A conspicuous Psaronius specimen from Döhlen Basin
deutsche Version
Polished tree trunk cross-sections consisting of a complex central stele surrounded by interconnected aerial roots, commonly known as Psaronius, are among the most beautiful fossils on display in museums. (A most remarkable collection of big polished Psaronius sections is on display at the Naturkunde Museum Chemnitz. [1]) Psaronius stems found in the Lower Permian cherts from Döhlen Basin near Dresden, Germany, are usually small, with diameters below 20cm, (with one big exception
of 30cm hitherto,) and squeezed. They are less abundant in the cherts than fern foliage. (See Permian Chert News 1, 2.) From the observation that foliage found together with Psaronius in the chert samples is always Scolecopteris, the "maggot fern", it is obvious that the two belong together.
Psaronius, Döhlen Basin
The specimen seen here is distinguished by an uncommon combination of advanced decay and excellent preservation: The collaps or squeezing process which had affected virtually all Psaronius stems in the Döhlen Basin while lying prostrate in the swamp did not at all affect a large part of the aerial root mantle of this stem. Only the centre and about half the aerial root mantle became squeezed. Apparently the tissues of the centre and adjacent roots became weakened by rot faster than influx of silica by diffusion could stabilize them so that the centre and the roots near the centre collapsed.

Fig.1: Psaronius, the stem of Scolecopteris, the "maggot fern", in cross-section: most beautiful specimen found hitherto in the Döhlen Basin. Width of the picture 9cm.

Preservation by silicification is governed by the competition between
tissue decay and silica uptake processes. These processes are interdependent, which complicates things:
Silica influx is precluded by diffusion barriers in healthy tissue and hence furthered by progressive decay. On the other hand, decay becomes increasingly delayed as silica accumulates in the tissue. Also the various tissues are differentially resistant to decay. Considering all this, one need not wonder why some features of the fossil are not easily understood.

Apparently the silica penetrated the strong walls of the aerial roots so slowly that the inner tisse decayed and vanished before it could become preserved by the mineral deposition. Silica subsequently entering into the cavity formed agates. Same as with the agates in small cavities in volcanic rock, it is always a cause of wonder that neighbouring ones may look completely different from each other, as in Fig.2.
Obviously, tiny differences of the chemical constitution of the water-filled cavities result in large effects on
the deposition of SiO2 and colourful minerals, mostly iron oxides. 

Psaronius roots with agatesFig.2, left: Agates in hollow aerial roots.
Psaronius root with complex agateWidth of the picture 1.2cm.

Fig.3, right: Complex agate formed by a sequence of deposition processes in a hollow aerial root after it had been split by a crack. Width of the picture 0.5cm.

As a peculiar fact, the xylem strand of the aerial roots with its characteristic "star-shaped" cross-section, which is usually preserved, has vanished here together with the soft tissue surrounding it. Contrary to such complete decay of the interior of the aerial roots, the cellular structure of their wall is completely preserved. (Not well seen with this magnification.)

Sample: Bu7/24.2 , found at the type locality of Scolecopteris, 2000;  Photographs: M. Barthel.

H.-J. Weiss       2011

[1] R. Rößler : Der versteinerte Wald von Chemnitz, Naturkunde-Museum Chemnitz, 2001.
Scolecopteris pinnule cross-section, Sardinia Permian Chert News 6

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