Two new species of sphenophyllalean strobili with in situ spores are proposed from the Radnice Basin of the western and central Bohemian Carboniferous continental basins of the Czech Republic. Bowmanites brasensis sp. nov. from Břasy (Matylda Mine) and B. pseudoaquensis sp. nov. from Ovčín locality are determined mainly on the basis of their spores, which are about 100 µm in diameter. The thick-walled exine of the miospores is laevigate or sometimes very finely scabrate on the proximal contact area. Spores resemble the dispersed species Punctatisporites obesus. Cones of B. brasensis and B. pseudoaquensis are organically connected with stems having prominent blade leaves and represent a new group of sphenophyllalean strobili.
The majority of Carboniferous sphenophylls and their spores are stratigraphically important fossils. Some also have palaeoecological importance and can be used in palaeoenvironmental reconstructions. Carboniferous sphenophylls have been classified based on sterile parts of plants (mainly leaves) for many years. Several authors have proposed a large number of sphenophyllalean species, even though heterophylly is widespread. The systematics of “leafy” sphenophyllalean taxa consists of several species that are probably synonymous. Most sphenophyllalean cones with different modes of preservation (petrifactions and compressions) have been assigned only to the genus Bowmanites. The main criterion for their classification is the morphology/anatomy of the cones, the significance of their in situ spores having been ignored for many years. This contrasts with modern concepts of the classification of fossil reproductive organs where in situ spores are as important in taxonomic diagnoses and descriptions as their parent plants (e.g., Thomas 1970; Bek and Opluštil 2004, 2006). Authors following this concept have developed a new approach to the classification of Carboniferous sphenophyllalean fructifications based on a combination of morphological/anatomical features of the cones and their in situ spores (e.g., Bek 1998, 2004; Libertín and Bek 2003, 2006; Bek et al. in press). This new systematics represents a more natural classification and recognises seven groups (e.g., Libertín and Bek 2006) characterised by different cones and in situ spores. The current paper adds to our knowledge of a new group of sphenophyllalean plants. Studies of several specimens of compression strobili of Bowmanites brasensis sp. nov. and B. pseudoaquensis sp. nov. and slides with their in situ spores reveal a new type of sphenophyllalean spore.
History of research
Parent plants of Punctatisporites obesus-producing spores belong to the genus Sphenophyllum which was erected by Brongniart (1822) to formally accommodate fossil plants formerly included in Sphenophyllites. Presl (1838) was probably the first to describe a compression specimen of a sphenophyllalean cone, as Rotularia marsileafolia. Binney (1871) proposed the name Bowmanites for structureless fragments of sphenophyllalean cones, while Seward (1898) erected Sphenophyllostachys for cones of Sphenophyllum.
Sternberg (1823) described the sphenophyllalean plants Rotularia and Volkmannia from the Carboniferous of the Czech Republic. Bek (1998) and Bek and Opluštil (1998) were the first to describe in situ sphenophyllalean spores from the Czech Republic. There are two approaches for the subdivision and classification of sphenophyllalean cones. The first divides them into three groups (Jugati, Conferti, and Simplices) based on the number and position of sporangia on each sporangiophore (Hoskins and Cross 1943). The second approach was suggested by Remy (1955), who proposed four new cone genera of sphenophylls, Koinostachys, Aspidiostachys, Tristachya, and Anastachys, based on similar criteria with special attention to the number of sporangia. However, he also included some non-sphenophyllalean taxa (e.g., Anastachys). Both of these divisions united parent plants that produced monolete, trilete, and trilete operculate (vestispores and Pteroretis) spores in one group or one genus.
Localities, material, and methods
The specimens WBMP-F 82, NMP-E 6124 (1484 in Bek and Opluštil 1998), and NMP-E 6293 are from Ovčín opencast mine while specimens WBMP-F 03760, WBMP-F 01334, and WBMP-F 00188 came from Břasy, Matylda Mine (both Radnice Basin, Czech Republic). The geographic positions of these localities are shown in Fig. 1. Digital photomicrographs and negatives of spores are stored in the Institute of Geology, Academy of Sciences v.v.i., Prague, whereas digital photographs of the cones and slides with spores are in the NMP. Spores were recovered by dissolving small portions of sporangia in 35 % hydrofluoric acid for 24 h and in nitric acid (40%) for 24–40 h, then cleared in KOH (5%) for 1 h. All spores were mounted in glycerine jelly for direct microscopic study. Some spores were examined with a CAMECA SX100 scanning electron microscope. Photomicrographs were taken using an Olympus C330s digital camera and a BX51 light microscope. The terms used for the descriptions of the in situ spores are the same as those in the latest edition of the Glossary of pollen and spore terminology (Punt et al. 2007). In situ spores were classified according to the system for dispersed spores suggested by Potonié and Kremp (1954, 1955), Dettmann (1963), and Smith and Butterworth (1967).
Order Sphenophyllales Seward, 1898
Family Sphenophyllaceae H. Potonié, 1893
Genus Bowmanites Binney, 1871
Type species: Bowmanites cambrensis Binney, 1871
Type locality: Pontypool, South Wales, Bensham, Seam, Jarrow; Viséan.
Etymology: After Břasy, the type locality.
Type material: Holotype: NMP-E 6293; paratypes: WBMP-F 03760, WBMP-F 01334, and WBMP-F 00188.
Type locality: Břasy (Matylda Mine), Radnice Basin, Czech Republic.
Type horizon: Base of the Whetstone volcanic horizon, directly overlying the Lower Radnice Coal, Radnice Member, Radnice Basin, Kladno Formation, Lower Bolsovian, Pennsylvanian, Carboniferous.
Material.—Specimens NMP-E 6293, WBMP-F 03760, WBMP-F01334 and WBMP-F 00188 from the Břasy (Matylda Mine) locality.
Diagnosis.—Monopodially branched sterile stems with internodes. Leaves divided into three or four lobes with sharp tips. Six leaves in whorls. Strobili borne terminally. Circular sporangia. The surface of sporangia grooved, terminal sporangia among the axis and sterile bracts. Trilete circular to subcircular spores. Rays of trilete mark three-quarters of the radius. Exine 3–6 µm thick.
Description.—Shoots are branched monopodially (Fig. 2C, E) and from 0.5 to 4 mm wide but thicker at the nodes. The internodes are 2–5 mm long. The narrow, elongate leaves can be divided into three or four segments, each with a prominent sharp tip (Fig. 2A, D). Leaves are arranged in verticills (Fig. 2E), up to six, deeply divided along single vein. The length of lobes is 2–5 mm. Reproductive organs are terminally borne (Figs. 2B, 3A). The strobili are 40–60 mm long and their width, including bracts, is 5–6 mm. The cone axis is relatively narrow, approximately 0.5 mm wide (Fig. 3D). Circular sporangia 1 mm in diameter (Figs. 2B, 3B–D) are connected to the cone axis by sporangiophores about 0.2 mm long (Fig. 3B, C). The surface of the sporangia is prominently grooved. Sporangiophores with terminal sporangia lie between the axis and sterile bracts (Fig. 3D). Sterile bracts are 4–5 mm long, hook-shaped, prominently deflected at the area with sporangiophores. Trilete spores are circular to subcircular 88(96)106 µm in diameter. Inner body 75(88)100 µm in diameter. The spores are three-dimensionally preserved (Fig. 3E–H). The outer exine layer, probably exospore, is almost always broken or is not preserved (Fig. 3E–H). Its thickness is 3–6 µm (Fig. 3I). Almost all spores are, therefore, preserved as isolated circular trilete inner bodies about 88 µm on average.
Discussion.—All the spores are closely similar and are comparable to the dispersed species Punctatisporites obesus.
Stratigraphic and geographic range.—Radnice Member, Lower Bolsovian, Carboniferous, the Radnice Basin, Czech Republic.
Etymology: The morphology of the holotype closely resembles cones of Sphenophyllostachys aquensis.
Holotype: NMP-E 6124.
Type locality: Ovčín opencast mine, near Radnice, Radnice Basin, Czech Republic.
Type horizon: Base of the Whetstone volcanic horizon, directly overlying the Lower Radnice Coal, Radnice Member, Kladno Formation, the Radnice Basin, lower Bolsovian, Pennsylvanian; Carboniferous.
Material.—Holotype (NMP-E 6124), and specimen NMP-E 6293 from Ovčín opencast mine, Radnice Basin.
Diagnosis.—Twelve lanceolate leaves per whorl. Tips of leaves pointed. Circular sporangia, sterile bracts arch-like bend apically. Subcircular to circular trilete spores. Thick-walled exine laevigate to finely scabrate.
Description.—Stems are monopodially branched, 1–2 mm wide with internodes 2–8 mm long and 1–3 mm wide (Fig. 6A, B, D). Leaves are 4–8 mm with narrow lanceolate margins (Fig. 6 C) and arranged in whorls (Fig. 4A, B2). Cones are 200 mm long and 4–5 mm wide and possess an apex with sterile leaves. Circular sporangia are about 1.5–2 mm in diameter (Figs. 4A3, 5A, G, 6E). Sporangia occur between the axis of the cone and sterile bract on reduced sporangiophores (Fig. 5A, G). The surface of sporangia is finely rugose. Sterile bracts are hook-shaped; the apical margins are slightly wider at the end with rounded tips. Sterile stems are 1.0–2.5 mm in diameter.
Trilete spores are 110(121)150 µm in diameter (Fig. 5B–F). Circular to subcircular amb, simple laesurae, 1/2–2/3 of the radius. Exine laevigate to finely scabrate at the proximal contact area, 5–9 µm thick (Fig. 5B–F).
Remarks.—All the spores are closely similar and can be identified as Punctatisporites cf. obesus because the sculpture of the exine is sometimes very finely scabrate (Fig. 5E, F) and not laevigate as given in original diagnosis of P. obesus (Potonié and Kremp 1954).
Stratigraphic and geographic range.—Radnice Member, Kladno Formation, Lower Bolsovian, Radnice Basin, Czech Republic.
Discussion of the spores
Dispersed Punctatisporites .—The dispersed spore genus Punctatisporites consists of several different spore types. Some species possess a relatively thin (e.g., P. minutus) or thicker exine (e.g., P. breviornatus). The exine of some species is laevigate (e.g., P. calvus), microgranulate (e.g., P. irrasus), vermiculate (e.g., P. vermiculatus), echinate-spinate (e.g., P. decorus), infrareticulate (P. nervatus), setate (P. setulosus) or possesses a ridge (P. sinuatus). The smallest spores of Punctatisporites are only about 14 µm in diameter, while the largest can reach 140 µm in diameter. Punctatisporites was proposed by Ibrahim (1933) and emended by Potonié and Kremp (1954) for spores with a variable exine sculpture (laevigate, punctate, microreticulate or microgranulate). For example, Guennel (1958) considered that all species with a sculptured exine (like P. punctatus, the type species of the genus) should be separated taxonomically. Punctatisporites today consists of several miospore morphotypes and it is evident that the genus is highly heterogenous and represents an artificial taxon. This presumption is supported by the extremely long stratigraphic ranges of the morphologically simplest spores. The first rare records of spores of this type are from the Silurian, more frequently occuring in the Devonian and becoming more abundant in the Carboniferous (Bek 1998).
Some dispersed Punctatisporites species are more or less similar to P. obesus, like P. limbatus, P. labiatus, P. pseudobesus, P. edgarensis, or P. bifurcatus. Laevigatisporites laevigatus and the Permian genus Callumispora may be also be similar.
In situ Punctatisporites.—In situ spores of the Punctatisporites-type have been described from fructifications of plant species of different stratigraphic ages and taxonomic positions. The oldest in situ record of similar simple spores was published by Hoeg (1967) from a Devonian member of the Trimerophytophytina. Boureau (1964) mentioned in situ spores of the Calamospora—Punctatisporites-type from a sphenopsid cone (Cheirostrobus pettycurensis). The most abundant Punctatisporites-producing plants were ferns and fern-like plants, mainly genera like Corynepteris, Stauropteris, Pecopteris, Scolecopteris, Asterotheca, and Radstockia (Balme 1995; Bek 1998). Similar in situ spores are reported also from parent plants of Triassic—Jurassic (Potonié 1962), Cretaceous (Krassilov 1982) and even Eocene (Balme 1995) age. Dispersed species of Punctatisporites are far more numerous than those reported in situ. Some spores of the Punctatisporites-type may even represent ontogenetic stages of marattialean microspores of the Cyclogranisporites—Verrucosisporites-type (Zodrow et al. 2006).
Comparison of measurements of Bowmanites brasensis sp. nov., Bowmanites pseudoaquensis sp. nov., and Sphenophyllostachys aquensis Remy, 1955.
The strobili of Bowmanites brasensis and B. pseudoaquensis are morphologically similar to Sphenophyllostachys aquensis Remy, 1955 (see Table 1) but differ mainly in their spore contents. In situ spores isolated from the type specimen of S. aquensis by Remy (1955) are laevigate, thin-walled and correspond with the dispersed genus Calamospora, while in situ spores of B. brasensis and B. pseudoaquensis are thick-walled and are assigned to a different spore genus, Punctatisporites.
In situ spores of both of the new Bohemian species correspond to the dispersed species Punctatisporites obesus. In situ spores isolated from the specimen assigned to B. pseudoaquensis (former S. aquensis) by Bek and Opluštil (1998) differ slightly in total diameter (120 µm on average) compared with spores of Bowmanites brasensis. It is evident that the Bohemian specimens, early wrongly assigned to S. aquensis by Bek and Opluštil (1998), differ from the holotype of S. aquensis and represent a new species.
The cone morphologies of Bowmanites pseudoaquensis, B. brasensis, and S. aquensis are only roughly similar. Sterile bracts of Bowmanites brasensis are distinctly dichotomously divided, but bracts of B. pseudoaquensis and S. aquensis are simple and lanceolate. The angle between the axis of the cone and bracts of S. aquensis is larger than in B. pseudoaquensis. Bracts of S. aquensis possess a prominent S-like shape, while bracts of B pseudoaquensis are straighter and those of B. brasensis curve apically two-thirds of their length from the point of attachment. Sporangiophores of B. pseudoaquensis and S. aquensis are relatively short, undivided and their sporangia are borne terminally. Sporangiophores of B. brasensis are relatively long (2 mm) and bear sub-terminal sporangia (Fig. 3C). Sphenophyllostachys aquensis is reported (Remy 1955) from the Duckmantian of Germany (Schacht Adolf locality) whereas Bohemian specimens are from the Bolsovian (Whetstone horizon) of the Radnice Basin.
There is no evidence of heterophylly comparable to that reported in Sphenophyllum tenerrimum and S. myriophyllum. The general habit is similar to S. trichomatosum, but B. brasensis does not possess trichome bases of the stem.
Similar laevigate, thick-walled spores have never been reported in situ from any Carboniferous fructifications.
Division of Punctatisporites spores into natural groups would be facilitated by a good knowledge of their parent plants and/or an accurate grouping of morphologically similar spore types into a few new independent spore groups/genera (according to their sculpture, diameter and exine thickness). This is needed because Punctatisporites is probably the most variable Carboniferous dispersed spore genus known.
It is possible to divide Carboniferous sphenophylls into seven groups based on their in situ spore types and different morphologies and anatomies of the cones (Libertín and Bek 2006). Spores isolated from Bowmanites brasensis and B. pseudoaquensis do not correspond with any members of these groups and represent a new group of Carboniferous sphenophyllalean plants. Their special position is based mainly on their in situ spores. This suggests that the previous classifications of sphenophyllalean cones given by Hoskins and Cross (1943) and Remy (1955) need basic revision because these classifications group together plants that produce different spores (e.g., monolete versus trilete) and overlook the significance of in situ spores. We are currently working on a new classification of Carboniferous fertile specimens of sphenophylls. This revision will be based not only on the morphology of the cones, but also on their in situ spores, underlining the necessity of collaboration between palaeobotanists and palynologists.
We acknowledge financial support from the Grant Agency of the Academy of Sciences v.v.i. of the Czech Republic (projects A 3013902 and A 3001130503). Special thanks are due to Jiřina Dašková (Laboratory of Paleobiology and Palaeoecology, Institute of Geology v.v.i., Academy of Science, Prague, Czech Republic) and Duncan McLean (University of Sheffield, UK).