Sawdonia
| Sawdonia | ||||||||||||
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Sawdonia ornata |
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| Temporal occurrence | ||||||||||||
| Pragium ( Devon ) to Frasnium (Devon) | ||||||||||||
| 411 to 375 million years | ||||||||||||
| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Sawdonia | ||||||||||||
Sawdonia is a genus of extinct early land plants that are only known from the Devonian and belong to the Zosterophyllopsida , relatives of the bear moss plants .
features
The best known species is Sawdonia ornata . It is reconstructed as a 30 cm high plant with a creeping rhizome from which the pseudo monopodial branched shoots rise. The side axes branch dichotomously and are curled up spirally at the ends. The branches have a diameter of one to four millimeters and are covered with numerous multicellular spines . The stele has a core of ring tracheids . The xylem is elliptical to stripe-shaped in cross-section and probably exarch (maturation from the outside in, protoxylem therefore outside).
Some epidermal cells have papillae, others form a structure of a central cell surrounded by several long radial cells. These are interpreted as hair bases, and secretion, ventilation and storage have been suggested as their function. There are stomata on the trunk epidermis , but defects on the spines.
The sporangia stand in loose ears in two rows at the branch ends. The individual sporangia stand on a short stalk , probably provided with a vascular bundle , and are kidney-shaped. The opening was made along the convex edge in two equal flaps. The spores are homogeneous (homospor), round to slightly triangular in shape and up to 64 micrometers in diameter.
The gametophyte of Sawdonia ornata is unknown.
distribution
Fossils assigned to the genus Sawdonia are known from North America, Scotland, England, Belgium, Poland and Southwest Siberia. The finds are chronologically classified from the pragium to the frasnium , most of them come from the pragium and emsium .
Systematics
Sawdonia is one of several genera in the Sawdoniaceae family, all of which are characterized by their multicellular spines.
In addition to the type species Sawdonia ornata , Sawdonia acanthotheca from New Brunswick , Canada, is also known.
Botanical history
Sawdonia ornata was first described in 1859 by John William Dawson from the Gaspé Peninsula as Psilophyton princeps var. Ornatum . The fossils with lateral sporangia were placed in a separate genus Sawdonia by Hueber in 1971 . In 1989 Dianne Edwards et al. some finds are outsourced to the genus Deheubarthia .
supporting documents
- Paul Kenrick, Peter R. Crane: The Origin and Early Diversification of Land Plants. A Cladistic Study . Smithsonian Institution Press, Washington DC 1997, v. a. P. 332f. ISBN 1-56098-729-4
- Thomas N. Taylor, Edith L. Taylor: The Biology and Evolution of Fossil Plants . Prentice Hall, Englewood Cliffs 1993, pp. 204f. ISBN 0-13-651589-4