A new genus for isolated bivalved sporangia with thickened margins from the Lower Devonian of the Welsh Borderland

Botanical Journal of the Linnean Society (2001), 137: 297–310. With 52 figures

doi:10.1006/bojl.2001.0490, available online at http://www.idealibrary.com on

A new genus for isolated bivalved sporangia withthickened margins from the Lower Devonian of theWelsh Borderland

D. EDWARDS∗, L. AXE and E. MENDEZ

Department of Earth Sciences, Cardiff University, P.O. Box 914, Cardiff CF10 3YE, U.K.

Received January 2001; accepted for publication July 2001

A new taxon Sporathylacium salopense gen. et sp. nov. is based on small isolated coalified sporangia from theLower Devonian (Lochkovian: micrornatus – newportensis spore zone) from the Welsh Borderland. The sporangiahave two equal valves with multi-layered walls and thickened borders, and contain trilete crassitate, non-curvaturateisospores that are completely covered by a microgranular ornament with possible distal verrucate/murornatestructures. They differ from zosterophyll sporangia in details of the presumed dehiscence zone extending aroundthe entire free convex margin, particularly in the presence of a wedge of amorphous material between the valves,and in spore characters. Absence of any information relating to water-conducting cells prevents further assignmentwithin the embryophytes. Spherical bodies associated with spores and a resilient sporangial lining are comparedwith similar structures in extant free-sporing plants and with Ubisch bodies. The mode of sporangial dehiscenceinvolving anatomical modifications of the valve margins and the novel wedge of tissue which connects them remainsspeculative. 2001 The Linnean Society of London

ADDITIONAL KEY WORDS: embryophyte – miospores – stomata – tapetum.

another fertile taxon. For information of material andINTRODUCTIONtechniques, see Rogerson et al., 1993; Edwards et al.,

Silurian and basal Devonian rocks from the Welsh 1995; Edwards et al., 1999. Locality and stratigraphicBorderland have yielded seminal information on the information is given in the latter and is summarizedearly diversification of land plants (e.g. Lang, 1937; in the Systematic Palaeobotany section below.Edwards, 1979, 1996; Edwards, Fanning & Rich-ardson, 1994; Fanning, Edwards & Richardson, 1992).Their importance has recently been reinforced by thediscovery of a number of localities from which frag-mentary coalified fossils showing exceptional ana-

DESCRIPTION OF SPORANGIAtomical preservation have been collected (e.g. Edwards,1996). These mesofossils, particularly in Lower Dev-

This account is based on at least ten sporangia. Fouronian (Lochkovian) strata, have demonstrated farwere originally more or less intact and clearly show twogreater diversity in vegetation than has been pre-equal valves. Three of the sporangia are transverselyviously appreciated (Edwards, 1996), have shown be-elliptical in face view (Figs 1, 6) and the fourth almostwildering disparity in conducting tissues in axial fossilscircular (Fig. 2). The sporangia are united in the con-(Edwards, 2000; Edwards & Axe, 2000) and have pro-struction of the sporangial wall, including tapetum,vided clues to the nature of the producers of permanentand in situ spore characters. Variation is in shapetetrads and dyads (Wellman, Edwards & Axe, 1998;and features associated with dehiscence. Some of theEdwards, Wellman & Axe, 1999). Here, using a com-remaining examples appear incomplete. They are al-bination of light microscopy, scanning and trans-most globular but with valves, broken, contorted andmission electron microscopy, we present details ofoverlapping, suggestive of fracturing and deformationsoon after death rather than during diagenesis orpreparation (Fig. 4).∗Corresponding author. E-mail: [email protected]

2970024–4074/01/110297+14 $35.00/0 2001 The Linnean Society of London

298 D. EDWARDS ET AL.

more or less circular outline to the stoma 29(35)41 �mMORPHOLOGYlong × 31(34)36 �m wide; N=6) (Figs 7–10). The len-In two cases, single sporangia terminate short curvedticular pore with narrow circumporal thickenings islengths of naked axis, possibly sporangial stalks, thatfrequently occluded. The outer periclinal, presumablyimposed an asymmetry on the specimen (Figs 1, 2).thinner walls of the guard cells are often partiallyThe sporangia in the more complete specimens aremissing (Figs 8, 10) and, where detached from theusually strongly compressed (Fig. 6) and bilaterallysurrounding unmodified epidermal cells, produce asymmetrical. The two intact elliptical specimens arebutterfly shape (Figs 7, 9). Such incomplete cells and1.35 and 1.33 mm wide and 1.08 and 1.25 mm high,fortuitous fractures show that the guard cells arerespectively; the less complete example, but with at-shallow and possibly cradled by the surrounding epi-tached axis, is c. 2.0 wide and 1.4 mm high. The ±dermal cells. There is no evidence for a substomatalcircular one is 1.14 mm wide and c. 1.15 mm high,cavity. Stomata have not been observed on the parallel-precise measurement of the latter being impossiblesided axes, nor on more distal parts of sporangia.because the valves taper gradually into the subtendingHowever, in some specimens, the borders and su-axis (Fig. 2). All valves possess a thickened borderperficially adjacent areas show pronounced de-that extends around the margin of the sporangiumpressions with bevelled, sometimes grooved, marginsexcept at the axis attachment. It tends to be more(Fig. 5), whose bases are features obscured by debris.pronounced on the more compressed specimens andThey were not conspicuous on the sporangium withusually has a smooth surface that lacks the epidermalstomata (holotype).imprints of the rest of the wall (Fig. 5). The latter is

sometimes obscured by drapings of presumed microbialfilms (arrows in Fig. 16). The thickened margins meet ANATOMY OF SUBTENDING AXISor are united at the base of a pronounced groove (Fig. Fracturing of specimens revealed two distinct zones.15), or a shallower depression (Fig. 19), by a flat, A peripheral one consists of 2–3 layers of longitudinallyparallel-sided strip. The latter has a smooth or gran- elongate tapering cells forming in transverse fracture aular exposed surface or one with a raised reticulum of reticulum of homogenized thick walls with the smallesttangentially extended rectangular to diamond shapes lumina in the epidermis (Fig. 23). The central regionthat may reflect the outlines of the underlying cells consists of similarly elongate cells but with walls of(Fig. 19). more variable thickness and often truncated ends that

Superficial features of the remaining sporangial wall are more friable such that intact cells are rarely seen.are best preserved in the almost intact elliptical speci- The internal surfaces of the walls in this region maymen with curved axis designated the holotype (Fig. bear adpressed transversely orientated strands (Fig.1). The proximally parallel-sided axis (230 �m wide) 25) that occasionally extend into the lumen. Thesewidens into the base of the sporangium, whose limit irregular thickenings may occur, but more sporadically,is marked on the periphery by the end of the marginal in the peripheral regions, where internal walls usuallythickening. The axis itself is irregularly and lon- show horizontal wrinkling (Fig. 24).gitudinally ridged at the base, but cell outlines grad-ually become more distinct distally and are wider

SPORANGIAL ANATOMYand shorter (± diamond-shaped) on widening andeventually more or less isodiametric (usually 5-sided) The walls of the valves are multilayered, the number

of layers and shape of the cells varying with position(Figs 3, 5) on the sporangial valve. The intermediatezone is marked by an obliquely transverse band of (Figs 26–31). Walls of adjacent cells are always ho-

mogenized. The bulk of the wall surrounding theslightly depressed stomata (Fig. 3).The two guard cells (with long axes roughly co- spores, broadly similar in appearance in fractures pro-

duced tangentially and radially (anticlinal to convexincident with that of the specimen axis) produce a

Figures 1–10. Sporathylacium salopense gen. et sp. nov. North Brown Clee Hill, Shropshire. Lochkovian. SEMs.Fig. 1. Holotype. NMW94.60G.11. Scale bar=500 �m. Fig. 2. Intact sporangium apart from apex. NMW96.11G.5. Scalebar=500 �m. Fig. 3. Reverse of specimen figured in 1, at junction between sporangium and axis. Arrows indicatestomata. Scale bar=100 �m. Fig. 4. Fractured contorted sporangium. NMW00.34G.1. Scale bar=250 �m. Fig. 5. Partof margin of a sporangium showing depressed area between valves (left), border with depressions (arrow) and surfacecells to right. NMW00.34G.2. Scale bar=50 �m. Fig. 6. Sporangium split such that most of the inner surface of onevalve is exposed. NMW00.34G.3. Scale bar=250 �m. Figs 7–10. Stomata on holotype. Spaces indicate absence of outerguard cell walls. For further explanation see text. Scale bars=10 �m.

NEW LOWER DEVONIAN BIVALVED SPORANGIA 299



margin) through the sporangium, consists of two layers surface features (Figs 12, 20–22). These marginal cellsvary considerably in lumen shape and size. Figure 22of tapering or truncated interdigitating cells, as dem-

onstrated by varying lumen shapes. Their long axes shows an exception where the cells are thin-walled.Cells to the inside of this layer are smaller, in two orare perpendicular to the surface, where their polygonal

outlines (4(5)6-sided) are sometimes visible (Fig. 26). three layers and even more variable in shape (Fig.21). However, the most conspicuous feature of theseAnticlinal walls are usually slightly narrower than

periclinal walls. The outer wall varies between speci- longitudinally fractured sporangia is the wedge-shapedmass of amorphous material (amorphous zone) unitingmens (cf. Figs 26 and 27), and may be slightly convex

where thin (Fig. 26). A distinct cuticle has not been the two valves (Figs 11, 16, 17, 20, 34). It decreases inwidth inwards and in one specimen has extensionsobserved. The two superficial layers are underlain by

a layer of tangentially extended, very thick-walled cells that extend into the thin anticlinal walls of a zone ofelongate cylindrical cells (Figs 16, 17, 34B,C). Thewith elliptical lumina, which are compressed to varying

degrees (Figs 31 and 33). In two specimens where amorphous zone is thus interpreted as initially formedfrom the fused (?deliquescent) outer periclinal wallsvalves are seen in inner face view, such cells appear

to radiate from the base of the sporangium (Fig. 6) or of these cells. The cells themselves are c. 70 �m insection and the outer periclinal walls c. 30 �m thick.may have transversely wrinkled inner surfaces (Fig.

32). However, their inner periclinal walls are often In a second specimen where the valves have separatedthrough the centre of this amorphous zone, the cutobscured by an irregularly folded film of amorphous

smooth material that lines the sporangial cavity (Fig. surface appears ‘pleated’ reflecting the overall co-lumnar shape of the constituent cells (Figs 13, 34F,G)39) and a possible further sheet to which numerous

spherical particles are attached or partially embedded and c. 60 �m thick. Here the lumina of the cells aremuch smaller and limited internally by a conspicuous(Figs 36, 37). The particles are 260–1280 nm in dia-

meter (x=880 nm), and at high resolution sometimes ridge (Fig. 14). However, in the majority of the spor-angia, this region between the two valve margins ishave a framboidal appearance. Rarer are the denditric

structures seen in Figure 36. marked by the amorphous zone with flat outer marginsand extended on the inside into short strands (FigsJust to the inside of the thickened margin, the spor-

angial wall thins to two layers of thick-walled cells 17, 34A,E,H–J, 35). Figure 18 shows the fracturedthickened marginal zone of cells where it abuts on the(Fig. 29). The outer has lumina that are square to

rectangular in tangential section (Fig. 29). The inner wedge.periclinal walls are the thickest and are continuouswith a layer of compressed and also thick-walled cells

SPORES(Fig. 29). Nearer the base of the sporangium, the wallhas two layers of cells more or less isodiametric in SEM. Most of the sporangia are packed with a large

number of trilete spores, of approximately the samesection (Fig. 28).In most examples, longitudinal sections at right size (Figs 31, 35, 40). The spores are usually flattened

to saucer-shaped (making measurement difficult), withangles to the plane of compression show the marginsto be dominated by very thick-walled elongate cells a marked, but narrow, equatorial thickening (cras-

situde) (Figs 40, 45). The simple trilete mark has armsthat are inclined towards the curved distal marginsuch that they may produce± isodiametric polygonal in the form of a fold of uniform diameter that are

Figures 11–22. Sporathylacium salopense gen. et sp. nov. North Brown Clee Hill, Shropshire. Lochkovian. SEMs.Fig. 11. Jagged fracture through sporangial margin; radial plane on right through the marginal thickening andamorphous zone; tangential on left, where cells to inside of thickened margin are visible on lower valve and the marginitself on upper. Holotype. NMW94.60G.11. Scale bar=50 �m. Figs 12, 13. Other valve of sporangium to that in Figure6. Outer periclinal cell walls of thickening are missing on the left. Section between valves has a smooth surface.NMW00.34G.3. Fig. 12. Scale bar=50 �m. Fig. 13. Part of margin. Scale bar=50 �m. Fig. 14. Section through part ofamorphous material at margin of sporangium in Figure 6, further magnified. Note spores to the bottom. Scale bar=50 �m. Fig. 15. Depression between sporangial valves. NMW00.34G.4. Scale bar=100 �m. Fig. 16. Distal region ofsporangium in Figure 2. Note cells below amorphous material (am) between valves, and micobial film (arrows) partiallycovering surface. Scale bar=100 �m. Fig. 17. Close up of cells below amorphous wedge in Figure 16 at different angle.Scale bar=100 �m. Fig. 18. Fractured thickened marginal zone. Note spores to top right. NMW00.34G.2. Scale bar=50 �m. Fig. 19. Depressed zone between valves showing raised reticulum. NMW00.34G.2. Scale bar=10 �m. Figs 20,21. Radial fractures through thickened marginal zone. NMW00.34G.5; NMW96.11G.5. Scale bars=10 �m. Fig. 22.Marginal cells with thinner walls. NMW00.34G.4. Scale bar=50 �m.



sinuous near the pole (Figs 38, 45), and extend almost 51, 52). In some sections the proximal exospore isslightly thicker than the distal; in others they are ofto the thickened margin (Figs 43, 45). Fractured spores

show no evidence of layering in the wall. All surfaces equal thickness. Small regions of increased thickness(arrowed in Fig. 50) may represent the crassitude,plus the apertural fold are covered by a microgranulate

ornament (Fig. 38). It is slightly coarser on the distal although where they are subequatorial and part of thedistal surface, they may represent the darker areassurface. Such ornament is barely visible in spores

observed by light microscopy (Figs 43, 44), and does noted in light microscopy. Further sections are requiredto clarify this. Larger globular structures between thenot modify the profile at the equator.

In interradial areas the proximal surfaces show spores are also composed of the tiny spheres (Figs50–52) and probably represent spherical particles,slight undulations (Fig. 45), but these are far more

pronounced on the distal surface where they are in some of which have a framboidal appearance, seen inSEM. Unfortunately we have no sections through thethe form of verrucae and short muri (Figs 40, 46). They

do not extend over the equatorial crassitude. Initially, sporangial wall lining itself.it was thought that this large scale ornament wascreated by pyrite within the body of the spore, but its

SYSTEMATIC PALAEOBOTANYpersistence in spores viewed by light microscopy (FigsEmbryophyta: Incertae sedis43, 44) following nitric acid treatment indicates that

Sporathylacium Edwards, Axe and Mendezthe spores may have been murornate. The combinationof this ornament and the microgranulate sculpture gen. nov.precludes the assignment of these spores to an existingtaxon. Derivation. Latinized from Thylakion (Grk.): a little

Scattered among the spores and adhering to their bag, sac or pouch.surfaces are isolated or clustered spherical particlesof similar appearance and dimensions to those lining Diagnosis. Isolated bivalved sporangia, circular to el-the sporangial wall (Figs 41, 42). liptical in face view, terminating smooth axes. Spor-

angium wall at maturity multilayered. Convex valvemargins thickened, comprising modified cells andTEM. Figures 47 and 48 are typical of transmissionunited at the base of a depression. Sporangium wallelectron micrographs of part of a compressed sporelined internally by smooth layer (remains of tapetum)mass. Individual spores may be folded, but while lu-with abundant adhering spherical particles (globules).mina are usually distinct (Figs 48–50), identificationGlobules also present between and on surfaces ofof proximal and distal walls is more problematic. How-spores. Abundant isospores with triradiate aperturalever, based on reference to SEMs that indicate smallerfolds extending to equatorial crassitude. Spores non-ornament on the proximal face, the latter may becurvaturate.identified as the very irregular surface with ‘out-

growths’ of similar electron density to the homogeneousNotes. Differs from Zosterophyllum sporangia in sporeexospore (arrows in Fig. 52). In contrast, the ornamentcharacters and from Resilitheca in possession of spor-of the presumed distal surface, although of similarangial border and spore characters.electron density, is separated from the distal wall by

a very narrow paler layer and appears composed ofclusters of variable diameter but composed of small Type species. Sporathylacium salopense Edwards, Axe

and Mendez.spheres of similar size and c. 350 nm in diameter (Figs

Figures 23–33. Sporathylacium salopense gen. et sp. nov. North Brown Clee Hill, Shropshire. Lochkovian. SEMs.Fig. 23. Transversely fractured end of axis. Holotype. Scale bar=20 �m. Fig. 24. Longitudinally fractured thick-walledcortical cells with transverse wrinkling. Holotype. Scale bar=10 �m. Fig. 25. Longitudinally fractured cell withtransverse thickenings. Holotype. Scale bar=10 �m. Fig. 26. Fractured sporangial wall with pronounced basal layer(arrows). NMW00.34G.2. Scale bar=20 �m. Fig. 27. Fractured wall: cell walls thicker than in Figure 26. NMW00.34G.1.Scale bar=50 �m. Fig. 28. Fractured wall near base of sporangium. NMW00.34G.2. Scale bar=50 �m. Fig. 29. Fracturedwall immediately to inside of thickened margin. Arrows indicate compressed inner layer. See also Figure 11. Holotype.Scale bar=20 �m. Fig. 30. Fractured wall. Holotype. Scale bar=50 �m. Fig. 31. Part of fractured sporangium. Notecasts of innermost layer of sporangial wall to bottom left. NMW00.34G.1. Scale bar=50 �m. Fig. 32. Surface view ofinnermost layer. NMW00.34G.1. Scale bar=50 �m. Fig. 33. Close up of transversely fractured inner part of sporangiumwall. Arrows indicate inner layer of cells. Holotype. Scale bar=10 �m.


Figure 34. A–J. Line drawings of marginal features in Sporothylacium salopense gen. et sp. nov. North BrownClee Hill, Shropshire. Lochkovian. Hatched areas=cell lumina; stippling=amorphous material of wedge. A, B. Radialsections (RS). NMW00.34G.5. Scale bars=10 �m. C. RS amorphous material, D. RS margin. NMW96.11G.5. Scalebars=10 �m. E. Oblique RS through margins of both valves and amorphous material. Holotype. NMW94.60G.11. Scalebar=10 �m. F. RS, G. TS (tangential section). NMW00.34G.3. Scale bars=20 �m. H. RS amorphous material and onevalve. NMW00.34G.2. Scale bar=10 �m. I. RS two valves and dislodged amorphous material. NMW00.34G.1. Scalebar=20 �m. J. RS amorphous wedge and margin of one valve. NMW00.34G.6. Scale bar=10 �m. K. RLS Zosterophyllumllanoveranum. V. 34301. Scale bar=50 �m. L. Generalized RLS Trichopherophyton teuchansii. Scale bar=100 �m.

subtending axis. Spores 23–30 �m diameter (x=27 �m:Derivation. From Salop: an alternative name for theN=10). Circular amb. Equatorial crassitude c. 1.6 �mcounty of Shropshire.wide. Narrow sinuous apertural folds. Spore ornament

Diagnosis. As for genus. Sporangial valves equal in composed of dense micrograna, evenly spaced oversize and shape. Sporangia 1.14–2.0 mm wide and entire surface; coarser on distal surface where coverc. 1.15–1.25 mm high. Subtending axes c. 250 �m wide. possible verrucae. Exospore homogeneous, but or-Superficial marginal thickening <c. 160 �m wide. Spor- nament attached to narrow less electron dense layer

on distal face.angium wall with band of stomata at junction with


Figures 35–46. Sporothylacium salopense gen. et sp. nov. North Brown Clee Hill, Shropshire. Lochkovian. Figs35–42, 45, 46=SEMs; Figs 43, 44=LMs. Fig. 35. Margin of fractured sporangium, with dispersed amorphous wedgebetween valves. Note spores to right. NMW00.34G.2. Scale bar=20 �m. Figs 36, 37. Close up of inner surface ofsporangium wall with globules. Holotype. Scale bars=5 �m. Fig. 38. Polar region of trilete. Holotype. Scale bar=1 �m.Fig. 39. Inner surface of sporangium wall showing cell outlines and partial covering with globules. Holotype. Scalebar=20 �m. Fig. 40. General view of spores; distal surfaces mostly visible. Holotype. Scale bar=20 �m. Fig. 41. Distalsurfaces of spores showing gross and micro-ornament with concentrations of globules on isolated films. Holotype. Scalebar=10 �m. Fig. 42. Globules between spores. Holotype after nitric acid treatment. Scale bar=5 �m. Figs 43, 44.Spores (after nitric acid treatment) showing equatorial thickening, verrucate and murornate ornament and simpletriletes. Holotype. Scale bar=10 �m. Fig. 45. Proximal face. Holotype. Scale bar=10 �m. Fig. 46. Distal surfaces.Holotype. Scale bar=10 �m.


Figures 47–52. Sporothylacium salopense gen. et sp. nov. North Brown Clee Hill, Shropshire. Lochkovian. TEMs.Holotype. Figs 47–49. Scale bars=1 �m. Figs 50–52. Scale bars=300 nm. All spores have had nitric acid treatmentand all are unstained apart from Figure 50 which is stained. Figs 47, 48. Sections through compacted spore mass. Fig.49. As above but atypical section showing pronounced spore lumina (L). Fig. 50. Sections with localized thickenings,possibly crassitudes (arrows). Fig. 51. Note associations of spherical particles between spores and attached to lighterlayer (arrows) on distal surface. Fig. 52. Magnified detail of left hand spore in Figure 51. Arrows indicate projectionson proximal surface.

Holotype. NMW94.60G.11. (1) those with sporangia terminating isotomouslybranching systems (e.g. Cooksonia caledonica (Ed-

Illustrations of holotype. Figs 1, 3, 7–11, 23, 24, 25, wards, 1970); Resilitheca salopensis (Edwards et29, 30, 33, 34E, 36–52. al., 1995) or similarly branching, but lateral, sys-

tems (e.g. Renalia hueberi (Gensel, 1976));Locality. Stream section on the north side of Brown (2) zosterophylls, where they are usually borne onClee Hill, near Monkhopton, Shropshire, England. short stalks;

(3) basal lycophytes (e.g. Drepanophycus qujingensisStratigraphy. Ditton Group, lower middle part of mi- (Li & Edwards, 1995), Halleophyton zhichangensecrornatus-newportensis spore zone, Lochkovian Stage, (Li & Edwards, 1997), Leclercqia complexa (KasperLower Devonian (see Richardson & McGregor, 1986). et al., 1988; Banks, Bonamo & Grierson, 1972).

A distinctive marginal thickening (usually termed asCOMPARISONS a border in compression fossils) is absent from the

lycophytes (e.g. Li & Edwards, 1995), although rel-Sporangia with two valves occur in a number of groupsof Lower Devonian plants including: atively little is known about their sporangia, and from


Resilitheca (Edwards et al., 1995). Details of any mar- in situ spores as Calamospora (Gensel, 1980). Its pres-ence in Sawdonia acanthotheca together with Retuso-ginal thickening or indeed of spores are not knowntriletes and ?Apiculiretusispora (Gensel, Andrews &from Cooksonia caledonica (Edwards, 1979; EdwardsForbes, 1975) may relate to state of maturation.et al., 2001). The distinctive sporangial/axis junction

On the basis of such comparisons we are reluctantin the impression fossils of C. caledonica has not beento place the new sporangia in an existing taxon, or toseen in the new plants, but this may be an effect ofsuggest affinity with zosterophyllums or plants similarpreservational differences. Until more is known ofto Cooksonia caledonica. We therefore erect a newthe anatomy of the type C. caledonica and the grossmorphotaxon for sporangia with two marginallymorphology of the new specimens, it would be unwisethickened valves, and non-retusoid, crassitate iso-to place the new sporangia in C. caledonica. The onlyspores.marginal feature described for Renalia hueberi

(Gensel, 1976) is a peripheral row of thick-walledrectangular cells, i.e. nothing of the complexity GENERAL DISCUSSIONdescribed here.

SPORE AND SPORANGIAL DEVELOPMENTIn that the type zosterophyll, Zosterophyllum my-retonianum, occurs in coeval rocks from Scotland Combined TEM and SEM observations on the lining

of the sporangial wall and spores demonstrated that(Lang, 1927; Edwards, 1975), and that an unnamedthe spherical particles found adhering to the liningZosterophyllum, believed close to Z. fertile Leclercqare similar to those associated with the spores and1942, was collected from a probably only slightlythat their basic construction is definitely similar toyounger horizon in a stream section near Abergavennythe distal ornament of the spore wall. The nature of(Kenrick, 1988; Edwards, 1990), it is possible thatthe proximal ornament where there is continuity withthese new sporangia are isolated examples from Zos-the underlying exospore is less readily resolved, al-terophyllum strobili. Indeed, in the two examples inthough in some regions individual spheres or hemi-which short lengths of axes are attached, these arespheres can be detected (Fig. 52). Similarities incurved, although their apparent orientation in theconstruction between the spherical particles and sporesame plane as that of sporangial compression is notornament suggest that the former may have beenthat expected of a lateral sporangium. In addition theinvolved, via a secretory tapetum, in the deposition ofoverall shape of the sporangia illustrated here is notthe spore wall. Similar spherical particles recorded intypical of either of the two coeval species. However, onzosterophylls (Gensel, 1980; Edwards & Richardson,the anatomical side, where three-dimensional or-1996) and Resilitheca (Edwards et al., 1995) are usuallyganization of the marginal thickening is known, albeitcalled tapetal residues and considered probably ho-in slightly younger Pragian Z. llanoveranum (Edwards,mologous to the Ubisch bodies of seed plants (Gensel,1969a) and Z. cf. fertile (Edwards, 1969b) per-1980; Shute & Edwards, 1989). The latter, as well asmineralizations in iron compounds, it is broadly sim-spherical structures in cryptogams such as Funariailar. Detailed comparisons at the cellular level areand Drynaria, became impregnated with sporopollenin

impossible because of poorer preservation in the Prag-at the end of tapetal activity. SEMs of Sawdonia

ian material. However, in Z. llanoveranum (Fig. 34K)acanthotheca (Gensel, 1980; Plate II.6) show ‘tapetal

there is also organic continuity at the base of a de- globules’ with ‘minute bumps’, and aggregations ofpression, but there is no evidence for the ‘wedge’ of small spheres are illustrated in TEM sections of Re-cells with massive thickening of the outer periclinal silitheca (Edwards et al., 1995; Fig. 39). Indeed, ex-walls. This latter feature is also absent from the Prag- ospore sections of the latter look remarkably similarian Rhynie and Windyfield Chert taxa Tricho- to those in the new plant in that in some examplespherophyton (Lyon & Edwards, 1991; Fig. 34L) and (e.g. Edwards et al., 1995: Fig. 44) the particles areVentarura (Powell, Edwards & Trewin, 2000) re- attached to the electron-dense exospore by a narrowspectively, where there is also continuity at the base pale layer. However, in neither Resilitheca nor Saw-of a depression. The Scottish taxa are also different donia is there ornament on the spore itself.because Trichopherophyton has unequal valves (with Similarly sized particles have been termed globulesspines) and Ventarura sporangia are sessile. Finally, and spherules by Lugardon (1981). The former occurand perhaps most importantly, the spores in the new in extant homosporous lycopods and certain ferns.sporangia are not retusoid and possess an equatorial They are small spherical bodies of sporopollenin andthickening. Curvaturae are a character of many of the are produced by the tapetum at the same time as thespores (e.g. Retusotriletes sp.) so far extracted from exospore. In section they sometimes appear layeredzosterophyll sporangia (Edwards & Richardson, 1996), with surfaces described as “plain, scabrate or gem-as is a loose, easily detached envelope, possibly a mulate” and resembling “exospore surface contours”

(Tryon & Lugardon, 1990). Some illustrated examplesperispore. The latter may explain identification of some


have solid centres with a peripheral layer of small with sporopollenin and adhering Ubisch bodies, is hy-pothesized to produce a non-wettable surface pre-spheres (Tryon & Lugardon, 1990: fig. 59 – Thelypterisventing water-logging and thus facilitating efficientvisicosa), but the structures appear never to consistdispersal of pollen (Heslop-Harrison & Dickinson,entirely of the latter. In some examples, globules are1969; Keijzer, 1987). This role might have been equallyattached to the exospore by a covering of perisporeimportant in early embryophytes and may account for(‘globules captifs’). In contrast, spherules said to bethe presence of a resilient layer in a number of lineagesindistinguishable from globules by LM or SEM consist(e.g. a new species of Cooksonia from this locality,entirely of perisporal material, i.e. they are depositedResilitheca and Psilophyton).in association with the exospore after the completion

of the latter. They are much rarer and confined toDehiscence. The above provides one of the most completecertain homosporous ferns. However, with the provisoaccounts at the cellular level of any Lower Devonianthat timing of depositional events or chemistry aresporangia preserved in clastic rocks, lacking data onlyimpossible to confirm in the fossils, should the spher-on the chemical composition of cell walls. In particularules be homologous to the fossil representatives, theit provides detail on the anatomical modifications pre-intriguing possibility arises that the ornamented layersumably relating to dehiscence of sporangia and dis-was a closely adhering perispore, and that the palersemination of spores. The most novel and intriguinglayer marks the disintegration of the innermost layerinformation centres on the tissue joining the two valves.of the perispore, where it was attached to the exosporeIt is clear that splitting did not occur between the

that would ultimately lead to its separation. While itthickened margins as such, but in the wedge of cells

must be admitted that there is no evidence whatsoever with extremely thick outer periclinal walls forming thefor separation in the new taxon, two layers are recorded amorphous zone (Figs 11–17). The process involvedin certain zosterophylls (e.g. Zosterophyllum and Saw- remains unknown. Cells of the bulk of the sporangiumdonia acanthotheca) and in trimerophytes. In the lat- wall may have lost water through their outer periclinalter, however, spherical particles are not recorded, and walls, although these are no thinner than the remainingGensel (1980) dismissed the presence of a perispore in walls and may have been cuticularized. Shrinkage incertain species of Psilophyton, because the two spore these areas would have resulted in stresses on thelayers had similar appearances and properties (but reinforced margins, with rupture between the valves.see Streel, 1967; Banks, Leclercq & Hueber, 1975). Did the split occur through the centre, as occurred whenHowever, even in exant plants, the detailed chemistry one sporangium was deliberately split open (Figs 13,of perispore remains uncertain and, while there is 14) or did the wedge remain intact? Is it possible thatthe likelihood that it contains cellulose as well as this wedge represents a mass of deliquescing materialsporopollenin (A. Hemsley: pers. comm., 2001), it might produced from the thick walls, whose eventual shrink-be anticipated that, following taphonomic processes, age on drying resulted in passive separation of the twoonly the latter would be preserved in fossils. A sep- valves? A role of the thickened margin is postulated toarating layer in Uskiella spargens was indeed in- have been the maintenance of sporangial shape fol-terpreted as a perispore and bears a granular lowing splitting, with the spores probably shaken outornament, similar to that on sheets and spheres oc- through a very extensive slit (see also Powell et al.,curring between the spores and on the lining to the 2000). Such conjectures clearly require more specimens

at various maturation stages: to date we have not foundsporangial wall (Shute & Edwards, 1989).any empty, only more or less intact, sporangia of theA resilient lining in the mature sporangial wallnew taxon.characterizes many Lower Devonian taxa and is also

found in certain extant cryptogams. In PsilophytonACKNOWLEDGEMENTSdawsonii such ‘membranes’ or ‘sacs’ have been re-

covered intact after maceration of the sporangia andUltrastructural studies were financed by NERC grantare postulated to have been ‘reinforced by end productsGR9/1441‘A’ which is gratefully acknowledged. E.M.of the disintegration of the tapetum’ (Banks et al.,thanks Consejo Nacional de Ciencia y Tecnologia, Mex-1975). Similar layers have also been described as ‘cu-ico for funding her M.Phil. We thank Mike Turner for

ticular’ (see Gensel, 1980). In living plants, persistentundertaking the sectioning and Dr Kate Habgood for

sporangial linings are described as peritapetal mem-the line drawings. We are also grateful to Dr J. B.

branes or ‘culture sacs’, the latter in angiosperms Richardson (NHM) for advice on spore identification.(Heslop-Harrison, 1968; Pacini, Franchi & Hesse,1985; Pacini, 1990). They occur in plants with both REFERENCESperiplasmodial and secretory tapeta (e.g. in Botry-chium (Pettitt, 1979) and Psilotum (Parkinson, 1987). Banks HP, Bonamo PM, Grierson JD. 1972. Leclercqia

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Documents

A new genus for isolated bivalved sporangia with thickened margins from the Lower Devonian of the Welsh Borderland