A peculiar habitat
A peculiar habitat |
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Superb preservation and colourful aspect made the psaronius tree trunks from Chemnitz widely known. Mainly the agate-filled cross-sections of large aerial roots are catching the eye. As a typical feature of the psaroniales, the stem produces roots in the air which grow downwards to form a root mantle tightly bound by parenchyma between the roots. This root mantle, which is thickest below, essentially makes up the trunk and provides mechanical strength. At some height above the ground, the outermost air roots leave the composite trunk and become free, forming a loose thicket suitable as a habitat for epiphytes. Several species of vines, for example, have been found within and without the root mantle of psaronius trunks from Chemnitz [1]. Psaronius root mantles are not rare in the Rotliegend cherts but their preservation is usually poor: Fallen trunks became constituents of peat and thus squeezed flat before silicification. Stumps became deformed as a result of either the trunk’s breaking off or compression by overlaying deposits. Nevertheless one can find chert samples bearing evidence of a sub-habitat “root mantle” in the habitat “swamp”, as the one found by H.SCHLESIER/Leukersdorf in the Nobitz gravel pit.
This sample is a gray-brown chert variety, conspicuous for its transparent matrix and areas with hematite and agate inclusions (Fig.1 & Fig.4). The tissue is well preserved in the light-coloured centre. Several Psaronius aerial roots with the typical star-shaped xylem can be recognized (Fig.2), also the loops of the sclerenchyma sections and the parenchyma within and between the roots. Among the confusing assemblage of structures one can find small diarch xylem strands with exarch protoxylem and surrounding cortex (Fig.3), which might be aerial roots of Tubicaulis or Ankyropteris [1]. As another tentative explanation, they might be aerial roots of Sphenophyllum. The latter option is suggested by the observation on a Permian fossil from Brazil where a triarch axis, possibly Sphenophyllum, is surrounded by small diarch aerial roots [2]. text & photographs: R.KRETZSCHMAR/Chemnitz
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