Pleuromeiales
| Pleuromeiales | ||||||||||||
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Pleuromeia sternbergii |
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| Temporal occurrence | ||||||||||||
| Triassic to Chalk | ||||||||||||
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worldwide |
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Pleuromeiales | ||||||||||||
The Pleuromeiales are an extinct group of the club moss plants (Lycopodiopsida) and occurred in the Mesozoic ( Triassic to Cretaceous ).
features
With a maximum height of 2 m, the representatives were much smaller than the coal-forming Lepidodendrales and herbaceous . They have characteristics of the Lepidodendrales like the Isoetales and were previously thought to be a link between these two groups.
The pleuromeiales are unbranched and have one or more terminal cones . Their root structures are lobed, kormus-like and have Stigmarien -like appendages.
The cones of some forms are bisporangiate ( heterosporia ), while others only form spores of one size ( homosporia ). The sporangia are somewhat sunk into the sporophyll . Most forms form triple megaspores and monolithic microspores. Pleuromeia rossica , however, has triple microspores.
Representative
Pleuromeia
The genus Pleuromeia is restricted to the Triassic and is known from sites around the world, including Germany, France, Spain, Russia, China, Japan, Argentina and Australia. Your representatives are likely to have settled in a wide variety of locations. Finds north of Sydney are interpreted as coastal halophytes . Specimens from China are interpreted as representatives of dry locations, perhaps near desert oases, others from Australia as plants in alternately humid locations. Finds from the red sandstone of the Eifel show that the species Pleuromeia sternbergii occurring here formed dense populations together with the fern Anomopteris mougeotii .
Pleuromeia appeared quite suddenly in the early Triassic. This is interpreted as the repopulation of many terrestrial ecosystems after the mass extinction at the end of the Permian .
Pleuromeia has an unbranched, upright trunk up to two meters high. The base ( rhizomorph ) is four-lobed, the roots go off in a spiral arrangement . At the tip of Pleuromeia longicaulis there is a crown of elongated leaves with ligules . Each leaf is supplied by two vascular bundles . Below the crown is a zone of persistent leaf bases, which merges down into a zone of widely spaced leaf scars. The trunk may have had a secondary growth in thickness , it is not known whether the tissue formed was bark or conductive tissue .
Pleuromeia has a relatively large cone at its tip. Some species could have had multiple cones. This is inferred from the occurrence of large numbers of small cones called cylostrobus , which occur in the same sites as Pleuromeia and may have been produced by these plants. Some species of Cylostrobus are bisporangiate, with the megasporangia sitting at the base of the cone. The megaspores are up to 700 µm in size, triple and covered with numerous long spines. The microspores are monolet and up to 30 µm in size. Because of their shape it is assumed that the Megasporophylle with the aid of water spread were.
- Pleuromeia sternbergii from Germany is the type species of the genus and had two types of leaves.
- Pleuromeia jiaochengensis from the early Shanxi Triassic is around 50 cm tall. The trunk is unbranched, the awl-shaped leaves were around 3 mm long. At the end of the trunk sits a large cone made of obovate sporophylls that carry disc-shaped sporangia. The megaspores are up to 500 µm in size.
- Pleuromeia epicharis from northern China is fairly well known.
Other genera
Pleuromeia rossica was transferred to its own genus Lycomeia . Micro- and megaspores have ultrastructural features that connect them to the isoetales .
Pleuromeia longicaulis is placed in its own genus Cylomeia by some authors ; the cones of the Skilliostrobus type are also assigned to it. This is an early Triassic pendulous bisporangiate cone from Australia and Tasmania. The sporophylls are wedge-shaped and sit in a screw-like arrangement. The obovate sporangia sit in an adaxial pit. The cone is up to 8 cm in diameter and about half as long. The microspores are monolet, about 40 µm in size and resemble Aratrisporites , the megaspores are trilet and up to 1.1 mm in size and resemble Horstisporites .
The fossilized trunks from North America described under the name Chinlea probably belong in the vicinity of Pleuromeia . The trunk has a siphonostele with an external phloem . The parenchyma is thin-walled. The leaf traces are very numerous, there are up to 165 in a cross-section, the vascular bundles are collateral.
Ferganodendron is known from trunks with a diameter of 20 to 30 cm, which is covered with numerous elliptical to rhombic, helically arranged leaf bases. The leaves are small and only sit on the distal areas of the trunk. Internal structure or reproductive organs are unknown.
Lycostrobus is a genus that was established for bisporangiate cones found in isolation. It is known from several Triassic sites. Among other things, it is placed near the pleuromales because of the monolithic microspores. The sporophylls are arranged helically and carry the sporangia adaxially.
For a long time Annalepis was only known from isolated sporophylls, today also cones up to 10 cm long are known. The sporangia sit adaxially on the sporophylls and are at least partially enclosed laterally by the leaf blades of the sporophylls.
supporting documents
- 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. 316-320.