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Cryptospores from the Katian (Upper Ordovician) of the Tungus basin: The first evidence for early land plants from the Siberian paleocontinent
Review of Palaeobotany and Palynology 224 (2016) 4–13
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Cryptospores from the Katian (Upper Ordovician) of the Tungus basin: The first evidence for early land plants from the Siberian paleocontinent Elena Raevskaya a,⁎, Andrey Dronov b,c, Thomas Servais d, Charles H. Wellman e a
FGUNPP “Geologorazvedka”, Knipovich str., 11, block 2, Saint-Petersburg 191019, Russia Institute of Geology, Russian Academy of Sciences, Pyzhevsky per. 7, Moscow 119017, Russia Kazan (Volga Region) Federal University, Kremlevskaya ul., 18, 420008 Kazan, Russia d Evo-Eco-Paleo, UMR 8198 CNRS, Université des Sciences et Technologies de Lille, Sciences de la Terre, Lille, France e Department of Animal and Plant Sciences, University of Sheffield, Alfred Denny Building, Western Bank, Sheffield S10 2TN, UK b c
a r t i c l e
i n f o
Article history: Received 12 May 2015 Received in revised form 14 September 2015 Accepted 9 October 2015 Available online 31 October 2015 Keywords: Cryptospore Upper Ordovician Katian Siberian Platform
a b s t r a c t A diverse assemblage of cryptospores is reported for the first time from the Upper Ordovician of Siberia. It was discovered during a palynological study of a sedimentary succession of about 100 m, exposed along the Bolshaya Nirunda River, a right tributary of the Podkamennaya Tunguska River. The succession is located on the southern margin of the extensive epicontinental Tungus basin on the Siberian Platform between the Katanga and Yenisei land masses. The cryptospore assemblage was recovered from the siliciclastic-carbonate Dolbor Formation and scarcely from the overlying more carbonate Bur Formation. Both formations belong to the Katian Global Stage (Upper Ordovician). The cryptospores occur together with marine remnants (acritarchs, prasinophytes, chitinozoans, scolecodonts). It is similar to all known Upper Ordovician cryptospore assemblages in comprising naked and envelope-enclosed monads, dyads, tetrads and polyads. Although preservation is moderate to poor, the cryptospore taxa Velatitetras laevigata, Tetrahedraletes medinensis, Abditusdyadus laevigatus, Dyadospora murusdensa, Pseudodyadospora laevigata, Segestrespora laevigata and Sphaerasaccus glabellus can be identified. This report represents the first record of spores of the earliest land plants from the paleocontinent Siberia and therefore extends the global paleogeographical coverage of Late Ordovician cryptospores. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Cryptospores are considered to have been produced by the earliest terrestrial flora that probably consisted of bryophyte-like plants (Gray, 1985, 1991; Richardson, 1996; Strother et al., 1996; Wellman, 1996; Edwards et al., 1999; Steemans, 2000; Wellman et al., 2013). They are considered to represent the earliest evidence for continental vegetation and thus document a fundamental phase of land plant evolution (Gray et al., 1982; Wellman et al., 2003; Steemans et al., 2009; Spina, 2014). The oldest cryptospores are known from the Middle Ordovician. More or less certain cryptospore taxa (i.e., ‘Tetrahedraletes cf. medinensis’) are recorded from the Dapingian of Argentina (Rubinstein et al., 2010). However, being represented by few specimens from one single sample this record still needs corroboration by further data before it can be unquestionably considered as the oldest evidence of cryptospores (Spina, 2014; Strother et al., 2015). Thus, for the present, generally accepted oldest cryptospores are from the Darriwilian of Saudi Arabia (Strother et al., 1996, 2015) since they are characterized by regular morphology and abundant population (Wellman et al., 2003; Spina, 2014). Numerous publications from many parts of the ⁎ Corresponding author. E-mail address: [email protected] (E. Raevskaya).
http://dx.doi.org/10.1016/j.revpalbo.2015.10.010 0034-6667/© 2015 Elsevier B.V. All rights reserved.
world show the uniformity of cryptospore assemblages, in terms of their taxonomic composition and diversity, from the Middle Ordovician to the Early Silurian, leading to a concept of the existence of a conservative cosmopolitan bryophyte-like flora that remained almost unchanged for about 40 million years (Wellman, 1996; Steemans, 2000; Wellman and Gray, 2000; Rubinstein et al., 2010; Vecoli et al., 2011; Spina, 2014). However, the steady increase of new data from different paleocontinents is beginning to reveal a more complex pattern of cryptospore origination and spatial/temporal distribution (Wellman et al., 2013) that may possibly lead to a slight modification of existing ideas. The reconstruction and interpretation of the early terrestrial vegetation mostly depend on the availability and adequacy of the fossil data (particularly of cryptospore records) that largely vary in both quantity and quality. While fairly representative data are now available from eastern North America (Laurentia), Western Europe (Laurentia, Avalonia, Baltica) and North Africa and Arabia (southern part of Gondwana), very little or nothing is known from other paleocontinents. As reviewed in Wellman et al. (2013) Ordovician (Dapingian–Hirnantian) cryptospore assemblages have been described from Gondwana (Libya, Saudi Arabia, South Africa, Australia, Argentina), Peri-Gondwana (the Czech Republic, southeastern Turkey), Avalonia (southern Britain, Belgium), Laurentia (USA and Canada), South China (Wang et al., 1997) and Tarim (Yin and He, 2000). The first records of Late Ordovician
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cryptospores from the paleocontinent Baltica have recently been added based on studies from Estonia (Vecoli et al., 2011) and Sweden (Badawy et al., 2014). More recently, further reports of Ordovician cryptospores are from Chad (Le Herisse et al., 2013), Iran (Mahmoudi et al., 2014) and Algeria (Spina, 2014). However, cryptospores have not been recorded from Russia so far. Thus, a significant lack of data concerns one of the major paleocontinents — Siberia. In recent years abundant research and field-work have been conducted in the Ordovician of the southern and western part of the Siberian Craton (e.g., Dronov et al., 2009, 2011; Kanygin et al., 2010a, b). Within the framework of a multidisciplinary research project intensive palynological investigations were initiated. In this pilot study the first preliminary palynological results concerning cryptospores are presented. This first discovery of cryptospores from Russia expands the current knowledge on the paleophytogeography of Late Ordovician early vegetation, extending their distribution to the paleocontinent Siberia.
in exposed sections (Kanygin et al., 2007), but possibly exists in the subsurface of the central part of the Tungus Syneclise (Dronov, 2013). The regional stages allow a correlation of Ordovician sections within the Tungus basin and across the Siberian Platform (Kanygin et al., 1984, 2007). However, due to the endemic character of the Siberian faunas, their direct correspondence to the global chronostratigraphic scale is not precise, although a provisional correlation was recently proposed by Bergström et al. (2009). So far, only four reliable levels have been identified for intercontinental biostratigraphic correlation of the Ordovician of the Siberian Platform: (1) the base of the Ordovician, which was established at the lower boundary of the Nyaian Regional Stage according to the most recent conodont investigations (Tolmacheva and Abaimova, 2009); (2) the base of the Volginian Regional Stage in the Middle Ordovician that correlates with the base of the Hustedograptus teretiusculus graptolite zone; (3) the base of the Chertovskian Regional Stage that correlates with the base of the Nemagraptus gracilis graptolite zone — the base of the Upper Ordovician; and (4) the base of the Silurian. The stratigraphic interval discussed in the present paper includes the uppermost Baksian, Dolborian, Nirundian and Burian Regional Stages, which correspond to the Katian Global Stage (Fig. 2). A precise correlation of the listed Upper Ordovician regional stages with the Global Scale is uncertain. However, recently discovered zircon crystals from K-bentonite beds in the upper part of the Mangazea Formation, attributed to the Baksian Regional Stage, provide a 206Pb/238U age of 450.58 ± 0.27 Ma confirming the Katian age (Huff et al., 2014). The studied Upper Ordovician sedimentary succession is represented by the upper part of the Mangazea Formation as well as the Dolbor, Nirunda and Bur Formations with the same name as the regional stages (Fig. 2). The whole succession of about 100 m is well exposed in a series of outcrops which can be traced with some certainty along both banks of the Bolshaya Nirunda River (Fig. 1). The upper part of the Mangazea Formation (Baksian Regional Stage) crops out in a small anticline fold (Fig. 3-A) in outcrop I (N61°58′23.83″; E95°16′22.53″). The exposed thickness is only of about 14 m. The formation is represented by a rhythmic intercalation of greenish-gray siltstone and gray coarse-grained bioclastic limestones that may reach 20–30 cm in thickness (Fig. 3-B). The base of these limestone beds is usually erosional while ripple marks may often be observed on their top. Bioclasts are mainly represented by fragments of brachiopod shells, trilobite carapaces and bryozoans. Ostracods and crinoids are also numerous. These rocks were interpreted as proximal cool-water carbonate tempestites deposited in middle-ramp settings (Dronov, 2013).
2. Geological setting and stratigraphy The studied outcrop area is located on the southern margin (in present day orientation) of the extensive epicontinental Tungus basin (Fig. 1). The Upper Ordovician (Katian–?lowermost Hirnantian) succession is exposed in four separated outcrops along the Bolshaya Nirunda River, a right tributary of the Podkamennaya Tunguska River about 60 km downstream from the village of Baykit (Fig. 1). Although being a large part of Eurasia today the Siberian Platform (Siberian Craton) was an individual paleocontinent during the early Paleozoic, located in low latitudes near the paleoequator (Cocks and Torsvik, 2007). During the Ordovician several more-or-less separate sedimentary basins were present in Siberia. Among the largest of them are the Irkutsk and Tungus basins (Fig. 1). The Tungus epicontinental basin represents a large, intracratonic, sedimentary sag basin embracing the central part of the Siberian Craton. It was surrounded by the Anabar, Turukhan, Yenisei and Katanga Lands. The biostratigraphy of the Ordovician succession in the Tungus basin is mainly based on faunal (trilobites, brachiopods and ostracods) distribution allowing a subdivision into twelve regional stages (Kanygin et al., 1984, 2007): two in the Lower Ordovician (Nyaian and Ugorian), five in the Middle Ordovician (Kimaian, Vikhorevian, Mukteian, Volginian and Kirensko–Kudrian) and five in the Upper Ordovician (Chertovskian, Baksian, Dolborian, Nirundian and Burian). The uppermost part of the Ordovician is eroded 96
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Fig. 1. Geographic location of the study area and outcrops of investigated Upper Ordovician strata. Legend: 1, Boundary of the Siberian platform. 2, Provisional boundaries of the Siberian platform and land areas. 3, Provisional boundary of the Tungus and Irkutsk basins. 4, Land areas without Ordovician deposits (1 — Turukhan Land, 2 — Yenisei Land, 3 — Katanga Land). 5, Ordovician deposits in subsurface areas. 6, Ordovician outcrop areas. 7, Studied outcrops along the Bolshaya Nirunda River.
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Laevigate monad IV-10 IV-9 IV-8 IV-7 IV-6 IV-5 IV-4 IV-3 IV-2 IV-1
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Fig. 2. Litho- and chronostratigraphy of the studied succession with sample locations and cryptospore distribution. Legend: 1, Silurian thick bedded dark gray limestone (mudstone to wackestone). 2, Greenish-gray siltstone. 3, Red siltstone. 4, Thin bedded bioclastic limestone and intercalation of limestone and siltstone. 5, Thick bedded bioclastic limestone. 6, Limestone beds with chert nodules. 7, Fine grained quartz sandstone. 8, K-bentonite layers. 9, Palynological samples. 10, Cryptospore occurrence (a — from 1 to 2 specimens, b — from 3 to 5 specimens, c — more than 5 specimens).
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Fig. 3. Representative Upper Ordovician strata cropping out along the Bolshaya Nirunda River. (A) The anticline fold in the outcrop I. Left bank of the Bolshaya Nirunda River. Upper part of the Mangazea Formation. (B) Intercalation of siltstone and bioclastic coarse-grained limestone (storm beds) of the Mangazea Formation. (C) The lower part of the Dolbor Formation with characteristic intercalation of thin-bedded limestone and siltstone layers (distal tempestites). Outcrop II on the right bank of the Bolshaya Nirunda River. (D) Middle part of the Dolbor Formation predominated by siltstone in the outcrop II. Right bank of the Bolshaya Nirunda River. (E) General view of the outcrop III on the right bank of the Yukteli creek (left tributary), about 100 m upstream from the point where it flows into the Bolshaya Nirunda River.
The Dolbor Formation roughly corresponds to the Dolborian Regional Stage (Dronov et al., 2009; Kanygin et al., 2010a,b). It is exposed in outcrop II (N61°59′29.58″; E95°15′44.68″) and is represented by a succession of about 63 m in thickness (Figs. 1, 2). In its lower part the Dolbor Formation consists of an intercalation of thin (0.5–2 cm) limestone layers that are interpreted as distal carbonate tempestites and greenish-gray siltstone layers of comparable thickness (Fig. 3-C). Thicker beds sometimes contain chert nodules. The middle part of the formation consists of almost pure greenish-gray siltstone of about 12 m thickness (Fig. 3D). The upper part of the formation is again more carbonate-rich and contains numerous coral colonies. The presence of pure terrigenous deposits in the middle part of the formation probably points to the proximity of Katanga Land. The Nirunda Formation corresponds to the Nirunda Regional Stage. It is exposed only in its lowermost part (6 m) on the very top of outcrop II (Fig. 1). The formation is represented by cherry-red and greenish-gray siltstone with fine-grained bioclastic limestone layers at some levels. Bryozoans and brachiopods dominate the macrofauna. The Bur Formation coincides with the Burian Regional Stage. In the Bolshaya Nirunda River valley, the formation is exposed in two separated outcrops (III and IV) that could not be directly correlated with each other (Fig. 1). In both outcrops the formation is represented by an intercalation of greenish-gray siltstone and gray bioclastic limestone beds. The outcrop III (N62°03′22.99″; E95°16′17.69″) contains a diabase sill on the top (Fig. 3-E). In outcrop IV (N62°04′31,82″; E95°14′37,88″) the “top” of the Bur Formation is represented by a regional unconformity and is directly overlain by Silurian deposits. The abundance of big (up to 20–30 cm in diameter) tabulate-coral colonies is typical of the Bur
Formation. Some of the colonies are overturned and obviously affected by storms. 3. Material and method In total 77 samples (Fig. 2) were processed following standard palynological procedures involving HF and HCl acid maceration, density separation of the organic residue and its filtration by sieving through a 15 μm mesh. Neither oxidative nor alkali treatments were applied. Almost all samples yielded more or less well preserved palynomorphs represented mainly by acritarchs and prasinophytes. The material is currently under ongoing investigation and a preliminary report of the acritarch diversity and distribution was recently presented (Raevskaya and Dronov, 2014). Cryptospores are abundant in 12 samples from the lower part of the Dolbor Formation and a few specimens occur in 3 samples from the upper part of the Bur Formation. All palynological slides are deposited in the Department of Stratigraphy in FGUNPP “Geologorazvedka”, Saint-Petersburg, Russia. 4. Results The cryptospores are mostly associated with marine palynomorphs including moderately richly diversified acritarchs (Raevskaya and Dronov, 2014), but also less numerous scolecodonts and chitinozoans, that remain unstudied so far. Abundant organic detritus is also very characteristic. The palynological content of the slides suggests a proximal shallow marine environment influenced by terrestrially derived sedimentation.
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The most diverse and rich cryptospores are from the lower part of the Dolbor Formation (Fig. 2). Almost all known morphotypes including naked and envelope-enclosed monads, dyads, tetrads and polyads have been recovered. However, many specimens are damaged by pyrite which makes it difficult to identify the true ornament of the envelopes if any is present. Therefore, taxonomic attribution in many cases remains uncertain, although some identifications can be provided. The diversity of the cryptospore assemblages is illustrated in Plates I–III. The discovered cryptospore assemblage include, among others, the following taxa: Velatitetras laevigata Burgess, 1991, Tetrahedraletes medinensis Strother and Traverse emend Wellman and Richardson, 1993, Abditusdyadus laevigatus Wellman and Richardson, 1996, Dyadospora murusdensa Strother and Traverse emend Burgess and Richardson, 1991, Pseudodyadospora laevigata Johnson, 1985, Segestrespora laevigata Burgess, 1991, Sphaerasaccus glabellus Steemans et al., 2000, and some other morphotypes that remain in open nomenclature or cannot be identified. The cryptospore taxa do not change noticeably throughout the section and their taxonomic diversity appears to be related to their abundance in general. 5. Discussion The reconstruction and interpretation of the early terrestrial vegetation are extremely important for the understanding of the early history of terrestrial ecosystems that had a significant impact in biosphere changes in the Early and Middle Paleozoic. However, plant megafossils are only recorded from one Ordovician locality (Wellman et al., 2003) and 23 localities in the Silurian (Wellman et al., 2013), whereas the fossil record of dispersed spores is much more abundant and extends back to at least the Mid Ordovician (Wellman et al., 2013). There is an ongoing debate about the understanding of the term cryptospores. However, there is a general consensus that the early Middle Ordovician (Dapingian) to Early Silurian (Llandovery) cryptospore assemblages (including monads, dyads and tetrads) appear over a broad geographic area and provide the first evidence of land plants following their evolution from charophycean green algae ancestors (e.g., Kenrick and Crane, 1997; Wellman et al., 2013). There is also a general agreement that the morphological evolution of cryptospores was rather limited during the Middle Ordovician–Early Silurian interval (Steemans, 2000; Wellman et al., 2013, etc.). The first records of Late Ordovician cryptospores from the paleocontinent of Siberia are of significance, as this discovery indicates that the typical cryptospore assemblages of the Late Ordovician interval
occurred on all of the major Ordovician paleocontinents: Gondwana, Baltica, Laurentia and Siberia. Due to the poor preservation of the palynomorphs the present study does not include a detailed taxonomic treatment. However, all of the identified taxa are already known from outside of Siberia and it is so far impossible to indicate if endemic components are present in the assemblages. Some taxa have a wide geographical distribution, as they are described from many other localities, such as for example the naked permanent tetrad Tetrahedraletes medinensis that is recorded globally (e.g., Vecoli et al., 2011). Other taxa recognized in the Siberian assemblages are known so far only from a few Ordovician localities, such as the true dyad Abditusdyadus laevigatus described from Estonia (Vecoli et al., 2011) and Belgium (Steemans, 2001). The naked true dyad Dyadospora murusdensa is known from southern Britain (Wellman, 1996), the Czech Republic (Vavrdová, 1988, 1989) and China (Wang et al., 1997), while the laevigate monad Sphaerasaccus glabellus has been previously recorded from southern Britain (Wellman, 1996), northwestern Argentina (Rubinstein and Vaccari, 2004; Rubinstein, 2005) and more recently from Estonia (Vecoli et al., 2011) and Algeria (Spina, 2014). The naked pseudodyad Pseudodyadospora laevigata was also recorded from many other localities, including the Czech Republic (Vavrdová, 1988, 1989), southern Britain (Wellman, 1996), southeastern Turkey (Steemans et al., 1996), China (Wang et al., 1997), Belgium (Steemans, 2001) and more recently from Estonia, Anticosti Island, Canada (Vecoli et al., 2011) and Algeria (Spina, 2014). The pseudodyad Segestrespora laevigata was previously found in South Wales (Burgess, 1991), southern Britain (Wellman, 1996), southeastern Turkey (Steemans et al., 1996), northwestern Argentina (Rubinstein and Vaccari, 2004; Rubinstein, 2005) and Estonia (Vecoli et al., 2011). The permanent tetrad Velatitetras laevigata was recorded from different localities, too, but not yet from Belgium, Saudi Arabia and the Czech Republic. Although the full spectrum of Siberian cryptospores is probably not yet recovered, it appears that the assemblages include many of the taxa that have been described previously from different parts of the world. The Siberian paleocontinent was located in the low latitude tropical area migrating slowly from the southern hemisphere in the Cambrian and Early Ordovician to the northern hemisphere in the Late Ordovician and Silurian (Cocks and Torsvik, 2007). The recent discovery of Late Ordovician cryptospores from Baltica, first by Vecoli et al. (2011) from Estonia and subsequently by Badawy et al. (2014) from Sweden, indicates that the two paleocontinents of Baltica and Siberia, both located in intermediate to equatorial positions, were thus also colonized by early land plants at least by the Late Ordovician.
Plate I. Cryptospore tetrads and pseudodyads from the Upper Ordovician (Katian) of Bolshaya Nirunda River, Siberian Platform. Scale bar represents 20 μm. 1, 2 3, 4 5–10, 13?
11–12 14 15, 16? 17
Permanent tetrad enclosed in a laevigate envelope (Velatitetras laevigata Burgess, 1991). Sample II-8, slide A-09-II-8/2, E.F. U43/3. Naked permanent tetrad (Tetrahedraletes medinensis Strother and Traverse emend Wellman and Richardson, 1993). Sample II-8, slide A-09-II-8/2, E.F. O43/1. Permanent tetrad enclosed in a laevigate envelope (?Velatitetras laevigata Burgess, 1991). 5 — Sample II-8, slide A-09-II-8/1, E.F. O35; 6 — Sample II-5, slide A-09-II-5, E.F. P34/ 4; 7 — Sample II-5, slide A-09-II-5, E.F. R26; 8 — Sample II-8, slide A-09-II-8/2, E.F. J,H41,42/4,3,2,1; 9 — Sample II-8, slide A-09-II-8/2, E.F. N33/4; 10 — Sample II-8, slide A-09-II8/1, E.F. P34/2; 13 — Sample II-4, slide A-09-II-4, E.F. A42/1. Naked permanent tetrad (Tetrahedraletes medinensis Strother and Traverse emend Wellman and Richardson, 1993). 11 — Sample II-8, slide A-09-II-8/2, E.F. O22/2; 12 — Sample II-8, slide A-09-II-8/1, E.F. O27/3. Cryptospore indet. Sample II-8, slide A-09-II-8/1, E.F. P30. Pairs of pseudodyads enclosed in a laevigate envelope (Segestrespora laevigata Burgess, 1991). 15 — Sample II-8, slide A-09-II-8/2, E.F. T32/1; 16 — Sample II-15, slide A-09-II15, E.F. V29/4. Cluster of cryptospores. Sample II-8, slide A-09-II-8/2, E.F. H37/1.
Plate II. Cryptospore dyads from the Upper Ordovician (Katian) of Bolshaya Nirunda River, Siberian Platform. Scale bar represents 20 μm. (see on page 10) 1, 2, 5, 6, 9, 13, 16, 17 Pseudodyad enclosed in a laevigate envelope (Segestrespora laevigata Burgess, 1991). 1 — Sample II-8, slide A-09-II-8/2, E.F. F19/3; 2 — Sample II-8, slide A-09-II-8/1, E.F. L26/4; 5 — Sample II-12, slide A-09-II-12, E.F. E40/2; 6 — Sample II-8, slide A-09-II-8/1, E.F. D,E18/3,4,1,2; 9 — Sample II-8, slide A-09-II-8/1, E.F. R37,38/4,3; 13 — Sample II-8, slide A-09-II-8/1, E.F. J40/4; 16 — Sample II-8, slide A-09-II-8/2, E.F. U20/4; 17 — Sample II-8, slide A-09-II-8/2, E.F. J34 3, 4, 7, 8, 11, 12 True dyad enclosed in a laevigate envelope (Abditusdyadus laevigatus Wellman and Richardson, 1996). 3 — Sample II-4, slide A-09-II-4, E.F. N45/3; 4 — Sample II-4, slide A-09-II-4, E.F. K33/4; 7 — Sample II-6, slide A-09-II-6, E.F. U42/3; 8 — Sample II-8, slide A-09-II-8/2, E.F. G42/1; 11 — Sample II-5, slide A-09-II-5, E.F. Q32; 12 — Sample II-4, slide A-09-II-4, E.F. G40/1,3. 10, 14, 15 Naked true dyad (Dyadospora murusdensa Strother and Traverse 1979 emend Burgess and Richardson, 1991). 10 — Sample II-6, slide A-09-II-6, E.F. X20/1; 14 — Sample II-16, slide A-09-II-16, E.F. J39/2; 15 — Sample II-8, slide A-09-II-8/2, E.F. M42/3. 18 Naked pseudodyad (Pseudodyadospora laevigata Johnson, 1985). Sample II-8, slide A-09-II-8/2, E.F. G45.
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Plate III. Cryptospore monads from the Upper Ordovician (Katian) of Bolshaya Nirunda River, Siberian Platform. Scale bar represents 20 μm. 1, 3, 4, 6–11
2 5, 14 12, 13, 15
Laevigate monad. 1 — Sample II-8, slide A-09-II-8/2, E.F. F27/4; 3 — Sample II-8, slide A-09-II-8/2, E.F. U45/1; 4 — Sample II-8, slide A-09-II-8/2, E.F. U27/3; 6 — Sample II-8, slide A-09-II-8/1, E.F. K18/1; 7 — Sample II-8, slide A-09-II-8/2, E.F. T38/2; 8 — Sample II-8, slide A-09-II-8/2, E.F. U38/2; 9 — Sample II-12, slide A-09-II-12, E.F. F22/1; 10 — Sample II-12, slide A-09-II-12, E.F. H20; 11 — Sample II-8, slide A-09-II-8/2, E.F. P21/3. Cluster of laevigate monads. Sample II-11, slide A-09-II-11, E.F. T32. Laevigate monad in a laevigate envelope (Sphaerasaccus glabellus Steemans et al., 2000). 5 — Sample II-8, slide A-09-II-8/2, E.F. K27/1,2. Laevigate monad in a ?laevigate envelope (?Sphaerasaccus glabellus Steemans et al., 2000). 12 — Sample II-11, slide A-09-II-11, E.F. Q27/2; 13 — Sample II-8, slide A-09-II-8/ 2, E.F. C25/2; 15 — Sample II-15, slide A-09-II-15, E.F. E34.
The picture of early land plant evolution is far from being complete. Wellman et al. (2013) pointed out that some areas remain unstudied. The present study closes a gap, indicating that the major paleocontinent of Siberia also was colonized by these types of early land plants. Nevertheless, much more data need to be collected from Ordovician strata to
test hypotheses on the timing of land plant origins and the first phases of colonization of the continents by early vegetation. It would not be surprising to find older occurrences of cryptospores from Siberia. Palynological investigations have been limited so far, and future studies are thus needed.
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6. Conclusions The present study reports the first record of land-plant derived spores from the Upper Ordovician of the paleocontinent Siberia. The cryptospores discovered in the studied interval of the Baksian– Dolborian Regional Stages (corresponding to the Katian Global Stage of the Upper Ordovician) are diverse throughout the section and relatively abundant at some levels in the lower part of the Dolborian. The cryptospores recovered are represented by almost all known morphotypes including naked and enveloped-enclosed monads, dyads, tetrads and polyads. These observations confirm the worldwide uniformity of cryptospore assemblages during the Late Ordovician to Early Silurian. However, in order to establish detailed comparisons of the recovered cryptospore assemblages from Siberia with those from other parts of the world a more accurate taxonomical study is necessary. Similarly, in order to understand if the Russian assemblages reflect a real diversification of the paleoflora or if the changes in the assemblages are the result of sampling effects or of increased input from land-derived clastic sedimentation, detailed sedimentological analyses need to be performed. It remains unclear if these records represent the oldest occurrence of cryptospores in Siberia, since in the Bolshaya Nirunda section they appear in the base of the Dolborian already in high number and morphological variety. It seems that their occurrence is most likely related to some environmental signal rather than having evolutionary meaning. Future studies in older sediments will probably reveal even older cryptospores. The obtained palynological material from the middle Late Ordovician of Siberia is a great additional source extending the paleogeographical coverage of the dispersed spores of the earliest land plants. The discovery of cryptospores in the Late Ordovician of Siberia extends the paleogeographical coverage of Late Ordovician cryptospore distribution confirming that early land-plants were probably globally distributed before the Silurian.
Acknowledgments This work is performed according to the Russian Government Program of Competitive Growth of Kazan Federal University. Financial support was also provided from the Russian Foundation for Basic Research Grant No. 13-05-00746, and the Agence National de Recherche project TERRES “Prospectives globales sur le processus de la terrestrialisation” (ANR-10-BLAN-0607). This study is a contribution to the IGCP Project 591 “The Early to Middle Paleozoic Revolution: Bridging the Gap Between the Great Ordovician Biodiversification Event and the Devonian Terrestrial Revolution.”
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Review of Palaeobotany and Palynology journal homepage: www.elsevier.com/locate/revpalbo
Cryptospores from the Katian (Upper Ordovician) of the Tungus basin: The first evidence for early land plants from the Siberian paleocontinent Elena Raevskaya a,⁎, Andrey Dronov b,c, Thomas Servais d, Charles H. Wellman e a
FGUNPP “Geologorazvedka”, Knipovich str., 11, block 2, Saint-Petersburg 191019, Russia Institute of Geology, Russian Academy of Sciences, Pyzhevsky per. 7, Moscow 119017, Russia Kazan (Volga Region) Federal University, Kremlevskaya ul., 18, 420008 Kazan, Russia d Evo-Eco-Paleo, UMR 8198 CNRS, Université des Sciences et Technologies de Lille, Sciences de la Terre, Lille, France e Department of Animal and Plant Sciences, University of Sheffield, Alfred Denny Building, Western Bank, Sheffield S10 2TN, UK b c
a r t i c l e
i n f o
Article history: Received 12 May 2015 Received in revised form 14 September 2015 Accepted 9 October 2015 Available online 31 October 2015 Keywords: Cryptospore Upper Ordovician Katian Siberian Platform
a b s t r a c t A diverse assemblage of cryptospores is reported for the first time from the Upper Ordovician of Siberia. It was discovered during a palynological study of a sedimentary succession of about 100 m, exposed along the Bolshaya Nirunda River, a right tributary of the Podkamennaya Tunguska River. The succession is located on the southern margin of the extensive epicontinental Tungus basin on the Siberian Platform between the Katanga and Yenisei land masses. The cryptospore assemblage was recovered from the siliciclastic-carbonate Dolbor Formation and scarcely from the overlying more carbonate Bur Formation. Both formations belong to the Katian Global Stage (Upper Ordovician). The cryptospores occur together with marine remnants (acritarchs, prasinophytes, chitinozoans, scolecodonts). It is similar to all known Upper Ordovician cryptospore assemblages in comprising naked and envelope-enclosed monads, dyads, tetrads and polyads. Although preservation is moderate to poor, the cryptospore taxa Velatitetras laevigata, Tetrahedraletes medinensis, Abditusdyadus laevigatus, Dyadospora murusdensa, Pseudodyadospora laevigata, Segestrespora laevigata and Sphaerasaccus glabellus can be identified. This report represents the first record of spores of the earliest land plants from the paleocontinent Siberia and therefore extends the global paleogeographical coverage of Late Ordovician cryptospores. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Cryptospores are considered to have been produced by the earliest terrestrial flora that probably consisted of bryophyte-like plants (Gray, 1985, 1991; Richardson, 1996; Strother et al., 1996; Wellman, 1996; Edwards et al., 1999; Steemans, 2000; Wellman et al., 2013). They are considered to represent the earliest evidence for continental vegetation and thus document a fundamental phase of land plant evolution (Gray et al., 1982; Wellman et al., 2003; Steemans et al., 2009; Spina, 2014). The oldest cryptospores are known from the Middle Ordovician. More or less certain cryptospore taxa (i.e., ‘Tetrahedraletes cf. medinensis’) are recorded from the Dapingian of Argentina (Rubinstein et al., 2010). However, being represented by few specimens from one single sample this record still needs corroboration by further data before it can be unquestionably considered as the oldest evidence of cryptospores (Spina, 2014; Strother et al., 2015). Thus, for the present, generally accepted oldest cryptospores are from the Darriwilian of Saudi Arabia (Strother et al., 1996, 2015) since they are characterized by regular morphology and abundant population (Wellman et al., 2003; Spina, 2014). Numerous publications from many parts of the ⁎ Corresponding author. E-mail address: [email protected] (E. Raevskaya).
http://dx.doi.org/10.1016/j.revpalbo.2015.10.010 0034-6667/© 2015 Elsevier B.V. All rights reserved.
world show the uniformity of cryptospore assemblages, in terms of their taxonomic composition and diversity, from the Middle Ordovician to the Early Silurian, leading to a concept of the existence of a conservative cosmopolitan bryophyte-like flora that remained almost unchanged for about 40 million years (Wellman, 1996; Steemans, 2000; Wellman and Gray, 2000; Rubinstein et al., 2010; Vecoli et al., 2011; Spina, 2014). However, the steady increase of new data from different paleocontinents is beginning to reveal a more complex pattern of cryptospore origination and spatial/temporal distribution (Wellman et al., 2013) that may possibly lead to a slight modification of existing ideas. The reconstruction and interpretation of the early terrestrial vegetation mostly depend on the availability and adequacy of the fossil data (particularly of cryptospore records) that largely vary in both quantity and quality. While fairly representative data are now available from eastern North America (Laurentia), Western Europe (Laurentia, Avalonia, Baltica) and North Africa and Arabia (southern part of Gondwana), very little or nothing is known from other paleocontinents. As reviewed in Wellman et al. (2013) Ordovician (Dapingian–Hirnantian) cryptospore assemblages have been described from Gondwana (Libya, Saudi Arabia, South Africa, Australia, Argentina), Peri-Gondwana (the Czech Republic, southeastern Turkey), Avalonia (southern Britain, Belgium), Laurentia (USA and Canada), South China (Wang et al., 1997) and Tarim (Yin and He, 2000). The first records of Late Ordovician
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cryptospores from the paleocontinent Baltica have recently been added based on studies from Estonia (Vecoli et al., 2011) and Sweden (Badawy et al., 2014). More recently, further reports of Ordovician cryptospores are from Chad (Le Herisse et al., 2013), Iran (Mahmoudi et al., 2014) and Algeria (Spina, 2014). However, cryptospores have not been recorded from Russia so far. Thus, a significant lack of data concerns one of the major paleocontinents — Siberia. In recent years abundant research and field-work have been conducted in the Ordovician of the southern and western part of the Siberian Craton (e.g., Dronov et al., 2009, 2011; Kanygin et al., 2010a, b). Within the framework of a multidisciplinary research project intensive palynological investigations were initiated. In this pilot study the first preliminary palynological results concerning cryptospores are presented. This first discovery of cryptospores from Russia expands the current knowledge on the paleophytogeography of Late Ordovician early vegetation, extending their distribution to the paleocontinent Siberia.
in exposed sections (Kanygin et al., 2007), but possibly exists in the subsurface of the central part of the Tungus Syneclise (Dronov, 2013). The regional stages allow a correlation of Ordovician sections within the Tungus basin and across the Siberian Platform (Kanygin et al., 1984, 2007). However, due to the endemic character of the Siberian faunas, their direct correspondence to the global chronostratigraphic scale is not precise, although a provisional correlation was recently proposed by Bergström et al. (2009). So far, only four reliable levels have been identified for intercontinental biostratigraphic correlation of the Ordovician of the Siberian Platform: (1) the base of the Ordovician, which was established at the lower boundary of the Nyaian Regional Stage according to the most recent conodont investigations (Tolmacheva and Abaimova, 2009); (2) the base of the Volginian Regional Stage in the Middle Ordovician that correlates with the base of the Hustedograptus teretiusculus graptolite zone; (3) the base of the Chertovskian Regional Stage that correlates with the base of the Nemagraptus gracilis graptolite zone — the base of the Upper Ordovician; and (4) the base of the Silurian. The stratigraphic interval discussed in the present paper includes the uppermost Baksian, Dolborian, Nirundian and Burian Regional Stages, which correspond to the Katian Global Stage (Fig. 2). A precise correlation of the listed Upper Ordovician regional stages with the Global Scale is uncertain. However, recently discovered zircon crystals from K-bentonite beds in the upper part of the Mangazea Formation, attributed to the Baksian Regional Stage, provide a 206Pb/238U age of 450.58 ± 0.27 Ma confirming the Katian age (Huff et al., 2014). The studied Upper Ordovician sedimentary succession is represented by the upper part of the Mangazea Formation as well as the Dolbor, Nirunda and Bur Formations with the same name as the regional stages (Fig. 2). The whole succession of about 100 m is well exposed in a series of outcrops which can be traced with some certainty along both banks of the Bolshaya Nirunda River (Fig. 1). The upper part of the Mangazea Formation (Baksian Regional Stage) crops out in a small anticline fold (Fig. 3-A) in outcrop I (N61°58′23.83″; E95°16′22.53″). The exposed thickness is only of about 14 m. The formation is represented by a rhythmic intercalation of greenish-gray siltstone and gray coarse-grained bioclastic limestones that may reach 20–30 cm in thickness (Fig. 3-B). The base of these limestone beds is usually erosional while ripple marks may often be observed on their top. Bioclasts are mainly represented by fragments of brachiopod shells, trilobite carapaces and bryozoans. Ostracods and crinoids are also numerous. These rocks were interpreted as proximal cool-water carbonate tempestites deposited in middle-ramp settings (Dronov, 2013).
2. Geological setting and stratigraphy The studied outcrop area is located on the southern margin (in present day orientation) of the extensive epicontinental Tungus basin (Fig. 1). The Upper Ordovician (Katian–?lowermost Hirnantian) succession is exposed in four separated outcrops along the Bolshaya Nirunda River, a right tributary of the Podkamennaya Tunguska River about 60 km downstream from the village of Baykit (Fig. 1). Although being a large part of Eurasia today the Siberian Platform (Siberian Craton) was an individual paleocontinent during the early Paleozoic, located in low latitudes near the paleoequator (Cocks and Torsvik, 2007). During the Ordovician several more-or-less separate sedimentary basins were present in Siberia. Among the largest of them are the Irkutsk and Tungus basins (Fig. 1). The Tungus epicontinental basin represents a large, intracratonic, sedimentary sag basin embracing the central part of the Siberian Craton. It was surrounded by the Anabar, Turukhan, Yenisei and Katanga Lands. The biostratigraphy of the Ordovician succession in the Tungus basin is mainly based on faunal (trilobites, brachiopods and ostracods) distribution allowing a subdivision into twelve regional stages (Kanygin et al., 1984, 2007): two in the Lower Ordovician (Nyaian and Ugorian), five in the Middle Ordovician (Kimaian, Vikhorevian, Mukteian, Volginian and Kirensko–Kudrian) and five in the Upper Ordovician (Chertovskian, Baksian, Dolborian, Nirundian and Burian). The uppermost part of the Ordovician is eroded 96
5
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Fig. 1. Geographic location of the study area and outcrops of investigated Upper Ordovician strata. Legend: 1, Boundary of the Siberian platform. 2, Provisional boundaries of the Siberian platform and land areas. 3, Provisional boundary of the Tungus and Irkutsk basins. 4, Land areas without Ordovician deposits (1 — Turukhan Land, 2 — Yenisei Land, 3 — Katanga Land). 5, Ordovician deposits in subsurface areas. 6, Ordovician outcrop areas. 7, Studied outcrops along the Bolshaya Nirunda River.
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Laevigate monad IV-10 IV-9 IV-8 IV-7 IV-6 IV-5 IV-4 IV-3 IV-2 IV-1
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Fig. 2. Litho- and chronostratigraphy of the studied succession with sample locations and cryptospore distribution. Legend: 1, Silurian thick bedded dark gray limestone (mudstone to wackestone). 2, Greenish-gray siltstone. 3, Red siltstone. 4, Thin bedded bioclastic limestone and intercalation of limestone and siltstone. 5, Thick bedded bioclastic limestone. 6, Limestone beds with chert nodules. 7, Fine grained quartz sandstone. 8, K-bentonite layers. 9, Palynological samples. 10, Cryptospore occurrence (a — from 1 to 2 specimens, b — from 3 to 5 specimens, c — more than 5 specimens).
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Fig. 3. Representative Upper Ordovician strata cropping out along the Bolshaya Nirunda River. (A) The anticline fold in the outcrop I. Left bank of the Bolshaya Nirunda River. Upper part of the Mangazea Formation. (B) Intercalation of siltstone and bioclastic coarse-grained limestone (storm beds) of the Mangazea Formation. (C) The lower part of the Dolbor Formation with characteristic intercalation of thin-bedded limestone and siltstone layers (distal tempestites). Outcrop II on the right bank of the Bolshaya Nirunda River. (D) Middle part of the Dolbor Formation predominated by siltstone in the outcrop II. Right bank of the Bolshaya Nirunda River. (E) General view of the outcrop III on the right bank of the Yukteli creek (left tributary), about 100 m upstream from the point where it flows into the Bolshaya Nirunda River.
The Dolbor Formation roughly corresponds to the Dolborian Regional Stage (Dronov et al., 2009; Kanygin et al., 2010a,b). It is exposed in outcrop II (N61°59′29.58″; E95°15′44.68″) and is represented by a succession of about 63 m in thickness (Figs. 1, 2). In its lower part the Dolbor Formation consists of an intercalation of thin (0.5–2 cm) limestone layers that are interpreted as distal carbonate tempestites and greenish-gray siltstone layers of comparable thickness (Fig. 3-C). Thicker beds sometimes contain chert nodules. The middle part of the formation consists of almost pure greenish-gray siltstone of about 12 m thickness (Fig. 3D). The upper part of the formation is again more carbonate-rich and contains numerous coral colonies. The presence of pure terrigenous deposits in the middle part of the formation probably points to the proximity of Katanga Land. The Nirunda Formation corresponds to the Nirunda Regional Stage. It is exposed only in its lowermost part (6 m) on the very top of outcrop II (Fig. 1). The formation is represented by cherry-red and greenish-gray siltstone with fine-grained bioclastic limestone layers at some levels. Bryozoans and brachiopods dominate the macrofauna. The Bur Formation coincides with the Burian Regional Stage. In the Bolshaya Nirunda River valley, the formation is exposed in two separated outcrops (III and IV) that could not be directly correlated with each other (Fig. 1). In both outcrops the formation is represented by an intercalation of greenish-gray siltstone and gray bioclastic limestone beds. The outcrop III (N62°03′22.99″; E95°16′17.69″) contains a diabase sill on the top (Fig. 3-E). In outcrop IV (N62°04′31,82″; E95°14′37,88″) the “top” of the Bur Formation is represented by a regional unconformity and is directly overlain by Silurian deposits. The abundance of big (up to 20–30 cm in diameter) tabulate-coral colonies is typical of the Bur
Formation. Some of the colonies are overturned and obviously affected by storms. 3. Material and method In total 77 samples (Fig. 2) were processed following standard palynological procedures involving HF and HCl acid maceration, density separation of the organic residue and its filtration by sieving through a 15 μm mesh. Neither oxidative nor alkali treatments were applied. Almost all samples yielded more or less well preserved palynomorphs represented mainly by acritarchs and prasinophytes. The material is currently under ongoing investigation and a preliminary report of the acritarch diversity and distribution was recently presented (Raevskaya and Dronov, 2014). Cryptospores are abundant in 12 samples from the lower part of the Dolbor Formation and a few specimens occur in 3 samples from the upper part of the Bur Formation. All palynological slides are deposited in the Department of Stratigraphy in FGUNPP “Geologorazvedka”, Saint-Petersburg, Russia. 4. Results The cryptospores are mostly associated with marine palynomorphs including moderately richly diversified acritarchs (Raevskaya and Dronov, 2014), but also less numerous scolecodonts and chitinozoans, that remain unstudied so far. Abundant organic detritus is also very characteristic. The palynological content of the slides suggests a proximal shallow marine environment influenced by terrestrially derived sedimentation.
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The most diverse and rich cryptospores are from the lower part of the Dolbor Formation (Fig. 2). Almost all known morphotypes including naked and envelope-enclosed monads, dyads, tetrads and polyads have been recovered. However, many specimens are damaged by pyrite which makes it difficult to identify the true ornament of the envelopes if any is present. Therefore, taxonomic attribution in many cases remains uncertain, although some identifications can be provided. The diversity of the cryptospore assemblages is illustrated in Plates I–III. The discovered cryptospore assemblage include, among others, the following taxa: Velatitetras laevigata Burgess, 1991, Tetrahedraletes medinensis Strother and Traverse emend Wellman and Richardson, 1993, Abditusdyadus laevigatus Wellman and Richardson, 1996, Dyadospora murusdensa Strother and Traverse emend Burgess and Richardson, 1991, Pseudodyadospora laevigata Johnson, 1985, Segestrespora laevigata Burgess, 1991, Sphaerasaccus glabellus Steemans et al., 2000, and some other morphotypes that remain in open nomenclature or cannot be identified. The cryptospore taxa do not change noticeably throughout the section and their taxonomic diversity appears to be related to their abundance in general. 5. Discussion The reconstruction and interpretation of the early terrestrial vegetation are extremely important for the understanding of the early history of terrestrial ecosystems that had a significant impact in biosphere changes in the Early and Middle Paleozoic. However, plant megafossils are only recorded from one Ordovician locality (Wellman et al., 2003) and 23 localities in the Silurian (Wellman et al., 2013), whereas the fossil record of dispersed spores is much more abundant and extends back to at least the Mid Ordovician (Wellman et al., 2013). There is an ongoing debate about the understanding of the term cryptospores. However, there is a general consensus that the early Middle Ordovician (Dapingian) to Early Silurian (Llandovery) cryptospore assemblages (including monads, dyads and tetrads) appear over a broad geographic area and provide the first evidence of land plants following their evolution from charophycean green algae ancestors (e.g., Kenrick and Crane, 1997; Wellman et al., 2013). There is also a general agreement that the morphological evolution of cryptospores was rather limited during the Middle Ordovician–Early Silurian interval (Steemans, 2000; Wellman et al., 2013, etc.). The first records of Late Ordovician cryptospores from the paleocontinent of Siberia are of significance, as this discovery indicates that the typical cryptospore assemblages of the Late Ordovician interval
occurred on all of the major Ordovician paleocontinents: Gondwana, Baltica, Laurentia and Siberia. Due to the poor preservation of the palynomorphs the present study does not include a detailed taxonomic treatment. However, all of the identified taxa are already known from outside of Siberia and it is so far impossible to indicate if endemic components are present in the assemblages. Some taxa have a wide geographical distribution, as they are described from many other localities, such as for example the naked permanent tetrad Tetrahedraletes medinensis that is recorded globally (e.g., Vecoli et al., 2011). Other taxa recognized in the Siberian assemblages are known so far only from a few Ordovician localities, such as the true dyad Abditusdyadus laevigatus described from Estonia (Vecoli et al., 2011) and Belgium (Steemans, 2001). The naked true dyad Dyadospora murusdensa is known from southern Britain (Wellman, 1996), the Czech Republic (Vavrdová, 1988, 1989) and China (Wang et al., 1997), while the laevigate monad Sphaerasaccus glabellus has been previously recorded from southern Britain (Wellman, 1996), northwestern Argentina (Rubinstein and Vaccari, 2004; Rubinstein, 2005) and more recently from Estonia (Vecoli et al., 2011) and Algeria (Spina, 2014). The naked pseudodyad Pseudodyadospora laevigata was also recorded from many other localities, including the Czech Republic (Vavrdová, 1988, 1989), southern Britain (Wellman, 1996), southeastern Turkey (Steemans et al., 1996), China (Wang et al., 1997), Belgium (Steemans, 2001) and more recently from Estonia, Anticosti Island, Canada (Vecoli et al., 2011) and Algeria (Spina, 2014). The pseudodyad Segestrespora laevigata was previously found in South Wales (Burgess, 1991), southern Britain (Wellman, 1996), southeastern Turkey (Steemans et al., 1996), northwestern Argentina (Rubinstein and Vaccari, 2004; Rubinstein, 2005) and Estonia (Vecoli et al., 2011). The permanent tetrad Velatitetras laevigata was recorded from different localities, too, but not yet from Belgium, Saudi Arabia and the Czech Republic. Although the full spectrum of Siberian cryptospores is probably not yet recovered, it appears that the assemblages include many of the taxa that have been described previously from different parts of the world. The Siberian paleocontinent was located in the low latitude tropical area migrating slowly from the southern hemisphere in the Cambrian and Early Ordovician to the northern hemisphere in the Late Ordovician and Silurian (Cocks and Torsvik, 2007). The recent discovery of Late Ordovician cryptospores from Baltica, first by Vecoli et al. (2011) from Estonia and subsequently by Badawy et al. (2014) from Sweden, indicates that the two paleocontinents of Baltica and Siberia, both located in intermediate to equatorial positions, were thus also colonized by early land plants at least by the Late Ordovician.
Plate I. Cryptospore tetrads and pseudodyads from the Upper Ordovician (Katian) of Bolshaya Nirunda River, Siberian Platform. Scale bar represents 20 μm. 1, 2 3, 4 5–10, 13?
11–12 14 15, 16? 17
Permanent tetrad enclosed in a laevigate envelope (Velatitetras laevigata Burgess, 1991). Sample II-8, slide A-09-II-8/2, E.F. U43/3. Naked permanent tetrad (Tetrahedraletes medinensis Strother and Traverse emend Wellman and Richardson, 1993). Sample II-8, slide A-09-II-8/2, E.F. O43/1. Permanent tetrad enclosed in a laevigate envelope (?Velatitetras laevigata Burgess, 1991). 5 — Sample II-8, slide A-09-II-8/1, E.F. O35; 6 — Sample II-5, slide A-09-II-5, E.F. P34/ 4; 7 — Sample II-5, slide A-09-II-5, E.F. R26; 8 — Sample II-8, slide A-09-II-8/2, E.F. J,H41,42/4,3,2,1; 9 — Sample II-8, slide A-09-II-8/2, E.F. N33/4; 10 — Sample II-8, slide A-09-II8/1, E.F. P34/2; 13 — Sample II-4, slide A-09-II-4, E.F. A42/1. Naked permanent tetrad (Tetrahedraletes medinensis Strother and Traverse emend Wellman and Richardson, 1993). 11 — Sample II-8, slide A-09-II-8/2, E.F. O22/2; 12 — Sample II-8, slide A-09-II-8/1, E.F. O27/3. Cryptospore indet. Sample II-8, slide A-09-II-8/1, E.F. P30. Pairs of pseudodyads enclosed in a laevigate envelope (Segestrespora laevigata Burgess, 1991). 15 — Sample II-8, slide A-09-II-8/2, E.F. T32/1; 16 — Sample II-15, slide A-09-II15, E.F. V29/4. Cluster of cryptospores. Sample II-8, slide A-09-II-8/2, E.F. H37/1.
Plate II. Cryptospore dyads from the Upper Ordovician (Katian) of Bolshaya Nirunda River, Siberian Platform. Scale bar represents 20 μm. (see on page 10) 1, 2, 5, 6, 9, 13, 16, 17 Pseudodyad enclosed in a laevigate envelope (Segestrespora laevigata Burgess, 1991). 1 — Sample II-8, slide A-09-II-8/2, E.F. F19/3; 2 — Sample II-8, slide A-09-II-8/1, E.F. L26/4; 5 — Sample II-12, slide A-09-II-12, E.F. E40/2; 6 — Sample II-8, slide A-09-II-8/1, E.F. D,E18/3,4,1,2; 9 — Sample II-8, slide A-09-II-8/1, E.F. R37,38/4,3; 13 — Sample II-8, slide A-09-II-8/1, E.F. J40/4; 16 — Sample II-8, slide A-09-II-8/2, E.F. U20/4; 17 — Sample II-8, slide A-09-II-8/2, E.F. J34 3, 4, 7, 8, 11, 12 True dyad enclosed in a laevigate envelope (Abditusdyadus laevigatus Wellman and Richardson, 1996). 3 — Sample II-4, slide A-09-II-4, E.F. N45/3; 4 — Sample II-4, slide A-09-II-4, E.F. K33/4; 7 — Sample II-6, slide A-09-II-6, E.F. U42/3; 8 — Sample II-8, slide A-09-II-8/2, E.F. G42/1; 11 — Sample II-5, slide A-09-II-5, E.F. Q32; 12 — Sample II-4, slide A-09-II-4, E.F. G40/1,3. 10, 14, 15 Naked true dyad (Dyadospora murusdensa Strother and Traverse 1979 emend Burgess and Richardson, 1991). 10 — Sample II-6, slide A-09-II-6, E.F. X20/1; 14 — Sample II-16, slide A-09-II-16, E.F. J39/2; 15 — Sample II-8, slide A-09-II-8/2, E.F. M42/3. 18 Naked pseudodyad (Pseudodyadospora laevigata Johnson, 1985). Sample II-8, slide A-09-II-8/2, E.F. G45.
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2
1
55
9
3
6
7
4
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12
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16 Plate I.
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5
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16 Plate II (caption on page 8).
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1 3 2
4 5
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Plate III. Cryptospore monads from the Upper Ordovician (Katian) of Bolshaya Nirunda River, Siberian Platform. Scale bar represents 20 μm. 1, 3, 4, 6–11
2 5, 14 12, 13, 15
Laevigate monad. 1 — Sample II-8, slide A-09-II-8/2, E.F. F27/4; 3 — Sample II-8, slide A-09-II-8/2, E.F. U45/1; 4 — Sample II-8, slide A-09-II-8/2, E.F. U27/3; 6 — Sample II-8, slide A-09-II-8/1, E.F. K18/1; 7 — Sample II-8, slide A-09-II-8/2, E.F. T38/2; 8 — Sample II-8, slide A-09-II-8/2, E.F. U38/2; 9 — Sample II-12, slide A-09-II-12, E.F. F22/1; 10 — Sample II-12, slide A-09-II-12, E.F. H20; 11 — Sample II-8, slide A-09-II-8/2, E.F. P21/3. Cluster of laevigate monads. Sample II-11, slide A-09-II-11, E.F. T32. Laevigate monad in a laevigate envelope (Sphaerasaccus glabellus Steemans et al., 2000). 5 — Sample II-8, slide A-09-II-8/2, E.F. K27/1,2. Laevigate monad in a ?laevigate envelope (?Sphaerasaccus glabellus Steemans et al., 2000). 12 — Sample II-11, slide A-09-II-11, E.F. Q27/2; 13 — Sample II-8, slide A-09-II-8/ 2, E.F. C25/2; 15 — Sample II-15, slide A-09-II-15, E.F. E34.
The picture of early land plant evolution is far from being complete. Wellman et al. (2013) pointed out that some areas remain unstudied. The present study closes a gap, indicating that the major paleocontinent of Siberia also was colonized by these types of early land plants. Nevertheless, much more data need to be collected from Ordovician strata to
test hypotheses on the timing of land plant origins and the first phases of colonization of the continents by early vegetation. It would not be surprising to find older occurrences of cryptospores from Siberia. Palynological investigations have been limited so far, and future studies are thus needed.
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6. Conclusions The present study reports the first record of land-plant derived spores from the Upper Ordovician of the paleocontinent Siberia. The cryptospores discovered in the studied interval of the Baksian– Dolborian Regional Stages (corresponding to the Katian Global Stage of the Upper Ordovician) are diverse throughout the section and relatively abundant at some levels in the lower part of the Dolborian. The cryptospores recovered are represented by almost all known morphotypes including naked and enveloped-enclosed monads, dyads, tetrads and polyads. These observations confirm the worldwide uniformity of cryptospore assemblages during the Late Ordovician to Early Silurian. However, in order to establish detailed comparisons of the recovered cryptospore assemblages from Siberia with those from other parts of the world a more accurate taxonomical study is necessary. Similarly, in order to understand if the Russian assemblages reflect a real diversification of the paleoflora or if the changes in the assemblages are the result of sampling effects or of increased input from land-derived clastic sedimentation, detailed sedimentological analyses need to be performed. It remains unclear if these records represent the oldest occurrence of cryptospores in Siberia, since in the Bolshaya Nirunda section they appear in the base of the Dolborian already in high number and morphological variety. It seems that their occurrence is most likely related to some environmental signal rather than having evolutionary meaning. Future studies in older sediments will probably reveal even older cryptospores. The obtained palynological material from the middle Late Ordovician of Siberia is a great additional source extending the paleogeographical coverage of the dispersed spores of the earliest land plants. The discovery of cryptospores in the Late Ordovician of Siberia extends the paleogeographical coverage of Late Ordovician cryptospore distribution confirming that early land-plants were probably globally distributed before the Silurian.
Acknowledgments This work is performed according to the Russian Government Program of Competitive Growth of Kazan Federal University. Financial support was also provided from the Russian Foundation for Basic Research Grant No. 13-05-00746, and the Agence National de Recherche project TERRES “Prospectives globales sur le processus de la terrestrialisation” (ANR-10-BLAN-0607). This study is a contribution to the IGCP Project 591 “The Early to Middle Paleozoic Revolution: Bridging the Gap Between the Great Ordovician Biodiversification Event and the Devonian Terrestrial Revolution.”
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