Scalariform tube wall patterns
While
most of the chert samples from Rhynie, which are famous for their often
well preserved Devonian fossils, offer only things found before, a few
of the samples may provide surprises, even multiple
ones. This is the case with the sample where Figs.1-5 have been taken
from.
The
nematophyte in Fig.1 seems to be rather similar to Nematothallus known
as compression fossils [1]. It is tentatively called Nematothallus
here, for simplicity. This specimen is a former lump of (probably
organic) gel with
worm-like tubes inside, seen
enlarged in Rhynie
Chert News 107.
Judging from the whitish
chalcedony
surrounding it, this Nematothallus
lump had got washed into a water-filled cavity in the (probably
anorganic) gel in the swamp matter to be silicified. Nothing similar
has been seen elsewhere, hence this tiny fragment is the only
specimen of Nematothallus
found in chert hitherto.
Fig.1: Worm-like tubes with patterned walls, width 19-23µm, in a
separate lump of gel:
Nematothallus, silicified
together with surrounding water.
Image width 2.5mm.
Figs.1-5 from same
sample.
Fig.2 (right):
Detail of Fig.1.
Fig.3 (left): Detail of Fig.1. Photograph: G. Schmahl.
Image width 0.8mm.
Same magnification
and width for Figs. 2-6.
Fig.4 (left): separate
Nematoplexus (?)
tube,
width 26-28µm.
Fig.5 (right): Nematoplexus
"knot", 3 types of tubes.
The
tubes with a more or less irregular mix of annular and helical wall
patterning found with nematophytes are strongly suggestive of an
evolutionary path leading to the tracheids of vascular plants [1].
Therefore they deserve attention. Tubes
of that type (Fig.4) are often associated, in small numbers, with the
smooth-walled tubes of Nematoplexus,
a nematophyte of quite different aspect (Fig.5). A
large tube with patterned wall, 25µm, is seen here only as a short
fragment not attached to but probably related to the tangle (known as
"branch-knot" owing to the obsolete idea of profusely branching tubes
inside).
There is no obvious reason why several
smooth tubes
and a patterned one should make a favourable combination. This is
only one of
several enigmas
encountered with
Nematoplexus.
Hence it will probably take some effort to find out the essential facts
about this enigmatic organism. 
Separate tube fragments with annular or helical
wall
patterns are not rare in the Rhynie chert. It can be difficult to tell
whether they are the
remains of Nematoplexus,
Nematothallus,
or even of tracheophytes. The dissolved xylem strand of Asteroxylon (Fig.6)
looks as if evolution had
simply made use of patterned tubes from nematophytes.
Fig.6: Tracheids, up to
28µm across, from disintegrated xylem strand of Asteroxylon.
Same
magnification and image width for Figs. 2-6.
Apparently, tracheid wall patterns have been a successful detail of
plant life up to now
although it is not quite clear to which purpose.
In the palaeobotany literature one may come across a quite absurd
explanation: It is stated that flowing water in the cells exerts an
underpressure which would cause the cells to collapse if the walls were
not reinforced [2]. This is utter nonsense.
Samples:
Rh9/86.1,2 (0.28kg), found in 2003: Figs.1-5.
Rh9/33.1 (0.07kg), found in
2004
by S.
Weiss: Fig.6.
H.-J.
Weiss 2020
[1]
P.K. Strother: Clarification of the genus Nematothallus Lang: J.
Paleont. 67(1993), 1090-1094.
[2] K.J. Niklas, V.
Smokovitis: Evidence for a
conducting strand in early Silurian plants.
Paleobiology
v.142 (1983), p.103-121.
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