Agate and fossils from watery habitat (4)
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black level
Picture: Silicified swamp matter including fungi and early land plants, with perfectly straight horizontal interface between former watery liquids.
Width of the level 7.6mm.

This picture together with this headline may seem incom-
prehensible: Where are the fossils and why is it called agate ? Admittedly, most agates do not show straight lines, which are really cut plane faces, but if there is a straight line in chalzedony, the whole may justly be called agate. However, it must be mentioned that most of the fossils seen in exhibitions or offered for sale are in no way related to agates. There are only a few fossil-bearing locations world-wide where small fossils are well preserved in chalzedony from silicified swamp water, eventually with small agates close by or even inside. The most famous location of such type is near Rhynie, Aberdeenshire, where this picture comes from.
This sample from the Lower Devonian may remind the observer of the fact that those were the times when plants had ventured onto dry land not long before, in the Upper Silurian, probably in symbiosis with fungi, according to results of recent research [1]. Lots of thin fungus hyphae are present in this sample, recognized in the black layer only by their thick coatings of bluish chalcedony.

Highly peculiar is the perfectly straight horizontal line bordering the black layer, which is liable to be misinterpreted as a former liquid surface. It is remarkable that the hyphae cross the border as if a liquid surface had never been there, which indicates that, instead of a surface, there had been a level interface between a dark watery suspension, possibly of dead microbes, and the water. This suspension must have been sufficiently liquid to form a perfectly plane interface, with the coated hyphae sticking through. (The aspect suggests a heavy suspension. In case the dark suspension would have been lighter than water, the picture should be turned upside down.)
What looks like a sea monster in the upper half of the picture is a squeezed specimen of the early land plant Aglaophyton (former Rhynia major), silicified while lying in the swamp water. What looks like an eye is a fungus organ known as chlamydospore. The xylem strand of the plant is preserved while other tissue is not. Another xylem cross-section, with several tracheids with black fill, is seen near the lower edge.
Sample: Rh14/3.2 (2005) 1.9kg, obtained from Barron.

[1] T.N.Taylor, M. Krings, E.L. Taylor: Fossil Fungi, Elsevier 2015.

H.-J. Weiss    2019
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