Cupulate seed plants from the Upper Devonian Upper

Review of Palaeobotany and Palynology 142 (2006) 137 – 151 www.elsevier.com/locate/revpalbo

Cupulate seed plants from the Upper Devonian Upper Old Red Sandstone at Taffs Well, South Wales Jason Hilton School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, B15 2TT, United Kingdom Received 8 September 2005; accepted 23 March 2006 Available online 17 August 2006

Abstract Two kinds of cupulate seed plants are described from the Upper Devonian aged Upper Old Red Sandstone at Taffs Well near Cardiff, South Wales. The first conforms to the characters of Xenotheca devonica (Arber and Goode) Hilton and Edwards previously recognised from the Upper Devonian of North Devon, to which it is assigned. The Taffs Well specimens provide additional information on intraspecific variation within this morphospecies that is now known to possess 4–8 preintegumentary lobes in the distal 50–66% of preovule length. This represents the first occurrence of this morphospecies outside the type locality and highlights the morphological similarities observed in the morphogenera Xenotheca and Moresnetia and the genus Elkinsia. Cupules of the second kind occur terminally on a cruciately dichotomous branching system and have distinctive long and thin cupule tips. Cupules bear at least one asymmetric preovule that comprises an entire integument in the proximal 90% of preovule length and distally possess ca. 4 short and wide preintegumentary lobes. Features of its cupule and integument are distinct from all other recognised early seed plants leading to the establishment of Glamorgania gayerii gen. et sp. nov. Glamorgania shows that by the onset of the Carboniferous some seed plants had attained an almost complete integumentary surround. These records add to the growing body of data on the earliest seed plants and their initial phase of phenotypic radiation in the Upper Devonian. This investigation also highlights the problems of accommodating preservational differences in schemes of palaeobotanical nomenclature, from which it is concluded that Glamorgania may correspond to the compression/impression equivalent of the anatomically preserved Mississippian morphogenus Eurystoma. © 2006 Elsevier B.V. All rights reserved. Keywords: Devonian; Famennian; gymnosperm; seed; cupule; Xenotheca; Glamorgania

1. Introduction The evolution of the seed during the Devonian is one of the most important single events in the evolutionary history of plants, changing the female gametophyte to develop an altogether new kind of reproductive strategy. This included a number of distinct botanical innovations that collectively characterise the seed habit, namely; (1) change from dehiscent to indehiscent megasporangium, E-mail address: [email protected]. 0034-6667/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.revpalbo.2006.03.022

(2) reduction of tetrad numbers in the megasporangium to one, (3) reduction to one functional megaspore per tetrad, (4) increase in the dimensions of the remaining single functional megaspore, (5) nucellar modification including development of a nucellus with functional apex consisting of pollen chamber for pollen reception and retention prior to fertilization, and (6) integumentation—development of an integumentary surround to the nucellus (e.g. Chaloner and Pettitt, 1987; Rothwell and Scheckler, 1988; Haig and Westoby, 1989; Bateman and DiMichele, 1994a). While several of these innovations

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J. Hilton / Review of Palaeobotany and Palynology 142 (2006) 137–151

also occur in other heterosporous plants either individually or as subsets from this list (for summary see Bateman and DiMichele, 1994a), only seed plants have the full compliment and have attained the level of reproductive sophistication seen in the seed (Bateman and DiMichele, 1994a; Hilton, 1998a). In terms of species diversity and ecological dominance the seed is a key factor in the overall success of seed plants. In particular the seed offered an increased level of

environmental independence foremost of which was independence from water for fertilization—a factor that severely limits free sporing reproductive strategies (e.g. DiMichele et al., 1989; Bateman and DiMichele, 1994a). Although vegetative characters of the earliest seed plants undoubtedly contributed to their success in water limited environments, reliable pollination was a key reproductive breakthrough (Bateman and DiMichele, 1994a,b). As a consequence, evolution of the seed was a crucial step to


the colonisation of land areas unattainable to plants of other groups, opening previously uninhabited ecological space and resources (DiMichele et al., 1989; Bateman and DiMichele, 1994a; Hilton, 1998a). Although Gerrienne et al. (2004) documented important Middle Devonian evidence for sequential character acquisition leading to the reproductive sophistication seen in the seed, the earliest stratigraphic occurrence of seed plants currently comes from the Famennian stage of the Upper Devonian. This evidence includes a number of seed morphotaxa that are united by the presence of a hydrasperman-type nucellar apex (Rothwell, 1986; Rothwell and Scheckler, 1988). This type of reproductive biology is characterised by an elaborate nucellar apex that includes a distinctive pollen chamber in which the initial development of the ovule post-pollination seals the opening to the pollen chamber (Rothwell, 1986, Rothwell and Scheckler, 1988). Although sharing the same kind of pollination biology, other features of these taxa vary considerably from which distinct phenotypic groups are increasingly being recognised (Hilton, 1998a). Of these the most frequently encountered are cupulate seed plants in which cupules contain multiple ‘preovules’–ovules with an incomplete integumentary surround consisting of distinct lobes. Examples of taxa with this organization include the oldest known seed plant, Elkinsia polymorpha Rothwell et al. (Rothwell et al., 1989; Serbet and Rothwell, 1992), as well as Moresnetia zaleskyi (Stockmans) Fairon-Demaret and Scheckler (Fairon-Demaret and Scheckler, 1987), Xenotheca devonica (Arber et Goode) Hilton and Edwards (Hilton and Edwards, 1999), Archaeosperma arnoldii Pettitt and Beck (Pettitt and Beck, 1968) and Kerryia matteni Rothwell and Wight (Rothwell and Wight, 1989). The distribution of these morphospecies indicate that seed plants with this kind of reproductive organization occurred across much of lowland Europe and North America during the latest Devonian (e.g. Scheckler, 1986; Hilton, 1996),

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while other comparable taxa that require substantiation as seed plants may extend this range further (e.g. Lenlogia Krassilov and Zakharova, 1995). Other groups are less common in the Devonian, and include preovules originally interpreted as acupulate such as Aglosperma quadrapartita Hilton and Edwards (Hilton and Edwards, 1996) and Aglosperma avonensis Hilton (Hilton, 1998b), the winged preovule Warsteinia paprothii Rowe (Rowe, 1997), and more unusual preovules such as Dorinnotheca streelii Fairon-Demaret (Fairon-Demaret, 1996) with a radically distinct cupule and preintegument morphology. The most recent recognised Devonian preovulate taxon follows reanalysis of specimens belonging to the genus Condrusia Stockmans (Stockmans, 1948), allowing Prestianni (2005) to determine another phenotypic group of Devonian seeds characterised by a ‘laminar’ bilaterally symmetrical cupule bearing a putative hydrasperman-type nucellus. Although the nucellus is remarkably consistent within these taxa, integumentary and cupular features vary considerably and show that by the onset of the Carboniferous seed plants were rapidly diversifying and undertaking their initial phases of adaptive as well as ecological radiation (Haig and Westoby, 1989; DiMichele et al., 1989, Bateman and DiMichele, 1994a; Rowe, 1997; Hilton, 1998a). In the current paper two kinds of cupulate seed plants are investigated from the Late Devonian aged Taffs Well fossil plant assemblage at Tongwynlais, Cardiff, south Wales. Fossil plants were first recorded from this locality by Gayer et al. (1973) who presented a preliminary synthesis of the assemblage. In this account four categories of plant remains were identified: cupulate seeds (cf. Xenotheca devonica Arber and Goode), sporangia containing microspores (similar to Telangium sp.), flabelliform leaves, and various isolated sporangia (Gayer et al., 1973). Edwards (in Gayer et al., 1973) concluded that the Taffs Well assemblage was closer to that from the Baggy Beds of North Devon (Arber and Goode, 1915; Rogers, 1926), but noted that further

Plate I. Cupules and preovules of Xenotheca devonica from the Late Devonian Taffs Well plant bed. 1.

2.

3.

4. 5.

Slab containing multiple specimens with specimen to right (NMW.94.75G.21a1) possessing two equally topped terminal cupules on the same dichotomous branching system, while that to the left (NMW.94.75G.21a2) has a single terminal cupule (enlarged in Fig. 2). This slab also contains an isolated dehiscent sporangium (bottom centre). Scale bar = 5 mm. Enlargement from Fig. 1 showing cupule dividing in the plane of photograph into two more or less equal halves, with the right half slightly overtopping the left. Details of the right half are unclear but paired cupule tips are visible. The left half contains two preovules (arrows) with 3–5 inwardly recurved integumentary lobes clearly distinguished from the surrounding cupule lobes. Scale bar = 1 mm. Cupule fractured parallel to basal dichotomy revealing complete cupule half with cupule quarters. Each cupule quarter bears a single preovule and possesses paired cupule tips (enlarged in Plate II, 1). Specimen figured by Bassett and Edwards (1982); NMW.94.75G.14; scale bar = 5 mm. Distal part of the cupuliferous branching system with two terminal cupules before preparation. Left hand cupule enlarged after dégagement in Plate II, 4 revealing preovules. NWM.94.75G.21b4, scale bar = 10 mm. External plane of fracture revealing outer cupule details and with twelve cupule tips visible. Basally cupule divides into two halves, with subsequent division forming cupule quarters. Cupule tips are paired, with two pairs (and 4 cupule tips) forming one cupule quarter. Internal features including preovules not observed in this plane of fracture. NMW.94.75G.18, scale bar = 2 mm.

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Plate II. Ovules of Xenotheca devonica from the Late Devonian Taffs Well plant bed. All scale bars = 1 mm unless otherwise stated. 1. 2. 3. 4.

Fracture through cupule revealing one half of cupule structure and showing single preovule with 5 preintegumentary lobes. NMW.94.75G.1a2. Cupule with slight overtopping with single complete preovule (arrow) with four integumentary lobes fused to each other in proximal 1/2 of preovule length. Second preovule occurs adjacent to the complete preovule on the right hand side of the same cupule half. NMW.94.75G.20a. Proximally incomplete cupule with one well preserved preovule in the left hand cupule quarter (arrow). NWM.94.75G.22a6, scale bar= 2 mm. Enlargement of Plate I, 3 showing tips of preintegumentary lobes from the right hand cupule (i) and partially preserved nucellus (n). Integumentary lobes are considerably shorter than those of the cupule. NMW.94.75G.14.


study was needed on both assemblages before they could be compared further. Specimens from the Taffs Well assemblage were subsequently illustrated and briefly described by Bassett and Edwards (1982) in a summary of fossil plants from Wales, and this account included a brief description and illustration of a specimen interpreted as a microsporangiate cupule. Hilton and Edwards (1996) undertook the first detailed investigation of specimens from the Taffs Well assemblage, re-examining specimens initially illustrated in Gayer et al. (1973) and established a new Devonian seed plant preovule, Aglosperma quadrapartita Hilton and Edwards (Hilton and Edwards, 1996). The aims of the present investigation are to determine the identity of the cupulate organs previously noted from the assemblage by Gayer et al. (1973) and Bassett and Edwards (1982), to consider their systematic relationships, and to further characterise the earliest radiation of seed plants during the latest Devonian. 2. Locality details and age The Taffs Well fossil plant assemblage occurs in the basal part of the Quartz Conglomerate Group of the

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Table 1 Integumentary characteristics of preovules of Xenotheca devonica from the Taffs Well assemblage Specimen

Length (mm)

Width (mm)

Lobe number (estimated in brackets)

% Fusion (ca)

NMW.94.75G.14 NMW.94.75G.15 NMW.94.75G.20a1 NMW.94.75G.22a6 NMW.94.75G.21b4 Summary

2.7 3.4 3.6 3.8 5.2 2.7–5.2

0.8 1.4 1.4 1.3 1.6 0.8–1.6

3 (4) 5 (6–8) 4 (4+) 5 (6–7) 5 (6) 4–8

b40 b33 b50 b33 b33 b50

Upper Old Red Sandstone cropping out at Taffs Well near Tongwynlais, 9.5 km north of Cardiff, Wales [O.S. Grid Reference ST 1302 8254]. Specimens were collected from the lowest two horizons exposed in the core of the Castell Coch anticline at the roadside section summarised by Gayer et al. (1973). Precise details of the plant bearing strata are given by Gayer et al. (1973) and elaborated by Hilton and Edwards (1996). Higgs et al. (1988) placed the Taffs Well plant bearing horizon within their proximal LL biozone based primarily on reinterpretations of the published miospore compositions of Gayer et al. (1973). However, Bless et al. (1992) and subsequently Hilton and Edwards (1996) questioned this precise stratigraphic conclusion because the LL, LE and LN miospore biozones are similar in composition except for the presence of a few key (marker) taxa. In this respect it is clear that a detailed palynological investigation is necessary before the biostratigraphic age is unambiguously assigned although, all three of the possible miospore biozones (LL, LE, LN) would place the Taffs Well plant beds in the uppermost part of the Famennian Stage of the Upper Devonian. 3. Materials and techniques

Fig. 1. Annotated camera lucida diagram of NMW.94.75G.14 (Plate I, 3) showing cupule and preovule morphology. Plane of fracture is between adjacent cupule halves, and only the distal regions of the preovules (stippled) observed, each with distally incurved integumentary lobes. A = cupule half forming dichotomy, B = cupule quarter forming dichotomy, C = cupule eighth forming dichotomy, D = cupule tip forming dichotomy, O = estimated position of preovule bases.

Specimens are preserved as coalified compressions/ impressions with occasional adpression (typically of preovulate organs). These fossils have proved difficult to investigate because of the clay and mica dominated composition of the matrix in combination with often poor preservation of the fossils (Hilton and Edwards, 1996). Dégagement (Fairon-Demaret et al., 1999) was employed to reveal the morphology of the individual specimens, using sharpened tungsten needles to successively remove either matrix or parts of the fossil to reveal the underlying segments. As previously noted this sediment proved difficult to prepare by dégagement; many specimens were insufficiently well preserved to successfully prepare (Hilton and Edwards, 1996) while

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Table 2 Comparison of preovule features of Xenotheca devonica from Taffs Well and the North Devon with other similar preovules Species

Xenotheca devonica (Baggy Beds)

Xenotheca devonica (Taffs Well)

Glamorgania gayeri

Moresnetia zaleskyii

Elkinsia polymorpha

Kerryia matenii

Source

Hilton and Edwards (1999) 3.4–5.5 2.1–2.5 4–7 b50

This paper

This paper

2.7–5.2 0.8–1.6 4–8 b50

1.8–3.1 1.1–1.8 ca. 4 ca. 90

Fairon-Demaret and Scheckler (1987) 1.0–5.0 1.0+ 8–10 Chalazal (b10)

Rothwell et al. (1989) 3.5–6.5 1.0–2.6 4–5 ca. 33

Rothwell and Wight (1989) 3.15–4.4 1.0–1.6 8–10 ca. 50

Length (mm) Width (mm) Integ. lobe no. % Fusion (ca)

others were unfortunately destroyed during preparation. Many of the best specimens were revealed by fortuitous fractures through the matrix. Specimens were reinforced during and after dégagement with an acetone soluble glue (Alvar and Paraloid) in varying concentrations depending on the level of reinforcement required. Photography was achieved with polarised light as described by Hilton and Edwards (1996) using an Olympus OM system camera with a 50–100 mm macro lens. Type and figured materials from this paper (along with those of Aglosperma quadrapartita from the same locality) are deposited in the National Museum and Galleries of Wales, Cardiff and are proceeded by the prefix NMW.94.75G. The earlier specimens illustrated in Gayer et al. (1973) are in the same repository and have the prefix NMW.72.12G. 4. Xenotheca devonica (Arber and Goode) Hilton and Edwards (Plates I and II, Figs. 1 and 2) 4.1. Description Approximately 45 specimens have been identified although fragmentary remains of this kind are frequently encountered in the assemblage. Cupules are recognised by their distinctive lobed appearance (Plate I, 1–5) and from the preovules that they contain having a prominently lobed preintegument (Plate II, 1–4). The cupuliferous branching system is proximally fragmented and only the ultimate two dichotomies have been observed and are cruciately organized (e.g. Plate I, 1). Cupules are terminal, and occur between 4 and 15 mm from the final dichotomy where axes range from 0.2 to 0.45 mm wide (Plate I, 1). Cupules on the same branching system range from those that are equally topped with individual cupules borne at approximately the same level (e.g. cupule to right in Plate I, 1), to those that are moderately overtopped in which different cupules are produced at different levels (e.g. cupule to left of Plate I, 1, 2, 4). Individual cupules are revealed through planes of fracture that either travel through the cupule (Plate I, 1–4)

and reveal the branching structure and internal organization of the cupule, or along the external surface of the cupule revealing only its external features (Plate I, 5). Cupules range considerably in size and appearance, and vary from 7 to 16 mm long and 6 to 12 mm wide, including individuals that are comparatively long and slender (Plate I, 1–2, 4) as well as others that are shorter and wider (Plate I, 3, 5). Despite this variation, individual cupules have the same basic organization. Individual cupules have a basal dichotomy that divides the cupule into two more or less equal halves (Plate I, 3 and Fig. 1). Continuing distally from the initial halfforming dichotomy, each cupule half divides to produce two more or less equal quarters. This second cupule dichotomy is at 90° to the initial (cupule half producing) dichotomy, as shown in Fig. 1. Above the second (quarter forming) dichotomy, cupule morphology is not particularly clear as it is usually obscured by preovules. Preovules are borne singly on the inner surface of the cupule above the level of the quarter forming dichotomy (Plate I, 2–3; Plate II, 1–4). With this organization a complete cupule would bear up to 4 preovules, although some cupule quarters do not contain preovules (e.g. cupule to right in Plate I, 4) and may be sterile. Cupule quarters that do not contain preovules typically have longer and thinner cupule segments than those containing preovules that are themselves generally shorter and wider. Preovule abscission scars have not been observed. Distal to the point of preovule attachment the cupule dichotomises twice more with the initial dichotomy occurring only slightly higher than the attachment of the preovules. However, this dichotomy is often obscured by the overlying preovules and for this reason distal cupule organization is best observed in specimens revealed by external fracture planes (Plate I, 5). The final cupule dichotomy occurs in the same plane as the penultimate dichotomy and approximately 2.6–3.9 mm above the fourth dichotomy (Plate I, 5; Fig. 1). This final dichotomy produces terete cupule tips each of which are approximately 0.3–0.5 mm wide proximally, tapering distally to a point, and up to 3.4 mm long. Cupule tips


are usually inwardly recurved (Plate I, 2–4; Plate II, 1–2) and range from equally topped (Plate I, 5) to weakly overtopped in different parts of the cupule (Plate I, 2–3). With this organization a complete cupule would comprise 16 cupule tips and bear up to 4 preovules, one in each cupule quarter. Most of the preovules observed are incomplete with few having chalazal portions observable. Where complete, preovules have a rounded chalaza (Plate II, 1–2) and range from 2.7 to 5.2 mm long and from 0.8 to 1.6 mm wide (Table 1). The width of the integument gently increases from the chalaza to reach its maximum dimension at approximately half of the length of the preovule (Plate II, 2–3). In all cases the chalazal extent of the integument is entire but distally the integument divides to form 4–8 lobes (Table 1), with the level of fusion varying from approximately 33% (Plate II, 1, 4) to 50% (Plate II, 2). Individual preintegumentary lobes are terete, slowly taper in diameter through their length, and have rounded apices. Lobes are inwardly curved (Plate II, 2–3), and in some specimens collectively bend inwards to form an arched apex (Plate II, 2). Distally the preintegumentary lobes do not connect with each other leaving a pre-micropylar opening (Plate II, 1–4). Within the preintegument a dark coloured central unit exists in several of the specimens. Higher magnification (n in Plate II, 4) shows this to be a single layer of tissue extending at least 2/3 of preovule length. The precise shape and structure of this unit are not observed in the available specimens. 4.2. Comparison with other species and taxonomic distinction between Devonian cupulate preovule morphotaxa In this species although the nucellus with hydrasperman pollen chamber have not been observed, this species is identified as a cupule bearing multiple preovules through comparisons with previously recognised taxa that show a comparable morphology. Most similar are the

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Devonian taxa Archaeosperma arnoldii (Pettitt and Beck, 1968), Moresnetia zaleskyi (Fairon-Demaret and Scheckler, 1987), Elkinsia polymorpha (Rothwell et al., 1989), Kerryia (Rothwell and Wight, 1989) and Xenotheca devonica (Hilton and Edwards, 1999) that have ovulate organs comprising telomic style dichotomous cupules bearing multiple preovules. Archaeosperma is distinguished by its spiny integument (Pettitt and Beck, 1968) which is unlike the glabrous preintegument in each of the other morphospecies. Preovule features of the remaining taxa are compared in Table 2, and Table 3 compares features of their cupules. While these characters undoubtedly intergrade between the taxa shown in Tables 1 and 2, the features of the Taffs Well specimens most closely conform to those of X. devonica from the Baggy Beds of North Devon (Arber and Goode, 1915; Rogers, 1926; Hilton and Edwards, 1999). The Taffs Well and the North Devon localities are geographically and stratigraphically close to each other and were deposited in the same depositional basin; they are 105 km apart and both occur within the LL miospore biozone of the Upper Devonian (Hilton and Edwards, 1999). Differences between Xenotheca devonica from North Devon and the specimens described from Taffs Well occur in terms of the dimensions of their cupules and preovules, the number of preovule lobes, and the degree of cupule overtopping. Of these Hilton (1996) and Hilton and Edwards (1999) questioned the reliability of size in establishing taxonomic distinction, and in particular noted that in examples such as these based on small sample sizes (n = 45 from Taffs Well, 41 from North Devon) the extent of polymorphic, ecophenotypic and ontogenetic variation cannot be reliably assessed. In each case dimensions overlap to such an extent that it is impossible to reliably separate these taxa based on their size. Likewise, while the number of preovule lobes varies from 4 to 7 in the North Devon specimens and from 4 to 8 in the Taffs Well specimens, these values overlap and this difference relates to a single specimen from Taffs Well in which 8 lobes have been interpreted (Table 1). This is an inferred

Table 3 Comparison of key cupule characters of Xenotheca devonica with other taxa with lobed preinteguments Species

Xenotheca devonica (Baggy Beds)

Xenotheca devonica (Taffs Well)

Glamorgania gayeri

Moresnetia zaleskyii

Elkinsia polymoprha

Kerryia matenii

Source

Hilton and Edwards (1999) 8–15 4–10 None–Strong 16 0–4

This paper

This paper

7–16 6–12 None–moderate 16 0–4

9–11 3.5–5 None 16 N1

Fairon-Demaret and Scheckler (1987) 8–10 7–12 Strong 8–16 0–4

Rothwell et al. (1989) 8–18 6–12 Slight 16 4

Rothwell and Wight (1989) 5–10 7–10 Slight 16–24 2–6

Length (mm) Width (mm) Overtopping Cupule lobe no. Preovule no.

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number and may not be accurate, and equally possible is that other preovules from North Devon assemblage had up to 8 preintegumentary lobes and have not been discovered. Again the small sample size and the reliability of this feature do not support taxonomic separation of the Taffs Well and North Devon specimens. The final difference relates to variation in the level of overtopping of the cupule, with some of the cupules from North Devon having a higher degree of overtopping (Table 3). Again the level of overtopping of the two sets of specimens overlaps, and as highlighted by Hilton and Edwards (1999) this may also be the effect of polymorphic, ecophenotypic or ontogenetic variation, or variation between cupules borne on different positions on the cupuliferous branching system. Considering these factors it is concluded that the Taffs Well specimens are indistinguishable from those of X. devonica from North Devon, and are here assigned to the same morphospecies. Of particular importance is that if specimens from one assemblage were placed among specimens from the other, based on their morphological structures they would not be distinguishable from each other. As evident in Tables 2 and 3, features of Xenotheca devonica from Taffs Well and North Devon also

intergrade with those of Moresnetia zaleskyi and Elkinsia polymorpha, as previously noted by Hilton and Edwards (1999). Of these Xenotheca has nomenclatural priority (Arber and Goode, 1915). Hilton and Edwards (1999) concluded that taxonomic validity of these taxa are subjective and to a large extent depend on the taxonomic criteria employed (see Hilton and Edwards, 1999, for summary). However, in accordance with the ICBN (St Louis code), Elkinsia is comprehensively characterised as a whole plant species (e.g. Serbet and Rothwell, 1992; Rothwell and Serbet, 1994) whereas both Xenotheca and Moresnetia represent morphospecies—a fossil taxon that comprises only a part of the life-history stages, or preservational states represented by the corresponding nomenclatural type; in this case only a part of the whole plant. Elkinsia, the genus characterising the order Elkinsiales, is therefore taxonomically distinct from both Xenotheca and Moresnetia. Furthermore, while the isolated cupules of Elkinsia are comparable to those of Xenotheca and Moresnetia, they are known in considerably more detail including details of their pollination biology, ontogeny, and cellular organization (Rothwell et al., 1989; Rothwell and Serbet, 1992; Serbet and Rothwell, 1992). As such

Plate III. Cupules of Glamorgania gayerii sp. nov. from the Late Devonian Taffs Well plant bed. Photographs previously taken by Edwards and since this time showed that partial degradation of the specimen has occurred. Scale bars = 1 mm unless otherwise stated. 1. 2.

3.

Cupuliferous branching system with two terminal cupules bearing preovules as illustrated by Gayer et al. (1973). NMW.72.12G.1, scale bar = 10 mm. Enlargement of cupules from Fig. 1. Upper cupule reveals basal half forming dichotomy (arrowed), and sediment (s) can be seen separating adjacent cupule halves distally. Lower cupule is revealed through external plane of fracture obscuring branching organization. A single preovule can be seen in each cupule, as shown in Fig. 3, and the counterpart of the upper cupule is shown in Fig. 3. Enlargement of counter-part of cupule from Fig. 2 showing margins of ovate preovule (arrow) and distal lobes.


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Moresnetia, on the other hand, are known in a comparable level of detail to Xenotheca, and as such comprehensive comparison of both morphospecies including multivariate and morphometric analyses are now required to evaluate their taxonomic status and to test the currently advocated generic level separation. For both Xenotheca and Moresnetia, complete specimens with extensive permineralisation as well as further studies are required to resolve these difficulties. 4.3. Discussion on Xenotheca devonica from Taffs Well

Fig. 2. (A) Cupule of specimen NMW.94.75G.20a1 exposing single, partially reclined preovule. (B) Cupule has been omitted showing the morphology of the preovule from which the tissues in the centre have become detached. Two entire integumentary lobes are visible with two more incomplete lobes to the left and right margin.

placing this well circumscribed genus into either of the comparatively poorly circumscribed morphogenera Xenotheca or Morenetia is inappropriate. Cupules of

The most informative specimen of Xenotheca devonica from Taffs Well (Plate I, 3; Plate II, 1; Fig. 1) was previously figured by Bassett and Edwards (1982) and interpreted as a “microsporangiate cupule”. This specimen lacks a counterpart but possesses 4 preovules (see above). Edwards noted that when wetted, numerous spores came from the specimen, leading her to coat the specimen in glue to prevent further loss (D. Edwards, personal communication, 1993). Approximately ten of the microspores noted by Edwards were lifted from the glue and studied under the SEM but revealed no diagnostic features. Subsequent preparations revealed nothing about their morphology. As such the presence of microspores in this specimen cannot be validated, although the presence of preovules can. It is uncertain if this cupule contained microsporangia with microspores within the cupule as previously observed in the Mississippian morphospecies Pullaritheca longii Rothwell and Wight (Rothwell and Wight, 1989) as originally noted by Long (1977). Preservation in the Taffs Well specimens precludes identification of

Fig. 3. Camera lucida diagrams of specimen NMW.72.12G.1. (A) Morphology of cupuliferous branching system with the two terminal cupules. (B) Enlargement of cupules from (A) showing position and morphology of preovules (stippled).

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internal structures such as in-situ microspores, and as such its microsporangiate nature must be considered as unproven. 5. Glamorgania gayeri gen. et sp. nov. (Plates III and IV, Fig. 3) 5.1. Description The following description is based on four articulated cupules along with numerous more fragmentary remains that allow details of cupule and preovule morphology to be determined. Specimens are recognised by their distinctive cupule morphology with elongated cupule tips, and their characteristic preovules (see below). Only the distal most parts of the cupuliferous branching system are known, and exhibit an isotomous branching pattern in which successive dichotomies occur at 90° to each other resulting in a cruciate organization. The most complete branching system is shown in Plate III, 1 and Fig. 3A, and has two dichotomies before cupules are borne. Distances between dichotomies decreases distally from 19.4 mm, 16.2 mm and with 4.8 mm between the final dichotomy and the base of the cupules (Plate III, 1). Individual cupules are cuplike and range from 9.2 to 11 mm long and 3.6 to 4.5 mm wide. Cupules widen gradually from their base to reach their maximum width 2/3 of the cupule length, and from this level gradually narrow and are apically incurved (Plate III, 1–2). The basal organization of the cupule is often unclear and in most cases appears to be proximally fused such that the bases of individual cupule segments are not discernable. However, closer examination reveals that the cupule divides basally to form two distinct halves (arrow in Plate III, 2; Plate IV, 3). This dichotomy is not apparent in all specimens; in those cupules exposed parallel to the basal (half forming) dichotomy the basal parts of the cupule appear to be fused (e.g. Plate III, 2, bottom), while those exposed perpendicular to it show the basal dichotomy (Plate III, 2, top). These two examples, with apparently different organizations, are borne on the same cupuliferous branching system (Plate III, 1; Fig. 3) with the variation apparently resulting from different orientation within the sediment. Above the basal dichotomy the morphology of the cupule becomes unclear, especially in those specimens in which the basal extent of the cupule appears to be fused rather than dichotomous. However, dégagement of one specimen (Plate IV, 3) reveals that the cupule has a subsequent dichotomy above the half forming dichotomy (q in Plate IV, 3). At this level the base of a single preovule

is evident (e.g. Fig. 3B; o in Plate IV, 3) obscuring much of the cupule organization (Fig. 3B). For this reason it is unclear if the next dichotomy (p in Plate IV, 3) occurs above or below the level at which preovules are borne— either way the two are closely spaced. Above this level the cupule divides a final time (u in Plate IV, 3) to produce individual ultimate cupule segments (Plate III, 2; Plate IV, 1–2, 4). Ultimate cupule segments are terete, 0.3 wide where they divide, long (up to 4.1 mm), and taper gradually until they are no longer observable in the matrix (Plate III, 2; Plate IV, 1–2). A maximum of 12 ultimate segments have been observed in a single cupule (Plate IV, 1–2), from which their total number is interpreted as 16 based on the dichotomous organization recorded in the cupule, and as inferred from the 4 cupule dichotomies observed. The penultimate cupule segments are prominent and easily distinguished by their paired ultimate segments (e.g. Plate IV, 3–4), and are clearly observed in specimens with planes of fracture through the inside of the cupule. Penultimate cupule segments are at least 0.5 mm long and extend up to 1.8 mm in length, and divide distally to produce 2 ultimate cupule segments. Preovules are borne on the inner surface of the cupule and are much darker in colour than the enveloping tissues of the cupule. Each half cupule bears at least one ovule, but it is uncertain how many preovules were born within individual cupules. Preovules are 1.8–3.1 mm long and 1.1–1.8 mm wide, and have a rounded chalaza (Plate III, 2–3; Fig. 3B) that is best seen in Plate IV, 3. In the specimen figured in Plate III, 2–3 a single asymmetric preovule with an irregular outline is present. The basal extent of the preovule is entire, and in this specimen two small longitudinally aligned cracks are visible. In this specimen the apical part of the ovule is not clearly observed but this is best seen in the preovule shown in Plate III, 3 and Fig. 3. In this specimen the preovule is distally constricted then becomes recurved towards the distal 10% of the preovule length (Fig. 3B). At its distal-most extreme, a single preintegumentary lobe is visible at either side of the preovule apex (Fig. 3B). Each lobe is approximately 0.2 mm long, comprising the distal ca. 10% of the preovule being lobate. From the position of the preintegumentary lobes it is considered unlikely that the preovule possessed only two as apparent in this specimen. From the size and position of the lobes it is anticipated that at least a further two more integumentary lobes would be present (on the unexposed surfaces of the preovule) to provide the same degree of cover around the apex. In the second (higher) cupule in Plate IV, 2 and Fig. 3B, 4, the preovule is also asymmetric in outline and the apex of the preovule is not as clear as that in the adjacent cupule. However, this


specimen has a distally constricted apex and part of a small preintegumentary lobe can be observed at the top left (Fig. 3B).

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A single specimen assigned to this species has a recurved cupule (Plate IV, 5) and is different from the other specimens in which the cupule is erect. In the

Plate IV. Cupules of Glamorgania gayerii sp. nov. from the Late Devonian Taffs Well plant bed. Scale bars = 2 mm unless otherwise stated. 1.

2. 3.

4. 5.

Distal part of cupuliferous branching system with two equally topped cupules. Cupules have fused proximal regions and have paired, long and slender cupule tips. Central cupule regions reveal dichotomies below the final (tip producing) dichotomy. NMW.94.75G.19c1, counterpart of Fig. 2. Counterpart of specimen from in Fig. 1 showing different feature of cupule morphology. Cupule showing approximate positions of branching including basal half forming dichotomy (h), quarter forming dichotomy (q), dichotomy forming ultimate pairs (p) and ultimate dichotomy (u) forming the slender cupule tips. In this specimen the levels of dichotomies varies across the cupule due to slight overtopping. A single asymmetric preovule (o) occurs centrally within the cupule, and has a different composition from the cupule indicated by its difference in colour and texture. Preovule has an entire integument but the distal parts are not visible. NMW.72.12G.1.5, scale bar = 1 mm. Enlargement from Fig. 1 showing indistinct proximal cupule region and the paired ultimate sections and elongated cupule tips. Scale bar=1 mm. Recurved cupule with clearly visible ultimate cupule tips. The cupule is more or less symmetrical, forming two halves (upper and lower images), and preovules are not visible. NMW.72.12G.2.5.

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recurved specimen the cupule organization is clearly visible and forms two halves and possesses the characteristic elongated cupule tips that characterise this species. 5.2. Comparison with other cupulate seed plants Edwards (in Gayer et al., 1973) first illustrated a cupule of this taxon (NMW 72.12G.1; this Plate III, 1) and assigned it to cf. Xenotheca devonica Arber and Goode (Arber and Goode, 1915). However, Edwards (in Gayer et al., 1973) noted that further study was necessary on X. devonica before any further specimens could be added to this species. The current investigation shows that this species has a cupule that follows the same basic organization as that of X. devonica (see above and also Hilton and Edwards, 1999), but is subtly different in its shape and in producing much longer ultimate cupule segments. These elongated ultimate cupule segments are most similar to those observed in the cupules of Archaeosperma arnoldii (Pettitt and Beck, 1968), and compare favourably with this species. However, preovules of the new cupule differ markedly from those of both Xenotheca and Archaeosperma. In Archaeosperma the preovule is radially symmetrical and basally entire, but has small integumentary lobes distally. The most distinctive feature of the preovules of Archaeosperma is their spiny preintegumentary covering. These features are very different from the ovules described here showing them to be taxonomically distinct. In Xenotheca the preovules are also radially symmetrical and have an entire chalazal integument dividing into distinct lobes in their distal 33–50% (see above). These are also distinct from the asymmetrical ovules of the present species that have small, diminutive preintegumentary lobes in the distal 10% or so of preovule length. In fact, this ovule morphology distinguishes the present specimens from all previously recognised Devonian preovule taxa and is closer in structure to taxa described from the Mississipian of Scotland by Long (1960, 1966, 1975, 1977). Preovules of the specimens described here are closest in morphology to those of Eurystoma angulare Long (Long, 1960) that possesses an almost complete integument and have 4–5 small lobes in the distal 0.25–2 mm of the preovules length (Long, 1960, 1966, 1975). Eurystoma is much larger than the preovules reported here, typically ranging from 4 to 8 mm long, although the length and number of the free preintegumentary lobes between the two is similar. Further similarity comes from the Taffs Well cupules (Plate IV, 5) including one with a recurved organization that are similar to detached cupules of E. angulare as reconstructed Long (1966, his Text-Fig. 5).

Cupules of E. angulare range from 10 to 15 mm long and 4 to 8 mm wide, and this species includes cupules that are bilaterally symmetrical and more typically radially symmetrical (Long, 1966, 1975); the reported bilateral symmetry is most likely explained through partial compression. In E. angulare cupules are lobate and have the same characteristic branching organization as those cupules described here (confirming the moresnetian architectural model sensu Hilton and Edwards, 1999) but E. angulare specimens lacks distally extended cupule tips present in the Taffs Well specimens. Although preservational differences prevent comprehensive comparisons, the cupule of E. angulare is distinct from the Taffs Well specimens, while the ovules are similar in their organization. Long's second morphospecies of Eurystoma, E. burnense Long [E. trigona of Long (Long, 1969) including ontogenetically mature specimens previously referred to as Anasperma burnense Long (Long, 1966), following revision by Long, 1975], is also distinct from the specimens described here (and E. angulare) in having an ovule that is triangular in outline with laterally extended integument lobes that is considered to be winged (e.g. Long, 1975). This preovule organization is distinct from the Taffs Well specimens, and again shows them to belong to different morphospecies. Morphological features show that the Taffs Well specimens are distinct from both morphospecies of Eurystoma, and preservational features preclude the assignment of the Taffs Well specimens to the morphogenus Eurystoma that is to a large extent founded on anatomical observations. Deltasperma fouldenense Long (Long, 1961) has asymmetrical preovules that resemble those described here. D. fouldenense possesses two short preintegumentary lobes and is on average 2.8 mm long and ellipsoid in cross section, with dimensions ca. 2.1 mm by 1.2 mm wide (Bateman and Rothwell, 1990). This size range is not inconsistent with those from the Taffs Well preovules, but D. fouldenense is distinct as it only possesses two preintegumentary lobes. Meaningful comparison of these taxa is prevented by the absence of anatomical information from the Taffs Well specimens, and as such it is impossible to assign these specimens to the morphogenus Deltasperma that is largely based on anatomical information. From the comparisons presented above the specimens from Taffs Well are distinct from all previously recognised taxa but have similarity with the cupules of the morphogenus Archaeosperma and the ovules of the morphogenus Eurystoma. The lack of anatomical presence precludes its assignment to Eurystoma, and its ovular features are clearly distinct from Archaeosperma. Its cupule has a similar organization to numerous other


Devonian and Mississipian taxa but its precise combination of characters does not match those of previously recognised morphospecies or morphogenera. It is here concluded that this necessitates the establishment of a new morphogenus and morphospecies, but in doing this it is recognised that this may represent the compression equivalent of the morphogenus Eurystoma Long, and the similarities with the cupules of Archaeosperma may indicate this to be closely related also. 5.3. Systematic palaeobotany Division SPERMATOPHYTA Class LAGENOSPERMOPSIDA sensu Cleal (1994). Glamorgania Hilton gen. nov. (Plates III and IV, Fig. 3). Diagnosis: Only reproductive axes known. Cupuliferous branching system with cruciate dichotomies and slender axes terminating in cupules borne singly or in pairs. Cupule with 4 dichotomies producing 16 terete, elongate, ultimate segments. Cupule dichotomies in cruciate organization except ultimate dichotomy occurring in same plane as penultimate. Entire cupule bearing at least one preovule. Preovules asymmetric, with integument basally entire dividing into short preintegumentary lobes in distal ca. 10% of preovule length. Etymology: The new genus is after Glamorgan, the former Welsh County from which the fossils were collected. Type morphospecies: Glamorgania gayeri Hilton Glamorgania gayeri Hilton sp. nov. (Plates III and IV, Fig. 3). Diagnosis: Cupules cuplike, approximately 9–11 mm long and 3–5 mm wide. Basal cupule dichotomy forming two more-or-less equal halves, distally dividing a further three times to produce 16 terminal lobes. Ultimate cupule dichotomy in same plane as penultimate dichotomy. Ultimate cupule segments terete, elongate, inwardly curved. Individual cupule with at least one preovule borne on inner surface of cupule near second cupule dichotomy. Preovules 1.8–3.1 mm long and 1.1–1.8 mm wide, asymmetric in outline and with distal constriction. Integument glabrous, basally entire, forming ca. 4 short, wide preintegumentary lobes in distal ca. 10% of preovule length. Holotype: NMW.72.12G.1 (Plate III). Synonym: cf. Xenotheca devonica of Gayer et al. (1973). Repository: National Museum and Galleries of Wales, Cardiff, Wales, UK. Type locality: Taffs Well, Mid Glamorgan, Wales. Grid reference [ST131825]. Type stratum: Quartz Conglomerate Group, Upper Old Red Sandstone.

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Biostratigraphic horizon: LL–LN miospore zones (Higgs et al., 1988). Age: Famennian, Late Devonian. Etymology: This morphospecies is named in recognition of the work on Welsh geology by Dr. R. A. Gayer (Cardiff University). 6. Discussion Although neither of the cupulate morphospecies described from Taffs Well have allowed the presence of the nucellus to be demonstrated, comparisons with numerous other Upper Devonian and Mississipian cupulate preovules support these being preovules bearing a nucellus, and therefore representing seed plants. The occurrence of Xenotheca devonica from Taffs Well is important as it extends the geographical range known for the morphospecies. It also provides important information on the morphology of this morphospecies not determinable from the specimens previously collected from North Devon. This morphospecies is now known to have preovules with 4–8 preintegumentary lobes (only 4–7 were observed in the limited materials from North Devon—see Hilton and Edwards, 1999), and fusion of the integument is now known to range from 33% to 50% of preovule length. These variations are here considered to vary within this species as either polymorphic, ontogenetic or ecophenotypic variation. This conclusion agrees with those made by Hilton and Edwards (1999) who considered these slight variations in morphology insufficient for distinguishing morphospecies. The conclusion presented here represents the continued development of taxonomic and nomenclatural treatments of Palaeozoic cupulate seed plants, from which it is abundantly clear that additional specimens or better preservation in certain previously recognised taxa are likely to necessitate further systematic and taxonomic revisions to previously recognised taxa. The establishment of Glamorgania gayeri is important as it indicates a higher diversity among Devonian cupulate seed plants and introduces integumental structures more typical of those observed in the Mississipian assemblages of Southern Scotland. This shows that the earliest phases of seed plant radiation and diversification were firmly established by the onset of the Carboniferous, and that these plants were increasingly successful through this period. A key factor in the success of seed plants in terms of their species diversity and ecological dominance was increased environmental independence offered by the seed. Foremost was increased independence from water for fertilization—a factor that severely limits free sporing reproductive strategies (e.g. DiMichele et al., 1989;

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Bateman and DiMichele, 1994a). The change from lobate preinteguments such as those characteristic of Xenotheca, Moresnetia and Elkinsia to taxa with a more complete integumentary surround such as Glamorgania and Eurystoma is likely to have made a significant contribution to this, providing additional protection to the developing megagametophyte. Here it is noted that lobate preinteguments only occur among the basal-most members of the seed plants, from which this condition has been inferred to represent the plesiomorphic condition within the clade (e.g. Rothwell and Serbet, 1994; Hilton and Bateman, 2006). This supports the theories of the evolutionary increase in preintegument fusion through evolution, and as advocated by Andrew's (1963) transformational series from Genomosperma kidstonii (Calder) Long–Genomosperma latens Long–Salpingostoma dasu Long– Physostoma elegans Williamson–Eurystoma angulare Long–Stamnostoma huttonense Long. Despite obvious problems with this transformational series not being restricted to a single evolutionary lineage and therefore not expressing evolutionary descent, additional problems come from species such as Glamorgania that have almost complete integumentary fusion but are stratigraphically older than the other members of this series. The increased level of integumentary fusion in Glamorgania shows that within a very short time from the earliest recognised seed plants some taxa had already evolved almost entire integuments. This either suggests extremely rapid rates of evolution towards a fused integument (within 1–2 million years judging from the stratigraphic differences between the Elkinsia and Glamorgania localities), or that stratigraphically older seed plants existed and have yet to be found and that a slower rate of change occurred. Recent investigations on the Givetian disseminule Runcaria Stockmans (Gerrienne et al., 2004) suggest that earlier seed plants are likely to have existed. The most recent cladistic analyses of lignophytes (progymnosperms plus seed plants) show that the change from heterosporous progymnosperms to the earliest seed plants equates to at least 9 unambiguous character changes at the node leading up to seed plants (Hilton and Bateman, 2006). This transition between progymnosperms and seed plants currently lacks viable intermediates that can be included into morphological cladistic analyses (Hilton and Bateman, 2006), and suggests that either key taxa have still to be identified from the fossil record, or, that this was an episode of saltational macroevolution (sensu Bateman and DiMichele, 1994b) and that intermediates did not exist (Hilton and Bateman, 2006). To determine which of these is the case, research focussing explicitly on this transition is required seeking to reduce the morphological distinction between

progymnosperms and seed plants. In this regard identifying new species within progymnosperms (e.g. Hammond and Berry, 2005) and basal-most seed plants (such as those described here) including the recognition of additional phenotypic variation can contribute significantly towards this goal. Acknowledgements This paper is dedicated to Muriel Fairon-Demaret in recognition of her contributions to Palaeobotany and especially her pioneering and meticulous research on Devonian seed plants and floral assemblages. This research was undertaken as part of a PhD investigation at the University of Cardiff supervised by D. Edwards, supported by NERC Research Studentship GT4/92/144. I thank D. Edwards, M. Fairon-Demaret, N. P. Rowe, R. M. Bateman, G. W. Rothwell, J. Galtier, P. Gerrienne and C. Prestianni for discussion throughout the course of this research, and N. P. Rowe and an anonymous reviewer for their helpful and constructive reviews. Photographic assistance was provided by L. Axe, J. Crawley and V. Williams. References Andrews, H.N., 1963. Early seed plants: recent fossil discoveries shed light on the evolution of the seed and on seed plant progenitors. Science 142, 925–931. Arber, E.A.N., Goode, R.H., 1915. On some fossil plants from the Devonian rocks of North Devon. Proceedings of the Cambridge Philosophical Society 18, 89–104. Bassett, M.G., Edwards, D., 1982. Fossil plants from Wales. National Museum of Wales, Geological Series, vol. 2, pp. 1–42. Bateman, R.M., DiMichele, W.A., 1994a. Heterospory: the most iterative key innovation in the evolutionary history of the plant kingdom. Biological Reviews 69, 345–417. Bateman, R.M., DiMichele, W.A., 1994b. Saltational evolution of form in vascular plants: a neoGoldschmidtian synthesis. In: Ingram, D.S., Hudson, A. (Eds.), Shape and Form in Plants and Fungi. Linnean Society, London, pp. 61–100. Bateman, R.M., Rothwell, G.W., 1990. A reappraisal of the Dinantian floras at Oxroad Bay, East Lothian, Scotland. 1. Floristics and the development of whole-plant concepts. Transactions of the Royal Society of Edinburgh B 81, 127–159. Bless, M.J.M., Becker, R.T., Higgs, K., Paproth, E., Streel, M., 1992. Eustatic cycles around the Devonian–Carboniferous boundary and the sedimentary and fossil record in Sauerland (Federal Republic of Germany). Annals of the Geological Society Belgique 115, 689–703. Chaloner, W.G., Pettitt, J.M., 1987. The inevitable seed. Bulletin de la Societé Botanique de la France 134, 39–94. Cleal, C.J., 1994. Gymnospermophyta. In: Benton, M.J. (Ed.), The Fossil Record, vol. 2. Chapman and Hall, London, pp. 795–808. DiMichele, W.A., Davis, J.I., Olmstead, R.G., 1989. Origins of heterospory and the seed habit: the role of heterochrony. Taxon 38, 1–11. Fairon-Demaret, M., 1996. Dorinnotheca streelii Fairon-Demaret, gen. et sp. nov., a new early seed plant from the Upper Famennian of Belgium. Review of Palaeobotany and Palynology 93, 217–233.

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