A Devonian plant that would not fit in
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Fossil finds which do not seem to fit to known species are not uncommon and keep the palaeontologists wondering. Most often it is found out where they fit in after all, otherwise they may indicate the presence of something new. Really intriguing are the nematophytes, which have got names but have not yet got a branch on the Tree of Life. The Lower Devonian Rhynie chert has provided some, including new ones. The Rhynie chert is expected to yield more surprises, and here is another one which awaits interpretation
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unknown plant section, Rhynie
Fig.1: Unknown fossil in Rhynie chert with granular aspect. Width of the picture 9.5mm.

Fig.2: Detail of Fig.1. Width of the picture 1.5mm.


unknown plant section, Rhynieunknown plant section, Rhynie
Fig.3: Angular grains representing casts of tiny cells, detail of Fig.1. Width of the picture 0.83mm.


The plant or plant part presented here looks like a bag filled with grains (Figs.1,2). Since the grains are angular but do not look like crystals, they are most probably casts of cell lumina. Cell casts can be formed by differential silicification of the tissue where some cells get filled, probably with chalcedony and triggered by some chemical difference, before the remaining tissue decayed and got replaced by slightly contiguous powdery quartz, which altogether makes the optical contrast. The dark grains show a slight tendency of being arranged in chains, as seen in Fig.3 at some place on the right, for example. This indicates that a process spreading from cell to cell may have triggered the silification of the interior. Phenomena of such kind, perhaps mediated by some rot fungus, have produced the cell-size clots or angular grains repeatedly misinterpreted as mite
coprolites.
What is quite uncommen with the present find is the grain size of 15...25Ám, which is much smaller than the cell sizes of any of the 7 vascular plants hitherto discovered in the Rhynie chert, and also much smaller than the "coprolites" found in petrified wood.
The black appearance is brought about by two effects: The more transparent grains let the light pass into depth and therefore look much darker than the white powdery quartz, and some
have black inclusions of decayed organic matter. Near the surface of the grain bag the dark grains are so crowded that one cannot see whether or not the plant had an epidermis.
cuticle (?) of unknown plant section, RhynieThere is a small spot of decayed and vanished tissue where the former surface is faintly indicated by a thin dark line (Fig.4). Such line is also seen in some places on the other half of the cut specimen. The thin lines along small parts of the contour might be the remains of a cuticle. detail of unknown plant section, Rhynie


Fig.4 (left): Cuticle (?) seen as a dark line covering a hole below the surface in Fig.1.

Fig.5 (right): Squeezed end of the bag in Fig.1 or end of stalk with holdfast ? Width of the picture 0.7mm.


There is one feature in Fig.1 which is either highly significant or an artifact by partial decay and compression: The bag of more than 1mm thickness suddenly narrows into an indistinctly seen extension to the right, poorly silicified and traversed by cracks. It resembles a stalk with a (tripartite ?) foot or holdfast, width 0.6mm, at its end (Fig.5).
As the bag is also seen on the other side of the cut gap of 1mm or more, with
1.5mm only slightly thicker, and with the surfaces of the half objects standing nearly perpendicular on the cut faces, one can conclude that what is seen in Fig.1 is a lengthwise section of a bag of about 1.5mm thickness and at least 3mm width. If the narrow extension really were a stalk, it would be ribbon-like.

Although a final interpretation of the fossil has to be left to those who might find more specimens of this kind sometime or perhaps have a bright idea just now, a few tentative conclusions suggested by the observations are presented here.
The fact that no indication of vascular tissue is seen on either half of the divided bag does not mean there is none but it suggests that possibly there is none. The angular grains most probably indicate the presence of a tissue of densely packed cells, thus being quite similar to tissues of vascular plants except for the conspicuously small cell size which distinguishes the present fossil from all vascular plants found in the Rhynie chert, and possibly from virtually all other palaeozoic vascular plants.
The absence of tubes excludes any affiliation with fungi or nematophytes. The presence of what seems to be a cuticle does not necessarily point to a real land plant. Organisms living in an aquatic habitat which repeatedly falls dry may develop a cuticle, as it is known from nematophytes. The habitat which turned into the Rhynie chert by silicification was of that type since it harboured aquatic plants, fungi, and creatures as well as land plants, fungi, and creatures. The well preserved fossil of Fig.1 stands out against the severely degraded and squeezed ones in its vicinity with microbial layers and the aquatic Palaeonitella. At a vertical distance of 3cm, lots of squeezed and a few well preserved Rhynia are seen, indicating subaerial conditions at times.
A primitive land plant growing on the mud in a prostrate way would most probably have unequal sides. Since the object in Fig.1 seems to have two equal sides, it is more probably a cm-size water plant of uncertain affiliation, attached to the ground with a holdfast, provided that the structure in Fig.5 is such. It will have to be checked if any of the various branches of green algae or even the fundamentally different branches of other algae in the phylogenetic tree are compatible with the above observations.
So it appears that there is a wide field of potential explanations of the find, and if it fits in nowhere, it will be interesting all the more.

The chert sample was found by Sieglinde Weiss at Castlehill in 2009.


H.-J. Weiss      2013
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