Fossil plant cuticle as easy crack path
deutsche Version

Land plants protect themselves against water loss with a thin cuticle containing several wax-like substances. Plant cuticles are seldom seen detached from the plant surface but their existence can be deduced from the fact that they provide easy crack paths since their waxy composition makes a mechanical discontinuity in the chalcedony: a face of low fracture toughness. A crack running along this face greatly increases the mechanical discontinuity and creates an optical discontinuity within the transparent chalcedony which reflects the incident light so that the cuticle may appear as a conspicuous boundary of illumination marking the plant surface.

Scolecopteris pinnules
Fig.1: Two pinnules  of the Permian tree fern Scolecopteris,
cross-sections with sporangia, with distinct optical effect of the crack along the cuticle on the pinnules. Image width 4.2mm.

Apparently a crack entering from above left, driven by tensile stress, incidentally met the cuticle with low fracture toughness, whereupon it changed its direction and followed the cuticle along the curved pinnule surface until the stress intensity at the crack tip had become so low on its downward path that propagation along the cuticle stopped. At that point, a horizontal path provided higher stress intensity so that the crack changed its direction again. After having traversed some debris, it met the cuticle on the other pinnule and followed it until it stopped again for the same reason as before: decreasing stress intensity on the downward path. Like before, the higher stress intensity on horizontal paths is apparent from vaguely seen horizontal cracks on the right.
Aglaophyton detached
Cracks running through the chert usually but not always cross the silicified plants. A cuticle acting as an obstacle to thorough silicification as suggested by the crack path in Fig.1 for Permian chert acts in the same way, as expected, for Devonian chert as seen in Fig.2. Here, cracks became deflected around the plant, thereby separating it from the surrounding chalcedony, with the resulting gap greatly increasing the visibility of the plant surface.

Fig.2: Cross-section of the early land plant Aglaophyton, slightly inclined, detached from the surrounding chalcedony by a crack along the cuticle. Height of the section 4mm.

Since the cuticle itself is not easily seen, indications of its presence can be useful in cases where one does not know whether a palaeozoic plant is aquatic or terrestrial. This applies to the enigmatic sphere Pachytheca, which once had been thought to be an alga but is probably a nematophyte. Nematophytes include both aquatic and terrestrial organisms, also aquatic ones adapted to occasional drying. The deflection of cracks at Pachytheca is apparently due to a cuticle covering the sphere.
Samples: B/51.2 (> 5.2kg), found near Bannewitz (Doehlen basin) in 1995;
                Rh3/3.2 (0.28kg), found near Milton of Noth (Rhynie) in 1998.

H.-J. Weiss       2021

Scolecopteris pinnule cross-section, Sardinia Permian Chert News 33
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