Specific definition

11 M. scheuchzeri (Order Trigonocarpales Meyen, Satellite morphogenus Macroneuropteris) is defined by gross morphology and epidermal characteristics (Cleal et al. 1990). The latter include large, irregularly shaped polygonal cells, 220 µm long and 60 µm wide on the adaxial surface, and somewhat smaller cells on the abaxial surface that also shows brachyparacytic stomata (two subsidiary cells); trichomes bases are present on both surfaces (Cleal and Zodrow 1989, text-fig. 3; Cleal et al. 1990).

12 The frond is bipinnate and has a basal dichotomy (Laveine 1967; Laveine et al. 1977; Laveine and Brousmiche 1982; Laveine et al. 1998). A comprehensive proposal for the reconstruction of the plant M. scheuchzeri is forthcoming (pers. comm. J.-P. Laveine, February, 2003).

Distribution

13 M. scheuchzeri is one of the very few azonal macrofossil plants in the Morien Group, as it ranges from the lowest strata at the Shoemaker Seam (Bolsovian) to the highest strata at the Point Aconi Seam (lower Cantabrian), see Bell (1938, Fig. 1). This is similar to the range in the Cantabrian in NW Spain (Wagner 1983, fig. 4). This contrasts with the distribution of the species in the Late Carboniferous Period of West and Central Europe, where it ranges from the Duckmantian to the top of the Westphalian D as an index for the Stephanian (summary: Josten 1991). In general, the species has a global distribution over palaeoequatorial tropical, Late Carboniferous, coal forests, and spans a time interval conservatively estimated to be at least 15 Ma, or three Westphalian Stages.

Palaeobiology

14 To date, organically attached male and female fructifications of M. scheuchzeri havenot been documented. From Sydney Coalfield, one pteridosperm seed attached to foliage of Neuropteris flexuosa is known (Zodrow and McCandlish 1980) which emphasizes the rarity of attached female organs to determinable foliage. However, Bell (1938) thought that the large, detached seeds of Trigonocarpus (Schizospermum) noeggerati Sternberg could represent ovules of the species. Ovules of this type are ubiquitous and are found throughout the strata of the Sydney Coalfield (see Zodrow 2002, Fig. 16c), and commonly in physical association with M. scheuchzeri foliage. Laveine (1967, fig 1d) suggested that Codonotheca caduca Sellard is the male organ.

Descriptions of foliar forms from Sydney Coalfield

15 In its most typical form in Sydney Coalfield, M. scheuchzeri (Fig. 2a) has large, lanceolate pinnules that reach a length of 12 cm or more and a width of 2.5 to 3.0 cm, rarely 5 cm, with a slightly round or acuminate apex. These measurements are larger than West and Central European measurements of 2 to 9 cm length and 0.6 to 2.5 cm width (summary: Josten 1991, p. 331). The pinnule base is cordate, or stalked, and the margin entire. The midvein is prominent and extends for most of the pinnule length. Venation is dense; lateral veins fork 3 to 5 times, and broadly arch to reach the lateral margin at an oblique angle, 70 to 80 degrees. Over most of the pinnule, abundant epicuticular hair (Fig. 2b) is present that can reach a maximum length of 2 mm. Hair is cross-aligned to the venation pattern, imparting a prominent pseudohexagonal pattern, and presents an important taxonomic feature (e.g., Bunbury 1847, pl. XXI; Laveine 1967; Kotasowa 1973; Cleal and Zodrow 1989;Josten 1991; present paper) for differentiating M. scheuchzeri from all of the other macroneuropterid species (Cleal et al. 1990; Josten 1991).

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16 M. scheuchzeri bears large terminal pinnules that reach a length of over 6 cm, a width of 2.5 cm, are elongate (Fig. 3a), or broadly oval (Fig. 3b), and are typically profusely covered with long, epicuticular hair.

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17 Based on the aforementioned gross morphological features, typical M. scheuchzeri is a most-easily recognizable species in the Late Carboniferous, worldwide.

18 Figures 4 and 5 illustrate a specimen of great interest in this discussion because it shows considerable morphological changes over its axial length of 30 cm. Its near-tip section is odontopteroid in form (Fig. 4a, detail 4b). These pinnules are attached to a rachis by their entire base, lateral veins are parallel to each other (no midvein is present), and they originate directly from the rachis. Neuropteroid morphology follows and pinnules have a cordate base, and are stalked-attached to a rachis. This morphology changes to the trifoliate form (Fig. 4c), with some transitional elements. In general, trifoliate pinnules are composed of a main pinnule of various lengths to which are basally attached two symmetrically-placed, smaller, orbicular pinnules, maximally 3 cm in length. Hair is absent in these pinnules, as confirmed in part by cuticular analysis. However, cuticular analysis of material illustrated in Fig. 4 reveals a stomatal structure, distribution of stomata, and shape of epidermal cells that are similar to those of M. scheuchzeri (see Cleal and Zodrow 1989, pl. 106). It is observed that trifoliate pinnules that reach a minimum length of 4 cm grew abundant hair, including on the orbiculars (Fig. 6; see also Bunbury 1847). Unless it can be demonstrated that the morphologies in Fig. 5 are reproducible over the distal parts of the frond, the usual assumption of self similarity (Heggie and Zodrow 1994) is called into question for M. scheuchzeri.

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19 The largest specimen of M. scheuchzeri (Fig. 7) from the Canadian Pennsylvanian originated from the historical coal-mining town of Sydney Mines, Sydney Coalfield. The primary rachis is preserved for a length of 22 cm, and is 1 cm wide with longitudinal striations that represent medullosalean sclerenchymatous fibres (imparting strength to the frond structure). The rachis also shows intercalated pinnules, as is typical for M. scheuchzeri (Bertrand 1930, pl. IX). Oppositely-placed ultimate rachides, 0.5 cm wide and striated, are preserved for a length of 17 cm, and are spaced ca. 11 cm apart; terminal pinnules are eroded. The fragmentary frond only bears trifoliate pinnules that are up to 6.5 cm long and 1.5 cm wide, and axially spaced at ca. 2 cm intervals. Abundant hair is present on all of the pinnules and orbiculars. This specimen fits Bunbury's (1847, pl. XXI, figs. 1A and 1B) concept of N. cordata var. angustifolia. The largest M. scheuchzeri specimen known with such morphology is 1 meter long, 50 cm wide, and is figured by Bertrand (1930, pl. IX).

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20 Apart from these forms, foliage of the M. scheuchzeri frondis furthermore characterized by laciniate, or deeply cut leaves that additionally show odontopteroid morphology (but it is not true odontopterid!). Bunbury (1847, pl. XXIII, fig. 1A, 1B) figured "Odontopteris subcuneata" from Sydney Mines, but he did not mention the presence of epicuticular hair on the holotype [that Bell in 1938 had recognized as being a synonym of M. scheuchzeri]. However, epidermal features of the 6-cm specimen of Bunbury's laciniate species include irregular, cluster-distributed trichomal bases on adaxial and abaxial surfaces, and on intercostal and costal fields. Epidermal cells are also similar in shape and size to those described by Cleal and Zodrow (1989) for M. scheuchzeri. Fig. 8 is one example of laciniate "O. subcuneata" that at the same time is the contiguous apical part of an otherwise typical M. scheuchzeri complete with epicuticular hair. This direct evidence confirms conspecifity between Bunbury's "O. subcuneata" and Hoffmann's species M. scheuchzeri, a proposition which Bell (1938) assumed all along.

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21 Detached "O. subcuneata" forms are shown in Fig. 9a, b. A deeply cut laciniate form, different from Bunbury's "O. subcuneata", is shown in Fig. 10a, whereas Fig. 10b compares with pl. XII of Bertrand (1930). Fig. 11a illustrates yet another atypical foliar form, haired, by its comparatively narrow, long and linear pinnule, without basal orbicular pinnules, and with a conspicuous acuminate tip. Smaller foliar versions of this form are shown in Figure 11b. This form is figured as Neuropteris angustifolia by Brongniart (1831, pl. 64, figs. 3 and 4), or as N. cordata var. angustifolia by Bunbury (1847). Fig. 12 documents a rather broad and short pinnule (compare with typical M. scheuchzeri, Fig. 2a). Transitional forms between being laciniate and typical pinnules are documented in Fig. 13 a,b; the lower parts of these leaflets are haired, whereas the apical parts are not.

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Palaeoecological aspects

22 M. scheuchzeri has all of the characteristics of a water-retaining, drought-adapted, arborescent pteridospermous plant which includes having a very thick cuticle, densely distributed trichomes and sunken stomata (observed on the cuticle; Cleal and Zodrow 1989), and epicuticular hair. Its larger leaf size presumably dissipated heat at a slower rate (summary: Wing and DiMichele 1992).

Foliar forms in the frond structure of M. scheuchzeri

23 The rachial architecture of the M. scheuchzeri frond is demonstrably bipinnate (Laveine and Brousmiche 1982), with intercalated pinnate structures (Bertrand 1930; this paper). The frond furthermore possesses the basal dichotomy of the neuropteroid-frond structure, and an apical bifurcation (Laveine and Brousmiche 1982, pl. 1, fig. 3; Laveine et al. 1998). Laveine (1967, 1989, p. 44) interpreted the frond as being retarded structurally, i.e., the typical or trifoliate pinnule is actually homologous to an imparipterid ultimate pinna that normally is composed of a number of lateral pinnules with one definite terminal pinnule.

24 Bertrand's largest specimen (1930, pl. IX) shows remarkable invariant-pinnule morphology in the proximal part of the one-meter long primary rachis. By contrast, Figs. 4 and 5 show remarkable variability in proximal-pinnule morphology in a 30-cm long specimen to suggest that it originated from the distal part of the bipinnate frond where morphology changes very rapidly. It is material schematized in Fig. 5 that would fit into a position above an apical bifurcation of a neuropteroid frond (see also Zodrow and Cleal 1988) which does not bear intercalated pinnate structures and whose terminal pinnules are comparatively smaller (compare Fig. 3a). Figure 7 would fit into the more central part of the bipinnate frond.

25 What is still poorly understood is the position of the many illustrated laciniate pinnule forms within the frond structure of M. scheuchzeri. In the first instance, Sydney Coalfield is characterized by an abundance of atypical, laciniate forms, originating particularly from the stratigraphical level of the Emery Seam (Fig.1), mid-Westphalian D. At an equivalent stratigraphical horizon in the Saarland, Germany (Cleal and Zodrow 1989; Josten 1991), these forms are absent, nor do they occur to such a level of abundance in the French Late Carboniferous (Laveine 1967; 1989). Either the significance of the differences has never been properly understood, or it could simply be sample bias! Laveine (1967) explained that these laciniate pinnule forms were attached near the base of the frond, below the frond dichotomy (such attachments are normal for medullosalean fronds). However, if such is the case, the evidence presented in this paper points to highly variable forms occupying the segment below the main dichotomy of the frond. An alternative explanation would be that some of these forms are actually intercalated on a primary pinna rachis.