustele leaf traces branch off in a spiral arrangement. They can also stand so close together that no arrangement can be recognized.
In bisporangiate cones, the microsporophylls are at the base of the receptacle and are pinnate or fleshy. The ovules stand distally between sterile interseminal scales. In each bract or microsporophyll there are several leaf traces. In the distal area, where the ovules start, the leaf tracks branch and unite. In each megasporophyll or interseminal scale there is only a single, circular leaf trace. Although each ovule is surrounded by several interseminal scales, these do not form a special structure like a cupula .
Microsporangia
The pollen-forming organs are whorled, free or overgrown, pinnate or simple. They carry numerous microsporangia . Several sporangia are grown together ( synangium ). In addition to some microsporangia that can be clearly assigned to the Bennettitales, there are also some microsporangia ( Morphotaxa ) found in isolation , which are often assigned to the Bennettitales, but cannot be clearly assigned due to their conservation status and isolated occurrence, such as Legumimanthus .
Ovules
The ovules stand individually upright at the end of reduced, narrow sporophylls . The sporophyll consists of a circular xylem cord, which is surrounded by several layers of thin-walled cells.
The ovule is radially symmetrical in cross section: circular in Williamsonia and Cycadeoidella , angular to star-shaped in Cycadeoidea . The ovule narrows to a narrow, round micropylene opening. Depending on the type, this may or may not protrude beyond the tips of the surrounding interseminal scales.
The ovule is oriented orthotropically and has a single integument that is several layers of cells and has no vascular bundles . The inner epidermis of the integument consists of radially elongated cells near the micropyle, further down they are isodiametric. The nucellus separates from the integument at the base of the ovule. It is supplied by a cup-shaped bundle of tracheids, which at the same time represents the end of the vascular bundle of the stalk. The tip of the nucellus forms a cylindrical to conical parenchymatic stopper. This sits close to the micropylene canal, there is no pollen chamber here, unlike practically all naked samers including the Gnetales . In the nucellus of many fossil specimens, the tissue of the megagametophyte can be seen, as well as an embryo . The latter has two cotyledons .
ecology
There is ample evidence that insect pollination was targeted . The resinous synangia in the pollen cones of Weltrichia are interpreted as a kind of attractant. Coproliths have been found in the cones of several species of Cycadeoidea .
Spatial and temporal distribution
Bennettitales fossils date from the Triassic to the Cretaceous and were found in both the northern and southern hemispheres.
Evolution and systematics
The systematic position of the Bennettitales within the seed plants is not clearly clarified. Were often near the in the past because of the very similar vegetative features cycads found (Cycadales), they have been associated with the emergence of since the work of Arber and Perkin 1,907 angiosperms seen. Practically all more extensive phylogenetic studies see the Bennettitales together with the Gnetales and Bedecktsamern. This group is known as anthophytes. Molecular genetic studies, which inevitably do not include the fossil Bennettitales, practically never show the Gnetales and Bedecktsamer in a common clade .
The Bennettitales order is now divided into two families:
- Cycadeoidaceae have bisporangiate cones
- Williamsoniaceae have monosporangiate cones that sit on one plant ( monocyte ) or on different plants ( diocyte ).
supporting documents
- Gar. W. Rothwell, William L. Crepet, Ruth A. Stockey: Is the anthophyte hypothesis alive and well? New evidence from the reproductive structures of Bennettitales . American Journal of Botany, Volume 96, 2009, pp. 296-322. doi : 10.3732 / ajb.0800209
- Thomas N. Taylor, Edith L. Taylor, Michael Krings: Paleobotany. The Biology and Evolution of Fossil Plants . Second Edition, Academic Press 2009, ISBN 978-0-12-373972-8 , pp. 722-741
