BotanicalJoumal o f t h e Linnean Sociely (1992),109: 161-188.With 68 figures
A diverse assemblage of early land plants from
the Lower Devonian of the Welsh Borderland
U. FANNING, D. EDWARDS, F.L.S.
Department of Geology, University of Wales College of Card& Cardzf CFl 3YE
AND
J. B. RICHARDSON
Department of Palaeontolog, Natural History Museum, London S W7 5BD
Received March 1991, accepted f o r publication December 1991
FANNING, U., EDWARDS, D. & RICHARDSON, J. B., 1992.A diverse assemblage of early
land plants from the Lower Devonian of the Welsh Borderland. Nine rhyniophytoid taxa are
described from a n early Gedinnian locality (middle micronzatus-newportensis spore Biozone) near
Ludlow, England. They include Cooksonia pertoni, C. hemisphaerica, C. cambrensis, Tortilicaulis
transwalliensis and three new taxa, Salopella marcensis sp. nov., Uskiella reticulata sp. nov.
and Tarrantia salopensis Ben. et sp. nov. Isolated sporangia of reniform shape, and those
subtended by short lengths of axis, contain spores of Apiculiretusispora type and may belong to
C. caledonica or Renalia. Morphologically distinctive forking, terminal sporangia lacking identifiable
spores are not placed in a new taxon, because evidence based on in situ spores from elsewhere
suggests they may belong to Salopella. Structures previously interpreted as clusters of sporangia of
Yarravin-type are shown to be globular sporangia longitudinally split into valves. Sterile axes are
dominated by smooth forms; although rare examples possess small enations. Tracheids have not
been seen in axes of fertile specimens nor in sterile coalified compressions. A single pyrite
permineralization contains tracheids of zosterophyll type, The assemblage demonstrates diversity
among rhyniophytoids in the early Devonian and the existence of low vegetation ‘alongside’ the
much larger zosterophyll dominated type,
ADDITIONAL KEY WORDS:---in situ spores - rhyniophytoids - tracheids
-
Gedinnian.
CONTENTS
Introduction . . . . . . . .
Geology, material and methods
. . .
Descriptions and systematic palaeobotany .
Cooksonia hemisphaerica Lang
. . .
C. pertoni Lang
. . . . . .
C. cambrensis Edwards . . . . .
cf. C. CaledonicalRenalia sp. . . . .
Tortilicaulis transwalliensis Edwards . .
Salopella marcensis sp. POV.
. .
Uskiella reticulata sp. nov. . . .
cf. 15’.reticulata
. . . . . .
Tarrantia salopensis gen. et sp. nov.
0024-4074/92/060161+
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U . FANNING E T AL.
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Bifurcating sporangia .
Indeterminate sporangia
Permineralized sporangia
Sterile axes . . .
Conclusions . . . .
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Acknowledgements
References
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INTRODUCTION
Assemblages from late Silurian and early Devonian localities in the Welsh
Borderland are of paramount importance to studies on early land vegetation.
Their significance was appreciated by Lang (1937) who provided the first
comprehensive account of diversity in organization exhibited by early land
vegetation and who described Cooksonia which he considered to be the earliest
tracheophyte. He recorded indisputable tracheids in sterile axes at the small
quarry in Targrove near Ludlow, from strata then included in the Downtonian
and hence Silurian, but now placed low in the Devonian Ditton Group. The
only fertile plant recorded at the locality was his new taxon, Cooksonia
hemisphaerica .and Lang was quite convinced that the sterile axis with tracheids
came from that plant. As a consequence Cooksonia has become generally regarded
as the earliest vascular plant.
Recent controversy relating to the timing of the colonization of land and the
nature of the pioneering plants has led to a reassessment and a greater
appreciation of Lang’s work and prompted us to recollect from many of his
localities. Our efforts at Targrove have been particularly rewarding both in
numbers of taxa and specimens. Here we present the results of detailed
investigations on plants with axial organization. A preliminary account was
given in Edwards & Fanning (1985).
GEOLOGY, MATERIAL AND METHODS
Targrove Quarry is a small overgrown exposure in the Lower Old Red
Sandstone situated on the north side of the drive to Downton Hall (SO 5254
7799). It is 500 m east of Whitbach and 3 km north-east of Ludlow. Ball &
Dineley (1961) placed it in the lower part of the Ditton Group about 60 m above
the main Psammosteus limestone horizon. The fossil fish, Traquairaspis symondsi and
Pteraspis rostrata var trimplyensis, in the nearby stream section suggest that the
exposure occurs towards the base of the crouchi Zone. The dispersed spore
assemblage belongs to the lower part of the micrornatus-newportensis spore Biozone
(Richardson, Ford & Parker, 1984). Both fish and spores thus suggest an early
Gedinnian age. The sediments themselves are typical of the external distal
alluvial facies of the Old Red Sandstone (Allen, 1979). At certain horizons near
the top of fining-upward sequences, bedding planes of buff to green, micaceous,
fine-grained sandstones are covered with fragmentary plant remains, both axial
and thalloid. Plants are so abundant that the sandstones appear laminated in
section. They include rhyniophytoids, such as Cooksonia and Tortilicaulis,
PrototaxiteslNematasketum (Burgess & Edwards, 1988), Pachytheca and
Nematothallus. Fragments of eurypterids are also present. Most of the material
described here comes from these fine-grained sediments. Larger indeterminate
axes and billets of PrototaxiteslNematasketum occur in the coarser, massive
EARLY LAND PLANTS
163
sandstones. Details of the sedimentology in relation to plant distribution in the
quarry are given in Edwards & Fanning (1985).
The plants are preserved as coalified compressions (adpressions sensu Shute &
Cleal, 1986) usually associated with limonite, and occasionally with traces of
pyrite. Limonite occupies internal spaces such as sporangial cavities and cell
lumens. When the rock is split along bedding planes, the bulk of the compression
typically adheres to one surface. The impression counterpart sometimes bears
traces of cells marked by coalified striations. Occasional axes and a few
sporangia are composed predominantly of limonite. One pyritized axis showing
cellular preservation has been found.
Although bedding planes are crowded with fossils, individual specimens,
particularly fertile ones, are almost impossible to detect with the naked eye.
Blocks were therefore split in the laboratory, thus ensuring retention of both part
and counterpart. Surfaces were scanned under a Wild dissecting microscope, and
all specimens were photographed before treatment. They were measured to
0.0 1 mm using an eyepiece graticule. Greater magnifications were obtained by
placing small blocks, uncoated, in a low vacuum chamber attached to an
International Scientific Instrument 60A SEM at the Natural History Museum,
London (Taylor, 1986).
Specimens were developed (dtgagement) by removing loosened grains with
tungsten needles, kept sharp by dipping into molten sodium nitrite. Extra care
was needed because removal of small fragments of coalified material could
radically alter sporangium shape. Specimens were reinforced by applying Alvar
dissolved in acetone with a fine brush.
In the search for spores, coalified material was loosened with tungsten needles.
Most promising were grainy, matt-black examples. Adhering rock grains were
dissolved in hydrofluoric acid, and, after washing, residues were either oxidized
in Schultze’s solution or mounted on double-sided Sellotape on stubs, dried and
coated. Cleared examples were mounted in Elvacite on glass sides for light
microscopy. Film pulls were prepared by coating compressions with a syrupy
solution of cellulose nitrate dissolved in amyl acetate.
The pyrite permineralization was embedded, sectioned and polished using the
techniques described by Kenrick & Edwards (1988). For scanning electron
microscopy 5600 and 5 180 Cambridge Instruments were used, the former in
Cardiff and the latter a t the Natural History Museum, London.
DESCRIPTIONS AND SYSTEMATIC PALAEOBOTANY
Cooksonia hemisphaerica Lung
Eighty-two fragmentary specimens of Cooksonia hernisphaerica (Fig. 2) were
collected from Targrove making this the commonest fertile plant in the
assemblage.
Morphology
The considerable range in sporangial shape is illustrated by the silhouettes in
Fig. 1. I n almost all cases the sporangial outline is entire and the junction with
subtending axis more o r less straight. I n life the sporangia were thus almost
hemispherical although some were oblate spheroids or ellipsoidal with long axis
164
U. FANNING ET AL.
Figure 1. Silhouettes of sporangia of Cooksonia ~ ~ ~ s ~ showing
~ u e range
~ c ain form and size; scale
bar = 1.5 mm.
of the ellipse continuous with the subtending axis. They are never more than 1.5
times higher than wide. Sporangial height ranges between 0.68 and 2.08 mm
(Z= 1.47 mm) and width is 0.83 mm to 3.25 mm (2 = 1.68 mm). The smallest
sporangium was 0.88 mm high by 0.83 wide and the largest 2.08 mm by
3.25 mm. The extent of the sporangium-axis contact is also variable ranging
from those where the subtending axis is almost as wide as the sporangium at its
widest point to some where it appears to be half this value. The latter are rare
and in re-examination were mostly those where the junction is poorly defined
because the entire coalified compression presents a uniform appearance. The
sporangia of such specimens also lack any evidence of the differentiation into two
regions that characterizes the majority. I n the latter a narrow peripheral band,
parallel-sided except where it tapers towards the junction, encloses a central
more heavily coalified region. Its maximum width ranges between 30 and 55 pm
(Z= 37 pm). It is prominent in some specimens, but in others only a faint trace
remains. We interpret it as the compressed sporangial wall and the central
region as wall plus sporogenous material. The axis itself tapers gradually before
becoming parallel-sided when it ranges between 0.27 and 1.07 mm (Z=
0.60 mm). Sporangia have not been found in isolation. Eighty-seven percent
terminate unbranched axes while isotomous branching occurs in the remaining
specimens at between 0.87 to 4.42 mm below the sporangia. Daughter branches
are of equal length and develop symmetrically from angles of between 30" and
100" ( 2 = 53"). I n all specimens the axis width increases slightly below a branch
and decreases after division. In the single specimen showing two branch points,
successive branches are arranged approximately at right angles to each other.
The longest specimen measures 13.7 mm.
EARLY LAND PLANTS
165
Anatomy
AXES: In reflected light, coalified compressions with traces of limonite either
appear structureless or striated where limonite occupies the cell lumens. The
walls of these cells are coalified and appear as strips of varying diameter
depending on the plane of fracture. Such cells, c. 30 pm wide, are clearly
elongate with tapering ends, but no single cell has been seen in its entirety. Distal
to the sporangium junction, these elongate cells are abruptly replaced by more
or less isodiametric forms. I n three specimens a more prominent coalified strip,
26-104 pm wide is located centrally on the axis (Fig. 2) and comprises
longitudinally coalesced fragments, but shows no further anatomical detail.
The preservation of the axis in Fig. 3 is atypical in that it is less compressed,
and examination of the fractured end shows a peripheral zone of thick-walled
cells, possibly a hypodermis, in which homogenization of adjacent walls has
occurred.
Cell outlines visible on the surface of the central region of the
sporangium may be isodiametric (25 pm in diameter) to elongate (52 by 25 pm),
with long axes parallel to that of the subtending axis. Similar outlines are
occasionally seen in the orange-stained matrix exposed when the central region is
removed. The latter sometimes detaches as a continuous film, but usually
fragments.
In two predominantly coalified but less compressed specimens the sporangia
are clearly split along their entire free margin into two halves, while in specimens
more conventionally preserved, the peripheral zone sometimes comprises two
discrete strips of equal width a t different levels in the matrix. I n an attempt to
determine whether such splitting was a fortuitous consequence of compression or
whether its regularity indicates that the sporangium possessed anatomical
modifications predetermining its dehiscence, fragments were examined with a
scanning electron microscope. A layer, one cell deep, of thick-walled cells was
discernable (Fig. 5) but evidence for a specialized zone for dehiscence was not
found.
Spores were recovered from three specimens (Fig. 4 ) . They are trilete, azonate
and retusoid, 23-44 pm in diameter (S = 39 pm) with circular to subcircular
ambs. The exine is thin-walled with taper-pointed folds. Contact areas are
covered by submicroscopic micrograna to microrugulae, but outside the contact
areas and distally the exine is laevigate, or sculptured with microgranulate to
microbaculate elements sometimes joined into microrugulae. Such sculpture is
seen only on the SEM. Irregular patches of larger grana sometimes present
possibly represent tapetal tissue. Trilete rays are labrate, sinuous, rarely c. 1/2R,
more usually c. 7/8R. They are joined by narrow, <0.7 pm wide, possibly
thickened ridges forming curvaturae perfectae, and contact faces are depressed.
T h e spores may be immature and resemble spores of the dispersed spore genus
Apiculiretusispora Stree1 ( 1964).
SPORANGIA:
Discussion
These specimens display all the morphological features, viz. more or less
circular sporangial outlines, almost flat, extensive sporangial/axis contacts,
gently tapering axes, and isotomous branching that Lang (1937) used to
characterize C. hemisphaerica. They also approximate to the same extensive size
EARLY LAND PLANTS
167
range. They differ in their greater morphological variability, particularly in
sporangial outline, but this might be anticipated in that we have collected a far
greater number. Similar diversity was noted in the Pridoli examples from
Freshwater East, Dyfed (Edwards, 1979) and we agree with her that sporangia
were probably sub-spherical, oblate spheroidal and ellipsoidal in life. However,
we have found no evidence to support her hypothesis that such variation may
have had an ontogenetic basis. We did on the other hand consider the possibility
that these sporangia were immature forms of C.pertoni but rejected it because
they are generally larger, possess the narrow marginal feature and show
indication of splitting into two halves. Further C. hemisphaerica has not been
recorded at Perton Lane, where C. pertoni predominates.
Ananiev & Stepanov (1969) radically altered the concept of C. hemisphaerica,
when they placed a pseudomonopodially branching plant from the Lower
Devonian of Siberia in that taxon. Their reconstruction shows lateral isotomous
branching systems terminating in sporangia which the authors claimed were the
equivalents of Lang’s more fragmentary material. O u r extensive collecting at
Targrove, the type locality, has failed to discover more complexly branching
plants, but bearing in mind that fragmentation and sorting would have occurred
before burial in the Dittonian fluviatile system, we cannot be certain of the
growth form of the Welsh Borderland specimens. However, examination of the
illustrations of the Siberian plant suggests that sporangial shape may be
different, and we consider it premature to modify the diagnosis of C. hemisphaerica
and would exclude the Siberian specimens (Banks, 1975; Edwards, 1979).
Edwards (1979) recovered spores from the Dyfed Downtonian C. hemisphaerica.
She described them as smooth, circular and ranging in diameter from 22.5 to
34.5 pm. They, therefore, appear similar in size to those recovered from the
Targrove specimens but are unlike them with respect to structure and sculpture.
Useful comparison is impossible as the Silurian spores were isolated on film pulls
which obscure detail.
Finally, it should be remembered that on the basis of finding tracheids in
sterile axes at Targrove, Lang concluded that C. hemisphaerica was a vascular
plant, a belief that has become entrenched in the literature (e.g. Stewart, 1983).
Figures 2-1 1. Species of Cooksonia from the Ditton Group, Lower Old Red Sandstone (micrornatusnewportemis spore Biozone), Targrove Quarry, Nr. Ludlow. Figs 2-5. Cooksonia hemisphaerica Lang.
Fig. 2. Typical sporangium with traces of a central line in the subtending axis. NMW 89. 49G. 6;
scale bar = 1 mm. Fig. 3. SEM of fractured end of subtending axis with thick-walled sterome and
homogenization ofadjacent walls. NMW 91. 42G. 1 (stub); scale bar = 30 pm. Fig. 4.SEM ofspore
isolated from sporangium. Globules may be of tapetal origin or bacterial contamination. NMW 91.
42G 3 (stub); scale bar = 2.56 pm. Fig. 5. SEM of sporangium wall and part of contents. Note the
thick outer and possibly inner periclinal walls of the outer limiting layer. NMW 91. 42G. 2 (stub);
scale bar = 50 pm. Fig. 6. Cooksonia pertoni, coalified compression. V. 62771; scale bar = 1 mm. Fig.
7. SEM of spore isolated from disc of comparable size and shape to C.pertoni and assignable to
Streelispora newportensis. NMW 91, 42G. 4; scale bar = 3 pm. Fig. 8. SEM of spore ?An~urospora/
Streelispora isolated from sporangium of C. pertoni. V. 62918; scale bar = 10 pm. Figures 9-1 1.
Cooksonia cambrensis, coalified compressions; scale bars = 0.5 mm. Fig. 9. Forma a. NMW 91. 42G. 5.
Fig. 10. TDehisced forma a. NMW 91. 42G. 6. Fig. 11. Forma fi. NMW 91. 42G. 7. Figures 12-15.
cf. Cooksonia caledonica: coalified compressions. Fig. 12. Reniform form showing curved contact area,
with only rim of the sporangium preserved. NMW 89. 49G. 7; scale bar = 1 mm. Fig. 13. Circular
form showing similar preservation to that in Fig. 12. NMW 91. 42G. 8; scale bar = 1 mm. Fig. 14.
4 x branched axis system with one remaining attached sporangium. NMW 91. 42G. 9; scale
bar = 2 mm. Fig. 15. ‘Overtopped’ sporangium. Axis shows central strand. NMW 91. 4% 10; scale
bar = 1.0 mm.
I68
U. FANNING ET AL.
We reiterate that tracheids have never been seen in C. hemisphaerica (Edwards,
Bassett & Rogerson, 1979) nor in any other Cooksonia, and we prefer to call these
plants rhyniophytoid.
Cooksonia pertoni Lung
Cooksonia pertoni (Fig. 6) is the second most abundant fertile taxon at Targrove.
The considerable variation in orientation resulting from compression recorded
from more than 70 specimens, combined with similar data from Perton Lane
permitted reconstruction of sporangia as essentially discoidal structures (Fanning
et al., 1988: fig. 1). Dimensions of sporangia from the two localities are almost
the same. At Targrove, height ranges between 0.08 and 1.22 mm ( 2 = 0.49 mm)
and width, 0.36 and 3.64 mm ( 2 = 1.22 mm). Axis diameter lies between 0.03
and 0.99 mm (2 = 0.23 mm). The isotomous branching seen in 25% of the
specimens is at angles of between 30" and 90" (2 = 63"). Anatomical information
on the sporangial wall, obtained from partially permineralized specimens
corroborated that from the far better preserved coalified material from a Clee
Hills Dittonian locality (Edwards, Fanning & Richardson, 1986). Only one
specimen yielded spores. These are equatorially crassitate and have a distal
ornament of rounded to pointed coni moderately and more or less regularly
spaced (Fig. 8). Since their proximal surfaces have not been observed it is
impossible to identify them as either Aneurospora or Streelispora. However, coalified
discs of similar size found isolated in the matrix at Targrove, and thus tentatively
related to C.pertoni, contain spores of similar size and form to those described
(Fig. 7). These can be assigned to Streelispora newportensis because their proximal
surfaces are smooth with the characteristic tangential folds, with indications of
inter-radial papillae, and sutures accompanied by lips.
Cooksonia cambrensis Edwards
Morphology
Of the ten specimens collected, nine are more or less circular in outline (Fig. 9)
and hence belong to forma a (Edwards, 1979). The sporangium of one specimen
appears split into two parts along a centrally located line (Fig. 10). The smallest
example of forma a is 0.47 mm in diameter and the largest 1.14 mm high by
1.12 mm wide. Subtending axes are parallel-sided immediately below the
sporangia, except in two cases where there is a slight taper. Axis width ranges
between 0.08 and 0.42 mm ( 2 = 0.33 mm). The sporangium-axis junctions are
flat and narrow when compared with maximum sporangium width (sporangium
width-junction width ratio = 2.38-5.88). The elliptical sporangium (forma p,
Fig. 11) is conspicuously larger (2.34 mm wide and 1.56 mm high). Its axis is
parallel-sided for most of its length and 0.65 mm wide, but widens slightly
immediately below the sporangium. The junction is again flat and narrow
(ratio = 3.6). Branching has not been observed, but the specimens are very
fragmentary. Maximum height (including axis) in forma a specimens is 1.68 mm
and forma p, 2.57 mm.
All the specimens are coalified, occasionally with traces of limonite: none show
cellular detail. Sporangia are not differentiated into zones, and although the thin
coalified layer is readily detached as large fragments we have been unable to
isolate spores.
EARLY LAND PLANTS
I69
Discussion
The Targrove specimens add nothing to our understanding of the morphology
of C. cambrensis based on specimens from Pridoli sediments. I n size and
morphology forma a examples are similar, although the elliptical one is larger.
However, they extend the species' stratigraphic range into the Lower Devonian
and its geographical distribution into the Welsh Borderland.
CJ
Cooksonia caledonica/Renalia sp.
We collected 68 fragmentary specimens in which the majority of the terminal
sporangia are described as reniform, because the sporangium-axis junction is
domed, the axis extending into the base of the sporangium. I n about two-thirds
of the specimens, a discrete band extends around the free margin of the
sporangium. Branching is isotomous but rarely observed. Such features
characterize C. caledonica, described from the Gedinnian of Scotland (Edwards,
1970). We would probably have unhesitatingly assigned these Targrove
specimens to that species had we not examined one (V. 58024) collected by Lang
from the same locality in which a similar sporangium is attached to a lateral
branching system (Fig. 16) and therefore referable to Renalia (Edwards &
Fanning, 1985).
Morphology
Outlines of most of the sporangia are transversely elliptical (Fig. 12), the rest
are almost circular (Fig. 13). Sporangium width ranges between 1.17 and
4.21 mm (X = 2.34 mm) and height, measured to lowest point of junction,
between 0.78 and 2.83 mm (2 = 1.72 mm). The peripheral band for the most
part is uniform in width but tapers into the sporangium-axis junction. Its
maximum width ranges between 0.08 and 0.26 mm (X = 0.13 mm). The extent
of the sporangium-axis contact region is variable: the sporangium width-junction
width ratio ranges between 1.51 and 3.53 (X = 2.22). Subtending axes taper and
when parallel-sided range from 0.18 to 0.91 mm wide (2 = 0.54 mm). Branching
was observed in ten specimens and occurred between 0.34 and 6.43 mm below
the sporangium. Axis diameter increases slightly below a branch and decreases
distally, the amount varying between specimens. Daughter axes are
symmetrically placed a t 40" to 45" (X = 42"). Typically the branched specimens
show one daughter axis terminated by a sporangium but the other is preserved
for only a short distance. Thus, although it is possible to state that daughter axes
are equal in diameter, it is usually impossible to determine if they were equal in
length. T h e most extensively branched specimen we found is 12.81 mm long and
shows four orders of branching, in which daughter branches are of equal length
with successive orders at right angles to each other (Fig. 14). However, we also
observed overtopping in a more fragmentary specimen where a sterile,
incomplete branch is higher than the daughter terminated by a sporangium
(Fig. 15).
Lang's specimen (V. 58024; Fig. 16) is somewhat differently preserved in that
it is mainly permineralized with some coalified material. I n the most compressed
parts of axes, a thin layer of limonite is associated with narrow streaks of
carbonaceous material, but elsewhere the limonite is lens-shaped in transverse
section and enclosed in a thin coalified layer sometimes in the form of narrow
EARLY LAND PLANTS
171
longitudinal bands. Two regions are distinguishable in the single flattened
sporangium. The central one is marked by a limonite stain associated with
localized areas of a silvery mineral (possibly pyrite) and coalified fragments. The
outer area comprises mainly the silvery mineral. Unfortunately, no cellular
details are preserved.
Anatomy
Most axes appear longitudinally striated because cells may be infilled with
limonite. Cell walls are represented by irregular thick or thin coalified lines
depending on the plane of fracture. Peripheral tissues thus appear to comprise
elongate tapered cells of indeterminate length but 25-50 pm wide. Cells are not
preserved in the axis immediately below the sporangium, a region typically
occupied by limonite.
A discrete, longitudinally orientated central strip (0.08 to 0.18 mm wide,
X = 0.13 mm) is a feature of many of the axes (Fig. 15). I t is parallel-sided, but
may expand distally, along with axis diameter. Cells are not preserved.
AXES:
SPORANGIA: Two to three regions can sometimes be distinguished in the
compressed coalified sporangia. The central reniform area, a sandwich of walls
and sporogenous contents, is often picked off as a continuous sheet or in large
fragments. Outlines of superficial cells are occasionally evident. They are slightly
elongate and tapered with long axes perpendicular to sporangium width. Similar
outlines are observed on the underlying orange-stained matrix on removal of the
sporangium. Spores were isolated from only one specimen. These are azonate,
trilete, 12-16 pm in diameter (X = 14 pm) with rounded ambs (Figs 17, 18).
Their contact areas show fine irregular folds and are bordered by curvaturae
almost coincident with the equator. Limited evidence on the distal surface
indicates granulate sculpture. Such spores are tentatively referred to
Apiculiretusispora Streel ( 1964).
Nearly two-thirds of the sporangia possess a narrow discrete marginal band
(Fig. 19) which is sometimes permineralized. Superficial cells in this region are
elongate (78-182 pm long), with the longer axis perpendicular to the margin
(Fig. 20). A third very narrow region (20-53 pm wide) is sometimes
discriminated between the central mass and the border. It is typically coalified,
Figures 16-33. Fig. 16. Renalia sp., coalified compression/impression/permineralization.V. 58024;
scale bar = 5 mm. Figs 17, 18. Spores isolated from sporangium ofcf. Cooksnnia caledonica. NMW 91.
42G. 1 1 (stub); scale bar = 10 pm. Fig. 19. Part of coalified sporangium of cf. C. calednnica with
narrow border apparently separated from spore-containing area. NMW 91. 42G. 12; scale bar =
500 pm. Fig. 20. Permineralized border with single layer of cells preserved. Viewed in
environmental chamber. NMW 91. 42G. 13; scale bar = 40 pm. Figs 21-27. Toortilicaulis
transwallensis. Fig. 21, 22. Specimen showing spiralling of cells in the sporangium (Fig. 22) and
twisting in axis immediately below (Fig. 21). NMW 91. 42G. 14; scale bar in Fig. 21 = 2 mm. Fig.
23. Branching permineralized specimen. NMW 91. 42C. 15; scale bar = 1 mm. Fig. 24. Two
sporangia lying parallel and close to each other suggesting they were borne on the same branching
system. NMW 91. 4%. 16; scale bar = 1 mm. Fig. 25. Sporangia showing dehiscence into two
valves, the split following the line of the spiralling cells. NMW 91. 42G. 16; scale bar = 1 mm. Figs
26, 27. Spore isolated from 1.transwalliensis. NMW 91. 42G. 18 (stub). Fig. 26; scale bar = 10 pm.
Fig. 27; scale bar = 3 pm. Figs 28-33. Salopella marcensis sp. nov. coalified compressions. Fig. 28.
Holotype NMW 91. 42G. 19; scale bar = 2 mm. Fig. 29. NMW 91. 42G. 20. Fig. 30. NMW 91.
42G. 21. Fig. 31. NMW 91. 42G. 22. Fig. 32. NMW 91. 42G. 23. Fig. 33. NMW 91. 42G. 24. I n
Figs 29-33, scale bar = 1 mm.
I72
U. FANNING E T AL.
Ir
Y
t
!
1
Figure 34. Silhouettes of sporangia of Tortilicuulir trunswalliensis showing range in form and size; scale
bar = 1.5 rnm.
but occasionally cell lumens are filled with limonite. Such semipermineralizations have been observed using the ‘environmental chamber’
(Fig. 20).
Discussion
Our dilemma in identifying small, isolated fragments is obvious and has been
discussed by Edwards, Fanning & Richardson (in press). Relationship with
Renalia is strengthened by the isolation of retusoid spores from one specimen,
although we have no information on the spores of C. caledonica. Further we have
found some evidence indicative of marginal cells modified for dehiscence, but
insufficient for useful comparison with those in R. hueberi. Again we have no
information on the anatomy of C. caledonica.
Isolated sporangia with similar morphology from a Pridoli locality in Dyfed
were compared with C. caledonica (Edwards, 1979). I n the light of the above,
these identifications must also be considered questionable. Indeed, to emphasize
our uncertainty it might be appropriate to erect a new form genus for such
sporangial fragments, but this could well lead to a proliferation of species
depending on state of preservation. Can we be sure that all the reniform
sporangia at Targrove possessed spores of Apiculiretusispora type? Does the
absence of a border reflect poor preservation or the absence of cells specialized
for dehiscence? Are the borders all anatomically similar? For the moment we
consider it appropriate to convey our reservations by the use of the prefix ‘cf.’,
while we await anatomical information on bonajide C. caledonica. Whether or not
the latter should be kept in the genus Cooksonia will be discussed elsewhere. We
have placed the Lang specimen in Renalia Gensel (1976), but consider present
information insufficient to merit the erection of a new species.
EARLY LAND PLANTS
173
Tortilicaulis transwalliensis Edwards
Morphology
This description is based on 23 fragmentary specimens in which the sporangia
themselves are frequently intact. The silhouettes in Fig. 34 show some similarities
in gross morphology with Salopella. Some have their maximum width occurring
a t mid-length while others appear more or less parallel-sided in the basal twothirds. Apices are acutely rounded or mucronate. Sporangia are 0.63 to 1.81 mm
wide (2 = 1.22 mm) and 1.63 to 7 . 1 7 mm high (2 = 5.31). T h e smallest
sporangium is 0.63 by 1.63 mm and the largest, 1.68 by 7.71 mm. T h e sporangia
are subtended by parallel-sided axes which appear constricted, indicative of
twisting, either immediately or some distance below the sporangium (Fig. 2 1). In
fusiform examples, the sporangium tapers imperceptibly into the axis and the
junction is also indistinct in the parallel-sided forms which are the same width as
the subtending axes. Two specimens exhibit branching. I n the first, division
occurs immediately below the sporangia, so that there is no axis above the
branch point as is seen in the second (Fig. 23). A third specimen exhibits two
sporangia lying side by side as if they had originated from a dichotomous branch
(Fig. 24).
Anatomy
The sporangia are typically preserved as coalified compressions with
traces of limonite although a single specimen is composed predominantly of
limonite. Grains of matrix are sometimes enclosed within the coalified layers.
The latter readily detach from the rock as films or larger fragments and reveal
oblique striations on the strongly orange-stained matrix beneath. Similar
striations are sometimes visible on the surface of the coalified layer itself (Fig. 22)
and are accentuated when limonite replaces cell lumens. The outermost layer of
the sporangium wall is thus interpreted as comprising spirally orientated
elongate cells, 26-52 pm wide, with tapering ends. As no single cell has been seen
in its entirety, cell length remains unknown. One of the specimens seems to have
dehisced along this spiral course so that it has a coiled appearance (Fig. 25).
Spores have been isolated from a number of sporangia. They have circular to
subcircular ambs and are 43-49 pm (2 = 47 pm) in diameter (Figs 26, 27). A
few spores have good trilete marks. These usually have simple sutures although
some are accompanied by fine folds or lips. Occasionally spores derived from
masses containing trilete spores appear to occur in pairs. The spores have grana
occurring in patches of closely spaced, isodiametric (c. 1 pm) elements which are
possibly fused at their bases with loose tapetal-like areas. They can be compared
with the dispersed spore genus Apiculiretusispora although no similar dispersed
spore taxon has been described.
SPORANGIA:
The subtending axes are similarly preserved with limonite in the lumens of
peripheral cells. These are elongate, c. 30 pm wide and tapering, but of unknown
length. They may be orientated vertically, but appear oblique due to the axis
twisting. We have found no evidence of a central strand.
AXES:
Discussion
These specimens conform to Edwards’ specific diagnosis for Tortilicaulis
transwalliensis (Edwards, 1979). They are similar in the range of sporangial
U. FANNING E'T AL.
174
shape, in organization of the sporangium wall and in the gross twisting of the
subtending axes. In addition they demonstrate spiralling cells in the sporangium,
prove that the terminal structures are actually sporangia, increase the upper
limits of sporangial dimensions and extend the plant's geographical distribution
into the Welsh Borderland and its stratigraphic range into the Lower Devonian.
Unfortunately, they do little to indicate the affinities of the plant, although the
demonstration of branching in the subtending axes makes bryophyte affinity less
likely, and T. transwalliensis is best included amongst the rhyniophytoids.
Incertae sedis: Salopella Edwards & Richardson
TYPE SPECIES:
Salopella allenii Edwards & Richardson, 1974.
Salopella marcensis Fanning, Edwards & Richardson, sp. nov.
From the Latin for the Marches (the area of country between
England and Wales).
DERIVATION:
Plant at least 6.38 mm high with elongate sporangia, 0.75-3.38 mm high and
0.25-0.88 mm wide. Sporangia terminating parallel-sided, smooth, isotomously
branched axes, 0.05 to 0.5 mm wide. Branch angle 30" to 60". Ultimate
dichotomy occurring immediately below to some distance below the sporangia.
HOLOTYPE:
NMW 91. 42G. 19 (Fig. 28).
ILLUSTRATIONS:
LOCALITY:
Figs 28-33.
Targrove Quarry, near Ludlow, Shropshire (SO 5254 7799).
HORIZON: c. 60 m above main Psammosteus limestone, lower part of Ditton Group,
Lower Old Red Sandstone. micrornatus-newportensis spore Biozone, early
Gedinnian.
Morphology
The new species is based on 37 fragmentary specimens, in which the coalified
sporangia, with traces of limonite, are usually incomplete. The silhouettes in Fig.
35 show the spectrum of sporangial shape. All are elongate with height:
maximum width ratio ranging between 2.59 and 5.0. Sporangial height ranges
from 0.75 to 3.38 mm (% = 1.6 mm) and width from 0.25 to 0.88 mm (2 =
0.54 mm). The largest sporangium is 0.81 by 3.38 mm and the smallest 0.25 by
0.75 mm. The tips of the sporangia are frequently missing. When preserved, a
few are rounded such that an angle of c. 90" can be accommodated within them
(i.e. obtuse): others are pointed. Rarely, sporangial tips show a deep vertical
cleft, suggestive of longitudinal dehiscence.
Ninety-three percent of the specimens show branching ranging from
immediately below the sporangia to up to 2.0 mm below. A relationship exists
between sporangium form and this distance. Generally when branching occurs
at some distance from the sporangium, the maximum width of the sporangium is
located more or less halfway along the length (Figs 28, 29). In these specimens
the sporangia are subtended by parallel-sided axes and the junctions between
sporangium and axis are clearly defined. However, when the sporangium occurs
immediately or very close to the branch it appears more parallel-sided at midlength (Figs 30, 31) and the distinction between sporangium and axis is more
c
EARLY LAND PLANTS
175
Y
Y
Figure 35. Silhouettes of sporangia of Salopella marcensis sp. nov. showing range in form and size; scale
bar = 1.5 mm.
difficult to determine. T h e distance between sporangium and branch point can
vary within a specimen. Hence, the two forms of sporangium can be borne by a
single plant (Fig. 33).
Branching is isotomous; the symmetrically inserted daughter axes are a t angles
of 30" to 60" (X = 42"). The orientation of the sporangia is such that they appear
as continuations of the subtending axes. Axes are 0.05 to 0.5 mm wide (X =
0.22 mm). Their width increases slightly below each branch point and decreases
following division. Axis length between branching also decreases distally. The
maximum number of successive branch points observed is three, and such
specimens suggest that branching was not planar. The longest specimen was
6.38 mm.
Anatomy
Neither cell outlines nor a central strand has been observed on axes. However,
darker striations on sporangia indicate a superficial layer of vertically aligned
cells. These cells are elongated (50-80 pm long and 30-40 pm wide) in proximal
regions and isodiametric (c. 30 pm) near the apices. Spores have not been seen.
.Comparison
The Targrove specimens clearly belong to the genus Salopella, which was based
on a single specimen with smooth, isotomously branching axes and terminal
fusiform sporangia, from the Lower Devonian (upper ?uppermost Gedinnian) of
the Welsh Borderland (Edwards & Richardson, 1974). The type species S. allenii
differs from the new material in its greater size and smaller branching angles. Of
the three species erected since 1974, S. xinjiangensis from the Pridoli of north-west
China (Dou Yawei & Sun Zhehua, 1983) is poorly known and will not be
I76
U. FANNING E T AL.
Figures 36-50. Figs 36-42, 44 and 45. Uskiella relicdata sp. nov. Fig. 36. Holotype: sporangiurn with
elliptical outline. NMW 91. 42G. 25; scale bar = 0.5 mm. Fig. 37. Part of 36 enlarged to show
reticulum produced by cells of sporangium wall. Fig. 38. Sporangium with elliptical outline and
border partly preserved. NMW 91. 42G. 26; scale bar = 0.5 mm. Figs 39, 40. Sporangia with f
parallel sides; scale bars = 1 rnm. Fig. 39. NMW 91. 42G. 27, Fig. 40. NMW 91. 42G. 28. Fig. 41.
Sporangium with elliptical outline NMW 91. 42G. 30; scale bar = 1 rnm. Fig. 42. Sporangium with
EARLY LAND PLANTS
177
considered further. The two Australian species, S. australis and S. caespitosa, were
based mainly on impressions (Tims & Chambers, 1984). The former, occurring
in both Ludlow and Siegenian sediments, is readiiy distinguished in that the
sporogenous region is confined to the basal two-thirds of the sporangium.
Salopella caespitosa, known only from a single specimen, is considerably larger
than the Welsh Borderland material with multi-branched axes, dichotomizing at
angles between 10" and 45", and terminating in ovate to fusiform sporangia.
Thus, sporangial shape is similar, but in the absence of data on the growth habit
in the English specimens, we prefer to erect a new species for the latter.
Incertae sedis: Uskiella Shute & Edwards
TYPE
SPECIES:
Uskiella spargens Shute & Edwards, 1989.
Uskidfa reticdata Fanning, Edwards & Richardson, sp. nov.
DERIVATION:
reticulate appearance of sporangium wall.
Plant with terminal elliptical sporangia, 0.53 to 2.00 mm high and 0.38 to
1.50 mm wide. Axes 0.05 to 0.50 mm wide, smooth, parallel-sided and
isotomously branched. Branch angle small to 60°, ultimate dichotomy
immediately below to some distance below sporangia. Sporangia with
equidimensional to slightly elongate cells, 25 to 50 pm long and 25 pm wide
presenting a reticulate appearance.
NMW 91. 42G, 25 (Figs 36, 37).
HOLOTYPE:
ILLUSTRATIONS:
LOCALITY:
Figs 36-42, 44, 45.
Targrove Quarry, near Ludlow, Shropshire (SO 5254 7799).
HORIZON:
c. 60 m above main Psammosteus limestone, lower part of Ditton Group,
Lower Old Red Sandstone, micrornatus-newportensis spore Biozone, early
Gedinnian.
Morphology
The new species is based on 18 coalified fragments, in which sporangia are
consistently longer than wide, but height is never more than twice the width
(Fig. 36). They are more or less elliptical in outline; most are ovate (Figs 36, 38),
while a few are obovate (Fig. 42) or have more or less parallel sides (Figs 39, 40).
All have obtusely rounded apices. T h e largest specimen is 1.50 mm wide by
2.00 mm high, and the smallest, 0.38 mm by 0.53 mrn. In all cases the junction
between sporangium and axis is well defined. The subtending axes are parallelborder and obovate outline. NMW 91. 42'3. 29; scale bar = 1 mm. Fig. 43. Tarruntia salopensis gen.
et sp. nov. Holotype: left hand sporangium possibly showing dehiscence. NMW 91. 42G. 31; scale
bar = 0.5 mm. Fig. 44. Uskiella reticdata. Bifurcating specimen with non-identical sporangial
outlines. NMW 91. 42G. 32; scale bar = I rnm. Fig. 45. SEM of specimen illustrated in Fig. 38
viewed uncoated in the environmental chamber. Reticulate surface is apparent but not spores.
NMW 91, 42G. 26; scale bar = 0.5 mm. Figs 46, 47. Turrantia salopmis gen. et sp. nov. SEM of
fractured permineralized oval sporangium with honeycombed appearance (environmental
chamber) NMW 91. 42G. 33; scale bar = 400 pm. Fig. 47. Close up of sporangial content showing
occasional permineralized casts of spores but no diagnostic characters. Figs 48-50. Bifurcating
sporangia; scale bars = 1 mm. Fig. 48. NMW 91. 42G. 34. Fig. 49. Branching specimen, NMW 91.
42G. 35. Fig. 50. Specimen with circular coalified bodies interpreted as spores. NMW 91. 42G. 36.
178
U. FANNING E T AL.
sided although in a very few specimens they appear slightly tapered in the region
immediately below the sporangia. The diameter of subtending axes are
0.05-0.50 mm. Isotomous branching occurs close to the sporangia, however,
since 72% of the sporangia are subtended by unbranched axes ranging up to
1.70 mm in length, branching must also have occurred at greater distances from
the sporangia. Branching angles vary from a few degrees, so that sporangia
overlap, up to 60". I n all specimens axis diameter increases slightly below a
branch point, and decreases above. Daughter branches are more or less equal in
diameter and symmetrically orientated.
Most of the sporangia are completely flattened and comprise a thin film of
coalified material associated with limonite, which is readily separated from the
rock. A few consist of coalified fragments tightly adhering to the underlying
matrix which is covered to a variable extent by limonite. The superficial cells of
the sporangium wall are visible in all of the specimens, but are only occasionally
well defined and then present a reticulate appearance (Figs 36, 37). Coalified
lines mark the thick cell walls and cell lumens are filled by limonite. In surface
view the cells are equidimensional (25 pm), or more or less rectangular in
outline and up to twice as long vertically as they are wide (50 by 25 pm).
Two regions are frequently identified in the encrusting specimens. A central
area with reticulate patterning, that derives from sporogenous tissues and walls,
is almost completely surrounded by a narrow region that merges proximally with
the axis (Fig. 38). This peripheral zone, the width of which varies between
specimens and ranges between 13 and 52 pm (2 = 28 pm) represents the
thickness of the compressed sporangium wall. The extent to which it is preserved
is also variable.
I n a few examples the central part of the sporangium (3 region of sporogenous
tissue) is filled with matrix so that its three-dimensional shape is maintained
(Fig. 45). From specimens so preserved, and from a single relatively
uncompressed, predominantly coalified specimen, it is concluded that the
sporangia were more or less ellipsoidal in life. Thus, some sporangia seem to have
split around their entire convex margin, or wholly or partly along a centrally
located vertical line, suggesting dehiscence into two equal parts. Splitting is most
commonly observed in the three-dimensionally preserved specimens (Fig. 45),
but is also seen in flattened encrusting examples where two valves are displaced
and separated by matrix. Anatomical modifications related to dehiscence have
not been seen.
Amongst the coalified and limonite fragments visible under the dissecting
microscope in two specimens are black, flat, rounded bodies c. 55 pm in
maximum diameter. Unfortunately, it was not possible to isolate and examine
them under higher magnification.
Comparison
Our limited evidence suggests these fossils are derived from plants of
morphological simplicity with isotomously branching axes and terminal,
vertically elongate, ellipsoidal sporangia that split around the longest
circumference into two equal valves. Such a description almost exactly fits the
generic diagnosis for Uskiella Shute & Edwards (1989) which was based on a
reinvestigation of the Lower Devonian Cooksonia sp. from South Wales (Croft &
Lang, 1942). The diagnosis of the type species, Uskiella spargens, was largely
EARLY LAND PLANTS
I79
based on permineralized material and includes details of the sporangial wall with
a well-defined zone of cells, thought to be modified for dehiscence into two equal
valves. Differences in preservation prevent detailed comparison at the cellular
level with our new material, but the absence of a compression border in the
latter suggests that the sporangium wall was thinner. However, thick cell walls in
the superficial layer, as evidenced by the coalified reticulum, also characterizes
U. spargens, where the cells are diamond-shaped in periclinal section not
isodiametric or sub-rectangular as in our specimens. Thus, we conclude that the
Welsh Borderland specimens should. be defined as a new species of Uskiella.
However, it should be noted that we have been unable to demonstrate tracheids
in our far smaller and more fragmentary fossils, but consider it likely that this
may well result from poor preservation.
Shute & Edwards (1989) compared U. spargens with a number of Lower
Devonian taxa showing three-dimensional preservation of the sporangium wall
(e.g. Rhynia gwynne-vaughanii Kidston & Lang, 1920) and showing sporangial
borders (e.g. Cooksonella Senkevich, 1978). Our specimens are better compared
with impression and compression fossils with vertically elongate elliptical
featureless sporangia. Taeniocrada decheniana (Schweitzer, 1980) has large
(3-7 mm long) elliptical sporangia which occur in clusters, and ribbon-like axes,
each with a narrow central strand. These are features not shared with our
specimens. Dutoitea maraisia Plumstead (Rayner, 1988) is based on very
fragmentary material from the Devonian of South Africa. Ellipsoidal sporangia
terminate curved axes so that they are described as pendulous, and thus differ
from the Targrove material. Finally, Eogaspasiea gracilis, one of the least familiar
plants from the extensive Emsian assemblages of the Gaspt. Peninsula, Canada
(Daber, 1960) has slender axes (90 mm long and less than 0.5 mm wide) that
occasionally dichotomize at an acute angle and terminate in ellipsoidal
sporangia, 2.3 mm high and 1.0 mm wide. These contain smooth spores. Its
growth habit was interpreted as tufted. Further investigation may well reveal
close similarities with the Targrove specimens but in the absence of anatomical
data for the sporangium, we consider our plants better placed in Uskiella.
cf. Uskiella
reticulata
Seven specimens recovered from the locality have sporangia which are no
longer than wide but have sporangium height/width ratios greater than 2.00 but
less than 2.59. They, therefore, fall outside the sporangium height/width ratios of
U. reticulata (125-1.83) and S. marcensis (2.59-5.00). Four of these specimens
have features very similar to those of U. reticulata. These include obtusely
rounded sporangial apices, reticulate cellular pattern on sporangial walls and the
more typical unbranched axes. Furthermore rounded black bodies c. 55 pm
diameter and similar to a few observed in U. reticulata are seen in two of the
specimens. When removed and examined under the light microscope the bodies
proved to be spores. They are thick-walled (distal wall 4-5 pm, equator wall
3-5 pm) with maximum diameters ranging between 60 and 68 pm. They are
laevigate or ?sparsely granulate on their distal surface. Their proximal surfaces
were unclear, and the specimens may have been alete and derived from dyads.
The poor preservation of the remaining three specimens prohibits detailed
comparisons with either U. reticulata or S. marcensis. Their tips are poorly
preserved and only faint traces of their sporangial cells are visible. All three
U. FANNING E'T AL.
180
branch isotomously between 0.13 and 0.88 mm below the sporangia and bear
the rounded black bodies. These were isolated from one specimen and examined
under the light microscope. They are poorly preserved and covered with pyrite.
Their walls are distally thickened and possibly patinate.
These specimens, as a group, exhibit many of the characters of U. reticulata
although their sporangial ratios fall someway outside those of this species. This
may be fortuitous. However, in view of the importance of in situ spore data in the
description of early land plants we prefer to be cautious and record this small
group as cf. U. reticulata, and await the discovery of additional specimens before
further subdivision.
Incertae sedis: Tarrantia Fanning, Edwards & Richardson, gen. nov.
DERIVATION:
In appreciation of help in field work from Peter Tarrant, artist and
amateur palaeontologist.
Plant with isotomously branching, naked axes. Sporangia solitary and terminal,
elliptical to ovate in outline (probably ellipsoidal in life) with height greater than
width. Subtending axes parallel sided. Homosporous.
TYPE
SPECIES:
Tarrantia salopensis Fanning, Edwards & Richardson, sp.
nov.
DERIVATION:
From Salop, an alternative name for the county of Shropshire.
As for genus: axis width 0.13-0.88 mm; sporangial height 0.38-2.63 mm;
sporangial width 0.25-2.00 mm. Sporangial height/width = 1.3-2.00. Spores
c. 23 pm diameter.
HOLOTYPE:
NMW 91, 42G. 31 (Fig. 43).
ILLUSTRATIONS:
Figs 43, 46, 47.
LOCALITY:
Targrove Quarry, near Ludlow, Shropshire (SO 5254 7799).
c. 60 m above main Psammosteus limestone, lower part of Ditton Group,
Lower Old Red Sandstone, micrornatus-newportensis spore Biozone, early
Gedinnian.
HORIZON:
Morphology
Twenty-seven specimens closely resembling those of U. reticulata in both size
and morphology were recovered from Targrove. The largest specimen is
2.00 mm wide by 2.63 mm high, and the smallest, 0.25 mm by 0.38 mm. They
exhibit the same range in sporangial outline (Fig. 51) as U. reticulata with
sporangial height/width ratios ranging from 1.30 to 2.00 and their subtending
axes are parallel-sided. Their axes range between 0.13 and 0.88 mm wide.
Eighty-one percent of the specimens are unbranched. When branching is evident
it is isotomous and close to the sporangia or up to 1.25 mm below it (Fig. 43).
Although the specimens are preserved in a manner similar to those of
U. reticulata, no evidence of the sporangial reticulum is visible. However, the
specimens exhibit the two regions (peripheral and central) identified in the new
species. The peripheral area is visible on the semipermineralized specimens
illustrated in Fig. 46. I n this flattened oval sporangium the featureless coalified
EARLY LAND PLANTS
181
Figure 51. Silhouettes of sporangia of Tarrantia salopensis showing range in shape and size; scale
bar = 1.5 mm.
layer c. 10 pm thick surrounds a quantity of limonite. Subspherical featureless
casts of spores are seen (Fig. 47) in the limonite. Their maximum diameter
ranges between 20 and 25 pm (2 = 23 pm), but no further details could be
elucidated.
Although the specimens described above bear a strong resemblance to those of
U. reticulata, in the absence of a sporangial reticulum and any other
distinguishing features we think it appropriate to assign them to the form genus,
Tarrantia.
Bifurcating sporangia
Morphology
This description is based on 26 fragmentary specimens preserved as coalified
compression with some limonite. Division occurs in the upper third of the
sporangium producing two more or less equal lobes (0.26-0.91 mm long, X =
0.58 mm and 0.18-0.83 mm wide, X = 0.53 mm). These taper to pointed (Fig.
48) or bluntly rounded apices (Fig. 50). The bifurcation angle is 85"-160"
(X = 95"). The sporangia are widest in the branching region and taper
proximally into the subtending axis such that distinction between the two is
often difficult to detect. Estimates of the overall height of the sporangia range
between 0.52 and 2.08 mm. Subtending axes, never observed to branch, are
parallel-sided (0.13 to 0.44 mm wide) or taper slightly below the sporangium.
They are narrow when compared with maximum sporangial width. Branching
may have been isotomous, this is seen in an isolated branch so orientated that it
was probably originally connected to adjacent sporangia (Fig. 49).
Anatomy
O n removal of the coalified matter representing the sporangium, strands of
coaly material adhere to the matrix, and occasionally cell outlines are visible on
the surface of the compression. Above the dichotomy, their cells are twice as long
as wide (39-52 pm long and 26 pm wide): proximally they are considerably
wider (39-52 pm) and longer but incompletely preserved. Examination under a
high-powered dissecting microscope revealed flat, rounded bodies, 43-52 pm in
maximum diameter (Fig. 50). They were removed on film pulls and appeared to
182
U. FANNING E 7 A L .
be tetrads, but further detail could not be elucidated. Coalified axes indicate the
presence of thick-walled peripheral tissues, but no information on individual cells.
Discussion
Bifurcating sporangia are known in two early land plant taxa, Horneophyton
lignieri from the Rhynie Chert and a new Pridoli genus, Caia (see discussion in
Fanning, Edwards & Richardson, 1990). I n both cases the sporangial lobes are
truncated and enations are present. Thus, on gross morphology the Targrove
specimens are unique. However, we are reluctant to erect a new taxon for them,
because we have unpublished data from another Lower Devonian locality on
Clee Hill, where better preserved sporangia of similar shape contain at least two
different kinds of spores. Moreover similar spores have been recovered from
unbranched elongate Salopella-like sporangia from the same locality. At the
Brown Clee Hill locality a continuous series may therefore be traced between
forms when branching occurs in axes some distance below the sporangium, or
where branching is immediately below the sporangium, and finally where
branching occurs within the sporangium. Although we have no evidence for
intergrading forms in Targrove we suspect that they may have existed and so for
the moment describe the sporangia as rhyniophytoids with bifurcating
sporangia.
Indeterminate sporangia
Descr$tion
We found nine structures with more or less circular outlines, which appear to
be divided vertically into three or four equal, discrete segments (Fig. 55). Such
segments may be almost completely free from each other or be united in the
basal half. The free tips may be pointed or rounded. The entire structures are
0.65 to 1.82 mm (2 = 1.10 mm) and 0.57 to 1.95 mm wide (2 = 1.20 mm). They
are interpreted as more or less spherical in life, three to four segments being
visible on the surface, the rest obscured by sediment filling the centre of the
structure. One specimen preserved in a finer matrix shows cellular detail, with
elongate thick-walled cells (78-130 pm long and 26-39 pm wide) orientated
vertically. Six of the structures terminate smooth axes (0.21-0.26 mm wide), and
in one case there is isotomous branching at c. 30°, and two lack any indication of
axes.
Discussion
Initially we believed these structures to be clusters of elongate, partially fused
sporangia, with superficial resemblance to the Australian genus Yarravia Lang &
Cookson (Edwards & Fanning, 1985). However, the larger and best preserved
specimens are better interpreted as ruptured spherical sporangia, splitting
regularly from apex to base into equal valves. They, thus, resemble capsules of
certain liverworts, although the number of segments in the fossils is larger. We
lack any unequivocal evidence for affinity, and await further specimens
preferably with anatomy and spores before erecting a new taxon.
EARLY LAND PLANTS
I83
Permineralized sborangia
(1) A three-dimensional, elongate sporangium, 1 mm wide and composed
predominantly of limonite, was divided into two parts on splitting a rock from
Targrove (Fig. 59). T h e contents of its sporangial cavity and its sporangial wall,
composed of a single layer of elongated cells (75 pm long and 30 pm wide) were
exposed (Fig. 60). Many spores have been preserved as limonite casts (their
maximum diameter ranged from 40 to 45 pm with X = 41 pm), some of which
may be possibly arranged in pairs. The affinity of this sporangium is uncertain
because its morphology is unknown. Its elongated form and the elongated nature
of the cells of the sporangial wall resemble those of Salopella. Unfortunately a
comparison of in situ spores cannot be made.
(2) A specimen recovered from the sediments of Targrove and composed
predominantly of limonite contains subspherical limonitic casts of spores, some of
which bear triradiate marks with sutures extending to the spores' equators (Fig.
62). The maximum diameter of these spores ranges between 20 and 26 pm
(2 = 23 pm). T h e impression left by the spores has a finely granulated texture.
unfortunately, the specimen is very incompletely preserved and, hence, we are
unable to provide an adequate description of the parent plant.
Sterile axes
Sterile, smooth, axial fragments are the commonest higher plant fossils in
Targrove. Most are unbranched; others belong to Hostinella. A few bear
emergences. Unbranched axes are parallel-sided, straight or flexuous, the latter
usually narrower. Some exhibit twisting. Circinate tips are rare; two examples
were recovered (Fig. 57). The majority of the Hostinella axes branch once. When
they branch more frequently, branching angles are similar, and axial width and
length decrease distally. A few specimens suggest that successive branches were
positioned a t right angles producing a bushy plant. Some of the larger axes
branch isotomously but asymmetrically, thus showing a tendency towards a
pseudomonopodial system such as is seen in the Renalia sp. (Fig. 16). We also
found branching examples with a 'lateral' axis of markedly smaller diameter
than the 'main axis'. I n these pseudomonopodial systems, the main axis remains
more or less consistent in length and does not deviate at the branch point, so that
lateral axes are described as being inserted at various angles (45"-go", 2 = 63").
The laterals taper distally for a short distance after branching, and then become
parallel-sided. I n one instance, a lateral axis branches isotomously. A few
specimens possess more than one lateral axis. They may be inserted on the same,
or on opposite sides of the main axis. In the latter they may be alternate and
inserted at 45" or, more usually, opposite forming a 't'-shaped structure. The
laterals often curve through go", so that they are parallel to the main axis and
taper markedly in their distal regions (Fig. 53). Close examination of fragments
showing apparent trichotomies, shows that this structure is produced by
successive closely-spaced dichotomies. I n one such specimen, a lateral axis
branches isotomously.
We found one example of 'k' branching (Fig. .56). It comprises a main axis
whose course and width remained unchanged when it produced (at 90") a
narrower lateral branch, that divides almost immediately giving rise to two axes
I84
U. FANNING ET AL.
Figures 52-62. Fig. 52. Part of a small coalified fragment with pairs of subopposite globular
outgrowths of unknown nature. NMW 91. 42G. 37; scale bar = 1 mm. Fig. 53. Trifurcating axis
(outlined in white) NMW 91. 42G. 38; scale bar = 1 mm. Fig. 54. Coalified disc with radiating
?axes or spines. NMW 91. 42G. 39; scale bar = 2 mm. Fig. 55. Segments of indeterminate
sporangium. NMW 91. 42G. 40; scale bar = 1 mm. Fig. 56. k-branch. NMW 91. 42G. 41; scale
bar = 2 mm. Fig. 57. Circinately coiled tip. NMW 91. 42G. 42; scale bar = 0.5 mm. Fig. 58. Axis
with conical projections. NMW 91. 42G. 43; scale bar = 0.5 mm. Fig. 59. Transversely fractured
elongate permineralized sporangium, viewed in environmental chamber. NMW 91. 42G. 44; scale
bar = 200 pm. Fig. 60. Cavities and spore casts from Fig. 59. No further details visible. Fig. 61.
Fractured end of semi-permineralized sterile axis viewed in environmental chamber. Note crystals of
iron oxides occupying lumens of cells. NMW 91 42G. 45; scale bar = 30 pm. Fig. 62. Area of
sporogenous tissue in a permineralized sporangium. Arrowed is a cast of the interior of a spore
showing the triradiate mark NMW 91. 42G. 46; scale bar = 20 pm.
EARLY LAND PLANTS
185
orientated at 90". One daughter axis lies in the same plane as the main axis and
the other plunges into the matrix. Thus, only one branch is visible on the
counterpart. The dimensions of this specimen lie in the upper range for Targrove
axes. The main axis is 1.81 mm and 14.74 mm long: lateral branches are
c. 1.01 mm wide and 2.35 mm long. Such branching is usually associated with
<osterop/yllum. We failed to find any convincing fertile specimens of this plant at
Targrove, although the specimen illustrated in Fig. 52 bears a superficial
resemblance to a <osterop/yllum spike.
The most unusual sterile specimens consist of an approximately isodiametric
central mass of coalified material, of irregular outline, from which one to six axes
radiate (Fig. 54). The central area is 1.04 by 1.56 mm in the smallest specimen
and 2.01 by 2.88 mm in the largest. Axes are generally 0.24 mm wide and at
most 2.00 mm long. The axes are inserted at the margin or on the surface of the
central area and taper before becoming parallel-sided distally. These structures
most closely resemble the problematic Lower Devonian genus Sciadophyton,
recently interpreted as a gametophyte (Remy, 1980; Schweitzer, 1980, 1983).
Our material is so fragmentary that assignation to that genus is considered
premature.
The vast majority of the axes are smooth. A few bear enations which are
triangular in profile and taper symmetrically or asymmetrically to blunt or
pointed tips (Fig. 58). Their heights are 0.13 to 0.26 mm ( 2 = 0.19 mm), with
basal widths, 0.26-0.65 mm (2 = 0.40 mm). We have never seen them on fertile
axes, but they occur on those showing a range of branching patterns. Their
density is variable, but usually greater on unbranched axes. They probably
occur all over the axes, although sometimes better preserved on one edge. We
assign all these spiny specimens to the form genus Psilophytites, and have failed to
find variation indicative of the presence of more than one type.
Anatomy
Axes are preserved as coalified compressions with varying amounts of
limonite. In less compressed specimens the latter is concentrated towards the
centre. These, and usually entirely coalified axes, were lifted off the rock more or
less intact and examined by scanning electron microscopy. Both forms of
preservation show a narrow peripheral zone, c. 70 pm thick and composed of two
to three layers of elongate thick-walled cells with narrow lumens (Fig. 61). The
cells are more or less isodiametric in fractured, transverse section. I n examples
where lumens are infilled with limonite, in longitudinal fracture the coalified
walls appear narrow or wide, depending on the plane of section. Flattened axes,
composed predominantly of limonite, contain superficial central strands
(0.05-0.2 1 mm wide), formed of concentrations of coalified fragments.
Examination under SEM failed to show tracheids. Film pulls were taken from
several hundred coalified compressions. None bore traces of tracheids like those
illustrated by Lang (1937: pl. 9, figs 35, 36).
Most information on xylem was obtained from a semipermineralized axis, in
which peripheral tissues are coalified and totally devoid of structure, and the
central strand is pyritized (Figs 63-66). T o our knowledge these are the oldest
permineralized tracheids. The strand is terete in cross section, and tracheids
appear of more or less uniform size. We can state with confidence only that the
xylem was not centrach!
186
U. FANNING E T AL.
Figures 63-66. Sterile, partially permineralized axis, Ditton Group, Targrove, Shropshire. NMW
91. 42G. 47. Fig. 63. Close up of tracheids with annular thickenings, sometimes interconnected; scale
bar = 20 pm. Fig. 64. Transversely fractured terete strand preserved in pyrite; scale bar = 40 pm.
Fig. 65. Fractured axis showing coalified peripheral tissues (at top) and pryritized xylem; scale
bar = 80 pm. Fig. 66. Close up of tracheids from Fig. 65; scale bar = 20 pm.
More detailed information on xylem anatomy was derived from a small clump
of tracheids recovered from a bulk maceration (Figs 67,68). I t is composed of (?)
helically thickened elements c. 20 pm in diameter, with the wall between the
very smooth thickenings perforated by more or less circular holes up to 1 pm in
diameter. Such detail was noted by Kenrick & Edwards (1988) in Gosslingia and
thought possibly typical of zosterophylls.
These permineralized and coalified tracheids will be investigated further as
part of a wider project on water-conducting cells in early land plants.
CONCLUSIONS
We believe detailed information such as we have presented here, with
emphasis on its limitations, to be an essential prerequisite to the development of
wider hypotheses concerning, for example, early land plant distribution, patterns
of evolution and ecology. The number of taxa listed is undoubtedly conservative.
We still rely heavily on sporangial shape in plants of such morphological
simplicity, and when we have better data on anatomy and in situ spores (see
Fanning, Richardson & Edwards, 1988), the list will certainly be extended.
Parallel studies on Pridoli assemblages, particularly a t Perton Lane near
Hereford, the type locality for the genus Cooksonia, support this expectation
(Fanning et al., 1990; Fanning et al., 1991). However, our preliminary studies
already suggest that there was an increase in the total number of rhyniophytes
(Edwards & Davies, 1990), accompanied by some turnover at the beginning of
EARLY LAND PLANTS
187
Figures 67 and 68. Isolated clump of coalified tracheids, Ditton Group, Targrove, Shropshire.
NMW 91. 42G. 48. Fig. 67. Tracheids with ?helical thickenings connected by perforated wall,
sometimes covered by continuous layer; scale bar = 30 prn. Fig. 68. Fractured end of tracheid
showing spiral thickening and perforated layer; scale bar = 5 pm.
the Devonian. We are also conscious that as a result of taphonomic sorting the
assemblage is not representative of the vegetation of the time, although we
believe that certain areas may have been dominated by swards of rhyniophytoids
(Edwards, 1990; Edwards et al., in press).
ACKNOWLEDGEMENTS
The research was supported by a Natural Environment Research Council
CASE studentship awarded to Una Fanning and held jointly at University
College, Cardiff and the Natural History Museum.
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