Late Devonian plant communities of North Russia

Review of Palaeobotany and Palynology 224 (2016) 94–107

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Late Devonian plant communities of North Russia Olga A. Orlova a,⁎, Aleftina L. Jurina a,1, Sergey M. Snigirevsky b,2 a b

Department of Palaeontology, Faculty of Geology, Lomonosov Moscow State University, Leninskie Gory, 1, Moscow 119991, Russia Institute of Earth Science, Saint Petersburg State University, 7-9, Universitetskaya nab., St Petersburg 199034, Russia

a r t i c l e

i n f o

Article history: Received 5 April 2015 Received in revised form 27 August 2015 Accepted 29 August 2015 Available online 21 September 2015 Keywords: Plant assemblages Northern Russia Frasnian Famennian Palaeovegetation

a b s t r a c t The article summarizes researches on the Late Devonian vegetation based on rich palaeobotanical collections from different localities of northern Russia (North Timan). The composition and the structure of the land plant communities are described. Changes in the vegetation are reported during the Frasnian and Famennian. The most diverse palaeovegetation included 29 taxa and occurred in the late Frasnian (Ust'besmozhitsa Formation). It was followed by a floral decrease (6 taxa) in the latest Frasnian (Kamenka Formation). The taxonomic composition of the Famennian vegetation (12 taxa) was much poorer than that of the Frasnian. Differences between the archaeopteridean assemblages of the studied region and the coeval assemblages of the surrounding areas (Russia, Ukraine and Belarus) are discussed. In North Timan, the highest archaeopterid diversity occurred in the Frasnian whereas, in the Donetsk Region of the Ukraine, it occurred in the Famennian. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Late Devonian plants of the European part of Russia are poorly known among the foreign palaeobotanists. It is difficult to compare and correlate data on the Late Devonian plants from Russia and Western Europe because correspondence between their respective Miospore Zones of is unknown. The Late Devonian floras from the north of the European part of Russia (especially North Timan) are hardly known because their monographic studies were not carried out. Our knowledge of these plants is restricted to uncorrected taxonomic lists and brief biostratigraphic analyses in different publications (see Kossovoj, 1959; Petrosjan, 1988, 1991, etc.) Sporadic researches on the Late Devonian floras of North Russia started in the 1930–40s. The first data about the Late Devonian plant remains of North Timan (NT3) were shortly published by Krishtofovich (1937, 1939) and Zalessky (1948). Later, after first broad-scaled geological field trips, the geologist Kossovoj (1959, 1966, 1971) reported lists of fossil plants from various Upper Devonian deposits and localities of the area studied in this paper. Since 1960, several new taxa of Late Devonian plants have been described from NT (Petrosjan and Radczenko, 1960; Senkevich, 1960; Lepekhina, 1968; Petrosjan, 1968). Special mention should be made of the finding of a unique new heterosporous lycopsid (Kossoviella timanica Petrosjan), which was described by

⁎ Corresponding author. Tel.: +7 495 9395642. E-mail addresses: [email protected] (O.A. Orlova), [email protected] (S.M. Snigirevsky). Tel.: +7 495 9395642. 2 Tel.: +7 951 6404700. 3 NT - North Timan. 1

http://dx.doi.org/10.1016/j.revpalbo.2015.08.009 0034-6667/© 2015 Elsevier B.V. All rights reserved.

Petrosjan (Petrosjan and Kossovoj, 1984) from the Frasnian deposits of several localities in NT. The Devonian plant assemblages of NT were previously proposed by Petrosjan (1988, 1991). Jurina (1988) summarized known Devonian floras from USSR and gave a list of the Late Devonian plants from the Northern part of Russia. Later Snigirevsky (1997a) attempted to revise the Late Devonian floras of NT. Reproductive organs with in situ spores were studied by Tschibrikova et al. (1998, 1999), Telnova and Meyer-Melikyan (2002) and Snigirevsky et al. (2007). Some new lycopsid strobili with in situ spores are currently studied (Orlova et al., in progress). Besides, some probable brown algae (Caudophyton fasciolus (Penhallow) S. Snig.) were described from the Upper Devonian deposits of NT (Snigirevsky, 2001). The anatomical structure of Frasnian (Orlova et al., 2011; Orlova and Jurina, 2014) and Famennian (Snigirevskaya, 2012) plants of the Northern part of Russia was studied recently. For the historical review of palaeobotanical researches in NT, see Snigirevsky (1997b). In the present paper, we provide updated Late Devonian plant lists and revise plant floras from NT with illustrations of some representative taxa. The identification and nomenclature of fossil plant macroremains and miospores are revised when necessary. 2. Materials and methods The present paper is based on the analysis of the palaeobotanical collections housed at the Palaeontological museum of the Sedimentary Geology Department, Institute of Earth Sciences, Saint Petersburg State University (Petrosjan and Snigirevsky collection); the Department of Palaeontology, Geology Faculty, Lomonosov Moscow State University; the Earth Science Museum of Lomonosov Moscow State University; and the Komarov Botanical Institute, St Petersburg.

O.A. Orlova et al. / Review of Palaeobotany and Palynology 224 (2016) 94–107

The material has been collected since 1931 by different geologists. The richest fossil plant collection from NT has been found by Kossovoj between 1957 and 1964. Some samples were also collected by Egorov, Chernov, Menner, and Petrovsky (Snigirevsky, 1997a). Jurina found several wood samples from the Volonga River locality of Famennian age

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(Snigirevskaya, 2012). The major collection of late Frasnian plants from various localities on the eastern coast of the Barents Sea (NT) was collected by Snigirevsky in 1993. The material was studied and photographed using a Leica MZ16 light stereomicroscope, and an Olympus CX 31 light microscope. Fragments

Fig. 1. Geographic position of the Late Devonian localities of NT. 1—the Vyuchejskij borehole; 2—the Sula River; 3—the Rassokha–Beznoshitsa River; 4—the Veselaya River; 5—the Suvojnaya River (Frasnian and Famennian localities); 6—Suvojnij field (area between the Suvojnyj Cape and the Krestovyj Cape); 7—Krestovyj field (area between the Krestovyj Cape and Ludovatyj Cape); 8—Ludovatyj field (area between the East Ludovatyj Nos Cape and the Velikaya River mouth); 9—the Velikaya River; 10—the Peschanka River; 11—the Travyanka River (1–4, 6–11—Frasnian localities); 12—the Volonga River (Frasnian and Famennian localities); 13—the Indiga borehole (Famennian deposits).

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of sterile axes and sporangia with in situ spores were investigated with a TESCAN scanning electron microscope (SEM) in the Laboratory of Electron Microscopy of the Borissiak Paleontological Institute of the Russian Academy of Sciences, Moscow (PIN). 3. Stratigraphy and miospore zonation of NT The Late Devonian localities with fossil plants are distributed on the eastern coast of the Barents Sea near the Cheshskaya Bay, in a zone between the Indiga Bay and the Suvojnyj Cape, and along several riversides in the Arkhangelsk region (Nenets Autonomous Area) (Fig. 1). Fossil plants come from the sandstones, siltstones and mudstones of five formations of the Frasnian Stage (see below) and one formation of the Famennian Stage. The stratigraphic scheme of the Frasnian Stage of NT was developed by Kossovoj (1959, 1963, 1966, 1971) and later supplemented by Snigirevsky (1997a). In previous stratigraphic schemes (Rzonsnitskaya and Kulikova, 1990), the Nadezhdin Formation and Kumushka Formation were at the base of the Frasnian Stage (see also Kossovoj, 1971; Petrosjan, 1988, 1991; Snigirevsky, 1997a, etc.). However, in the present paper, they are not considered within the Frasnian Stage. The Nadezhdin Formation corresponds to the Yaranian Regional Stage of the Timan–Pechora Province; the latter is an analogue of the lower part of the Pashiyan Regional Stage of the East European Platform (Rzonsnitskaya and Kulikova, 1990) and is now related to the upper Givetian Substage (Menning et al., 2006; Sobolev and Evdokimova, 2008). The Kumushka Formation corresponds to the upper part of the Pashiyan Regional Stage and, now, is also related to the upper Givetian Substage (Menning et al., 2006). Consequently, in the present study, the Frasnian Stage of the area consists of the following formations (successively upward): Vyuchejskij Formation, Grubyj Ruchej Formation, Rassokha Formation, Ust'bezmoshitsa Formation and Kamenka Formation (Table 1). In regional stratigraphic schemes (Rzonsnitskaya and Kulikova, 1990) the deposits of the Vyuchejskij Formation were correlated to the deposits of the upper Timanian Regional Substage and to most part of the Sargaevian Regional Stage of the East European Platform (Table 1). The latter corresponds to the base of the Frasnian (Sobolev

Table 1 Correlation chart of the Frasnian of European Russia. Two middle columns show regional units. Local formations of North Timan are in the right column.

Substage

Regional stage of East European Platform (Menning et al., 2006)

Regional stage of Timan-Pechora Province (Sobolev et al., 2006)

Formation of NT (Rzonsnitskaya and Kulikova, 1990)

Upper Frasnian

Livnian

Livnian

Kamenka

Evlanovian

Evlanovian

Voronezhian

Sirachoian

Petinian

Vetlosyanian

Semilukian

Domanikian

Sargaevian

Ust’yaregian

Middle Frasnian

Lower Frasnian

Ust'bezmoshitsa

Rassokha

Grubyj Ruchej Vyuchejskij

Upper Givetian

Timanian

Timanian

Pashiyan

Dzherian

Kumushka

and Evdokimova, 2008). The miospore assemblage of the Vyuchejskij Formation is represented by Geminospora rugosa (Naumova) Obukhovskaya, G. micromanifesta (Naumova) Arkhangelskaya, G. macromanifesta (Naumova) Obukhovskaya, Gravisporites basilaris (Naumova) Pashkevich, and Archaeozonotriletes variabilis Naumova (Rzonsnitskaya and Kulikova, 1990) (Table 2). The overlying deposits of the Grubyj Ruchej Formation correspond to the upper part of the Sargaevian Regional Stage (lower Frasnian) of the East European Platform (Table 1). The palynological assemblage of the Grubyj Ruchej Formation is represented by Apiculatisporites dentatus (Naumova) Obukhovskaya, Ancyrospora arguta (Naumova) Lu, Ancyrospora incisa (Naumova) M. Raskatova et Obukhovskaya, Calyptosporites krestownikovii (Naumova) Oshurkova, Calyptosporites bellus (Naumova) Oshurkova, Geminospora rugosa, Tholisporites variabilis var. insignis Sennova, Archaeoperisaccus ovalis Naumova, A. concinnus Naumova, and A. menneri Naumova (Rzonsnitskaya and Kulikova, 1990) (Table 2). The Rassokha Formation designates the middle Frasnian and lower part of the upper Frasnian deposits of NT. It corresponds to the Domanikian and Vetlosyanian Regional Stages of the Timan–Pechora Province (Rzonsnitskaya and Kulikova, 1990) (Table 1). The Domanikian Regional Stage of the Timan–Pechora Province corresponds to the Semilukian Regional Stage of the East European Platform, and the Vetlosyanian Regional Stage of the Timan–Pechora Province to the Petinian Regional Stage of the East European Platform (Table 1). According to Kossovoj and Pashkevich (1963) the palyno-assemblages of the Rassokha Formation are characterized by Geminospora rugosa, G. semilucensa (Naumova) Obukhovskaya et M. Raskatova, Ancyrospora laciniosa (Naumova) Mantsurova, Gravisporites basilaris, Apiculatisporis eximius (Naumova) Oshurkova, Diaphanospora aff. zadonica (Naumova) Arkhangelskaya, Calyptosporites bellus, Archaeoperisaccus ovalis (Table 2). The upper Frasnian deposits of NT are also represented by the Ust'bezmoshitsa Formation and the Kamenka Formation (Table 1). The first one corresponds to the Sirachoian Regional Stage of the Timan–Pechora Province (Rzonsnitskaya and Kulikova, 1990) and to the Voronezhian Regional Stage of the East European Platform (Menning et al., 2006). According to Jurina and Raskatova (2005), the miospore assemblages of the Ust'bezmoshitsa Formation are represented by Geminospora rugosa, G. sеmiluсеnsa, G. notаtа (Naumova) Obukhovskaya, G. nalivkinii (Naumova) Obukhovskaya, Arсhaеozonotrilеtes variabilis, Cymbosрorites aсanthaсеus (Kedo) Obukhovskaya, Convolutispora subtilis Owens, Kеdoesporis imperfectus (Naumova) Obukhovskaya, Cyrtospora explеta Arkhangelskaya, Bulbosisporites bulbosus (Obukhovskaya) Obukhovskaya, Lophozonotrilеtes torosus Naumova, L. tylophorus Naumova, L. grandis Naumova, Verruсosisporites grumosus (Naumova) Obukhovskaya, V. evlanensis (Naumova) Obukhovskaya, V. sp., Vеrrucirеtusispora pallidа Owens, Cristatisporites trivialis (Naumova) Obukhovskaya, C. deliquesсens (Naumova) Arkhangelskaya, C. sp., Diduсites radiatus (Kedo) Obukhovskaya, Мembrabaсulisporis radiatus (Naumova) Arkhangelskaya, Anсyrospora voronensis (Arkhangelskaya) Arkhangelskaya, A. sp., Aurorаsрora spесiosa var. 1, Stenozonotriletes сonformis Naumova, Hystricоspolites reflехus Owens, Arсhaeoperisaссus conсinnus, A. mirаndus Naumova, A. eсhinatus Raskatova and A. ovalis (Table 2). They correspond to the upper Frasnian MR Miospore Subzone (Membrabaculisporites radiatus) of the OG Miospore Zone (Table 2) adopted for the Timan–Pechora Province (Sobolev et al., 2006). The Kamenka Formation corresponds to the deposits of the Evlanovian and Livnian Regional Stages of the East European Platform (Table 1). According to the regional stratigraphic schemes (Rzonsnitskaya and Kulikova, 1990), the Kamenka Formation is represented by the miospores Auroraspora speciosa (Naumova) Obukhovskaya, Apiculatisporites dentatus, Diducites hopericus (Nazarenko) Obukhovskaya, Verrucosisporites grumosus, and Geminospora rugosa that characterize the AS Miospore Subzone (Auroraspora speciosa) of the DE

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Table 2 Miospore composition of the Upper Devonian deposits of NT. Stage

Formation

Famennian Pokayama

Frasnian

Miospore Zone of Eastern Europe Avkhimovich et al. (1993)

Miospore assemblage

Reference

C. varicornata CVa

Cornispora monocornata, C. bicornata, C. bicornata var. krilovkensis, C. tricornata, Cyrtospora cristifera, Grandispora famenensis, G. facilis, Diaphanospora rugosa, D. macrovaria, Archaeozonotriletes turgenevii, Auroraspora varia, Calyptosporites papilosus, Retispora lepidophyta, Laevigatosporites ovalis, Lophozonotrilеtes ledebianensis, L. proscurrus, L. curvatus, Stenozonotriletes сonformis Geminospora rugosa, Auroraspora speciosa, Apiculatisporites dentatus, Verrucosisporites grumosus, Diducites hopericus Geminospora rugosa, G. sеmiluсеnsa, G. notаtа, G. nalivkinii, Archaeozonotriletes variabilis, Cymbosрorites aсanthaсеus, Convolutispora subtilis, Kеdoesporis imperfectus, Cyrtospora explеta, Bulbosisporites bulbosus, Lophozonotrilеtes torosus, L. tylophorus, L. grandis, Verruсosisporites grumosus, V. evlanensis, V. sp., Vеrrucirеtusispora pallidа, Cristatisporites trivialis, C. deliquesсens, C. sp., Diduсites radiatus, Мembrabaсulisporis radiatus, Anсyrospora voronensis, A. sp., Aurorаsрora spесiosa var. 1, Stenozonotriletes сonformis, Hystricоspolites reflехus, Arсhaeoperisaссus conсinnus, A. mirаndus, A. eсhinatus, A. ovalis Geminospora rugosa, G. semilucensa, Ancyrospora laciniosa, Gravisporites basilaris, Apiculatisporis eximius, Diaphanospora aff. zadonica, Calyptosporites bellus, Archaeoperisaccus ovalis Geminospora rugosa, Apiculatisporites dentatus, Ancyrospora arguta, A. incisa, Calyptosporites krestownikovii, C. bellus, Tholisporites variabilis var. insignis, Archaeoperisaccus ovalis, A. concinnus, A. menneri Geminospora rugosa, G. micromanifesta, G. macromanifesta, Gravisporites basilaris, Archaeozonotriletes variabilis

Mantsurova (2014)

Kamenka

C. deliquescens–V. evlanensis DE Ust`bezmoshitsa A. ovalis–V. grumosus OG

Rassokha

G. semilucensa–P. donensis SD

Grubyj Ruchej

C. optivus–S. krestovnikovii OK

Vyuchejskij

Miospore Zone of the East European Platform (Sobolev et al., 2006) (Table 2). The Famennian deposits with fossil plants are restricted to the localities of the Pokayama Formation (Snigirevsky, 1997a). The miospore assemblages of the Pokayama Formation from the Volonga river locality (NT) were studied by Mantsurova (2014) and correspond to CVa (Cornispora varicornata) Miospore

Rzonsnitskaya and Kulikova (1990) Jurina and Raskatova (2005)

Kossovoj and Pashkevich (1963) Rzonsnitskaya and Kulikova (1990) Rzonsnitskaya and Kulikova (1990)

Zone of the Timan–Pechora Province (Sobolev et al., 2006) (Table 2). According to Avkhimovich et al. (1993), the abovementioned Miospore Zone is correlated with the VCo (Rugospora versabilis-Grandispora cornuta) Miospore Opel zone for Western Europe (Streel et al., 1987). In the Ardenne–Rhenish region, the VCo Miospore Zone is assigned to the middle Famennian.

Table 3 Taxonomic composition of the Late Devonian floras of NT. Asterisks indicate taxa reported in previous researches but absent in the collections studied in this paper. Stage

Formation

Plant assemblage

Localities

Famennian Pokayama

Outcrops along the Volonga River banks (Fig. 1.12); the Suvojnaya River banks (Fig. 1.5); the Indiga borehole (Fig. 1.13)

Frasnian

Outcrops along the Volonga river banks (Fig. 1.12)

Caudophyton fasciolus, Jurinodendron kiltorkense, Pseudobornia sp., Sphenophyllum subtenerrimum*, Archaeocalamites radiatus*, Rhacophyton condrusorum, R. incertum, Dimeripteris gracilis, Archaeopteris archetypus, A. halliana, Svalbarbia fissilis, Callixylon timanicum Kamenka Lepidodendropsis theodorii*, Knorria sp., Pseudobornia ursina, Pteridorachis sp., Archaeopteris sp., Heterangium sp.* Ust`bezmoshitsa Caudophyton fasciolus, Taeniocrada timanica, Timanophyton lorum, Gutzeitia timanica, Kossoviella timanica, Ludovatia mirabilis, Pseudobornia schweitzeri, Pseudobornia sp. 1, Kosychia nenetzkiana, Pseudosporochnus sp. 1, Cephalopteris mirabilis, Dimeripteris gracilis, Flabellofolium brownianum, F. gilkineti, Flabellofolium sp., F. williamsonii, F. ginkgophylloides, Rhacophyton incertum, R. condrusorum, Pteridorachis sp., Archaeopteris sphenophyllifolia, A. halliana, A. archetypus, Archaeopteris sp., A. macilenta, Svalbardia fissilis, S. osmanica, Eddya sullivanensis, Sphenopteridium lebedevii*. Rassokha Taeniocrada timanica, Kossoviella timanica, Knorria sp.*, Sphenophyllum subtenerrimum*, Rhacophyton incertum, Rhacophyton sp., Pteridorachis sp.*, Aphlebiopteris boegendorfiana, Dimeripteris gracilis, Svalbardia elschanica, S. fissilis, Archaeopteris archetypus, A. halliana, Archaeopteris sp., A. sphenophyllifolia. Grubyj Ruchej Kossoviella timanica, Lepidodendropsis theodori*, Knorria sp.*, Pseudobornia sp., Sphenophyllum sp.*, Meristopteris laciniata, Rhacophyton sp., Rhacophyton cf. incertum, Aphlebiopteris aff. boegendorfiana*, Dimeripteris gracilis, Pietzschia timanica, Calamophyton sp.*, Flabellofolium brownianum, F. gilkineti, Pteridorachis sp.*, Aneurophyton sp.*, Svalbardia osmanica, S. acuta, Archaeopteris sphenophyllifolia, Archaeopteris sp., A. archetypus, A. halliana, Sphenopteridium lebedevii*, Moresnetia cf. zalesskyi* Vyuchejskij Knorria sp.*, Rhacophyton sp., Flabellofolium brownianum, F. gilkineti, Dimeripteris gracilis, Meristopteris laciniata, Svalbardia acuta, Archaeopteris sphenophyllifolia, Aneurophyton sp., Heterangium sp.*

Outcrops of the Suvojnyj field (Fig. 1.6); Krestovyj field (Fig. 1.7); Ludovatyj field (Fig. 1.8); outcrops along the Velikaya river banks (Fig. 1.9); the Peschanka river banks (Fig. 1.10); the Travyanka river banks (Fig. 1.11)

Outcrops along the Rassokha–Bezmoshitsa river banks (Fig. 1.3); the Vyuchejskij borehole (Fig. 1.1)

Outcrops along the Rassokha–Bezmoshitsa river banks (Fig. 1.3); The Veselaya river banks (Fig. 1.4); the Sula river banks (Fig. 1.2)

Outcrops along the Sula river banks (Fig. 1.2); the Vyuchejskij borehole (Fig. 1.1)

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12 10 8 6 4 2

probable algae lycopsids sphenophytes fern-like plants progymnosperms probable seed ferns

0

Fig. 2. Changes in diversity of main plant groups in the Late Devonian of NT. The number of species is indicated on the vertical axis.

4. Results 4.1. Frasnian floras from NT Fossil plants from the Vyuchejskij Formation were found in the drill samples of the borehole near the Vyuchejskij Settlement (Fig. 1.1) and in the outcrops along the Sula river banks (Fig. 1.2). According to Kossovoj (1966), Petrosjan (1988), and Snigirevsky (1997a), Knorria sp., Rhacophyton sp., Flabellofolium brownianum (Daws.) Stone (Platyphyllum brownianum in Kossovoj, 1966), F. gilkineti (Leclercq) comb. nov. (Ginkgophyton gilkineti in Petrosjan, 1988), Dimeripteris gracilis Schmalh., Meristopteris laciniata Zal., Svalbardia acuta (Tschirk.Zal.) comb. nov. (Archaeopteris acuta in Petrosjan, 1988), Archaeopteris sphenophyllifolia Lesq. (A. sibirica in Kossovoj, 1966), Aneurophyton sp., and Heterangium sp. (according to Petrosjan, 1988) were found here. Among the ten species of the flora of the Vyuchejskij Formation (Table 3), lycopsids are rare and represented by stem fragments of Knorria. Sphenopsids are absent (Fig. 2). This flora shows a lot of leaf adpressions assignable to ferns (Flabellofolium, Rhacophyton, Meristopteris, and Dimeripteris) and archaeopteridaleans (Svalbardia and Archaeopteris), together with aneurophytalean (Aneurophyton) plant remains. The first appearance of leaf fragments of Rhacophyton sp., F. brownianum, and A. sphenophyllifolia is noted here. Fossil plant localities of the Grubyj Ruchej Formation occur in outcrops along the Rassokha–Bezmoshitsa river banks (Fig. 1.3), Veselaya river banks (Fig. 1.4), and Sula river banks (Fig. 1.2). Fossil plants of this formation were also met in drill samples of the Vyuchejskij borehole (Fig. 1.1). Lists of fossil plant remains are given in Kossovoj (1959, 1966), Petrosjan (1988) and Snigirevsky (1997a). The Grubyj Ruchej flora is represented by the following taxa (Table 3): Kossoviella timanica Petrosjan, Lepidodendropsis theodori (Zal.) Jongm., Knorria sp., Pseudobornia sp. (Asterocalamites sp. in Kossovoj, 1966), Sphenophyllum sp. (according to Petrosjan, 1988), Meristopteris laciniata Zal., Rhacophyton sp., R. cf. incertum (Daws.) Kr. et Weyl., Aphlebiopteris aff. boegendorfiana Gothan, Dimeripteris gracilis, Pietzschia timanica Lepekhina (1968), Calamophyton sp. (according to Petrosjan, 1988), Aneurophyton sp., Flabellofolium brownianum (P. brownianum in Kossovoj, 1966), Flabellofolium gilkineti (G. gilkineti in Rzonsnitskaya and Kulikova, 1990), Svalbardia osmanica Radczenko et Petrosjan, S. acuta (A. acuta in Rzonsnitskaya and Kulikova, 1990), Archaeopteris sphenophyllifolia (Plate I, 1) (A. sibirica in Kossovoj, 1966), A. archetypus Schmalh., Archaeopteris sp., A. halliana (Goeppert) Dawson (= A. roemeriana in Petrosjan, 1988), Pteridorachis sp., Sphenopteridium lebedevii (Scmalh.) Anan. and Moresnetia cf. zalesskyi Stockm. (latter two species—according to Petrosjan, 1988). The taxonomic plant composition of the Grubyj Ruchej Formation is more diverse (24 species) than that of the Vyuchejskij flora (Fig. 3). Stem fragments of lycopsids assignable to Lepidodendropsis and Knorria are observed. Kossoviella remains are represented by sterile axes and strobili with sporangia full of mega- and microspores. The remarkable fact here is the first appearance of Kossoviella (Table 4). Sphenophytes are represented

by adpressions of stems and leaf whorls of Pseudobornia and Sphenophyllum; the latter was found here for the first time. Archaeopteridalean remains in the Grubyj Ruchej flora are also diverse: two species of Svalbardia and three species of Archaeopteris. Like in the previous flora, fern plant remains are abundant and represented by Meristopteris, Rhacophyton, Aphlebiopteris, Pietzschia, Calamophyton, and Flabellofolium (Fig. 2). Seed ferns (Moresnetia and Sphenopteridium) are first found here (according to Petrosjan, 1988). Fossil plants of the Rassokha Formation (middle Frasnian and lower part of the upper Frasnian) were found in outcrops along the Rassokha– Bezmoshitsa river banks (Fig. 1.3) and in the drill samples of the Vyuchejskij borehole (Fig. 1.1). The flora consists of Taeniocrada timanica Tschirkova-Zalesskaya, Kossoviella timanica, Knorria sp., Sphenophyllum subtenerrimum Nathorst (according to Petrosjan, 1988), Rhacophyton incertum, Rhacophyton sp., Pteridorachis sp., Aphlebiopteris boegendorfiana, Dimeripteris gracilis, Svalbardia elschanica (Tschirk.-Zal.) comb. nov., S. fissilis (Schmalh.) Jurina and Raskatova (2012) (Archaeopteris fissilis in Snigirevsky, 1997a), A. archetypus (Plate II, 9), A. halliana (A. roemeriana in Petrosjan, 1988), Archaeopteris sp., and A. sphenophyllifolia (A. sibirica in Petrosjan, 1988). The taxonomic composition (15 species) of the flora is heterogeneous (Table 3, Fig. 3). There are both typical Frasnian species (A. boegendorfiana, K. timanica, and D. gracilis) and Famennian ones (S. subtenerrimum and A. halliana). Generally, the composition of the Rassokha Formation is similar to the Grubyj Ruchej flora, differing in smaller diversity of the ferns and lycopsids (Fig. 2). In contrast with the Grubyj Ruchej flora, leaf remains of Flabellofolium and stems of Lepidodendropsis are absent in the Rassokha formation (Table 4). The archaeopteridalean genus Svalbardia is represented by other species (S. fissilis and S. elschanica), the latter being first met in the Frasnian deposits. Like in the previous floras, archaeopteridaleans and ferns dominate while lycopsids (Kossoviella, Knorria) and sphenophytes (Sphenophyllum) are rarely observed (Fig. 2). Fossil plants from the Ust'bezmoshitsa Formation (upper Frasnian) were collected in the following localities: near the creek of the Suvojnaya River (Fig. 1.5), Suvojnyj field = area of the Eastern coast of Cheshskaya Bay between the Suvojnyj Cape and Krestovyi Cape (Fig. 1.6), Krestovyj field = area of the Eastern coast of Cheshskaya Bay between the Krestovyi Cape and the Eastern Ludovatyj Nos Cape (Fig. 1.7), Ludovatyj field = area of the Eastern coast of Cheshskaya Bay between the Eastern Ludovatyj Nos Cape and the Velikaya River mouth (Fig. 1.8), outcrops along the Velikaya river banks (Fig. 1.9), the Peschanka river banks (Fig. 1.10), and the Travyanka river banks (Fig. 1.11). The unique preservation of the fossil plants is particularly noteworthy. In addition to adpressions and coal remains, petrifactions were revealed, and also numerous sporangia with micro- and megaspores (Plate II, 1–7). The taxonomic composition of the Ust'bezmoshitsa flora includes Caudophyton fasciolus, Taeniocrada timanica, Gutzeitia timanica (Petrosjan) S. Snig. (Snigirevsky et al., 2007) (Plate I, 5) (Protobarinophyton timanica in Petrosjan, 1988), Kossoviella timanica (Plate III, 1–7, Plate IV), Ludovatia mirabilis Jurina et al. (in progress) (Plate III, 9), Pseudobornia schweitzeri Jurina et O. Orlova (Plate I, 6; Plate III, 8), Pseudobornia sp. 1 (Plate I, 4), Kosychia nenetzkiana (Snigirevsky et al., in progress) (Plate I, 3), Pseudosporochnus sp. 1 (Plate II, 1), Aphlebiopteris boegendorfiana, Cephalopteris mirabilis (Nathorst) Nathorst, Dimeripteris gracilis (Plate II, 7), Flabellofolium brownianum (P. brownianum in Snigirevsky, 1997a), Flabellofolium gilkineti (G. gilkineti in Petrosjan, 1988), Flabellofolium sp. (Ginkgophytopsis sp. in Snigirevsky, 1997a), Flabellofolium williamsonii (Nath.) comb. nov. (Platyphyllum williamsonii in Snigirevsky, 1997a), Flabellofolium ginkgophylloides (Lee) Jurina (Plate II, 5) (Platyphyllum ginkgophylloides in Snigirevsky, 1997a), Rhacophyton incertum, R. condrusorum, Archaeopteris sphenophyllifolia (A. sibirica in Petrosjan, 1988), A. halliana (A. roemeriana in Petrosjan, 1988), A. archetypus (Plate II, 6, 8, 9) (A. obtusa in Snigirevsky, 1997a), Archaeopteris sp., Archaeopteris macilenta Lesquereux (Plate II, 3, 4), Svalbardia fissilis (Plate I, 2, 7) (A. fissilis in Petrosjan, 1988), S. osmanica, Eddya sullivanensis Beck (Plate II, 2), Timanophyton lorum Senk., Sphenopteridium lebedevii

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(according to Petrosjan, 1988), and Pteridorachis sp. This is the richest flora (29 species) for the Upper Devonian of NT (Fig. 3). Besides the higher plants, remains of probable brown algae (C. fasciolus) are observed (Snigirevsky, 2001). Lycophytes of the Ust'bezmoshitsa flora are represented by three genera. Notably, there are zosterophyllean sterile and fertile structures (genus Gutzeitia), heterosporous strobili of Kossoviella, and also an endemic genus of probable lycopsid affinities, Ludovatia (Plate III, 9). Archaeopteridean plants are also represented by three genera: Archaeopteris, Svalbardia, and Eddya (Table 3). Findings of Eddya (Plate II, 2) are recorded here for the first time. The probable aneurophytalean genus T. lorum is observed (Table 3). Besides, there are a lot of fern adpressions (Fig. 2) such as D. gracilis, R. incertum, R. condrusorum, etc. Localities with fossil plants of the Kamenka Formation (uppermost Frasnian) are situated in the outcrops along the Volonga river banks (Petrosjan, 1988) (Fig. 1.12). The flora is represented by Lepidodendropsis theodorii, Knorria sp., Pseudobornia ursina, Archaeopteris sp., Heterangium sp., and Pteridorachis sp. (Table 3). Only stem fragments were found here. Fertile structures of the plants were not met in the assemblage. The Kamenka flora is much poorer than the Ust'bezmoshitsa one (Fig. 3). In summary, the total list of the fossil plants revealed in the Frasnian deposits of NT is following: Caudophyton fasciolus, Taeniocrada timanica, Gutzeitia timanica, Kossoviella timanica, Lepidodendropsis theodorii, Knorria sp., Ludovatia mirabilis, Pseudobornia schweitzeri, P. ursina, Pseudobornia sp. 1, Sphenophyllum sp., S. subtenerrimum, Kosychia nenetzkiana, Pseudosporochnus sp. 1, Cephalopteris mirabilis, Dimeripteris gracilis , Flabellofolium brownianum, F. gilkineti, Flabellofolium sp., F. williamsonii, F. ginkgophylloides, Meristopteris laciniata, Rhacophyton incertum, R. condrusorum, Rhacophyton sp., Pietzschia timanica, Calamophyton sp., Aneurophyton sp., Timanophyton lorum, Archaeopteris sphenophyllifolia, A. halliana, A. archetypus, Archaeopteris sp., A. macilenta, Svalbardia fissilis, S. acuta, S. elschanica, S. osmanica, Eddya sullivanensis, Sphenopteridium lebedevii, Heterangium sp., Pteridorachis sp., and Moresnetia cf. zalesskyi. 4.2. Famennian flora from NT Famennian fossil plants from the Pokayama Formation were found in several localities of the Eastern coast of Cheshkaya Bay: in the outcrops along the Volonga river banks (Fig. 1.12), in the Suvojnaya river banks (Fig. 1.5), and in the drill samples of the Indiga borehole (Fig. 1.13). According to Petrosjan (1988, 1991), Snigirevsky (1997a, 2001), Jurina (1988), and Snigirevskaya (2012), the Pokayama flora (Table 3) comprises Caudophyton fasciolus, Jurinodendron kiltorkense (Haught.) Doweld (Cyclostigma kiltorkense Haught.; see in Doweld, 2001), Pseudobornia sp., Sphenophyllum subtenerrimum, Archaeocalamites radiatus (Brongniart) Stur; Rhacophyton condrusorum, R. incertum, Dimeripteris gracilis, Archaeopteris archetypus (A. obtusa in Snigirevsky, 1997a), A. halliana (A. laxa in Snigirevsky, 1997a; A. roemeriana in Petrosjan, 1988), Svalbarbia fissilis (A. fimbriata Nath. in Petrosjan, 1988), and Callixylon timanicum Snigirevskaya (2012). The taxonomic composition (12 species) is poor in comparison with some Frasnian plant assemblages (Fig. 3). One single species of heterosporous lycopsids (J. kiltorkense) was observed here. Sphenophyte remains are more diverse (Fig. 2) and

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represented by three taxa: Pseudobornia sp. (stem and leaves), S. subtenerrimum (leaves), and A. radiatus (stems). Besides, three fern species and three archaeopteridalean species were found (Table 3): R. condrusorum, R. incertum, D. gracilis, and A. archetypus, A. halliana, S. fissilis, respectively. Petrifactions of C. timanicum Snig. trunks were also described from the Pokayama formation (Table 3). The Famennian flora of NT is poorer than the Frasnian one in generic and specific composition: 12 species and 10 genera in the Famennian plant assemblage against 43 species and 26 genera in the Frasnian assemblage. Only three new genera and their respective species, Jurinodendron kiltorkense, Callixylon timanicum, and Archaeocalamites radiatus, appeared in the Famennian whereas 20 genera and 35 species from the Frasnian disappeared. Notably, typical Famennian plants are recorded here: Archaeopteris halliana (leaves), Sphenophyllum subtenerrimum (leaves), and J. kiltorkense (stems). 5. Analysis of the palaeovegetation from NT 5.1. Frasnian vegetation of NT In the beginning of the Frasnian time, the area of NT was basically a lagoonal shallow-water basin, but the axial region still persisted as land (Snigirevsky, 1997a). Sometimes short-run volcanic activity occurred. The land plant cover was slightly monotonous. Rare lycopsids (Knorria sp.) preferred wet floodplains. Bushy Aneurophyton (Meyer-Berthaud et al., 2010) and Rhacophyton (Andrews and Phillips, 1968) together with the probable ferns Flabellofolium brownianum, F. gilkineti, Meristopteris laciniata and Dimeripteris gracilis dominated in poordrained margins of a coastal environment forming scrub vegetation. First archaeopterideans Svalbardia acuta and Archaeopteris sphenophyllifolia are thought to have grown in well-drained soils and to form closed forests. In general, archaeopteridean evergreen forests covered huge areas preferring various conditions, from poorly to well-drained lowlands (Le Hir et al., 2011). Archaeopteridean forests started to cover the area of NT already in the Middle Devonian. This is evident from the fact that findings of Svalbardia acuta, S. fissilis, S. osmanica, Archaeopteris macilenta, and A. halliana were met in Givetian time here. The comparison of archaeopteridean forests in the Givetian and early Frasnian showed that archaeopteridean diversity was less in the early Frasnian than in the Givetian (2 versus 5 species). Only S. acuta crossed the Givetian–Frasnian boundary. Later in the middle late Frasnian, most Givetian species of Archaeopteris and Svalbardia reappeared (see below). Plant vegetation was more diverse in the second half of the early Frasnian up to the bottom of the late Frasnian, although the deposits of the Grubyj Ruchej and Rassokha Formations are considered as relatively marine. A short-run volcanic activity was occasionally recorded. The lagoons were periodically flooded by the waters of the Timan Sea (Snigirevsky, 1997a). It is assumed that the high herbaceous halophytic sphenopsid Pseudobornia sp. covered coastal settings. Woody lycopsids (Kossoviella timanica, Lepidodendropsis theodori, and Knorria sp.) dominated in the wettest places of a floodplain. Rare Sphenophyllum vines and herbaceous Taeniocrada occupied an understorey. The shrubberies

Plate I. Frasnian plants of the Grubyj Ruchej and Ust'bezmoshitsa formations from NT. 1. 2, 7. 3. 4. 5. 6.

Archaeopteris sphenophyllifolia Lesq. PM SPbGU-21-356. Leafy branch. Sula River. The lower Frasnian deposits. Grubyj Ruchej Formation. Scale bar = 5.5 mm. Svalbardia fissilis (Schmalh.) Jurina et Raskatova. PM SPbGU-21-394-1. 2—general view of dissected leaves; 7—in situ microspores. Krestovyj field. The upper Frasnian deposits. Ust'bezmoshitsa Formation. Scale bar for 2 = 5.7 mm; for 7 = 25 μm. Kosychia nenetzkiana (Snigirevsky et al., in progress) PM SPbGU-21-335. General view of specimen. Velikaya River. The upper Frasnian deposits. Ust'bezmoshitsa Formation. Scale bar = 2.1 cm Pseudobornia sp. 1. PM SPbGU-21-351-6. Leaf whorl. Suvojnyj field. The upper Frasnian deposits. Ust'bezmoshitsa Formation. Scale bar = 1.25 cm. Gutzeitia timanica (Petrosjan) S. Snig. PM SPbGU-21-361-3. Fertile structure. Krestovyj field. The upper Frasnian deposits. Ust'bezmoshitsa Formation. Scale bar = 8 mm. Pseudobornia schweitzeri Jurina et Orlova. PM SPbGU-21-346-2. Fragment of the axis with leaf whorl. Ludovatyj field. The upper Frasnian deposits. Ust'bezmoshitsa Formation. Scale bar = 1 mm.

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29 24

15 12 10

18

18 6

10

9

10

6 Vyuchejskij

Grubyj Ruchej

EARLY

Rassokha

MIDDLE FRASNIAN

Ust`bezmoshitsa

Kamenka

Pokayama

LATE FAMENNIAN

Fig. 3. Changes in species and genus diversity from the early Frasnian to the Famennian of NT. Species curve is in red; genus curve is in blue. Numbers of genera and species are indicated by Arabic numerals on the curves.

Table 4 Stratigraphic distribution of the main plant genera of NT from the early Frasnian to the Famennian.

consisting of Aneurophyton sp., Rhacophyton sp., R. incertum, Meristopteris laciniata, Aphlebiopteris boegendorfiana, Dimeripteris gracilis, Flabellofolium brownianum, and F. gilkineti occupied the river banks. It is likely that the rare cladoxylopsids (Pietzschia timanica and Calamophyton sp.) grew in the immediate vicinity, although the latter genus may have formed monospecific forests (Giesen and Berry, 2013). In the lowlands far from the water margins broad-leaved trees of Archaeopteris sphenophyllifolia,

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A. archetypus, Archaeopteris sp., A. halliana, and smaller trees with dissected-leaved such as Svalbardia osmanica, S. elschanica, and S. acuta formed dense forests. Probable seed ferns of S. lebedevii and Moresnetia cf. zalesskyi could have complemented the shrubbery community or, to the contrary, perhaps grew as an understorey in the archaeopteridean forests. The Ust'besmozhitsa (late Frasnian) palaeovegetation was more diverse than the older ones. Among the 29 species, twelve (Caudophyton fasciolus, Gutzeitia timanica, Ludovatia mirabilis, Pseudobornia schweitzeri, Pseudobornia sp. 1, Kosychia nenetzkiana, Pseudosporochnus sp. 1, Cephalopteris mirabilis, Flabellofolium williamsonii, F. ginkgophylloides, Eddya sullivanensis, and Timanophyton lorum) are first recorded in the Frasnian time. It's hard to say why such a rich diversity (Fig. 2) appeared at this particular time. Preservation of the palaeobotanical material is good, with numerous reproductive organs full of in situ spores (Plate IV). It is therefore unlikely that the fossil plants had been transported far from their habitats. At that time an effusive activity ceased in the area of NT. The ancient Ust'besmozhitsa River flowed in south-south-eastern direction. Along the river banks, scrubs and vines formed dense fern communities consisting of Aphlebiopteris boegendorfiana, Cephalopteris mirabilis, Dimeripteris gracilis, Flabellofolium brownianum, F. gilkineti, Flabellofolium sp., F. williamsonii, F. ginkgophylloides, Rhacophyton incertum and R. condrusorum. It is quite possible that the probable aneurophytes of Timanophyton lorum, Pseudosporochnus cladoxylopsids, and Eddya sullivanensis (possible juvenile stage of Archaeopteris) have grown here. The latter two genera are preserved closely together. The new woody plants – the Duisbergia-like plant assignable to Kosychia nenetzkiana, the Kossoviella timanica arborescent lycopsid and the probable tree lycopsid Ludovatia mirabilis - have potentially grown in the wettest parts of floodplains. The understorey of this community was covered by herbaceous zosterophylls of Gutzeitia timanica, and Taeniocrada timanica. In coastal environments, at least two sphenopsid species of Pseudobornia were abundant. A littoral belt is thought to have been covered by the probable brown algae of Caudophyton fasciolus. Apparently, archaeopteridean forests covered the massive land areas during the time corresponding to the Ust'besmozhitsa Formation. Four species of Archaeopteris (A. sphenophyllifolia, A. halliana, A. archetypus, and A. macilenta) and two species of Svalbardia (S. fissilis and S. osmanica) represented the main components of the forests. The Sphenopteridium lebedevii seed ferns supposedly grew in the understorey. In the latest Frasnian, the NT area was covered by the numerous lagoons of the shallow-water basin of the Timan Sea and was periodically drying out (Snigirevsky, 1997a). The previous diverse palaeovegetation (29 species) underwent a floral crisis. Only six taxa are recorded for the time corresponding to the Kamenka Formation (Fig. 3). Plant remains have an allochtonous origin as indicated by the fragmentary preservation

Plate II. Frasnian plants of the Rassokha and Ust'bezmoshitsa Formations of NT. 1. 2. 3, 4. 5. 6, 8, 9.

7.

Pseudosporochnus sp. 1. PM SPbGU-21-344-9. General view. Suvojnyj field. The upper Frasnian deposits. Ust'bezmoshitsa Formation. Scale bar = 7 mm. Eddya sullivanensis Beck. PM SPbGU-21-371-2. Narrow leafy branch. Suvojnyj field. The upper Frasnian deposits. Ust'bezmoshitsa Formation. Scale bar = 6.5 mm. Archaeopteris macilenta Lesquereux. 3—PM SPbGU-21-385-4 General view. 4—PM SPbGU-21-385-5. Isolated leaves with pattern of lamina laceration. Suvojnyj field. The upper Frasnian deposits. Ust'bezmoshitsa Formation. Scale bar for 3 and 4 = 6.5 mm. Flabellofolium ginkgophylloides (Lee) Jurina. PM SPbGU-21-368. Isolated leaf. Suvojnyj field. The upper Frasnian deposits. Ust'bezmoshitsa Formation. Scale bar = 8 mm. Archaeopteris archetypus Schmalh. 6—PM SPbGU-21-334-1-01. Fragments of sterile leafy branches. 8—PM SPbGU-21-334-1-02. Fertile structure; for 6 and 8—Ludovatyj field. The upper Frasnian deposits. Ust'bezmoshitsa Formation. 9—PM SPbGU-21-163. General view of broad-leaved branch. Vyuchejskij borehole. The middle-upper Frasnian deposits. Rassokha Formation. Scale bar for 6 = 2.2 cm; for 8 = 6.5 mm; for 9 = 12 mm. Dimeripteris gracilis Schmalh. PM SPbGU-21-340-4. General view. Suvojnyj field. The upper Frasnian deposits. Ust'bezmoshitsa Formation. Scale bar = 1.9 mm.

Plate III. Late Frasnian plants (Ust'bezmoshitsa Formation) of NT (see on page 10) 1-7.

8. 9.

Kossoviella timanica Petr. 1, 4, 5—MGU-341-50. Megasporangiate strobilus. 1—general view; 4—detail of the structure of megasporangia; 5—individual megaspore tetrad. 2—PM SPbGU-21-418-4. Upper part of bisporangiate strobilus. 3—PM SPbGU-21-418-2b. Megasporangiate strobilus with numerous megaspores; a sterile stem is seen at the base. 6—PM SPbGU-21-415. Part of the megasporangiate strobili and stem. 7—PM SPbGU-21-418-2a. Dichotomous bisporangiate structure: right axis mostly with microspores, base of left axis with megaspores. 1–5, 7—Ludovatyj field; 6—Suvojnyj field. Scale bar for 1–3, 6, 7 = 1 cm, for 4 = 2 mm, for 5 = 50 μm. Pseudobornia schweitzeri Jurina et Orlova. MGU-341-51. Fragment of axis with leaf whorl. Peschanka River. Scale bar = 1 cm. Ludovatia mirabilis (Jurina et al., in progress). General view of the specimen from the Arkhangelsk Regional Museum collection. Ludovatyj field. Scale bar = 1 cm.

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Plate III (caption on page 8).

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Plate IV. Late Frasnian plant Kossoviella timanica Petr. (Ust'bezmoshitsa Formation of NT) in SEM. 1. 2. 3. 4, 5. 6.

MGU-341-50. Megaspore tetrad. Ludovatyj field. Scale bar = 470 μm. PM SPbGU-21-418-2a. Aggregation of microspores. Ludovatyj field. Scale bar = 86 μm. PM SPbGU-21-418-2a. Microsporangia full of microspores. Ludovatyj field. Scale bar = 1 mm. PM SPbGU-21-418-2a. Megasporangia with megaspore tetrads and individual megaspores. Ludovatyj field. Scale bar for 4 = 1.2 mm; for 5 = 1 mm. Megasporangia with megaspores, the upper left sporophyll with a crenulated margin. PM SPbGU-21-415. Suvojnyj field. Scale bar = 2.5 mm.

of the stems and axes (Lepidodendropsis, Knorria, Pseudobornia, Archaeopteris, Heterangium, and Pteridorachis) devoid of any leaf structures. Changes in the land plant cover probably resulted from

minor climatic changes. It is necessary to pay attention to the red beds that significantly differ from other Frasnian deposits. The distinctive red color of the matrix probably indicates some aridification

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Table 5 Distribution of the Frasnian archaeopteridean plants in NT, Donetsk region, Voronezh region, and Volga-Ural Province. Species/region

North Timan (Russia)

Donetsk region (Ukraine)

Voronezh region (Russia)

Volga-Ural Province (Russia)

Archaeopteris halliana Archaeopteris archetypus Archaeopteris sphenophyllifolia Archaeopteris macilenta Archaeopteris sp. Eddya sullivanensis Svalbardia acuta Svalbardia osmanica Svalbardia elschanica Svalbardia fissilis

+ + + + + + + + + +

+ + + − − − − − − +

− + − − − − − − − +

− − − + − − + − + +

(Krassilov, 1972) that resulted in the significant decrease of the land plant cover. 5.2. Famennian vegetation of NT The Famennian vegetation strongly differs from the Frasnian one by a poor taxonomic composition and by vegetation conditions. The Famennian deposits of NT are restricted to the Pokayama Formation. Besnosov et al. (2012) noticed that the lithological section of the Pokayama Formation in localities of the Volonga River is characterized by layers with cross-bedded sandstones, siltstones, and coal seams. Accordingly, the deposits of the Pokayama Formation were formed in the conditions of a broad tidal valley, shallows, and channels with sporadic floods. The development of the palaeovegetation can be traced in the area of NT up to the middle Famennian. Two types of plant communities can be distinguished in the middle Famennian. The first one covered small uplands and was represented by archaeopteridalean forests with the broad-leaved Archaeopteris archetypus and A. halliana, and the dissected-leaved Svalbarbia fissilis. The second type of plant community inhabited coastal lowlands. Remarkably, archaeopteridalean wood petrifactions are found here. Snigirevskaya (2012) studied the anatomical structure of Callixylon timanicum and distinguished growth rings supporting possible changes in the ratio of humidity/aridity in the climate (Snigirevskaya, 2012). We also considered that heavy seasonal rainfalls may have influenced the occurrence of growth rings in Callixylon. As is well-known Callixylon trunks are associated with Archaeopteris leaves (Beck, 1960). Leaf fragments of A. archetypus or A. halliana found in the Pokayama Formation are probably associated with C. timanicum. Heterosporous woody lycopsids of Jurinodendron kiltorkense grew at some distance from the archaeopterideans. They did not densely cover the land. Because of a huge Stigmaria root-like system (Schweitzer, 1969), Jurinodendron trees preferred to grow at some distance from other plants. Rare arthrophytes of Archaeocalamites radiatus and Sphenophyllum subtenerrimum, and ferns of Rhacophyton condrusorum, R. incertum, and Dimeripteris gracilis (Leclercq, 1951) probably grew close to the forest edge. Coastal margin settings and lowlands were inhabited by herbaceous sphenopsids of Pseudobornia. Two hypotheses exist concerning the habit of Pseudobornia. According to Schweitzer (1967) P. ursina Nathorst from the Famennian of Bear Island was a woody plant (tree) up to 20 m high. We (Orlova and Jurina, 2014) studied a large number of P. ursina

from the Schweitzer collection kept at the Swedish Museum of Natural History in Stockholm. Unfortunately, all the “wood” pieces of P. ursina studied in SEM did not show any anatomical structure. At that time the axes of a new species P. schweitzeri were studied using SEM and tracheids of a primary xylem were found. It showed the absence of secondary tissues in the axes of different orders and therefore secondary growth was not developed in P. schweitzeri from NT. It allowed us to reconstruct P. schweitzeri as a herbaceous plant up to 2–3 m high. We suppose that the Famennian Pseudobornia from NT was also herbaceous and that, like the Frasnian one, it preferred wet salt-water conditions and densely covered the coast. 6. Discussion and conclusion It has become common knowledge that the first forests appeared in the Middle Devonian, that they were strongly structured, with giant cladoxylopsid trees (Wattieza/Calamophyton/Duisbergia), shrubs of rhizomatous woody aneurophytaleans and arborescent lycopsids in the vicinity, and that they preferentially grew in coastal areas (Stein et al., 2012; Giesen and Berry, 2013). In the Late Devonian, archaeopterideans were widespread and dominant, forming huge forest areas. A lot of variously preserved archaeopteridean remains (adpressions of branches, leaves, fertile structures; stem and root petrifactions and mega- and microspores) were found all over the world. This resulted in many publications on their morphology, anatomy, palaeoecology, etc. (Beck, 1960; Snigirevskaya, 1984, 2012; Orlova and Jurina, 2011; Meyer-Berthaud et al., 2010, 2013, etc.) Comparison of the Late Devonian floras of NT with several surrounding ones is based on the analysis of archaeopterideans. We found that all above-described floras of NT contain various archaeopteridean remains. The late Frasnian time was the culmination period for the archaeopterideans in NT. A large variety of taxa ( Archaeopteris sphenophyllifolia, A. halliana, A. archetypus, A. macilenta, Svalbardia fissilis, S. osmanica, and Eddya sullivanensis) was observed at this time (Table 5). The Famennian archaeopterideans (Table 6) were less diverse although they were represented both by leaf remains (A. archetypus, A. halliana, and S. fissilis) and stem petrifactions (Callixylon timanicum). It is interesting to note that the diversity and amount of dissectedleaved archaeopterideans (such as Svalbardia) gradually decreased from the Givetian (Middle Devonian) to the Famennian. The largest diversity of such archaeopterideans in NT occurred in the late Givetian (S. osmanica, S. acuta, and S. fissilis). In the early, middle, and late

Table 6 Distribution of the Famennian archaeopteridean plants in NT, Donetsk region, and Pripyatian depression. Species/region

North Timan (Russia)

Donetsk region (Ukraine)

Pripyatian depression (Belarus)

Archaeopteris archetypus A. halliana Archaeopteris macilenta Callixylon timanicum Callixylon trifilievi Svalbarbia fissilis

+ + − + − +

+ + + − + +

+ − − − − −

106

O.A. Orlova et al. / Review of Palaeobotany and Palynology 224 (2016) 94–107

Frasnian, two species of Svalbardia were observed in each substage and only one species (S. fissilis) was found in the Famennian. A similar tendency in archaeopteridean changes is observed in the areas surrounding NT. A comparison of the Late Devonian floras of NT with coeval floras of cross-border regions reveals similarities between them, especially in the predominance of archaeopterideans. Let's compare the Late Devonian flora of NT with several coeval floras. Up to now, Donbass floras (Ukraine) are considered as the best known and richest floras of the Late Devonian. Indeed, there are four archaeopteridean species (Archaeopteris archetypus, A. halliana, A. sphenophyllifolia, and Svalbardia fissilis) in the Frasnian deposits (Table 5). All these species are recorded also in the Frasnian of NT, but the Frasnian archaeopteridean floras of NT are more diverse, particularly in the number of Svalbardia species. The Famennian archaeopterideans of the Donetsk region (Donbass, Ukraine) are similar in generic composition with the coeval ones of NT. Previous studies on leaf fragments of A. archetypus, A. halliana, A. macilenta, and S. fissilis, and on stem and root petrifactions of Callixylon trifilievi Zal. (Snigirevskaya, 1984) were based on Donetsk material (Table 6). The late Frasnian flora from Voronezh region (near the Petino village) was recorded by Nikitin (1934), then by Tschirkova-Zalesskaya (1957) and Jurina (1988). According to their data Archaeopteris archetypus and Svalbardia fissilis were observed in the Petinian Regional Stage (late Frasnian). This flora is much poorer that the late Frasnian one of NT (Table 5). Notably, Jurina visited the indicated locality near Petino village in the early eighties and did not find any fossil plant remains. Besides, the collection of the Petinian plants of Tschirkova-Zalesskaya (1957) was lost. Rare fossil plants from Famennian deposits were observed in Belarus (Pripyatian depression). Archaeopterideans (Table 6) are represented only by Archaeopteris archetypus (Jurina, 1988). The Late Devonian flora of the Volga-Ural Province (Table 5) was studied by TschirkovaZalesskaya (1957). She defined several plant assemblages among which two of them are dated of the Frasnian. Three archaeopteridean species (A. macilenta, Svalbardia acuta and S. elshanica) were reported in the lower Frasnian deposits in contrast to coeval plant assemblages of NT where six species were found. The Late Frasnian archaeopteridean flora of Volga-Ural Province was extremely poor (A. macilenta and Svalbardia fissilis). Therefore, the Late Devonian floras of NT, by their taxonomic composition and diversity, could be considered as the richest ones among the coeval surrounding floras except for the Famennian flora of Norway (Schweitzer, 1969, etc.) Acknowledgments We thank Dr. Roman Rakitov (Paleontological Institute, Moscow) for the assistance with SEM, Dr. Irina O. Evdokimova (A.P. Karpinsky Russian Geological Research Institute) for advices on the Upper Devonian stratigraphy, Dmitry A. Mamontov (Lomonosov Moscow State University) for some corrections of the miospore lists, and Aleksandr V. Yudin and Sergey Yu. Kharitonov (Lomonosov Moscow State University) for technical assistance. We are thankful to Prof. Aleksandr S. Alekseev (Lomonosov Moscow State University), Dr. Maria V. Tekleva (Paleontological Institute, Moscow), and Dr. Natalia S. Snigirevskaya (Komarov Botanical Institute, St Petersburg) for helpful suggestions. We thank Dr. Brigitte Meyer-Berthaud and two anonymous reviewers who provided useful comments and suggestions on the manuscript, and corrected the English of the manuscript. This work is supported by the Russian Foundation for Basic Research, project no. 15-04-09067. References Andrews, H.N., Phillips, T.L., 1968. Rhacophyton from the Upper Devonian of West Virginia. J. Linn. Soc. Bot. 61 (384), 37–64.

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