A Devonian plant that would not fit in
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
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.
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.
Fig.3: Angular grains representing casts of tiny cells, detail
of Fig.1. Width of the picture 0.83mm.
plant or plant part presented here looks like a bag filled with grains
Since the grains are angular but do not look like crystals, they are
most probably casts of cell lumina. Cell casts can be formed
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
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
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"
black appearance is brought about by two effects: The more transparent
grains let the light pass into depth and therefore look much
than the white powdery quartz, and some have
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.
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
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
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.
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
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
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,
and creatures as
well as land plants, fungi, and creatures. The well preserved fossil of
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
are seen, indicating subaerial conditions at times.
A primitive land plant growing on the mud in a prostrate way would most
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
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
The chert sample was found by Sieglinde
Castlehill in 2009.