Late Carboniferous palaeobotany of the upper Bideford Formation,north Devon: a coastal setting for a Coal Measures flora

Christopher J. Cleal1 & Barry A. Thomas"

CLEAL, C. J. & THOMAS, B. A. 2004. Late Carboniferous palaeobotany of the upperBideford Formation, north Devon: a coastal setting for a Coal Measures flora. Proceedingsof the Geologists' Association, 115, 267-281. The Culm Seams are thin coals in the UpperCarboniferous upper Bideford Formation of north Devon. Clastic sedimentary rocks associ­ated with the coals have yielded a fossil macroflora dominated by the remains of theCalamostachyales and Medullosales, together with subsidiary lycophytes, sphenophylls, ferns,lagenostomaleans, rare cordaites, and a possible early cycad. The flora is probably earlyLangsettian in age, which is in agreement with the evidence of the non-marine bivalves andmarine bands. It is broadly similar in composition to contemporaneous macrofloras from SouthWales. It is unlikely to represent an assemblage formed from plant remains subject to long­distance transportation. Rather, it was probably preserved in the lower reaches of a 'bird-foot'delta that had temporarily transgressed into the Culm Basin. The distal margin of this deltawould represent a comparable habitat to the levees of the rivers further inland, and thus wouldhave supported vegetation similar to that which generated the more usual Coal Measuresmacrofloras.

Key words: Carboniferous, palaeobotany, biostratigraphy, palaeoecology

'Departmen: of Biodiversity and Systematic Biology, National Museums and Galleries ofWales, Cathays Park, Cardiff CF10 3NP, UK (e-mail: chris.cleal@nmgw.ac.uk)2Jnstitute of Rural Sciences, University of Wales Aberystwyth, Llanbadarn Fawr, AberystwythSY23 2EX, UK

1. INTRODUCTION

The palaeobotany of the mainly non-marine West­phalian (Late Carboniferous) 'Coal Measures' ofBritain has been extensively studied and there havebeen many monographs and reviews (e.g. Kidston,1923-1925; Crookall, 1955-1976; Cleal & Thomas,1994, 1995). However, the Westphalian of southwestEngland is in a different, mainly marine facies andconsequently has a much poorer palaeobotanicalrecord. The main exception are the beds associatedwith thin coals known as the Culm Seams, whichoccur in a narrow strip of land between the coast atGreencliff (Bideford Bay) and Hawkridge Wood nearChittlehampton (Edmonds et al., 1979). Even here, theplant fossils are generally sporadic and fragmentary,but are nevertheless of interest for the light they throwon the biostratigraphy and palaeoecology of these beds.

The first description of these macrofloras was byLindley (in De la Beche, 1834, 1839; and in Sedgwick &Murchison, 1840). The definitive study on them was byE. A N. Arber (1904a, b, 1907, 1911a, b), who alsoreviewed the earlier history of research. Rogers (1910)gave a detailed account of the different plant-bearinglocalities but no specimens were illustrated. In somecases, the published description of the localities ispoor, but M. A. Arber (1982) has managed to identify

Proceedings of the Geologists' Association, 115, 267-281.

several of them from Rogers' field-maps and notes.Since E. A N. Arber's work, the only publishedpalaeobotanical work on these beds has been a critiqueof his biostratigraphical conclusions by Crookall(1930), and the reclassification by Leary (1980) ofArber's Megalopteris. Hofmann (1992) reported ad­ditional plant remains from beds associated with theCulm Seams, but neither illustrated them nor recordedthem beyond 'axes and leaves of calamites, with rarerfern pinnules'. In view of the recent reawakening ofinterest in these beds (e.g. Xu Li, 1990; Hofmann,1992; Eagar & Xu Li, 1993) and the considerablechanges that have occurred in the taxonomy of LateCarboniferous fossil plants, a review of the evidence istimely. An attempt by one of us (RAT.) to collectplant fossils from these beds exposed along the coastyielded only a little additional material. The presentstudy is therefore based on existing collections, princi­pally in the Natural History Museum, London(BMNH), the Sedgwick Museum, Cambridge (CSM),the Royal Albert Memorial Museum, Exeter(RAMM), and the Museum of Barnstaple and NorthDevon (MBND; this includes the collections formerlyin the Athenaeum Museum of Barnstaple).

The etymology of 'culm' has been the subject ofsome debate, with the consensus being that it is derivedfrom 'coal' and that the Culm Basin and Culm Group

0016-7878/04 $15.00 © 2004 Geologists' Association

268 C. J. CLEAL & B. A. THOMAS

N

1lkm

1-----1

~L.:.J

E:ilD

Budc Fm

Bidcford I'm(showing Comborough Sst)

Crackington Fill

Fig. 1. Simplifiedgeological map of the area near Bideford, North Devon, showing the location of the main localities that haveyielded fossils of the Culm Seams flora. The number of each locality is given in the text and in Table 1. Based upon BGS1:50000 Geological Map Sheets 292, 293 and 309, by permission of the British Geological Survey.

were so named because they contained the Culm Seam.However, this seems unlikely, as the coal-bearinginterval is not a typical part of the Culm Group orBasin. Edmonds et al. (1979) has instead suggested thatit was derived from the Welsh word cwlwm, meaningknot, and referring to the contorted nature of the beds.Hence, the Culm Seam was named after the interval ofrocks in which it occurs, and not the other way around.

2. GEOLOGICAL BACKGROUND

The Upper Carboniferous deposits of Devon wereformed in the Culm Basin, on the foreland that devel­oped between the cratonic Wales-Brabant Massif andthe northwards migrating Variscan Orogen (Hartley &Warr, 1990). The relationship between the Culm Basinand the contemporaneous South Wales Basin has beenthe subject of some speculation (Hartley, 1993). Cur­rent opinion is that there has been significant crustalshortening between the two basins, as a result ofthrusting along the Bristol Channel Fault Zone. How­ever, the amount of crustal shortening is unknown andso the original distance between the two basins duringCarboniferous times is uncertain.

Edmonds (1974) classified the Late Carboniferousdeposits of the Culm Basin into three formations, inthe following ascending sequence.

(I) Crackington Formation. Turbidites of mainlyNamurian age deposited in an east-west-trendingbasin (Edmonds et al., 1979), but which according toThomas (1988) were ultimately derived from a sourceto the north of the basin.

(2) Bideford Formation. A localized sequence, con­fined mainly to an elongate area between Bidefordand Chittlehampton, where it overlies the CrackingtonFormation. It consists of nine coarsening-upwardscycles probably formed in an elongate 'bird-foot'-type

delta (De Raaf et al., 1965; Walker, 1966, 1969;Elliott,1976). The upper eight cycles were formed by a pro­grading delta front (Xu Li, 1990), culminating inrooted beds and, in cycles 5 and 9, a coal. The cycle 5coal is only thin; but that at the top of cycle 9 is moresubstantial (sometimes more than 4 m thick; Edmondset al., 1985, p. 116) and is underlain by a prominentsandstone (the Comborough Sandstone). The sedi­ment was derived from the north, probably from anuplifted area that had resulted from the inversion ofthe Bristol Channel Fault Zone (Hartley, 1993).

(3) Rude Formation. Shallow-water deposits, formedin either lacustrine or near-shore marine conditions(Higgs, 1983, 1984, 1986; Melvin, 1986;Hartley, 1993).The base of the Bude Formation is at the first appear­ance of massive sandstone beds, although the contactmay be visually gradational (British Geological SurveyLexicon of Named Rock Units). Near Bideford, theBude Formation overlies the Bideford Formation,but further south it lies directly on the CrackingtonFormation.

Plant fragments occur throughout the UpperCarboniferous of the Culm Basin, but the bulk ofthe identifiable remains are in the upper BidefordFormation, in the area between Westward Ho! andUmberleigh (Fig. 1). The succession here is generallysouth dipping. However, a major east-west-trendingfault about 1 km south of Bideford brings theCrackington Formation back to the surface. On thesouth side of this fault, the Bideford Formation isabsent and the Bude Formation immediately overliesthe Crackington Formation.

3. PROVENANCE OF SPECIMENS

Significant assemblages of plant fossils were found(mainly by Rogers) at the following localities. The

PALAEOBOTANY OF UPPER BIDEFORD FORMATION

Table 1. Distribution of species in the main plant localities in the upper Bideford Formation.

269

2 3 4 5 6 7 8 9 10 II 12 13

Lepidodendron aculeatum Sternberg xLepidodendron dichotomum Sternberg ?Lepidodendron fusiforme Corda xLepidophloios acerosus Lindley and Hutton x xSigillaria scutellata Brongniart x

Sigillaria tessellata Brongniart xSphenophyllum cuneifolium (Sternberg) Zeiller x x x xCalamites undulatus Sternberg x x xCalamites suckowii Brongniart x x x xCalamites cistii Brongniart xCalamites sp. x x xAsterophyllites charaeformis (Sternberg) G6ppert x ? xRenaultia crepinii (Stur) Zeiller x ? xCorynepteris angustissima (Sternberg) Nernejc ?Karinopteris acuta (Brongniart) Boersma x x x x x xLyginopteris hoeninghausii (Brongniart) Gropp x xEusphenopteris sp.Neuralethopteris schlehanii (Stur) Laveine x x ?Neuralethopteris rectinervis (Kidston) Laveine x ? x x x xNeuralethopteris jongmansii Laveine ?Alethopteris lonchitica Sternberg ? ? x ?Alethopteris decurrens (Artis) ZeillerNeuropteris obliqua (Brongniart) Zeiller x xTrigonocarpus sp. xLesleya cf. cheimarosa Leary & Pfefferkorn xArtisia sp. x

1, Greenc1iff;2, Pit Quarry; 3, Bideford Wharf; 4, Bideford Station; 5, Robert's Quarry; 6, Broadstone Quarry; 7, Pollard's Quarry; 8, PillheadCopse; 9, Warmington Farm; 10, Webbery Wood; 11, Alverdiscott; 12, Somers, Hiscott; 13, Ford Quarry.

numbering of the localities is the same as that used onthe map (Fig. I) and Table I.

(I) Greencliff (or Cornborough Cliff), Bideford BayISS 405271 j. The cliffs west of Bideford provide thebest available outcrops of the Bideford Formationand are a Site of Special Scientific Interest (Cleal &Thomas, 1996). Edmonds et al. (1979) published adetailed measured stratigraphical section. Rogers(1909) identified the main fossil horizons as occurringin shales within what are now called Cycles 8 and 9 inthe upper Bideford Formation, with soft black shalesbelow the coal of Cycle 9 yielding the best material.

(2) Pit Quarry, Abbotsham [SS 421270 j. This is theclassic site from which Sedgwick & Murchison (1840)obtained plant fossils. Rogers (1910) stated that thefossils occur in the thick sandstone (now known as theCornborough Sandstone) associated with the CulmSeams. Locality details were not given but M. A. Arber(1982) has traced it to a disused and overgrown quarrynorthwest of Abbotsham.

(3) Bideford Wharf Neither Arber (l904a) norRogers (1910) mentioned this locality, but there is asingle specimen from here in the Rogers Collection atthe Sedgwick Museum. However, it does not given anyfurther locality details other than that the fossil waspreserved in shales exposed on the east side of the river.

(4) Bideford Railway Station, East-the-Water ISS457263 j. Rogers (1910) recorded plants from vertical

beds exposed alongside the road from the stationleading to Chudleigh Fort. These lie immediatelyabove the Cornborough Sandstone.

(5) Robert's Quarry, East-the- Water ISS 461265 or462265J. M. A. Arber (1982) listed this as one of twopossible sites immediately east of Chudleigh House,East-the-Water. Exposed here were bluish 'ovoidallysplitting' shales that lie a short distance above theCornborough Sandstone. As well as yielding plantfossils, Rogers (1910) reported that the shales containnon-marine bivalves. However, from evidence pro­vided by Eagar & Xu Li (1993) it seems more likelythat the shells originated from below the sandstone(these shells were also discussed by Simpson (1933»).

(6) Broadstone Quarry, East-the-Water. The plantfossils from here were collected from a spoil heap ofshale and impure coal, which had resulted from adrainage adit being driven through the north side ofthe quarry (Rogers, 1910). The coal is probably thatwhich overlies the Corn borough Sandstone, as thereare several old workings of that seam in the vicinity(Arber, 1904a). The exact position of the quarry isunknown but according to M. A. Arber (1982) wasnear Pollard's Quarry (see next).

(7) Pollard's Quarry, East-the- Water ISS 469264].Arber (1904a) reported only poorly preserved speci­mens from here but Rogers (1910) gave a list of speciescollected from one of the coals exposed here in 1910.

270 C. J . C L E A L & B . A . THOMAS

Ta ble 2. Percentage composition of the Culm Seams macroflora, compared with tha t o f two Lan gsettian-aged macroflor as fromSouth Wales (based on data from Davies (1929» .

Culm macroflora Gellideg Seam Five Feet Seam

Lepidocarpales 0.74(2.30)5.33 1.48(1.64)1.82 0.03(0.16)0.47Sigillar iostrobales 0.49(1.83)4.67 0.03(0.06)0.10 0.00(0.06)0.31Sphenophyllales 3.60(6.52)10.72 0.01(0.02)0.05 0.00(0.00)0.20Calamostachyales 24.33(30.43)37.09 38.96(39.61)40.26 28.58(30.69)32.86Ferns 5.05(8.40)12.97 0.00(0.00)0.02 0.00(0.00)0.20Lagenostomales 9.66(14.03)19.43 0.00(0.00)0.02 1.57(2.19)2.97Medullosalcs 28.30(34.67)41.48 53.79(54.45)55.11 20.65(22.55)24.53?Cycadales 0.01(0.47)2.59 0.00(0.00)0.02 0.00(0.00)0.20Co rdaitantha les 0.27(1.35)3.97 3.96(4.22)4.49 42.06(44.35)46.66No. of specimens 213 22056 1831

Values are given with their 95% two-sided confidence bound s, calculated as recommended by Howarth (1998).

M . A. Arber (1982) identified the exact position of thissite, which she reported was now a rubbish dump. Th estratigraphical position was not stated but the localityis immed iately south of the Cornborough Sandstone,and so was presumably the seam that immediatelyoverlies that sand stone.

(8) Pillhead Copse. Rogers (1910) stated that hecollected fossils from the eastern end of this wood,three-quarters of a mile (1.2 km) east of Bideford. Thefossils were from spoil tips near old coa l workings, butno such tips or workings can be identified on theGeol ogical Surveyor Ordnance Survey maps (1:50 000).

(9) Warmin gton Farm i S S 478260] . Rogers (1910)repo rted finding abundant plant fragments in a hedgebank 300 yards (280 m) north of the farm .

( 10) Webbery Wood iSS 501260j. Rogers (1910)sta ted that he collected fossils from one of the old spoiltips at a disused coal working, about 270 m west ofWebbery House. It is very near a thick sandstone thatis probably the Cornborough Sandstone (Edmondset al., 1985).

(II) Alverdiscott. Arber (1904a) noted that fossi lsfrom near a coal seam exposed at this village, 7 kmESE of Bideford, were collected by Townshend Hall,but further locality details were not given .

(12) Somers, Hiscott Down iSS 551255]. Rogers(1910) collected from another old spoil tip from adisu sed coa l working, about 450 m west of SomersHouse. It is, again , near the Cornborough Sandstone.

(13) Ford Quarry , Umberleigh iSS 605237]. Shalesassociated with a 0-45 m thick coal bed were describedby Rog ers (1910) as yielding abundant fossils, althoughnone were described by Arber (1904a) .

It is noti ceable from Figure I that all of theselocalities are close to the Cornborou gh Sand stone, andthere can be little do ubt that they all represent thesame stra tigraphical level, i.e. the Paint Seam justabove that sandstone. Rogers (1910) mentioned plantfragments from other stratigraphical levels, lower inthe Bideford Formation and in the Bude Formation,from severa l localities in this part of Devon, nearWestward Ho!, Torrington, Hartland and Appledore,

but these mostly yield on ly fragments of Calamites anddecorticated Iycophyte stems, and will not be dealtwith further here.

4. PALAEOBOTANY

The following is not a detailed systematic review ofthe Cu lm Seams macroflora, which has been welldo cumented elsewhere, principally by Arber (1904a).Rather, we have onl y made comments on the identifi­cation of the species. especially where the species havebeen revised since the time of Arber's study. Table Ishows the distribution of the species in the 12 identi­fied localities yielding this macroflora, as well as th eauthorship of the species. Table 2 summarizes thebroad composition of the Cu lm Seam s macroflora interms of the major plant groups used below .

Lepidocarpales

There are very few Lepidocarpalean Iycoph yte remainsknown from the Culm Seams and all are fragments ofstems or of the rooting rhizophore Sigmar ia fi coides(Sternberg) Brongniart. Lepidodendron aculeatum isknown from Somers, Hisc ott Down (F ig. 2a) . Anotherspecimen has much longer and narrower leaf cushionsthan Lepidodendron aculeatum and leaf scars that areas long as broad , and Arber (1904a, plate 19, fig. 5)correctly identified it as L epidodendron fusiforme.

Lepidophloios acerosus is known from Somers (F ig.2b) and Pit Quarry (F ig. 2c). This species is character­ized by do wnward drooping leaf cushion s with leafscar s at their lower edges, and these are clearl y visibleon the Somers specimen. There is another specimenthat probably belo ngs to this species (Fig. 2f), fromGreenclilT. Arber (1904a, p. 308) referred it to Lepido­dendron obovatum Sternberg, but the cushions aremore similar to those of Lepidophloios acerosus, havinga similar central raised keel and very faint longitudinalstriations.

One small branching specimen from Pit Quarry hasslightly laterally elongated diamond-shaped leaf sca rs,

PALAEOBOTANY OF UPPER BIDEFORD FORMATION 271

e

Fig. 2. Lycophyte remains from the Culm Seams flora. (a) Lepidodendron aculeatum Sternberg, Somers, Hiscott (BMNH,V.I077I). (b, c, I) Lepidophloios acerosus Lindley & Hutton: (b) Somers, Hiscott (BMNH, V.lI076); (c) Pit Quarry (RAMM,Rogers ColI. No. 135); (I) Greencliff (MBND, 1994.7.1254). (d) Sigillaria tessellate Brongniart, Somers, Hiscott (BM, V.II 077).(e) Halonia sp., Pit Quarry (RAMM). (g) Sigillaria scutellata Brongniart, Broadstone Quarry (RAMM, Rogers Coli. No. 137).(h) 'tl.epidodendron dichotomum Sternberg, Pit Quarry (RAMM, Rogers Coll. No. 136). All x I, except for (e), which is x 0.5.

about 2 mm long and 3 mm broad (Fig. 2h). Theseare superficially similar to those of Lepidodendrondichotomum but we do not definitely refer it to thisspecies.

There are also a number of specimens of decorti­cated stems of the Knorria type from stems up toc. 15 em in diameter and others of the Halonia typewith many raised scars marking the position of former

272 C. J. CLEAL & B. A. THOMAS

branches (Fig. 2e). As these are merely preservationalforms of the lycophyte stems, they are not listedseparately in Table 1.

Sigillariostrobales

A very few specimens of sigillarian stems are knownthat are referable to the Sigillariostrobales. Twospecimens from Broadstone Quarry were identified byArber (1904a), in our view correctly, as Sigillariascutellata. One was figured by Arber (l904a, plate 19,fig. 4) and clearly shows the form of leaf scars that arecharacteristic of this species: they are slightly longerthan broad, shield-shaped, and on vertical, smooth­sided ribs. The second specimen (Fig. 2g) shows thesame smooth-sided ribs, although the scars are notquite as well preserved. Other similar but decorticatedstems are known from several other locations (Arber,1904a, p. 309) but are impossible to name.

One other specimen from Somers has leaf scars asbroad as long that are crowded together and lessthan a leaf scar length apart vertically (Fig. 2d). Theinterpretation by Arber (1904a) of this as S. tessellatais again acceptable.

Sphenophyllales

A number of leafy shoots were found with whorls ofwedge-shaped leaves, which undoubtedly belong toSphenophyllum (Fig. 3a and b). The leaves are no morethan 10 mm long and 7 mm wide, and have shallow butvery acute teeth on the distal margin. The only specieswith which they compare closely is Sphenophyllumcuneifolium, which is found widespread in early West­phalian macrofloras in Britain.

Calamostachyales

About one-third of the fossils examined in this studywere horsetail remains, mainly stems (Calamites) andfoliage (Annularia, Asterophyllites). The Calamitesstems usually have no branch scars and we assign themto Calamites suckowii, C. undulatus or C. cistii, depend­ing on the width and apical form of the ribs (seeCrookall (1969) and Cleal & Thomas (1994) for thecriteria for distinguishing these species). There is onlyone Calamites in the Rogers Collection with a branchscar, 6 mm in diameter (V.10773), from a cliff aboveBideford railway station. Arber (1904a, plate 19, fig. 2)figured this as C. ramosus Artis, and Crookall (1969)later re-identified it as C. paleaceus Stur, but withoutgiving a reason. There in fact seems little reason forseparating it from C. undulatus, with which it isassociated, and which has occasional branch scars.

The vegetative shoots belong to the species Astero­phyllites charaeformis, and are particularly abundantand well preserved at Greencliff (Fig. 3d). It is note­worthy that Hemingway (in Crookall, 1969, p. 711)suggested that A. charaeformis was probably the foli-

age of the same plant that has C. undulatus stems,which occurs so abundantly in the Culm Seams macro­flora. The shoots consist of at least three orders ofbranching, each node bearing a pair of subsidiaryshoots apparently in a planar arrangement. The ulti­mate branches consist of slender stems with smallleaves arranged in whorls spaced 1-2 mm apart. Thewhorls tend to be preserved side-on, which makes itdifficult to determine the shape of the individual leaves.However, rare examples of whorls preserved face­down or face-up to the bedding plane show that theyconsist of six or seven lanceolate leaves with a bluntapex, and are 1-2 mm long and about 0.5 mm wide.The leaves are attached at nearly right angles to thestem, run straight for about half their length, thencurve sharply so that the distal end is roughly parallelto the stem. One specimen shows the leaves bunchedtogether and probably represents part of a young shoot(Fig. 3c).

The angle of attachment of the ultimate branchesvaries from 40° to right angles, presumably dependingon its overall position in the shoot. At each node onthe penultimate branch, the pair of ultimate branchesoccurs together with four or five slender leaves, up to4 mm long. These leaves are angled proximally whenthey leave the stem but then arch broadly so at theirapex they tend to point towards the stem apex.

Associated with the shoots with small leaves areoccasional shoots with more elongate leaves, 5-10 mmlong and up to 1 mm wide (Fig. 3e). They wereidentified by Arber (l904a) as Annularia radiataBrongniart. However, they also resemble the leavesfound attached to the nodes of the penultimatebranches of A. charaeformis and it seems more likelythat they represent leaves from a proximal positionin one of these shoots. A similar situation has beenshown by Stur (1887, plate 14, fig. 13) in Asterophyl­lites roehlii Stur, which is now generally acceptedas a later synonym of A. charaeformis (e.g. Crookall,1969).

Two cones were found with A. charaeformis shootsat Somer. One is more or less complete, and is 19 mmlong and 4 mm wide (Fig. 3f); the other, which isincomplete, is 25 mm long and 7 mm wide (Fig. 3g).The bracts are virtually identical in shape and size tothe leaves, and there can be little doubt that theybelong to the same species. The bract whorls arespaced 1 mm apart in the smaller cone, 2 mm part inthe larger one. The bracts are attached to the coneaxis at about right angles, and then they curve so thatthey lie parallel to the axis in their distal part. Thedistal part of each bract just overlaps the bracts inthe next most distal whorl. The sporangia betweenthe bracts are mostly not well preserved but in atleast one place can be seen to be attached in aCalamostachys style (i.e. directly to the cone axis,rather than in the axil between the axis and a bract asin Palaeostachya).

Very similar cones to those found at Somer havebeen described by Zalessky (1907, plate 13, fig. 4) in

PALAE OBOTANY O F UPPER BIDE FORD FORMATION 273

Fig. 3. Sphenophytes and sphenophylls from the Culm Seams flora . (a, b) Sphenophyllum cuneifolium (Stern berg) Zeiller: (a)leafy shoot, Somers, Hiscott (BMN H, V.10717), x I; (b) leaf-whorl , Broadstone Quarry (BMN H, V. I0714), x 1. (e- g)Asterophyllites charaeformis (Sternberg) Goppert: (e) young shoo t with tufted leaves, Greencliff (BMN H, Y.11184); (d) typica lleaves, Gree ncliff (BMN H, V.l11 83); (e) leaves from proximal position in shoot, Greencliff (BMN H. VI I165); (0 cone. Somers,Hiscott (BM NH, V.10711); (g) slightly larger cone, Somers, Hiscott (BMNH. Y.I0712). All x 2.

274 C. J. CLEAL & B. A. THOMAS

association with A. charaeformis in the Donets Coal­field (Ukraine). However, A. charaeformis is normallyassociated with rather smaller cones, 2-3 mm wide(e.g. Stur, 1887, plate 14, fig. 13). Cones of morecomparable size to the Devon and Donets exampleswere described by Nemejc (1953, plate 9, figs 1-5)associated with Asterophyllites grandis (Sternberg)Geinitz. Nemejc stated, however, that the cones ofthese two species are very difficult to distinguish unlessthere is associated foliage. In our view, it is almostcertain that the Devon cones belonged to the associ­ated A. charaeformis leafy shoots. It means that eitherthere was significant variation in cone size within theplant (possibly ontogenetic) or there was more thanone biological species that had A. charaeformis typefoliage.

Arber (1904a) had identified the wider of the twoDevon cones as Calamostachys longifolia Weiss, whichaccording to Crookall (1969) is a synonym of thebetter-known Calamostachys ludwigii (Carruthers)Weiss. However, C. longifolia cones are rather morerobust and usually occur associated with Asterophyl­lites longifolius (Sternberg) Brongniart leafy shoots(Crookall, 1969), of which no sign has been found inthe Culm Seams macroflora.

Ferns

Ten pinna fragments from Broadstone Quarry andGreencliff are of a delicate ferny sphenopteroid withblunt-lobed pinnules up to 3 mm long and 1·5 mmwide (Fig. 4b). In one place there are sporangia,c. 250 urn in diameter, attached to what is presumablythe lower (abaxial) surface of the pinnules, mainlyaround the margins. Some of these specimens wereidentified by Arber (1904a) as Renaultia footneri(Marratt) Kidston but the pinnules are too slender andwidely spaced for that species (compare, for instance,the specimen in our Fig. 4b with that figured byBrousmiche, 1983, plate 37, fig. 1). Arber (1904a)compared others with Renaultia schatzlarensis (Stur)Danze but this species has more deeply incised pin­nules (e.g. Brousmiche, 1983, plates 44 and 45). Thenearest comparison is with fossils from the Langsettianof Scotland figured by Kidston (1923) as Sphenopterislanarkiana Kidston and Renaultia gracilis (Brongniart)Zeiller, which Brousmiche (1983) has transferred toR. crepinii.

Arber (1904a, plate 19, fig. 6) figured a drawing of asingle specimen from Robert's Quarry as Urnatopteristenella (Brongniart) Kidston. Danze (1956) acceptedthe identification, although he appears not to have seenthe specimen. Unfortunately, the drawing is rathermisleading in that it shows the specimen as being betterpreserved than it actually is. It is in fact probably anexample of Renaultia crepinii in which much of thelamina has been lost during fossilization, leaving onlythe veins. There was no other evidence of U. tenella inthe Culm Seams macroflora.

Although not mentioned in his 1904a paper, Arber(1907) listed Corynepteris sternbergii (Ettingshausen)Arber as occurring here. The three specimens, collectedby Rogers from Somers, Hiscott Down, are now in theNatural History Museum collections and consist ofpinnae some 100 mm long (Fig. 4a). The 2 mm longpinnules on these pinnae tend to be poorly preservedbut some are clearly asymmetrical with three or fourmarked teeth on the forward-facing side, with one cleftusually deeper than the others. Although the preserva­tion prevents a definite identification, the specimensshow some comparison with pinnae figured by Kidston(1923, plate 74, and plate 78, figs 1 and 2) as C. stern­bergii. Brousmiche (1983) regarded this species as alater taxonomic synonym of Corynepteris angustissima.

Lagenostomales

The most abundant of the lagenostomalean pterido­sperms in the Bideford Formation is Karinopterisacuta, which occurs in most of the known localities(Fig. 4c and d). Although represented by only smallfragments, they all have the characteristic slenderpinnuies with acute lobes. Arber (1904a, plate 20, fig.17) had identified this material as Mariopteris muricata(Schlotheim ex Sternberg) Zeiller, which also has pin­nules with acute lobes. The two species can be difficultto distinguish in the absence of larger specimens show­ing whether the fronds had the characteristic Karinop­teris or Mariopteris frond architecture (Boersma,1972). However, K. acuta tends to have more delicatepinnules that are constricted at the base, whereas M.muricata pinnules have a thicker limb and are broadlyfused to the rachis. The Bideford Formation specimensnearly all conform to the former type.

Two specimens have been seen of Lyginopterishoeninghausii, one of which is shown in Fig. 4e. Theyhave the small, rigid pinnules that are characteristic ofthis species, and compare closely with the specimensfigured by Pattiesky (1957, plate 3, figs 4-8). Arber(1904a) did not record this species from Devon, butit was presumably responsible for the record of'Neuropteris Haninghausi' by Rogers (1910, p. 551).

Three fragments from Broadstone Quarry have oval,vaulted pinnules that resemble Eusphenopteris, but it isimpossible to be certain to what species they wouldbelong. They were neither figured nor recorded byArber (l904a).

Medullosales

The most abundant pteridosperm remains in the CulmSeams flora are frond fragments of medullosaleanpteridosperms (Neuralethopteris, Alethopteris, Neurop­teris). There are two or possibly three species ofNeuralethopteris here, all of which Rogers and Arberassigned to N. schlehanii. In fact, N. schlehanii is ratheruncommon in the Devon macrofloras, occurring at

P A L A E OBOTA N Y O r U P P E R BIDE F ORD rOR MATI ON 275

Fig. 4. Fern s and Lagenostomales from the Culm Seams flora. (II) Corynepteris anguslissima (Sternberg) Nernejc, Somers,Hiscot t (BMNH, V.l 1141). (b) Renault ia crepinii (Stur) Zeiller, Broadstone Quarry (BMNH, V.10749). (c, d) Karinopteris acuta(Brongniart) Boersma, Robert' s Quarry (CSM, c, M.lI49a; d, M.ll46). (e) Lyginopteris hoeninghau sii (Brongniart) Gropp,shales at wharf on east side of River, Bideford (CSM, M.l 8l 7). All x 2.

276 C. J . C L E A L & B. A . THOMAS

only two or possibl y three localities. It can be recog­nized by its characteristic ovoid to sub triangular pin­nules , usually with a rounded apex , and arched, widelyforking veins (vein density of about 20 per em) (Fig.Sb).

Mo st of the Devon neuralethopterids differ fromtrue N. schlehanii in having pinnules with a more acuteapex (Fig . 5a). They also mostly have lateral veins thatarch sharp ly near the midvein and then extend at nearto right angles to the pinnule margin for the rest oftheir length, producing a very dense venation (SO-60veins per em on pinnule margin). This type of venationis characteristic of N. rectinervis. Only one specimenhas been seen that has the more broadly arched veinsnormally associated with N. jongmansii. It occurs at alocality that also has yielded abundant N. rectinervisand it is possible that it simply represents an unusualfragment of that species.

True alethopterids are also abundant in these Devonmacrofloras. They tend to have similar-shaped pin­nules to N. rectinervis except that they are broadlyattached to the rachis. The problem in trying todistinguish them is epitomized by a specimen figuredby Arber (l904a, plate 20, fig. IS), which has con­stricted, neuralethopteroid pinnules attached to thepenultimate rachis , and broadly attached, alethop­teroid pinnules on the ultimate rachi ses. In this case ,it is likely that the specimen belon gs to N. rectinervis,as the pinnules on the ultimate rachi s are very shortand do not have a fully developed morphology. Thespecimen figured by Arber (1904a , plate 20, fig. 11)as A. lonchitica clearly has the con stricted pinnulesof N. rectinervis. There are , however, cases of truealethopterids that sometimes develop slightly con­stricted pinnul es and so some care must be exercised.

Arber (1904a, plate 20, fig. II) figured anotherspecimen as A. lonchitica, which looks more compat­ible with that species (compare with specimens figuredby Bertr and (1932, plates 31-34) as A. lonchitifolia; seeZodrow & Cleal (1998) for discussi on on the taxo­nomic problems surrounding these species). Arber(I904a, plate 20, fig. 14) figured a third small specimenas Alethopteris serlii (Brongniart) Goppert, but thevenation is not as flexuous as seen in that species(Zodrow & CleaI, 1998). The pinnules are a littlebroade r than is typical for A. lonchit ica, but thisprobably represents natural variation within thespecies.

Alethopteris lonchitica most typically occurs in theupper Duckmantian to Westphalian D . In Langsettianmacrofloras, Alethopteris urophylla (Brongniart) Preslis the more normally found repre sent ative of thisgroup of alethopterids. The two species can be difficultto distinguish, the most reliable character being themorphology of the pinnules towards the pinna termi­nal. In A. lonchitica such pinnules tend to be barrel­shaped and more or less symmetrical about themidvein, whereas in A. urophylla they tend to be sub­lan ceolate and more asymmetrical, with a significantba sal con striction on the acroscopi c side (Zodrow &

Cleal , 1998). The pinnae from the Bideford Formationhave barrel-shaped pinnules near the terminal and thusare similar to A. lonchitica. However, as the y are allsmall fragmen ts and are from a rather low stratigraphi­cal hori zon for that species, they are placed there witha 'cf.' .

A single specimen in the Sedgwick Mu seum(M.4S46) has very slender alethopterid pinnules resem­bling those of A. decurrens . It also shows a smalljuvenile circinately coiled leaf Ccrozier' ) similar to thatfound at other localities yielding A. decurrens.

Much rarer in the Bideford Formation are pinnafragments with more delicate pinnules that have a lesspronounced mid vein . These rarel y show the venationclearly, but the general appearance of the pinnulesindicates that they almost certainly belong to Neurop­teris obliqua. Figure Sd shows a specimen with linguae­form pinnules, typical of those found in the Devonmacroflora, but there are also rarer examples withsmall, broadly attached pinnules Cpecopteroid') fromhigh in the frond, and much larger pinnules with aconstricted base (known as forma impar) from thebasal pa rt of the frond. The presence of pecopteroidpinnules and the flexuous vena tion ind icate that theydo not belong to Neu ropteris heterophylla Brongniart,which can otherwise be very similar to N. obliqua. Theonly other neuropterid species of this general gro upthat occurs in strata of this age is N. ghaye i Stockmansand Williere , but this has much den ser veins and larger,more opulent pinnules than seen in the BidefordFormation specimens.

Finally, mention should be made of a large ovule orseed, 17 mm long and 9 mm wide. It shows three clear,longitudinal ribs and undoubtedly belongs to Trigon o­carpus. It probably was attached to the same plant thathas either the neuralethopterid or alethopterid fronds.

?Cycadales

Arber (1904a, p. 307, plate 20, fig. 16) described asingle fragmentary specimen from Alverdi scott andtentatively referred it to the genus Megal opteris, whichhad previously been known only from North America.The midvein is thick, and the secondary veins are denseand oblique (Fig . 5e). The veins frequently dichoto­mize and occasion ally anastomose. Leary (1980) re­exam ined the specimen and referred it to Lesleya ba sedon its vena tion. This would make it a fragment of anentire simple leaf rather than one of a compo und leafwith th ree or more lobe s. Leary compared it closelywith the North America Lesleya cheimarosa Lear y &Pfefferk orn, bu t restricted his identification to a poss­ible 'cf.' because of the specimen's fragmenta ry nature.The affinities of Lesleya have been a matter of somedebate, although it is normally accepted as gymnosper­mous. Rem y & Remy (1978) tentatively suggestedcycadalean affinities and this idea was strengthenedby the discovery of attached ovules (Lear y, 1990).Cuticle evidence doe s not contradict a cycadalean

PALAEOBOTANY OF UPPER BIDEFORD FORMATION 277

Fig. 5. Medullosales and ?Cycadales from the Culm Seams flora. (a) Neuralethopteris rectinervis (Kidston) Laveine, PillheadCopse (BMNH, V.21552). (b) Neuralethopteris schlehanii (Stur) Laveine, Greencliff (BMNH, V.1l155). (c) Alethopterislonchitica Sternberg, Greencliff (CSM, M.l850b). (d) Neuropteris obliqua (Brongniart) Zeiller, Greencliff (BMNH, V.11194). (e)Lesleya cf. cheimarosa Leary & Pfefferkorn, Alverdiscott (MBND 929). All x 2.

278 C. J. CLEAL & B. A. THOMAS

affinity for the morphogenus (Simunek, 1996). How­ever, whether or not the fragmentary Devon specimenis also cycadalean is impossible at this stage to say.

Cordaitanthales

Although it is generally thought that cordaite remainsare relatively robust and can survive significant trans­portation, they are remarkably rare in the Culm Seamsflora. The only unequivocal specimen that we haveseen is a piece of stem pith-cast (Artisia sp.) fromBroadstone Quarry (BMNH V.I0708). The two frag­ments from Pit Quarry, in the Rogers Collection inthe Royal Albert Memorial Museum, are too poorlypreserved to identify with certainty.

Plant petrifactions

One small piece of wood, from the beach at Greencliff,shows internal anatomy of the gymnosperm type withtracheids (specimen in the Royal Albert MemorialMuseum). It is probably a fragment of cordaite stem.

Also, among the banks of rolled pebbles along thecoast south of Hartland Point, occasionally can befound some that contain goniatites and other marineorganisms. An even smaller number of pebbles arefilled with small black water-worn fragments thatArber (1911a, plate 22, fig. 2) thought to be plants.More recently, they have been identified as fish copro­lites (P. Davis, pers. comm. 2003).

5. Discussion

Age of the Culm Seams macroftora

Since De la Beche (1834), most workers have acceptedthat the beds now known as the Bideford Formationare Late Carboniferous in age. As pointed out byArber (1904a, p. 311), where an older age was sug­gested, it was based on reports of a Late Devonianmacroflora from the Barnstaple area. However, thismacroflora came from the Baggy Formation (Arber &Goode, 1915), which is stratigraphically much lowerthan the Bideford Formation.

Ussher (1892, 1901) suggested that the beds nowknown as the Bideford Formation might correlate withthe Millstone Grit of Wales and the rest of England(Namurian Series). However, this was not based onbiostratigraphical evidence, and Ussher himself wasnot adamant on the point. Simpson (1933) describednon-marine bivalves from just below the CornboroughSandstone, which Calver & Eagar (in Edmonds et al.,1979) compared with the Carbonicola proxima­extenuata faunas in the upper Carbonicola lenisulcataZone (lower Langsettian). Subsequently, however,Eagar & Xu Li (1993) compared them with bivalves ofthe Carbonicola torus faunas from the lower Carboni­cola communis Zone (middle Langsettian). This iscompatible with the discovery at the base of Cycle 6

(sensu De Raaf et al., 1965) of marine faunas similar tothose found in the lower Langsettian Amaliae MarineBand of the British coalfields.

Based on the macrofloras, Arber (1904a, b) sug­gested a rather younger age, equating it with what hecalled the Middle Coal Measures (broadly equivalentto the Duckmantian and lower Bolsovian Stages in thechronostratigraphy used today). This was based onthe presence of Asterophyllites charaeformis, Neur­alethopteris schlehanii and Alethopteris serlii, none ofwhich he believed occur in the 'Lower Coal Measures'(= Langsettian Stage). The report by Arber (1907) ofLaveineopteris tenuifolia (Sternberg) Cleal et al. fromthese macrofloras appeared to corroborate this view.Hofmann (1992) agreed with Arber's conclusions, re­portedly based partly on newly collected specimens,although whether these included any examples of thecritical taxa was not stated.

Crookall (1930) disagreed with Arber's interpret­ation, and correlated the strata yielding the CulmSeams macroflora with the 'Lower Coal Measures'. Hedismissed A. charaeformis as being a poorly definedspecies of little stratigraphical significance. Whether ornot one accepts this view, the species is now knownfrom Langsettian-aged macrofloras (e.g. Crookall,1969; Josten, 1991; Kotasowa & Migier, 1995).Crookall also correctly pointed out that N. schlehanii isessentially a 'Lower Coal Measures' (i.e. Langsettian)species. Arber's record of A. serlii from this macro­flora is undoubtedly a misidentification of Alethopterislonchitica, a species that ranges throughout the West­phalian Series. In the present study, we have beenunable to verify the record of L. tenuifolia. Of thespecies mentioned by Hofmann (1992) as indicatinga Duckmantian age, Neuropteris obliqua and Sigillariascutellata are also both well known from Langsettian­aged macrofloras (Laveine, 1967; Josten, 1991;Kotasowa & Migier, 1995).

In our view, the macrofloral evidence unequivocallysupports the faunal evidence for a Langsettian age forthe upper Bideford Formation, and almost certainly anearly Langsettian age. As correctly pointed out byCrookall (1930), this is clearly indicated by the pres­ence of Neuralethopteris schlehanii and Lyginopterishoeninghausii. Neuralethopteris rectinervis is most typi­cally a middle Langsettian species (Laveine, 1967;Cleal & Thomas, 1994) although it can range down tothe lower Langsettian (Josten, 1991). The absence ofLaveineopteris tenuifolia or L. loshii (Brongniart) Clealet al., which are normally extremely common in lateLangsettian to Bolsovian macrofloras, would tend toindicate an early Langsettian age.

Palaeoecological interpretation

Table 2 gives the percentages of each of the maingroups of plant fossils in the Culm Seams macroflorain the collections that we have studied (see Introduc­tion). For comparison, Table 2 also includes the data

PALAEOBOTANY OF UPPER BIDEFORD FORMATION 279

of Davies (1929) from two Langsettian horizons inSouth Wales. Scott (1977) criticized Davies's data asbeing subjective, making it impossible to compare hisfigures from different stratigraphical levels and, byimplication, with data from elsewhere. Scott's com­ments are undoubtedly valid if we were to try totranslate Davies's figures directly into the compositionof the parent vegetation. However, if we think ofDavies's figures as being more comparable to palyno­logical spectra, to be interpreted in the context of thetaphonomic filter through which they have passed aswell as the composition of the original vegetation, thenthey should not be so readily dismissed. Each ofDavies's samples was, after all, collected in essentiallythe same way and was derived from similar roof­shale facies (i.e. clastic deposits formed during theinitial flooding events that destroyed the lycophyte­dominated forest) (North, 1935). Furthermore,Davies's sampling methods are comparable with theway that our figures for the Culm Seams macroflorawere compiled. Davies counted every specimen that hefound, assigning it to a particular morphogenus andmajor plant group. The plant fossils in the BidefordFormation are sufficiently scarce that they representpractically every specimen that was ever found there.

Table 2 shows that the composition of the threemacrofloras is broadly comparable. The figures for theMedullosales are somewhat higher for the GellidegSeam macroflora, compensated for in the Culm Seamsmacroflora by slightly higher values for the Spheno­phyllales, ferns and Lagenostomales, and in the FiveFeet Seam macroflora by increased number ofCordaitanthales. Nevertheless, the overall balance ofthe three fossil macrofloras is comparable, with theCalamostachyales and Medullosales each making upabout one-third of the total number of specimens. Thissuggests that the Culm Seams macroflora was pre­served in essentially the same way as the two Welshmacrofloras.

This might seem surprising as the Culm Seamsmacroflora is preserved in a more basinal settingcompared with the Welsh macrofloras. One possibilityis that the Culm Seams macroflora was a transportedsample of the same mainly riparian vegetation repre­sented by the Welsh macrofloras, but this seems un­likely as the sorting effects of the transportation wouldbe expected to be more striking (compare with modernexamples discussed by Spicer (1980, 1981)).

Alternatively, the Culm Seams might represent atemporary southwards transgression of a delta, bring­ing the coal-swamp environment into north Devon. Itcannot have been a full development of this habitat, asthere is little evidence of the lycophyte-dominatedwetlands that dominated the Welsh Basin. Hofmann(1992) reported some evidence of palaeosols in theupper Bideford Formation, but they were mostly pro­duced by Calamostachyalean vegetation. However,the distal reaches of the type of 'bird-foot' deltas thathave been postulated by the source of the Bideford

Formation sediments may have provided similar sub­strates to the levees and sandbanks in the South Walesbasin, and would have supported a similar type ofvegetation, probably dominated by Medullosales andCalamostachyales.

It might be tempting to correlate this southwardsexpansion of the delta with the southwards influx ofsands into the South Wales basin, resulting in theFarewell Rock Formation (Bluck, 1961). Although theSouth Wales and Culm Basins were partly separatedfrom each other by an area of net erosion at this time(Hartley, 1993), it is possible to envisage a common,perhaps tectonic, cause to both the influx of theFarewell Rock sediment and the expansion of theCulm delta. The sediment slumping, probably ofseismic origin, seen in the basal Westphalian sequenceof Pembrokeshire (Kuenen, 1949) would also supportthis model. However, the Farewell Rock sandstonesand the Pembrokeshire slumping represent a timeequivalent to the early C. lenisulcata Zone. This pre­dates the formation of the Culm Seams, which formedsome time after the Amaliae Marine Band transgres­sion, and probably represents a time equivalent to theearly C. communis Zone. In the contemporaneous partof the South Wales succession, conditions seem to havebeen relatively quiescent, with abundant ironstonesbeing formed in lacustrine conditions (Evans et al.,2003). It would seem, therefore, that the temporarysouthwards transgression of the Culm delta in earlyLangsettian times was not a response to regionalchanges, but was an essentially local event.

The only significant problem with this interpretationof the Culm Seams flora is the presence of Lesleya.This possible early cycad normally occurred in extra­basinal, 'upland' settings (Leary & Pfefferkorn, 1977;Leary, 1980, 1990). It is possible that such plants werealso adapted to coastal habitats, but then why do wenot see more of it in macrofloras preserved in lowlandsettings? As only one specimen is known from theCulm Seams flora, it may simply be the remains of aleaf that had floated down-river from an extra-basinalarea. However, this still does not explain why we donot see similar drifted leaves in places such as SouthWales, as similar if not the same rivers also fed thedeltas here. At present, we have no explanation for thepresence of this species here.

ACKNOWLEDGEMENTS

We are grateful to the Natural History Museum(London), the Sedgwick Museum (Cambridge), theRoyal Albert Memorial Museum (Exeter) and theMuseum of Barnstaple and North Devon for allowingaccess to their collections of the Culm Seams flora andfor the loan of specimens. Thanks go to Bill Chalonerand Cedric Shute for constructive reviews of the manu­script, and to Richard Howarth for advice on analys­ing the statistical data. National Grid coordinates areused with kind permission of the Ordnance Survey.

280 C. J. CLEAL & B. A. THOMAS

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Manuscript received 27 January 2004; revised typescript accepted 26 May 2004