Psaronius /Scolecopteris – a floating tree fern ?
deutsche Version
An idea suggested by fossil evidence

Stems and fronds of Palaeozoic tree ferns of the family Psaroniaceae [1] have got separate names for practical reasons: Usually they are not found together so that it is virtually impossible to establish a species covering both foliage and stem. Or else there can be several species present in the same petrified biotope, which likewise makes it difficult to find out which foliage belongs to which stem. The stems are called Psaronius, the foliage is called Pecopteris if preserved as a compression fossil, otherwise it is subdivided into several genera including Scolecopteris. Nowadays the name Psaronius is also used for the whole plant but the other names are still valid. The genus Scolecopteris has been subdivided by Millay [1] into 26 species.
A drawing by J. Morgan [2] thought to be more or less representative for some or all Psaronius /Scolecopteris  species has been published repeatedly in monographs [3,4]. These drawings, however, begin always above ground level although there are reasons to assume that a good deal of the tree is hidden below.
Any attempt to reconstruct the lowest part of Psaronius has to start from the peculiar structure of the tree trunk: Most of it consists of apparently strong aerial roots running down the stem, connected by soft tissue. The stem proper, without the roots, is widest at the top where it bears the fronds but very narrow near the ground. It is as narrow there as it had been in the juvenile stage because there is no subsequent lateral growth. There is no primary root left, hence the whole tree rests on its aerial roots.
Far-reaching conclusions can be drawn from one feature of the aerial roots: As they get near the ground while growing downward, they develop air-filled tissue, the aerenchyma, thereby largely increasing their cross-section. They become more and more detached from each other, hence their lower parts are called free aerial roots.
The free aerial roots are seen beautifully preserved on cross-sections of the lower part of the large Psaronius tree trunks displayed in museums [5] but usually they are not found fossilized in the ground. This may be due to the event which supposedly led to the fossilisation of the conspicuous trunks: a volcanic eruption causing a pyroclastic flow moving down the slopes at high speed (typically about 400km/h) and spreading over level ground, thereby tearing the trees from their base and possibly blowing away their habitat as well, scattering the roots together with the soft ground or mud.
So it can be understood that the big Psaronius specimens displayed at the Naturkunde Museum Chemnitz had not been found silicified together with the related parts of the tree.

Luckily, the fossilisation of tree ferns was not always preceded by catastrophic events so that occasionally all parts of the tree, namely the free aerial roots in the ground, the stems with the fused aerial roots, and the foliage, are found in the swamp matter turned into chert.
Numerous chert samples representing a wet habitat with layers of peat and mud silicified while at or near the surface have been found lately in the Lower Permian Döhlen basin. Moults of the aquatic crustacean Uronectes and extended microbial layers found among the remains of Psaronius / Scolecopteris indicate that there was not only wet ground but free water as well. Part of the chert samples contain aerial roots (Figs.1,2), some of which are preserved in a non-collapsed state.
Psaronius roots in ground
Fig.1: Psaronius "free" aerial roots in the ground, more or less squeezed before silicification, aerenchyma (air-filled tissue) poorly visible here, layered peat consisting of collapsed roots below.
Döhlen basin (Lower Permian), type locality of Scolecopteris. Width of the picture 9cm.
Psaronius aerial root below ground
Fig.2 (right): Psaronius free aerial root cross-section with aerenchyma,
originally air-filled tunnels up to 0.5mm across.
Width 1.5cm.

Site: Type locality of the "Maggot fern" Scolecopteris elegans at the boundary between Kleinnaundorf and Burgk, Döhlen basin, Saxony, Germany.
Samples: Fig.1: Bu13/31.3 ;  Fig.2: Bu13/35.2 ; found in 1998 on the property Kohlenstr. 8. The recovery of these and numerous  other samples has been furthered by the interest of the owners, family

The idea suggests itself that the mass of tangled and branching air-filled free aerial roots would have enough buoyancy in soft mineral mud or even in water or organic mud to support the whole tree. This would be doubtless an advantage or even a precondition for the growth of trees on wobbly ground. Since Nature usually realizes favourable options, it is worth while considering the implications of such design. One implication is evident from Fig.3.
floating tree in storm
Fig.3: Advantage of a floating tree in strong winds: Does not get rooted up or broken off.

If there were floating trees among the several Psaronius species, what could be predicted about their successive growth ? The answer is visualized in Fig.4.
The tree sinks in as it grows, with buoyancy increasing such that it keeps equilibrium with the increasing weight. This may explain why the lowermost part of the stem with its tiny centre dating back to the earliest growth stadium is never seen on the conspicuous polished stem cross-sections displayed in museums [5]: This oldest part of the plant had most probably been dead and gone before the tree became big.
Psaronius as a floating tree
Fig.4: Hypothetical design of floating Psaronius:
Equilibrium of the growing tree is maintained by successively sinking in, as indicated by the arrow supposed to be fixed to the trunk.
Stability against upsetting is brought about by the large raft of air-filled roots.

                H.-J. Weiss       2011

[1]  M.A. Millay: A review of permineralized Euramerican Carboniferous tree ferns. Rev. Palaeobot. Palyn. 95(1997), 191-209.
[2]  J. Morgan: The morphology and anatomy of American species of the genus Psaronius. Illinois Biol. Monogr. 27(1959), p1-108.
[3]  W.N. Stewart, G.W. Rothwell:  Paleobotany and the Evolution of Plants. Cambridge Univ. Press 1993, p228.
[4]  T.N. Taylor, E.L. Taylor, M. Krings:  Paleobotany, Elsevier 2009, p418.
[5]  R. Rössler: Der versteinerte Wald von Chemnitz. Museum für Naturkunde Chemnitz, 2001.
Scolecopteris pinnule cross-section, Sardinia Permian Chert News7

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