Asteroxylon aspects
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Asteroxylon
is the most complex plant in the Lower Devonian Rhynie chert. It has been described thoroughly before [1,2]. Nevertheless, some additional information may be useful. First, the xylem strands can appear white instead of the usual black, which is not an essential feature but merely a detail of preservation. Second, the plant does not always look like a recent clubmoss as much as current reproductions [3] try to make believe.Asteroxylon cross-sections
What makes it look like a clubmoss is the small leaf-like appendages called enations. As another peculiar detail, the xylem strand is of a highly complex build. Its cross-section is surprisingly variable [1], and it splits off thin branches which sharply bend radially outwards, then upwards again, and end within the stem in a patch of "transfusion tissue" [2].
According to own finds but contrary to the drawing in [3], the main upright axis of Asteroxylon does not seem to branch profusely, although smaller aerial axes usually called branches are often seen among the Asteroxylon main axes in larger numbers (Fig.1). This suggests that they possibly emerge immediately from an extended rhizome. Incidentally these "branches" resemble the aerial axes of Nothia, which has made them a source of continuing confusion, all the more so since the two plants may be found in the same piece of chert. (Nothia may be recognized by tube-like "giant cells" running along the epidermis.)

Fig.1: Asteroxylon, cross-sections of aerial axes on a cut chert face, xylem bleached after silicification. Width of the picture 17mm.
Asteroxylon xylem sections
Normally the xylem strands appear black in the chert but not seldom they are white when on the surface of the sample. Hence, the white aspect must be the result of some bleaching process penetrating for a small distance. The present sample is different as the xylem strands are bleached throughout. (Figs.1,2 show cut chert faces.)
There is a possible explanation of the selective through-bleaching:  Xylem strands in the chert have been observed which easily soak up liquid, which means they still consist of bundles of tubes, which makes easy diffusion paths. The bleaching by diffusion of oxygen could have occurred while the strands were still in such state. By continuing deposition of silica the bleached strands became compact chert as they are now in the present sample.

Fig.2: Asteroxylon, two sections of yellowish xylem strands. (Same scale as Fig.1, for enlarged detail see Fig.3.)

Asteroxylon xylem inclined cut
Fig.3: Asteroxylon tracheids seen as separate tubes on an inclined section of yellow xylem in transparent chalzedony.
Detail of Fig.2, width 2.3mm.

Note that bleaching during and after silicification can affect the visibility of fossil structures. Black tracheids would not stand out against the dark background as the yellow ones do in Fig3.
While the xylem strands of Asteroxylon are nearly always clearly seen, also in the usual state of preservation where they had not passed a stage of bleaching, the tissue of the aerial axes is nearly always not seen at all, and if seen it is in a poor state of preservation and most often not suitable for reproduction.
Asteroxylon xylem, sorangium nearby
Fig.4: Asteroxylon on the raw surface of the chert sample: cross-section of xylem within decayed stem, sporangium with spores nearby.
Width of the image 5.5mm.

The ususal aspect of Asteroxylon with well-preserved xylem but decayed other tissue is seen here combined with a sporangium which is not well compatible with the widely accepted reconstruction in [3]. Another case of dubious sporangium is reported in Rhynie Chert News 53 but neither is well compatible with the rather big and flat sporangia in [1-3]. It appears that Asteroxylon is more variable than previously thought [4]. It has even been suggested that there may be more than one Asteroxylon species in the Rhynie chert [5].

Apparently there are no other fossil plant species in this chert sample so that one can assume that the sporangium belongs to Asteroxylon.


H.-J. Weiss      2016


[1]  R. Kidston, W.H. Lang: On Old Red Sandstone plants showing structure, Part III.  Trans. Roy. Soc. Edinburgh 52(1920), 643-680.
[2]  A.A. Bhutta: Studies on the Flora of the Rhynie chert. Ph.D. thesis, University of Wales, Cardiff, 1969.
[3]  T.N. Taylor, E.L. Taylor, M. Krings : Paleobotany. Elsevier 2009.
[4]  D. Edwards: Embryophytic sporophytes in the Rhynie and Windyfield cherts. Trans. Roy. Soc. Edinburgh Earth Sci. 94(2004 for 2003), 397-410.
[5]  W. Remy, H. Hass: Langiophyton mackiei, ... . (Addendum p103). Argumenta Palaeobot. 8(1991), 69-117.
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