Introduction

Pteridosperms with the callipterid foliage are an important group of late Palaeozoic plants. In West and Central Europe, callipterids are known from the upper Stephanian (Němejc 1951; Doubinger 1956; Havlena 1958; Doubinger and Langiaux 1982) and they are regarded as characteristic elements of Autunian flora (Gothan and Gimm 1930; Barthel 1976, 1982; Haubold 1985; Kerp 1988). Callipterids are represented by five form-genera and the natural genus Autunia Krasser, 1919 emend. Kerp, 1988 of the family Peltaspermaceae Thomas, 1933 (Kerp and Haubold 1988).

In the Donets Basin, callipterids were found in upper Gzhelian deposits at Luganskoye village (Stschegolev 1960, 1965). Investigation of this site was initiated by Stschegolev (1965, 1975) and continued by the present author (Boyarina and Stschegolev 1989; Boyarina 1994, 2008). Three new taxa have been described, i.e., Lodevia luganica (Boyarina and Stschegolev, 1989) Boyarina, 1994, Raminervia mariopteroides Boyarina, 1994 and Dichophyllum cuneatum Boyarina, 1994. Some other callipterid species have only been named informally, but they have neither been described nor illustrated.

The aim of the present paper is to provide descriptions for eight callipterid species from the locality Luganskoye and to give a review of the main morphological characters of callipterid pteridosperms. Based on a revision of the type material of the previously established genera and species and a careful analysis of a series of the differently shaped pinnule remains illustrating heteroblastic development of these callipterids, revised diagnoses of some taxa are provided. The palaeoecology of callipterids is discussed, based on taphonomic and sedimentological data.

Geological setting

Callipterids were collected from six plant-bearing strata that are exposed in the gully situated on the west bank of the Lugan River Reservoir opposite Luganskoye village (Fig. 1). The section in the gully reaches a thickness of 50 m and represents the middle part of the Araukaritovaya suite in the interval between the limestones and P₆. This suite represents a part of the Moscovian-Gzhelian paralic succession and consists predominantly of lithological units interpreted as fluvial, lagoonal, bay, marine and peat-swamp deposits (Zhykaljak 1984). The plant-bearing strata (Fig. 2) occuring at Luganskoye are listed below.

Plant-bearing strata no. 1.—The strata with seven plant-bearing beds are exposed in the mouth part of the gully. The lowermost bed, consisting of claystone being up to 24 cm thick, yielded a rich plant assemblage. The following taxa have been identified from this bed: Autunia conferta (Sternberg, 1826) Kerp, 1988, A. naumannii (Gutbier, 1849) Kerp, 1988, Lodevia nicklesii (Zeiller, 1898) Haubold and Kerp, 1988, L. luganica (Boyarina and Stschegolev, 1989) Boyarina, 1994. A few small fragments of the Autunia naumannii foliage together with the pelecypods were collected in the 7 cm thick siltstones. A single specimen of Dicranophyllum sp. was found in a 5 cm thick siltstone bed. The rare plant remains and seeds from four overlying beds belong to Odontopteris subcrenulata (Rost, 1839) Zeiller, 1888, Neuropteris sp., Samaropsis bachmutiensis Boyarina, 2004.

Fig. 1.

Location of the outcrop at Luganskoye (starlet) in the Donets Basin.

Plant-bearing strata no. 2.—The strata with a thickness of 35 cm are situated in the lower part of the gully and consist of an alternation of claystone and siltstone. The plant assemblages from the siltstone beds include: Lodevia suberosa (Sterzel, 1918) Haubold and Kerp, 1988, Neuropteris cf. planchardii Zeiller, 1888, Neurodontopteris auriculata (Brongniart, 1830) Potonié, 1893, Calamites suckowii Brongniart, 1888, Cordaites sp., Walchia sp., Samarospermum moravicum (Helmhacker, 1871) Arber, 1914, Samaropsis bachmutiensis, and S. spinifera Boyarina, 2004.

Plant-bearing strata no. 3.—This claystone bed with a thickness of 45 cm yielded abundant plant fossils. The flora consists almost exclusively of the callipterids Raminervia mariopteroides Boyarina, 1994 as well as rare occurrences of Pecopteris bredovii Germar, 1845, Sphenopteris fayolii Zeiller, 1888 and Asterophyllites equisetiformis (Sternberg, 1825) Brongniart, 1828. The common plant remains from the 25 cm thick siltstone interval with thin claystone intercalations are assigned to the following taxa: Autunia conferta, A. naumannii, Lodevia nicklesii, L. luganica, L. suberosa, Dichophyllum cuneatum Boyarina, 1994, Odontopteris subcrenulata, Neuropteris planchardii, Sphenophyllum cf. angustifolium (Germar, 1845) Göppert, 1848.

Plant-bearing strata no. 4.—The strata reaching 45 cm in thickness consist of alternating clay-, silt-, and fine-grained sandstones. The plant remains found in the siltstones and the sandstones are frequently fragmented. The recognisable plant fossils were collected in the claystone beds, e.g., Autunia naumannii (foliage, small fragments of ovuliferous organs and many ovules), Lodevia luganica, Neurodontopteris cf. auriculata, Samarospermum moravicum.

Plant-bearing strata no. 5.—Two plant-bearing beds contain plant material and pelecypods. The bed with a thickness of 25 cm, consisting of an alternation of siltstone and fine-grained sandstone beds, yielded a rich plant assemblage comprising the following elements: Autunia naumannii (foliage, ovuliferous organs and ovules), Alethopteris rubescens (Sternberg, 1825) Němejc, 1929, Odontopteris osmundaeformis (Schlotheim, 1820) Zeiller, 1879, Walchia cf. frondosa Renault, 1885, Samaropsis spinifera, Trigonocarpus sp. Among the rare plant remains from the overlying 20 cm thick siltstones, some taxa were recognised, e.g., Autunia naumannii (foliage), Neurodontopteris cf. auriculata, Calamites sp., Cordaites sp., Samaropsis sp.

Plant-bearing strata no. 6.—These strata are exposed in the uppermost part of the gully. In this outcrop, rich assemblages of the plant remains were collected from three plant-bearing beds. The 15 cm thick sandstone bed yielded plant material belonging to Autunia naumannii (foliage, ovuliferous organs, polliniferous organs), Lodevia nicklesii, L. cf. luganica, Samaropsis sp. However, the richest plant assemblage was found in the siltstone bed that is 30 cm thick. The following taxa have been identified from this bed: Autunia naumannii (foliage, many fragments of ovuliferous and polliniferous organs, ovules), Lodevia nicklesii, L. luganica, L. suberosa, Dichophyllum cuneatum, D. flabelliferum (Weiss, 1879) Kerp and Haubold, 1988, Odontopteris osmundaeformis, O. subcrenulata, Cordaites sp., Walchia cf. whitei Florin, 1939, Samaropsis spinifera, Samarospermum moravicum. Among the plant fossils from the 15 cm thick claystone bed are Autunia conferta and Lodevia nicklesii.

Material and methods

The collection of studied callipterids includes more than 340 specimens. The plant remains are mainly preserved as compressions and impressions. The compressions are very thin and difficult to macerate. The specimens have been studied with a binocular dissecting microscope MBS-9.

Fig. 2.

Stratigraphic position, lithological successions, depositional environments interpretations and plant assemblages of the plant-bearing strata in the locality Luganskoye. ISS, International Stratigraphic Scale

Systematic palaeontology

Fig. 3.

Callipterid pteridosperms Autunia (Krasser, 1919) Kerp, 1988 from late Gzhelian (Late Carboniferous) of Luganskoye, Ukraine. A–F, H, J. Autunia conferta (Sternberg, 1833) Kerp, 1988. A–D, F, J. Plant-bearing strata no. 6. A. Basal portion of a frond with small closely spaced pinnules, SNM PMC 2216/77. B. Fragment of a pinna with entire-margined pinnules, SNM PMC 2216/78. C. Fragment of a frond showing the overtopping of the primary rachis, figured by Boyarina and Stschegolev (1989: fig.1a, b), SNM PMC 2216/2. D. Apical portion of a frond with lobed and pinnatifid pinnules, SNM PMC 2216/66. F. Fragment of a frond with small closely spaced pinnules showing the overtopping of the primary rachis in the apical part, SNM PMC 2216/81. J. Fragment of a frond with prolonged pinnules, SNM PMC 2216/23. E. Plant-bearing strata no. 1, fragment of an immature frond showing the very small pinnules folded along the midvein, SNM PMC 2216/79. H. Plant-bearing strata no. 3, middle portion of a small frond with entire-margined pinnules, SNM PMC 2216/80. G, I, K. Autunia naumannii (Gutbier, 1849) Kerp, 1988. G. Plant-bearing strata no. 6, apical portion of a frond showing the overtopping of the primary rachis in combination with prolonged and pinnatifid pinnules (“polymorpha”-aspect), SNM PMC 2216/24. I. Plant-bearing strata no. 5, pinnae with pinnatifid pinnules, SNM PMC 2216/52. K. Plant-bearing strata no. 2, upper portion of a frond with lobed pinnules in combination with the overtopping of the primary rachis, SNM PMC 2216/67. Scale bars 5 mm.

Fig. 4.

Callipterid pteridosperm Autunia naumannii (Gutbier, 1849) Kerp, 1988 from late Gzhelian (Late Carboniferous) of Luganskoye, Ukraine. A, L. Plant-bearing strata no. 1. B–D, H–K, M. Plant-bearing strata no. 6. E–G. Plant-bearing strata no. 5. A. Apical portion of a frond showing the overtopping of the primary rachis, SNM PMC 2216/43. B. Portion of a rachis with two basal parts of the ovuliferous organs, SNM PMC 2216/68. C. Upper portion of a frond showing the overtopping of the primary rachis in combination with prolonged and pinnatifid pinnules (“polymorpha”-aspect), SNM PMC 2216/25. D. Ovuliferous organ with closely spaced megasporophylls and attached ovules, SNM PMC 2216/71. E. Pinna in association with an isolated megasporophyll, SNM PMC 2216/82. F. Longitudinal section of a megasporophyll with two attached ovules, SNM PMC 2216/64. G. Isolated megasporophylls and seeds, SNM PMC 2216/83. H. Ovuliferous organ with closely spaced megasporophylls and attached ovules, SNM PMC 2216/69.1. Apical portion of a frond showing the overtopping of the primary rachis in association with an isolated seed, SNM PMC 2216/41. J. Isolated polliniferous organ, SNM PMC 2216/72. K. Fragment of a small frond with pinnatifid pinnules, SNM PMC 2216/58. L. Upper part of a frond showing the repeated overtopping of the primary rachis, SNM PMC 2216/42. M. Fragment of a frond most likely above the overtopping of the primary rachis showing stronger developed exterior pinnatifid pinnules than interior one, SNM PMC 2216/28. Scale bars 5 mm.

Fig. 5.

Callipterid pteridosperms of morphogenera Dichophyllum Elias ex Andrews, 1941 and Lodevia Haubold and Kerp, 1988 from late Gzhelian (Late Carboniferous) of Luganskoye, Ukraine. A, D, F, H–J. Plant-bearing strata no. 6. A, D–J. Dichophyllum cuneatum Boyarina, 1994. A. Asymmetrically developed pinna of the penultimate order with lobed to incised pinnules, figured by Boyarina (1994: fig. 8A), holotype SNM PMC 2216/11, natural size. B. Dichophyllum flabelliferum (Weiss, 1879) Kerp and Haubold, 1988. Plant-bearing strata no. 6, small fragment a pinna, SNM PMC 2216/30. C. Lodevia luganica (Boyarina and Stschegolev, 1989) Boyarina, 1994. Plant-bearing strata no. 6, fragment of a frond above a bifurcation of the primary rachis showing the overtopping of the penultimate order axis in combination with lobed to dissected pinnules, figured by Boyarina and Stschegolev (1989: fig. 2a, b), holotype SNM PMC 2216/1. D. Fragment of a pinna with dissected pinnules, figured by Boyarina (1994: fig. 9B), SNM PMC 2216/14. E. Plant-bearing strata no. 3., fragment of a penultimate order pinna with lobed to incised pinnules, SNM PMC 2216/51. F. Middle portion of a frond with a bifurcation of the primary rachis showing the pinnate pinna below the bifurcation, SNM PMC 2216/60. G. Plant-bearing strata no. 3., fragment of a pinna with dissected pinnules, SNM PMC 2216/38. H. Fragment of a penultimate order pinna with incised pinnules, SNM PMC 2216/37. I. Fragment of a penultimate order pinna with dissected pinnules, figured by Boyarina (1994: fig. 9C), SNM PMC 2216/15. J. Fragment of a penultimate order pinna with dissected pinnules, SNM PMC 2216/65. K, L. Lodevia suberosa (Sterzel, 1918) Haubold and Kerp, 1988. K. Plant-bearing strata no. 6, SNM PMC 2216/54. K₁. Fragment of penultimate order pinna with dissected pinnules and spines on the axes. K₂. Detail, showing a spine on the primary rachis. L. Plant-bearing strata no. 3, pinnae with dissected pinnules, SNM PMC 2216/55. Scale bars 5 mm.

Morphological analysis of the frond architecture and the foliage

The results of the study of the morphology of callipterids are summarised in the Table 1. The reconstructions of the callipterid fronds presented here are based on the specimens representing the middle or upper parts of fronds.

In Autunia conferta fronds, the overtopping in the apical part and the terminal pseudodichotomy have been noticed by Gothan (1907a), Barthel and Haubold (1980) and indicated as diagnostic species characters by Kerp (1988). In our collection there are some fragments of A. conferta fronds with an apical overtopping of the primary rachis (Fig. 3C, F). The Autunia naumannii fronds show the overtopping of the primary rachis either once (Figs. 3G, 4A) or twice (Fig. 4L). It should be noticed, that fronds with small and frequently fused pinnules exhibit the overtopping of the primary rachis in combination with a terminal pseudodichotomy (Fig. 4A). The fronds with larger and loosely spaced pinnules interpreted to be of more mature fronds have the repeated overtopping (Fig. 4L).

Fig. 6.

Callipterid pteridosperm Lodevia nicklesii (Zeiller, 1898), Haubold and Kerp, 1988 from late Gzhelian (Late Carboniferous) of Luganskoye, Ukraine. A, C–H. Plant-bearing strata no. 6. A. Fragment of a penultimate order pinna with lobed pinnules, SNM PMC 2216/44. B. Plant-bearing strata no. 1, fragment of a frond showing the bifurcation of the primary rachis and lobed pinnules, SNM PMC 2216/46. C. Fragment of a penultimate order pinna showing the overtopping of the penultimate order axis in combination with lobed pinnules, SNM PMC 2216/48. D. Fragment of a penultimate order pinna in association with a seed, SNM PMC 2216/61. E. Fragment of a penultimate order pinna showing the overtopping of the penultimate order axis in combination with lobed pinnules, SNM PMC 2216/34. F. Pinna with incised pinnules, SNM PMC 2216/40. G. Fragment of a frond with the bifurcation of the primary rachis and pinnate pinnae below the bifurcation, SNM PMC 2216/63. H. Portion of a pinna with incised pinnules, SNM PMC 2216/53. Scale bars 5 mm.

The callipterid fronds belonging to the form-genera Lodevia, Dichophyllum, and Raminervia display a bipartite blade in the upper part (Figs. 5C, F, 6B, G, 7H, 8A). Their bifurcated primary rachis results in two equal parts being the penultimate order axes. The penultimate order pinnae are asymmetrically developed. They bear interior pinnae of ultimate order being reduced and less well developed in order to avoid overlapping. This general appearance of the fronds is typical for pteridosperms with a bifurcation of the primary rachis (Laveine 1997). The studied callipterids show some differences in the architecture in the upper parts of the frond above the bifurcation.

The penultimate order pinnae of Dichophyllum cuneatum show the gradual steps of progressive decrease in a differentiation of the ultimate order pinnae toward the frond apex, viz., the penultimate order pinnae have the normal shape of a pinnate pinna (Fig. 5A). Below the bifurcation, the primary rachis bears the pinnate pinnae (Fig. 5F; Boyarina 1994: fig. 9a). The same type of frond architecture is seen in Dichophyllum flabelliferum frond in Gothan (1907b: fig. 1). Therefore, the frond of callipterids of the genus Dichophyllum may be designated as bipartite in the upper part, and bipinnate with the pinnate pinnae below the bifurcation.

Three fragments of Lodevia nicklessii fronds (Fig. 6B–D) and one fragment of a L. luganica frond (Fig. 5C) show the overtopping of the penultimate order axes. Information about the proximal part of the frond is based on the frond fragment of L. nicklessii that shows the pinnate pinnae below the bifurcation (Fig. 6G). This fossil material suggests that fronds in two species of the genus Lodevia were bipartite in the upper part and bipinnate, with an overtopping of the penultimate order axes. The species of the genus Lodevia are also distinguished by the surface structure of the rachis (Table 1).

The frond and pinnae fragments of Raminervia mariopteroides (Figs. 7G, F, 8F) show the gradual steps of progressive decrease in a differentiation of the ultimate order pinnae toward the frond apex and the penultimate order axis terminating in a pseudodichotomy. Therefore, the R. mariopteroides frond is designated as bipartite in the upper part, and bipinnate with an apical pseudodichotomy of the penultimate order axes.

The foliage of the described callipterids demonstrates also considerable heterophylly that is an expression of the heteroblastic development already documented for some callipterids (Kerp 1988; Boyarina 1994). Heterophylly in the callipterids with the segmented lamina of the ultimate order elements is expressed in the progressively increasing segmentation of the pinnules correspondently to increasing its sizes (Table 1). The fact that the appearance of the pinnule segments, dissected in a different degree, is identical, the shape of lobes or segments is accepted as a diagnostic species character, whereas, the degree of segmentation of the pinnules and their sizes are considered to be intraspecific variability of the pinnules reflecting successive growth stages of the foliage.

Palaeoecological implications

The callipterids are, on the whole, considered as elements of the non-peat-forming mesophilous vegetation grown on sandy river banks and natural levees above the ground water level (Gothan and Gimm 1930; Barthel 1976, 1982; Remy and Remy 1977), although there are a few but extremely rich occurrences of Autunia conferta from peat-forming environments (Haubold 1985; Kerp 1988). Only the ecology of Autunia conferta has been interpreted in detail (Roselt 1980; Haubold 1985; Kerp 1988).

The fossil plants from the locality Luganskoye give additional information about the ecology of callipterids. At Luganskoye callipterid remains are found in fluvial deposits that are represented by greenish-grey clay-, silt-, and sandstone beds, occurring a few meters above the alluvial, grey fine-, and middle-grained sandstones with the cross and sinuous lamination (Zhykaljak 1984). The fluvial deposits alternate with the lagoonal strata. The latter are represented by barren siltstone and fine-grained sandstone beds. The continuous succession, consisting of alternating the fluvial and lagoonal sediments is interpreted as a sequence of a delta plain setting. Three types of depositional environments have been recognised based on the facies of these deposits and the taphonomical analysis of the plant remains reconstructing their transport history. It includes: lacustrine, floodplain-lacustrine, and floodplain settings (Fig. 2).

Fig. 7.

Callipterid pteridosperms of morphogenera Lodevia Haubold and Kerp, 1988 and Raminervia Boyarina, 1994 emend. nov. from late Gzhelian (Late Carboniferous) of Luganskoye, Ukraine. A, B, E. Lodevia nicklesii (Zeiller, 1898), Haubold and Kerp, 1988. A, E. Plant-bearing strata no. 6. C, D, F–H. Plant-bearing strata no. 3. A. Portion of a pinna with incised pinnules, SNM PMC 2216/45. B. Plant-bearing strata no. 3, pinna with lobed pinnules, SNM PMC 2216/50. C, D, F–H. Raminervia mariopteroides Boyarina, 1994. C. Fragment of a pinna showing its asymmetrical development in combination with slightly lobed pinnules, SNM PMC 2216/76. D. Fragment of a penultimate order pinna with lobed to slightly incised pinnules, figured by Boyarina (1994: fig. 2C), holotype SNM PMC 2216/4. E. Pinna with dissected pinnules, SNM PMC 2216/73. F. Fragment of a penultimate order pinna terminating in a pseudodichotomy, SNM PMC 2216/49. G. Fragment of a pinna showing its asymmetrical development in combination with slightly lobed pinnules, SNM PMC 2216/35. H. Fragment of a frond above the bifurcation of the primary rachis showing the asymmetrical penultimate order pinna and slightly lobed pinnules, SNM PMC 2216/118. Scale bars 5 mm.

Fig. 8.

Callipterid pteridosperm Raminervia mariopteroides Boyarina, 1994. Plant-bearing strata no. 3. from late Gzhelian (Late Carboniferous) of Luganskoye, Ukraine. A. Middle portion of a frond showing the bifurcation of the primary rachis, figured by Boyarina (1994: fig. 3A), paratype SNM PMC 2216/6. B. Fragment of an asymmetrical penultimate order pinna with lobed pinnules, SNM PMC 2216/36. C. Fragment of a penultimate order pinna showing the strongly developed basal basiscopic pinnule as in mariopterids, SNM PMC 2216/39. D. Fragment of a penultimate order pinna with entire-margined and slightly lobed pinnules, SNM PMC 2216/32. E. Fragment of a frond with small entire-margined and slightly lobed pinnules, possibly a basal part of an immature frond under the bifurcation, SNM PMC 2216/27. F. Fragment of an asymmetrical penultimate order pinna with entire-margined pinnules, SNM PMC 2216/47. G. SNM PMC 2216/10, figured by Boyarina (1994: fig. 2A). Fragment of a pinna with dissected pinnules (G₁), detail, showing insect damage of the pinnule (G₂). Scale bars 5 mm.

Table 1.

Main characters of the sterile foliage of the described callipterids.

The lacustrine deposits are represented by massive greenish-grey claystone with a thickness of 45 cm (lower part of the strata no. 3), which yielded a rich plant assemblage consisting of the dominant callipterid Raminervia mariopteroides and rare pecopterids. The claystone facies indicates a very low-energy environment like an oxbow lake. The association of callipterids and ferns indicates that the plants grew on lowland shores with humid ecological niches.

The floodplain-lacustrine deposits consist of an alternation of clay-, silt-, and fine-grained sandstone beds (lower part of the strata no. 1, upper part of the strata no. 3, strata nos. 4 and 6), which contain the abundant callipterids, rare neuropterids and odontopterids. The fine-grained deposits with well-preserved large and small fragments of callipterid fronds indicate that the depositional environments ranged from low-energy to moderately high-energy settings and that the plant remains have been transported over a minimal distance. These strata are interpreted as delta lake levee deposits, and the fossils occurring in them may have derived from the plants that spread on delta levees. The sandy silt- and sandstone beds within this sequence yielded the fragmented pteridosperm remains. These deposits may have formed by minor sheetfloods during brief flooding episodes.

The floodplain deposits are composed of the horizontally or vaguely laminated siltstones and the fine-grained sandstones (upper part of the strata no. 1 and strata nos. 2 and 5), that contain rare callipterids, neuropterids, odontopterids, cordaites, conifers, seeds as well as plant detritus and bivalves. All plant material consists of fragmented pinnae and isolated pinnules, which were likely to have been transported to the site of deposition during floods from delta levees and valley-side slopes.

It is necessary to note that the co-occurrence of pteridosperms and cordaites, both in the floodplain-lacustrine and floodplain deposits, suggests that these fossils may have originated from plants, which could grow both within a floodplain and on valley-side slopes. Conifers are only rarely concentrated in floodplain deposits related to high-energy environments, therefore they are interpreted as plants exclusively occurring on slopes.

Taphonomical features of the plant remains from the different facies indicate some ecological differentiation among individual callipterid populations within the delta plain.

Autunia conferta.—A. conferta foliage is only known from floodplain-lacustrine deposits represented by claystone beds (strata nos. 1 and 6) and thin claystone interlayers within the siltstone bed (strata no. 3). The occurrence of relatively well-preserved fragments of apical and middle portions of fronds in lacustrine deposits of low-energy settings indicates that A. conferta may have principally spread on shores of delta levee lakes.

Autunia naumannii.—This is one of the dominant elements of the plant assemblages in this locality. Frond fragments of A. naumannii are abundant in the floodplain-lacustrine siltstone (strata no. 6) and common in the clay- and sandstone beds (strata nos. 1, 3, and 4). The well-preserved fragments of the fronds in the clay- and siltstone beds, which represent rather low-energy facies, indicate that the fossil material was transported over a minimal distance. In addition, A. naumannii rarely occurs together with other pteridosperms and conifers in the silt- and sandstone beds interpreted as the floodplain deposits (strata no. 5). The small portions of fronds and pinnae from the sandstone beds represent plant material drifted during floods. Thus, A. naumannii may be considered as a widespread element of the delta plain vegetation growing both on shores of delta levee lakes and on delta levees.

Lodevia nicklesii.—Frond and pinnae fragments of L. nicklesii are abundant in the floodplain-lacustrine silt- and sandstones (strata no. 6), and common in the claystone (strata nos. 1 and 6) and the siltstone beds (strata no. 3). Consequently, L. nicklesii may be regarded as a dominant element of the floodplain vegetation like A. naumannii that grew within delta levees and near delta levee lakes.

Lodevia luganica.—Lodevia luganica foliage was found in the floodplain-lacustrine deposits, viz., in the claystone (strata nos. 1 and 4) and the siltstone beds (strata nos. 3 and 6). The occurrence of a few frond and pinnae fragments in the deposits of not very high-energy depositional environments indicates that L. luganica was a rare element of the floodplain vegetation and could spread within delta levees together with dominant the forms Autunia naumannii and Lodevia nicklesii.

Lodevia suberosa.—A few small fragments of L. suberosa were found in the floodplain-lacustrine siltstone (strata nos. 3 and 6) and the floodplain siltstone (strata no. 2). The division of the pinnules into the narrow segments and the presence of the spines on the axes may suggest a mesophytic nature. Rare small remains of L. suberosa in the rich plant-bearing beds (strata nos. 3 and 6) indicate that this species was a rare element of the plant communities and that these callipterids grew at some distance from lakes within the floodplain. It is possible, that L. suberosa, regarding its mesophytic nature, could grow on delta levees as well as on valley-side slopes.

Dichophyllum cuneatum.—The common well-preserved remains of the D. cuneatum foliage from the floodplain-lacustrine siltstone (strata nos. 3 and 6) point at a short distance of transportation. Therefore, D. cuneatum may be considered to have been a widely distributed element of the floodplain vegetation growing mainly on delta levees.

Dichophyllum flabelliferum.—A single specimen of D. flabelliferum was found in the siltstone interpreted as the floodplain-lacustrine deposit (strata no. 6) together with other callipterids that occur abundantly. These data are not sufficient for ecological interpretations. By analogy with the associated callipterids, D. flabelliferum was possibly a rare element of the floodplain or valley-side slopes vegetation.

Raminervia mariopteroides.—The abundant frond fragments of R. mariopteroides were found in lacustrine claystone (strata no. 3). Raminervia mariopteroides is accompanied by rare remains of the hydrophilous and hygrophilous plants, i.e., Pecopteris bredovii Germar, 1845, Sphenopteris fayolii Zeiller, 1888 and Asterophyllites equisetiformis (Sternberg, 1825) Brongniart, 1828. Consequently, R. mariopteroides can be considered to be the dominant element of the plant community that occupied lowland shores of oxbow lakes.

Conclusions

Late Gzhelian pteridosperms with callipterid foliage in the Donets Basin are represented by eight species including three endemic taxa and belong to four genera. The callipterids of the morphogenera Lodevia, Dichophyllum, and Raminervia are characterised by a bifurcation of the primary rachis in the upper part of the bipinnate fronds and can be distinguished by the morphology of the penultimate order pinnae. The penultimate order pinnae of Dichophyllum cuneatum have the normal shape of pinnate pinnae. Lodevia nicklessii and L. luganica are characterised by the overtopping of the penultimate order axes. The penultimate order axes of Raminervia mariopteroides terminate in the pseudodichotomy.

The form of segments and a degree of segmentation of pinnules are used for the specific delimitation of the callipterids with dissected pinnules. The shape of the lobes or segments of the pinnules is considered as the main diagnostic species character. The degree of segmentation of the pinnules and their size represent the intraspecific variability related to the heteroblastic development of the foliage.

In the upper Gzhelian, the callipterids occur in the floodplain, floodplain-lacustrine and lacustrine deposits and they are interpreted as elements of the floodplain vegetation within a delta plain. They were spread within delta levees and on the shores of delta levee lakes. Raminervia mariopteroides a species that was associated by ferns grew on oxbow lake shores. The floodplain vegetation was dominated by Autunia naumannii and Lodevia nicklesii.