Asteroxylon
| Asteroxylon | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
Asteroxylon |
||||||||||||
| Temporal occurrence | ||||||||||||
| Lower Devon to Middle Devon | ||||||||||||
| 416 to 385 million years | ||||||||||||
| Locations | ||||||||||||
|
Scotland, Germany, Norway |
||||||||||||
| Systematics | ||||||||||||
|
||||||||||||
| Scientific name | ||||||||||||
| Asteroxylon | ||||||||||||
| Kidston & WHLang |
Asteroxylon is an extinct genus of plants from the Devonian . It is systematically based on the bear moss plants .
Asteroxylon mackiei
Asteroxylon mackiei is only known from the Rhynie Chert in Scotland and can be found there in the lower Old Red Sandstone .
The main axes branch out isotomically, side branches also arise anisotomically, which leads to smaller side branches. The microphylls are simple and spiral on the axes. A simple vascular bundle arises from the xylem of the stem axis, but ends at the base of the microphyll, so it does not lead into it. The microphylls have no ligules . The xylem is star-shaped in cross-section, from which the generic name is derived. Large axes have seven xylem lobes, the protoxylem is in the ends of these lobes. In the inner cortex there are large intercellulars, which are formed by special cells (trabeculae). The outer bark is rather thin, around six cells thick. A hypodermis is not known. The epidermis is made up of elongated cells, some of which have papillae . The stomata consist of two cells.
The branching occurs in two ways: In the case of isotomous branching, the star-shaped stele divides into two small, identical star-shaped steles. In the anisotropic branching, one of the lobes of the stele divides, forming a small round stele in the side branch, which distally develops again into a star shape.
The sporangia are large, up to 7 mm wide, kidney-shaped and open with two identical valves. They stand on the axes and have a short stem with vascular bundles. They are not in the leaf axils. The spores are trilet (have a three-pronged scar) and have few ornaments.
The rhizomes are bare, isotomically branched, lying axes. Your xylem is cylindrical and surrounded by a tissue interpreted as a phloem , followed by an inner cortex made up of narrow, fine cells, an outer cortex made up of large cells and an epidermis. Root hairs are not known.
The gametophyte is unknown.
Asteroxylon elberfeldense
This second species comes from the Central Devonian region of Germany, Scotland and Norway. The axes are dichotomous to sympodial, have a diameter of over 5 mm and are up to one meter long. Their appendages are arranged helically and densely and closely resemble microphylles . The species resembles Asteroxylon mackiei , but its systematic position is still unclear. Circinate vernation and pseudomonopodial branching indicate a relationship with the Zosterophyllophyta .
Another species described, Asteroxylon setchellii , is only known from fossilized axes from the New Albany Shale, is now more regarded as a rhachis of a fern and is carried under the name Stenokoleos setchellii .
Systematics
Asteroxylon stands at the base of the bear moss plants. In the cladistic analysis of Kendrick and Crane (1997), Asteroxylon mackiei formed a clade with Drepanophycus and Baragwanathia .
supporting documents
- Thomas N. Taylor, Edith L. Taylor, Michael Krings: Paleobotany. The Biology and Evolution of Fossil Plants . 2nd Edition. Elsevier / Academic Press, Amsterdam et al. 2009, ISBN 978-0-12-373972-8 , pp. 270–271 ( limited preview in Google Book Search).
- Paul Kenrick, Peter R. Crane: The Origin and Early Diversification of Land Plants. A Cladistic Study . Smithsonian Institution Press, Washington / London 1997, ISBN 1-56098-729-4 , pp. 339 .
Web links
- Asteroxylon on the Rhynie-Chert side of the University of Aberdeen