Cleal, C.J. & Thomas, B.A. 1995. Palaeozoic Palaeobotany of Great Britain. Geological Conservation Review Series No. 9. JNCC, Peterborough, ISBN 0 412 61090 6.
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R.M. Bateman, G. W. Rothwell and C.J. Cleal
Oxroad Bay has yielded a wide variety of plant petrifactions that give a remarkable insight into Early Carboniferous plants, particularly among the lycopsids, sphenopsids and pteridosperms. They are yielding the first rigorous whole-plant reconstructions for the Early Carboniferous, as well as establishing details of the plant communities and their environments. This is clearly a palaeobotanical site of international importance
This remarkable palaeobotanical locality in Tournaisian volcaniclastic strata on the coast near North Berwick
The Oxroad Bay sequence is about 38 metres thick and belongs to the North Berwick Member of the Garleton Hills Volcanic Formation (
All but the most recent species list published for this locality (Barnard and Long, 1973; Long, 1984; Scott et al., 1984) included taxa identified from loose blocks, which were transported into Oxroad Bay from the nearby Castleton Bay and Gin Head plant beds (described by Scott and Galtier, 1988; see also
Lycopsida:
Oxroadia gracilis Alvin
O. conferta Bateman
Equisetopsida:
Protocalamites longii Bateman
Protocalamostachys farringtonii Bateman
Filicopsida:
Cladoxylon cf. waltonii Long
Stauropteris cf. berwickensis Long
Gymnosperms:
Amyelon bovius Barnard
Tetrastichia bupatides Gordon
Bilignea cf. solida D.H. Scott
Eristophyton beinertianum Zalessky
Buteoxylon gordonianum Barnard and Long
Triradioxylon primaevum Barnard and Long
Oxroadopteris parvus Long
Calathopteris heterophylla Long
aff. Tristichia ovensii Long
Lyginorachis waltonii Calder
Lyginorachis sp. nov.
Calathospermum fimbriatum Barnard
Pullaritheca longii Rothwell and Wight
Salpingostoma dasu Gordon
Dolichosperma cf. sexangulatum Long
Tantallosperma setigera Barnard and Long
Hydrasperma tenuis Long
cf. Spbaerostoma sp. nov.
Stamnostoma oliveri Rothwell and Scott
cf. Eurystoma burnense Long
Eosperma oxroadense Barnard
cf. Eccroustosperma langtonense Long
Deltasperma fouldenense Long
cf. Telangium sp.
cf. Melissiotheca sp.
Two species of lycopsid megaspore have also been described (Alvin, 1966). Bateman (1988) recognized at least eight other (probably new) species of petrifactions from Oxroad Bay that have yet to be named. He also described, but did not name, several adpression taxa, including a lycopsid (stem), a probable filicopsid (stem and foliar organ) and pteridosperms (one stem, two petioles, four foliar organs, four ovules and one pollen-organ).
According to Bateman and Scott (1990) the vegetation was growing in a hostile, volcanic environment in which the substrate was unstable. It resulted in a patchwork of subcommunities that consequently yielded an unusually wide range of plants.
Oxroadia axes are distinguished from most pene-contemporaneous lycopsids by their consistently small size and prominent protoxylem strands
Alvin's (1965) original description of Oxroadia gracilis Alvin was based on a loose block containing a strobilus, seven megaspores and about 12 axes. Additional loose-block material from both here and Berwickshire was described by Long (1971, 1986) to provide details of the leaf bases and rhizomorphs. A further several hundred axes and 43 strobilus fragments from Oxroad Bay were used by Bateman (1992) to reconstruct the bizarre 'pseudoherbaceous' gross morphology of the plant, which possessed a very short stem, strongly and basally concentrated secondary thickening and wide-angle axial dichotomies that suggest a scrambling mode of growth. The leaves were ligulate but lacked sophisticated abscission zones (details of their anatomy are shown in
Oxroadia conferta has a similar rhizomorph, but the vegetative axes exhibit fewer protoxylem strands and more closely-spaced branching (Bateman, 1988, 1992). The single known strobi-lus was shorter (c. 30 mm long) and its sporangia contained more megaspores (?16) of smaller diameter (
Stems of a third lycopsid were described by Barnard (1960a) as cf. Paurodendron; small fragments occur sparsely throughout the sequence here (Bateman and Rothwell, 1990). The xylem lacks wood and the stele is deeply incised into 7–10 rounded lobes. It thus resembles Asteroxylon (from the Lower Devonian Rhynie Chert — see Chapter .4) and Leclercqia (Banks, 1960; Grierson and Bonamo, 1979), rather than supporting Barnard's comparison with Paurodendron.
Petrified equisetopsid remains were first reported here by Bateman (1988). Vegetative axes were named Protocalamites longii and reproductive structures Protocalamostachys farringtonii (Bateman, 1991). They represent a single whole-plant species with short, slender stems. The tuberoid stem bases generated dense root-balls and were interconnected by rhizomes. The woody stems subtended at least one order of branching and bore repeatedly dichotomizing leaves. Strobili were probably small, with three vertical rows of paired sporangiophores. The sporangia were small and the plant heterosporous, this being the earliest evidence of heterospory amongst the sphenophytes
Two filicopsid species have been reported from here (Bateman, 1988; Bateman and Rothwell, 1990). Cladoxylon cf. waltonii Long (1968b) is represented by a few polystelic stem bases that are greatly expanded and exhibit weakly-developed secondary xylem. Other stem fragments are smaller than the largest petiole-like primary branches, which are adaxially concave and appear to undergo initial equal dichotomies. The more distal branches exhibit typical Hierogramma-typeanatomy and bear pinnae with clepsydropsid vascular traces. Possible stem and foliar correlatives occur in the adpression assemblages, and resemble those of Cladoxylon scoparium (Leclercq, 1970) and Pseudosporochnus nodosus (Leclercq and Banks, 1962). These cladoxylalean fragments re-assemble to form a short, stocky, infrequently branched, upright plant.
Several rachises with a characteristic four-lobed stele have been identified as Stauropteris berwickensis Long (1966). Also tentatively placed in this species was a single fragment of a larger axis with a five-lobed stele. No filicopsid reproductive organs have been found at Oxroad Bay.
There has been much speculation concerning possible whole-plant restorations of pteridosperms from the assemblages of isolated, petrified organs at Oxroad Bay. A hypothetical Calathopteris–Calathospermum–Salpingostoma plant postulated by Long (1976a) is in danger of becoming enshrined in the literature as a genuine reconstruction (cf. Retallack and Ditcher, 1988). Other postulated vegetative-reproductive (stem-ovule) correlations were Buteoxylon with Tantallosperma (Barnard and Long, 1973), and Tetrastichia with Eosperma (Barnard, 1959, 1960a). Unfortunately, these hypotheses are based only on histological similarity and comparisons with preconceived bauplans. The abundance of pteridosperm organs of all types at Oxroad Bay permits an enormous number of theoretical combinations.
More recent studies (Bateman, 1988; Bateman and Rothwell, 1990) have yielded much information on how to interpret associations of organs from different exposures and different horizons within exposures, thus reducing the number of likely combinations of organs. Individual plants are being reconstructed by organic connection, though the most difficult correlation (megasporan-giate and microsporangiate reproductive organs to stem via petiole) has yet to be achieved unequivocally for any Lower Carboniferous pteri-dosperm. However, sufficient evidence has accumulated to suggest that five or six species of whole plant were present. This is more consistent with the numbers of petioles (4), pollen-organs (4) and ovulate cupules (4) currently recognized at Oxroad Bay. The reported number of taxa of stem (9) has probably been inflated by the use of different names for different ontogenetic stages of the same species (Long, 1984; Wight, 1987; Bateman and Rothwell, 1990). Despite many previous assertions to the contrary, all of these reconstructed plants will probably prove to be lagenostomalean. The large number of recorded isolated ovules (10) probably reflects greater dispersal potential of disseminules (Bateman and Rothwell, 1990; Bateman and Scott, 1990; Bateman in Cleal, 1991). In the absence of an unequivocal correlation between them, the following discussion will treat each of the pteridosperm organ-types separately.
Such evidence has stimulated ideas concerning early cupule organization and structure (Matten and Lacey, 1981), the reproductive biology of early pteridosperm ovules (Andrews, 1940; Walton, 1954; Smith, 1964), and the ontogeny of these ovules (Bateman, 1988; Rothwell and Wight, 1989). An important result of this work has been the development of the 'hydrasperman reproduction' concept, in which pollination was facilitated by an elongate extension of the pollen chamber wall (the lagenostome or salpinx) rather than by a micropyle formed by the integument, and the presence of a specialized structure (the central column) that sealed the pollen chamber after fertilization had occurred (Rothwell, 1986). This work also prompted the controversial ideas on angiosperm origins, put forward by Long (1966, 1975, 1977b, 1985; see discussion on the Whiteadder, above).
Barnard (1962) attributed certain gymnosperm roots to the form-species Amyelon bovius Barnard. The diversity of pteridosperms at the locality obscures their affinities.
Bateman and Rothwell (1990) list nine pteridosperm stem species, which probably represent five or six whole-plant species. The first four are clearly distinct:
Tetrastichia bupatides Gordon (1938) has a distinctive, cruciform protostele (
Bilignea cf. solida Scott, consisting of stems up to 70 mm in diameter with a distinctive pith of short tracheids, was first reported by Long (in Scott et al., 1984). Attached lyginopterid petioles reported by Bateman (1988), confirm the lagenostomalean affinities of this pteridosperm tree.
The assignment of Eristophyton beinertianum Zalessky to the lagenostomalean pteridosperms similarly resulted from studies of Oxroad Bay material. Originally described as calamopityalean by Gordon (1941), it was first recognized as Eristophyton by Barnard (1960a). Long (1987) demonstrated that the stems reach 100 mm in diameter and possess a parenchymatous pith that extends into the lateral branches. Associated Lyginorachis waltonii petioles are large, with several stelar corrugations that separate near the base; Long (1987) suggested correlation with the adpression frond genus Diplothmema.
Stem I of Bateman and Rothwell (1990) resembles a similarly unnamed pteridosperm axis with three sympodia recovered from Visean strata at Kingswood End by Scott et al. (1986). A single short length of narrow fusainized axis exhibits parenchymatous pith in intimate association with five mesarch sympodia. They are surrounded by a secondary xylem cylinder of narrow tracheids interspersed with abundant small, uniseriate rays. In the absence of attached petioles, its familial affinities are unclear.
It is uncertain whether Buteoxylon gordonianum Barnard and Long represents an additional whole-plant species or should be synonymized with the taxa listed below. The single specimen described from here by Barnard and Long (1973, 1975) was the apical portion of a small stem with a mixed pith and numerous attached petioles. The petioles possessed single protoxylem strands in the centre of both arms of the characteristic Y-shaped stele, and apparently did not dichotomize. A much larger axis of this species with well developed wood was reported by Bateman and Rothwell (1990). Barnard and Long (1975) used Buteoxylon and Triradioxylon to erect a new family, the Buteoxylonaceae, which they suggested might belong to the progymnosperms. The discovery of protostelic, triradiate stems associated with pteridosperm ovules (Rothwell and Erwin, 1987) renders this less probable.
Although originally described as separate genera, the four remaining pteridosperm stem taxa probably belong to only one whole-plant species (Bateman and Rothwell, 1990; Rothwell and Scott, 1992b). Although not yet fully reconstructed, it has been tentatively interpreted as scrambling or lianascent.
Only four isolated, pteridospermous foliar organs have been named from Oxroad Bay (Bateman and Rothwell, 1990):
Compressed petioles and frond segments have not been successfully correlated with the more informative petrified material.
Two types of ovulate cupule found at Oxroad Bay have generated more discussion than any other species present. Calathospermum fimbriatum Barnard (1960b) is a large, apparently solitary and bilaterally symmetrical cupule that is dissected into many lobes and probably contained numerous ovules. It was interpreted by Barnard (1960b) and Smith (1964a) as a modified frond borne on an apparently unbranched but otherwise Lyginorachis-like petiole. Long (1975) interpreted Calathospermum as a 'megacupule', resulting from the fusion of a pair of radially symmetrical cupules. He also argued that it is a possible precursor of the angiosperm carpel (Long, 1966, 1975, 1977b, 1985). Matten and Lacey (1981) interpreted branching of the cupule lobes distal to the basal-most dichotomy as pseudomonopodial. In situ ovules reported by Barnard (1960a, b) and Long (1975) resemble Salpingostoma dasu Gordon in pattern of organization but are much smaller; they may have been abortive. However, Bateman and Rothwell (1990) report in situ ovules more closely resembling Tantallosperma setigera Barnard and Long.
A.G. Long (pers. comm. 1988) argued that specimens with unusually long axes that appear to lack pinnae may constitute a second species of Calathospermum here. Study of similar compressed cupules from Oxroad Bay may help resolve this question.
Pullaritheca longii Rothwell and Wight (1989) represents cupule aggregates containing Hydrasperma tenuis Long ovules (
Long (1977a, b, 1979b) recognized two modes of arrangement of the cupules (paired 'hemicupules' and unpaired 'megacupules') that were both considered to represent an entire mega-sporophyll, and necessitated revision of his 'cupule-carper theory. However, Rothwell and Wight (1989) and Bateman and Rothwell (1990) reported a gradation between these two extremes. Comparison of Pullaritheca with adpressions assigned to Sphenopteris bifida Lindley and Hutton led Long (1979b) to suggest that both bore cupules in similar fashion, as aggregates attached to the median axes of trichotomous petioles. Indeed, clusters of up to eight Pullaritheca-like cupules occur in adpression assemblages at Oxroad Bay (Bateman, 1988). One petrified Pullaritheca cupule found by Long (1977a) contained both ovules and deformed sporangia. This suggests a developmental anomaly reflecting the homologous architecture of pollen-organ aggregates that were probably borne on other trifurcating petioles of the same plant (Bateman, 1988; Bateman and Rothwell, 1990; Bateman and DiMichele, in press).
An open, frequently and irregularly dichotomous, hirsute cupule contains Dolichosperma cf. sexangulatum Long ovules (Bateman, 1988). Its branching pattern resembles that of the cupules bearing the ovule Eurystoma burnense (Long, 1960b, 1965).
A similarly open, but pseudomonopodially branched, cupule bears Stamnostoma oliveri ovules in aggregates of up to four (Bateman and Rothwell, 1990; Rothwell and Scott, 1992a).
Bateman (1988) and Bateman and Rothwell (1990) list ten species of isolated petrified ovule. All appear to be distinct, though an ontogenetic relationship between Tantallosperma and Dolichosperma is credible. Some potential correlatives occur in the adpression assemblages, notably the ovule megaspore studied by Pettitt (1969), which probably represents Eosperma oxroadense Barnard (Bateman, 1988).
Salpingostoma dasu Gordon (1941) is the largest known Lower Carboniferous ovule, up to 50 mm long and 6 mm wide (Bateman and Rothwell, 1990). It usually had six integumentary vascular bundles extending into lobes that project well beyond the tall, narrow lagenostome. The integument also bore large, antapically concentrated trichomes. Gordon (1941) erroneously endowed Salpingostoma with an atypical pollen-receiving apparatus (Rothwell, 1986).
Dolichosperma cf. sexangulatum was first reported from Oxroad Bay by Bateman (1988). It typically had six integumentary bundles and lobes that extend well beyond the lagenostome and was covered with evenly distributed, coarse tri-chomes. Very similar but rather smaller is Tantallosperma setigera, which possessed four (rarely five or six) vascular bundles, integumentary lobes that extend well beyond the lagenostome, and an even covering of coarse trichomes. Since the two are essentially distinguished only on size and number of integumentary lobes, they may conceivably be ontogenetic stages of the same species.
Hydrasperma tenuis Long was reported by Long (1977a, b, 1979b). It had 8–12 integumentary lobes that extended only slightly beyond the lagenostome, which is the widest part of the ovule. Dispersed H. tenuis ovules exhibit a wide range of ontogenetic variation at Oxroad Bay, and were erroneously segregated as H. longii by Matten et al (1980).
Cf. Sphaerostoma sp. nov. (Bateman, 1988; Bateman and Rothwell, 1990) is characterized by an integument with eight vascular bundles, but no lobes. It differs from S. ovale Benson (1914; see also Long, 1961b) in its thicker, denser integument and sparser, but coarser, trichomes.
The 'cf. Sphaerostoma sp.' of Barnard (1960a) differs from bona fide Sphaerostoma in having a morphologically distinct pollen chamber, and an integument that forms an open collar surrounding the lagenostome. It was therefore assigned to Stamnostoma huttonense Long by Barnard and Long (1973; see also Rothwell, 1986). However, it differs from S. huttonense Long in having a smaller length:breadth ratio and papillae on the integument. Consequently, it has been referred to a new species, Stamnostoma oliveri Rothwell and Scott (1992a).
Cf. Eurystoma burnense Long has been reported by Bateman (1988). Elsewhere, Long (1966, 1969) has recognized both pre-prothallial and prothallial ontogenetic phases. Its distinctive, triangular transverse section reflects its three vascular bundles and associated wide lateral keels, which terminate as short lobes projecting slightly beyond the lagenostome.
Eosperma oxroadense Barnard (1959) has only two integumentary vascular bundles and lacks integumentary lobes; short spiny trichomes occur on some specimens. Pettitt (1969) demonstrated attachment of three abortive megaspores to a compressed functional megaspore that probably belonged to Eosperma.
Bateman (1988) has demonstrated the presence of cf. Eccroustosperma langtonense Long from in situ strata, the report by Long (1987) having been based on a loose block that was probably transported from nearby Castleton Bay. It has two vascular traces terminating in fairly short integu-mental lobes that do not extend beyond the lagenostome. Bateman (1988) observed that Oxroad Bay specimens are more strongly curved than those from elsewhere (cf. Long, 1961b, 1975), and may represent the pre-prothallial ontogenetic stage of Camptosperma berniciense Long (1961a).
Deltasperma fouldenense Long (1961a) was first recognized as fusainized material macerated by Bateman (1988) (
Three of the four pollen-organs listed by Bateman (1988) and Bateman and Rothwell (1990) appear to be synangiate: cf. Telangium sp., cf. Melissiotheca sp. and 'pollen-organ C'. The Telangium-like synangia terminate repeatedly dichotomous axes. They typically comprise eight, basally-fused, bilaterally-arranged synangia containing finely tuberculate pre-pollen. They therefore share characters with several Telangium-like form-genera (e.g. Eggert and Taylor, 1971; Long, 1979a; Millay and Taylor, 1979; Meyer-Berthaud and Galtier, 1986a).
The pedicellate synangia of cf. Melissiotheca sp. are shallowly divided into at least two lobes, each consisting of 30–100 basally-fused sporangia that are embedded in a parenchymatous cushion, and contain rugulate pre-pollen (Bateman, 1988; Bateman and Rothwell, 1990). The sporangia are smaller and exhibit less profound lateral fusion than those of Melissiotheca longiana Meyer-Berthaud (1986). Numerous compressed synangia from Oxroad Bay are difficult to apportion between cf. Telangium sp. and cf. Melissiotheca sp. (Bateman, 1988).
Pollen-organ C of Bateman and Rothwell (1990) is a large, hirsute, oviform organ that superficially resembles the ovulate cupule Calathospermum fimbriatum Barnard in size and branching pattern. Large sporangia form asymmetrical synangia and enclose dimorphic pre-pollen; the larger pre-pollen morph is considered fertile and the smaller abortive (Bateman, 1988). Similar pre-pollen has frequently been found in the lagenostome of Salpingostoma dasu Gordon (Long, 1975; Bateman, 1988). Pollen-organ C was incorrectly referred to cf. Telangium by Barnard (1960a) and subsequent workers, and should not be confused with the cf. Telangium described above.
Pollen-organ D of Bateman and Rothwell (1990) is a single terete branching axis terminated by a small, thick-walled sporangium. It is associated with abundant foliar organs showing similar histology. Spores found in the sporangium have sparse, blunt-tipped spines, and compare with forms found in the lagenostome of Deltasperma (Bateman, 1988).
At least 11 whole-plant species of tracheophyte grew at Oxroad Bay, and these have produced one of the most diverse assemblages of Early Carboniferous petrifactions. In Britain, it is second in estimated partial-plant diversity only to the Whiteadder (see above, and Scott et al., 1984; Bateman in Cleal, 1991). Other taxonomically similar petrifaction assemblages occur at nearby Castleton Bay (Scott and Galtier, 1988); the upper assemblage at Loch Humphrey Burn (see below); Kingswood (see below); and Ballyheighue, Irish Republic (Matten et al., 1980; Matten et al., 1984b). These occurrences vary considerably in age (Strunian to possible early Asbian), sedimentological regime and geographical location, but evidently reflect similar ecological and taphonomic constraints. However, none of the above localities matches Oxroad Bay in whole-plant diversity, or in the potential for correlating petrifaction, adpression and fusain fossils of the same plant organ.
Oxroad Bay is one of a series of basal Carboniferous sites in southern Scotland that yield anatomically-preserved plant fossils (about 350 million years old). Taken together, they are the most important global source of information on vegetation of this age. This site is especially important for allowing the first rigorous reconstructions for Early Carboniferous representatives of these plant groups. Studies of the club-mosses and horsetails have been completed, but work continues on the more diverse (and thus more problematic) early seed plants and on the ferns. Integration of the reconstructions with palaeoenvironmental data from this unstable volcanic setting has led to recognition of several distinct plant communities. Increasing knowledge of their structure and dynamics will allow comparison with extant vegetation.