Veining type - a distinct feature of Psaronius tree fern pinnules
Veining type - a distinct feature of Psaronius tree fern pinnules |
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An introductory remark is appropriate here to avoid confusion since there are several valid names for different parts of the same plant: The conspicuous trunks with their intricate internal structure are known as Psaronius. Their foliage has got the name Pecopteris when preserved as a compression fossil but Scolecopteris ("Maggot Fern") when preserved 3-dimensionally with sporangia present. The ambiguities introduced in this way shall not concern us here further. With increasing amounts of Scolecopteris from Doehlen basin (Lower Permian) becoming available since 1993 it has become apparent that some features vary within wide bounds: The pinnules may be nearly straight or curved up to half-circular shape. Their margins may be nearly smooth or beset with more or less long and tapering fringes. The number of sporangia fused into a synangium varies from 3 to 6 or rarely 7, and even for a given number their pattern of arrangement may differ on one pinnule [1]. It will be difficult to find out to which degree such variability is due to environmental factors (sun or shade, more or less wet habitat), age (juvenile, adult, wilting), etc., and which are inherited features. Finding out this might give clues concerning the response to a changing environment, or it might serve as a basis for differentiation between various strains or species. There is one feature which is most likely inherited: It is the presence of forking or branching lateral veins on the pinnules. According to Millay [2], forking veins are absent in Scolecopteris elegans and several related species. It seems that most of the chert samples from Doehlen basin do not show forking veins but it can be stated that samples with forking veins are not rare. There are chert samples with largely decayed plant matter where virtually nothing is left but a few forking veins revealing the presence of Scolecopteris.
Although much more structure information can be preserved in chert compared to coalified compressions, the vein pattern of the chert specimens is less conspicuous because the fronds and their parts are usually not lying flat in the chert but in tilted and distorted positions. Even in the rare cases where pinnules form a surface relief the vein pattern is less distinctly seen than in the sometimes large frond fragments from locations of compression fossils. With some experience one can reveal a vein pattern as that one obtained by GERT MUELLER by carefully chosing the cut plane of his sample (found in 1985) in such a way that as many pinnules as possible were seen in plane (Fig.1). In this connection it is worth mentioning that he was the first to find more "Maggot Stones" after nearly a century of no finds, thus essentially contributing to the recent upsurge of interest in the subject. Numerous chert samples investigated lately show that forking veins usually come along with unbranched veins as in Fig.1. Unbranched veins with angles of about 45° with respect to the midvein are seen in Fig.2, thus differing from the veining of Scolecopteris elegans with its angle of about 60°. Apparently the Maggot Fern in chert samples from the type locality of Scolecopteris elegans somehow varies between this species and deviating strains whose relations to this and other known species are still obscure. text: H.-J. WEISS/Rabenau, photographs: M. BARTHEL/Berlin, H. SAHM/Dresden
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