Another type of Rhynia gametophyte ?

deutsche Version

The gametophyte* of Rhynia gwynne-vaughanii was first pictured in 2000 without being recognized as such [1], then thoroughly described by H. Kerp et al. as Remyophyton delicatum and presented at the Rhynie Chert Conference in 2003 [2].
Also in 2003, a small (0.4kg) piece of chert found well away from the site of the type specimen revealed a few globular objects laterally attached to an axis (Fig.1). Although their lateral position does not fit to any of the known gametophytes, their aspect, and their attachment to the shoot by a short pedicel in particular, suggest a tentative interpretation as antheridia. A granular structure which might be the remains of sperm cells in rows is seen in the globule on the right in Fig. 2.

Fig.1: Globules resembling antheridia, laterally attached to an axis by a short pedicel; section of Rhynia sporangium with black abscission tissue nearby.

Fig.2 (right): Detail of Fig.1, granular structure in the globule on the right.

The chert sample also contains axes which would doubtless be assigned to Rhynia because of their typical wart-like bulges and their diameter of 1 to 2 millimeters. Also there are sporangia of the Rhynia type. Incidentally, a tilted section of such is seen in Fig.1. The black tissue is thought to be the pre-formed rupture site [3].
So we are confronted with the fact that a chert sample with Rhynia contains something suggesting an interpretation as a gametophyte clearly differing from Remyophyton [2] (see table below and Fig.3).

Sample: Rh6/36.1 (0.4kg whole), found in 2003 by S. Weiss.

	Remyophyton (male), after [2]	this sample
axis diameter                       [mm] antheridium (globule) shape                                height                                width                                position                                pedicel length	0.4 ...0.6, seldom larger slightly wider than high 0.21 ...0.38 0.24 ...0.42 on the flat top of the axis 0.04 ...0.09	1.2 slightly higher than wide 0.6 0.54 laterally attached to the axis < 0.04

 
Fig.3: Antheridia on the top of Remyophyton delicatum (left), after [2];  globules (antheridia or what else ?) arranged on an axis as seen on the cut face of this sample (right).
 
In the attempt to interpret the above observations, several options should be considered:
(1) The definition of Remyophyton delicatum in [2], apparently based on only one chert sample, may not be adequate to the variability of the species.
(2) What is seen in this sample might be a gametophyte belonging to a hitherto unknown sporophyte species.
(3) The globules attached to the axis may be caused by some fungus incidentally mimicking antheridia of early land plants.
(4) added in 2021: The globules may be propagules, as emphasized for other chert samples in [4].

* Note that the Rhynie chert plants, like extant ferns, bring forth alternating spore-bearing and gamete-producing generations.
While the gametophyte of ferns is tiny, that of the early land plants may be not so much smaller than the sporophyte.

Annotation 2021: The interpretation of laterally attached globular bodies as propagules in [4] is acknowledged here as highly interesting with respect to the present fossil.
As minor mistakes in [4], Fig.3B is supposed to differ from Fig.3A in view and magnification but is really the same picture, except for slightly different tilt and depth of focus.
If 100µm is the right size of the scale bar for Fig.3C, it should be 300µm for Fig.3D.

H.-J. Weiss         2007, revised version 2010,  2021

[1]  C.L. Powell, N.H. Trewin, D. Edwards : Palaeoecology and plant succession …,
    Geol. Soc. London Special Publication 180(2000), 439-57.
[2]  H. Kerp, N.H. Trewin, H. Hass : New gametophytes from the Early Devonian Rhynie chert,
    Trans. Roy. Soc. Edinburgh, Earth Sciences 94(2004 for 2003), 411-28.
[3]  D.S. Edwards : Evidence for the sporophytic status of the Lower Devonian plant Rhynia gwynne-vaughanii,
    Rev. Palaeobot. Palynol. 29(1980), 177-88.
[4]  P. Kearney, H. Kerp, H. Hass : Whole plant regeneration via epidermal cells in the ... Rhynie chert plant Rhynia ...
    Int. J. Plant Sci. 177(2016) (6) .

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