Tracheid-like tubes in Rhynie chert
deutsche Version

While most of the chert samples from Rhynie, which are famous for their often well preserved Devonian fossils, offer only things already known from that site, 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.
Nematothallus in chert
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 becoming silicified. Nothing similar has been seen elsewhere, hence this tiny fragment may be the only specimen of Nematothallus found in chert hitherto.

Fig.1: Worm-like tubes with patterned walls, width 19-23Ám, in a separate lump of gel:
Nematothallus
silicified together with surrounding water. Image width 2.5mm.
   Figs.1-5 from same sample.


Nematothallus in Rhynie chertNematothallus tracheids
Fig.3: Detail of Fig.1.
Photograph: G. Schmahl.

Image width 0.8mm.

Same magnification
   and width for Figs. 2-6.

Nematoplexus tube, scalariform
Fig.2 (left): Detail of Fig.1. 


Fig.4 (right): separate
   Nematoplexus
(?) tube,
   width 26-28Ám.
Nematoplexus clot10
Fig.5 (left): 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. Their existence might be easily accepted as a peculiar invention serving some purpose in the course of evolution if there were not the disturbing fact that 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, 25Ám, is seen here only as a short fragment not attached to but probably related to the clot (known as "branch-knot" owing to the obsolete idea of profusely branching tubes inside). The tube of intermediate width in Fig.5, 17Ám, seems to be smooth-walled but a very faint texture might indicate a narrowly spaced wall pattern as it is known from tubes of this size on other Nematoplexus specimens.

There is no obvious reason why many smooth tubes and a few patterned ones, as in Fig.5, should make a favourable combination. This is only one of several enigmas encountered with Nematoplexus. Hence it can be stated that it will take some effort to find out the essential facts about this enigmatic organism.
tracheids
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 detached Nematoplexus tubes or the remains of decayed Nematothallus, or even of tracheophytes. The decayed xylem strand of Asteroxylon (Fig.6) could be mistaken for> Nematothallus, as if the patterned tubes of Nematothallus had simply been applied for water-conducting components known as tracheids in tracheophytes.

Fig.6: Tracheids, up to 28Ám 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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