Alleged fossil wood burrows
deutsche Version

V-shaped cracks on petrified treeA fossil tree trunk section with furrows on its cylindrical surface which resemble arthropod burrows below the bark of extant coniferous trees (Fig.1) is shown in [1], there Fig.442, and interpreted there as evidence of arthropod activity in the Lower Permian. That interpretation has been refuted in [2] by means of details from Fig.1.

Fig.1: Lateral view of a fossil tree trunk section, surface area with furrows formed from shrinkage cracks, misinterpreted as arthropod burrows in [1], (there mirror image, erroneous scale). Height 16.5cm, Palaeontological Museum Nierstein.

Closer inspection of the same specimen confirms the early doubts and thus contradicts the interpretation in [1] once more. Some of the furrows are V-shaped at the bottom, often with a slightly protruding ridge along the bottom, as clearly seen below right and elsewhere. This provides a clue to an explanation: Furrows of this type could never have been made by crawling creatures, which suggests that they may have been brought about by a combination of a few simple effects involving colloid chemistry and solid state physics, as outlined in the following.  
At an advanced stage of silicification of the tree trunk, the solid silica gel shrunk as a result of restructuring from amorphous to partially microcrystalline. Inhomogeneous shrinkage gave rise to local tensile stress and narrow cracks. Later the cracks became filled with silica while deposition continued, a phenomenon well known as "healing" of the cracks. (An uncommon but impressive example of crack fills is seen
 in Fossil Wood News 17 , Fig.3.)
Apparently the petrified tree trunk had been laid free by erosion of the sediment where it had lain, then broken into pieces which became tossed about on the beach together with other pebbles and boulders.
It seems that the healing had not quite restored the strength of the compact material but has left weak boundaries along the crack flanks, hence the healed cracks responded in particular ways to the impact of pebbles and boulders: Repeated impacts at or beside a healed crack possibly crushed the crack fill and caused the nearly parallel crack flanks to recede into V-shapes, rounded above, as a result of spalling off the edges. In this way the sculptured surface was formed from the originally inconspicuous shrinkage crack pattern extending downward from the surface.
The arrangement of the furrows is not quite compatible with an expected distribution of shrinkage cracks in plain wood, which may have contributed to the misinterpretation as arthropod burrows in [1]. This arrangement must have been governed by some irregularity in the wood which is irrelevant here. For comparison, shrinkage cracks compatible with the regular wood structure are seen on another area of this trunk surface (Fig.2).  
healed cracks in petrified wood
Fig.2: Surface detail of Permian tree trunk, same sample and same scale as Fig.1, furrows less distinct but compatible with the structure of plain wood, narrow crack fills seen as ridges along horizontal furrows: no arthropod borings. Height of the image 6.8cm,

Annotation: The example considered here is one of many others where lack of due care has led to misinterpretation of fossils.

H.-J. Weiss     2018   

[1]  R. Rößler: Der versteinerte Wald von Chemnitz. Museum f. Naturkunde Chemnitz 2001, p.176.
[2]  H.-J. Weiss: Elusive creatures in fossil wood – Clean-up in the wake of a waning obsession., Fossil Wood News 16 .

quartz crystal with wood inside
Fossil Wood News  29

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