Kinky fossils
deutsche Version
confusing structure

This combination of headline and image may either scare off readers interested in fossils or arouse their curiosity. It must be admitted that what is seen here inside an aerial root of a Permian tree fern is related rather to chemistry and physics than to palaeontology. The presence of mineral platelets, probably baryte crystals, arranged near the conspicuous line with several kinks can mislead to the suspicion of a hidden reason behind it but is purely incidental here (Fig.1).
Simply ignoring the platelets may be a good guess to start with. The thin dark line is really a former narrow crack traversing the partially solidified outer aerial root (Fig.2). Since cracks tend to move straight ahead but never take a kinky path, there must have been hidden processes at work. Apparently the silicification process was still going on after crack formation so that the gap became filled with silica gel which hardened into chalcedony while the partially silicified aerial root as a whole had still retained some ductility. While the aerial roots became gradually compressed by the overlying sediment, the incompressible crack fill responded with the formation of kinks near the middle of the root where the whitish matter apparently had still been less hard.

Psaronius aerial root traversed by kinky crack
Fig.1: Peculiar structure inside a Permian fossil but unrelated to palaeobotany. Image width 2.2mm.


Fig.2: Outer aerial root of the tree fern Scolecopteris, slightly compressed, with kinked crack fill seen as a thin dark line. Image width 11mm.

Closer consideration reveals that the phenomenon of kink formation is fraught with problems of continuum mechanics. The crack now seen in cross-section as a thin line must have formed in a soft elastic solid, probably silica gel. For reasons unknown, the silica-rich water in the gap turned into gel and brittle siliceous matter while the surrounding gel still retained some degree of ductility. This follows from the absence of secondary cracks beside the kinks. Hence, the kinked line in Figs.1,2, which is really a section of a kinked laminar crack fill, is the non-trivial outcome of the common action of elasticity and ductility.
By leaving mechanics aside and turning to palaeobotany again, a better preserved root from the same sample is shown in Fig.3.

Fig.3: Outer aerial root of the tree fern Scolecopteris with aerenchyma. Image width 11mm.Psaronius aerial root cross-section

Unlike the particular crack in Figs.1,2, the "normal" cracks in Fig.3 were formed when all was hard and brittle and thus are uninteresting. The huge aerenchyma cells with diameters up to 0.5mm are supposed to have been filled with air and thus have kept the tree afloat. Many are seen well preserved here but the comparatively tiny tracheids in the central strand seem to be all collapsed.
Rhynia with kinked xylemLess wondrous than the kinked hard sheet in Permian chert (Fig.1) is the
kinked stiff rod in Devonian chert (Fig.4), included here for comparison. It is the xylem strand of Rhynia acting like a laterally confined rod unable to bear an increasing load. While most of the plant responded by gradual deformation, the stiff silicified xylem became mechanically unstable and suddenly relieved the compressive stress by a sideward jerk making the two kinks seen in Fig.4 on the left.

Fig.4: Lengthwise cut of two Rhynia, with and without kinked xylem strand.
Image width 5mm.

Samples: Bu13/35 (0.15kg) Parts
1,2, found at the classical Scolecopteris- site, Doehlen Basin, in 1999: Figs.1-3.    Rh4/57 (0.51kg) Part2, found near Rhynie in 2009: Fig.4.

H.-J. Weiss     2021
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