Prickly frond stalks of Permian tree ferns
deutsche Version

Chert fossils usually provide tiny details not seen on compression fossils, like hairs on tree fern pinnules or book lungs of trigonotarbids seen on cut and polished chert faces, for example. Fossil structures may also appear on crack faces
if the fossiliferous chert is not homogeneously silicified but contains poorly silicified layers serving as faces of easy crack propagation. Even a plant cuticle, thin as it is, can become an easy crack path in chert. Apparently this has been realized in the present sample where the prickly surface of a frond stalk is exposed. As a surprising fact, it is not seen from the outside only, as expected, but also from within.
prickly frond stalkprickly frond stalk

Figs. 1,2: Surface of Permian tree fern frond stalk with tiny prickles, seen also from within as depressions. Image widths 8mm.

This is one of the rare occasions where a crack face in chert reveals the essential structure which would not be seen adequately on a cut face.
The peculiar fact that both the outside and inside of the frond stalk surface are seen on the same image requires an explanation.
The tissue of this frond stalk  apparently had decayed so that the stalk had collapsed to a thickness of about 0.2mm before silicification. A crack had propagated along one of the flat sides of the collapsed stalk so that it is now seen exposed on the sample surface with randomly distributed prickles. Subsequently, part of the 0.2mm fill of the collapsed stalk broke off whereby the opposite surface of the stalk became visible from within, hence with depressions instead of prickles, as seen on the left of Fig.1 and in the middle of Fig.2. (The slighty yellowish hue on part of Fig.1 indicates that this area had been laid bare later.)
This sample does not provide features which would allow to assign it to a tree fern species. It is from a site where hundreds of Scolecopteris and Psaronius fossils have been found so that the prickly fronds probably belong there. However, the possible presence of other tree ferns at the same site should be considered.

Bu7/42 (0.71kg), Freital, Kohlenstr., Döhlen Basin, Lower Permian, found in 2000.

H.-J. Weiss      2020
Scolecopteris pinnule cross-section, Sardinia Permian Chert News29
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