Drepanophycales

Asteroxylon sp.

The Drepanophycales are the oldest group of Bärlapppflanzen ( Lycopodiopsida ) and known only from the Devonian.

Features

The Drepanophycales were probably herbaceous plants. The axes upright have a exarche maturation of the primary xylem. On the axes are spirally arranged appendages that are sometimes nearly whorled. These tags are either free of vascular bundles or partially traversed by vascular bundles: the bundle extends into the proximal part, but does not draw through the entire structure. They are therefore Mikrophyllen rather than true leaves. The annexes are in contrast to those of Protolepidodendrales not branched dichotomously at the tips.

The sporangia were probably not together in cones. They are also not on the adaxial sporophylls as with other Bärlapppflanzen, but spring directly to the stem axis just over an appendage, ie axially. All representatives are homospor, therefore form only one type of spores.

System

The Drepanophycales are the oldest known group of Bärlapppflanzen. Sometimes they are also regarded as a transitional group of the Zosterophyllophyta to the genuine Bärlapppflanzen. You do not yet have all the typical characteristics of Bärlapppflanzen. In the cladistic analysis of Kendrick and Crane ( 1997) which they studied representatives are at the base of Bärlapppflanzen.

Taylor, Taylor and Krings (2009) lead to the following genera:

• Asteroxylon is one of the characteristic plants of the Rhynie Chert.
• Baragwanathia 's common in the Silurian - Devonian rocks of Australia and one of the oldest Bärlapppflanzen.
• Drepanophycus is an ill -defined genus with sickle-shaped appendages.
• Halleophyton similar morphologically Drepanophycus, the leaf bases are rhomboid to hexagonal, the sporangia open with two equal-sized flaps. It dates from the early Devonian of China.
• Haplostigma with Haplostigma baldisii has Mikrophyll -like appendages, which are simple and have approximately hexagonal bases.
• Haskinsia was earlier asked to Drepanophycus. The spirally arranged appendages are crescent- shaped and about 3 mm long. The metaxylem consists of various types of bordered pits. In Haskinsia hastata the tags are arranged in Pseudoquirlen and about 5 mm long. The sporangia are spherical.
• Hestia eremosa is quite original, their systematic position is uncertain. It was described from the Mississippian of Oxroad Bay, East Lothian (Scotland). The Stems have a star-shaped stele Leitertracheiden
• Smeadia from the Upper Devonian Cleveland Shale of Ohio combines features of several club moss Groups: She was herbaceous with a siphonostele and spirally arranged leaves. At the distal end stood an upright peg in whose sporangia trilete spores from 40 to 80 microns in diameter were formed.

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. 268-271.