INTRODUCTION

The study of the Ordovician conodonts of Kazakhstan started more than 40 years ago with the first finds in the siliceous–terrigenous sections of the Atasu region (Gridina and Mashkova, 1977). Thereafter, conodonts were actively used to determine the age of sedimentary strata varying in the age from the Cambrian to the Upper Ordovician (Resheniya…, 1991), although taxonomic descriptions of conodonts are given only in a few works (Dubinina, 2000; Kurkovskaya, 1985, Zhylkaidarov, 1998).

The recent publications examining the taxonomic composition of conodonts and their stratigraphic distribution are devoted only to the sections of the Ordovician siliceous deposits of Kazakhstan (Tolmacheva, 2014; Tolmacheva and Degtyarev, 2012; Tolmacheva et al., 2004). To date, there are no data on the taxonomic composition of Ordovician conodonts in the carbonate rocks of Kazakhstan in the scientific literature. There are only monographs devoted to this issue (Dubinin, 2000; Tolmacheva, 2014).

The last monograph included preliminary data on the composition of the conodont assemblage from the Middle Ordovician limestones, which compose a thin layer in a stratotype section of the siliceous–terrigenous–tuffaceous deposits of the Naiman Formation in the Naiman valley, located in the central part of the Chingiz Ridge (East Kazakhstan).

This conodont locality has been known since the 1990s (Orlova, 1993), but only recently a representative collection of conodonts was extracted from it. This collection has provided type specimens of the new genus and species Naimanodus degtyarevi Tolmacheva (Tolmacheva, 2013), which has only recently been described.

This work presents the first complete taxonomic description of the conodont assemblage from limestone of the Naiman Formation. The age, biodiversity, and biogeographic features of distribution of this assemblage are specified. These systematic data on conodonts from carbonate rocks make it possible to characterize the relatively shallow-water conodont fauna of Kazakhstan and they are of great importance for the reconstruction of the paleogeographic position of a paleobasin and its remoteness from the continental blocks in the Middle Ordovician.

GEOLOGICAL STRUCTURE OF THE STUDY AREA AND MATERIAL

East Kazakhstan is characterized by the wide distribution of Lower and Middle Paleozoic volcanogenic-sedimentary, tuffaceous, and tuffaceous–terrigenous strata and related granitoids, which were formed in different parts of the oceanic island arcs (Degtyarev, 2012; Degtyarev et al., 1999; Tektonika…, 1982). In the central part of the Chingiz Ridge, Lower Paleozoic complexes of this type, having a complex cover–folded structure, are most complete. In this area, the Middle Cambrian island-arc volcanogenic-sedimentary and granitoid complexes and overlying Middle Cambrian–Middle Ordovician terrigenous–carbonate and siliceous–tuffaceous strata are tectonically juxtaposed with Late Cambrian–Early Ordovician island-arc volcanics and overlying Middle Ordovician siliceous–tuffaceous–terrigenous strata (Degtyarev, 2012; Degtyarev and Tolmacheva, 2005). The para-autochthonous complex is composed of volcanics and tuffs of moderate and moderate–felsic composition; basalts, tuffaceous sandstones and siltstones, tuffosilicites, jasperoids, and lenses of organogenic limestone with trilobites of the Amgian Stage of the Middle Cambrian are less common. The thickness of volcanogenic-sedimentary sections is about 1500 m. The volcanic rocks are intruded by small complex massifs of granodiorites and granites. Volcanic rocks and granitoids are overlain unconformably by the carbonate–siliceous–tuffaceous strata, covering the interval from the Middle Cambrian Mayan Stage to the lower Middle Ordovician (Degtyarev et al., 1999; Degtyarev and Tolmacheva, 2005; Tolmacheva et al., 2008). The allochthonous complex is represented by a thick (up to 2000–2500 m) stratum of andesites, andesite-dacites, dacites, their tuffs, and tuffaceous sandstones with limestone lenses. The Late Cambrian and Early Ordovician trilobites (Karagutui, Mamat, and Saryshok formations) were collected in these lenses (Nikitin, 1972; Tolmacheva, 2014). The Saryshok Formation is overlapped with a gradual transition by the siliceous–tuffaceous–terrigenous Naiman Formation of the Middle Ordovician, which is composed of tephroids, siliceous and calcareous tuffites, quartz–feldspar sandstones, siltstones, and conglomerates with a total thickness of 900–1200 m. The middle part of the section includes a limestone member varying in thickness from 5 to 50 m (Nikitin, 1972; Orlova, 1993) (Fig. 1).

Fig. 1.
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Schematic (a) cross section of the Naiman Formation and (b) geological scheme of the stratotype area of the Naiman Gully (Chingiz Ridge, Eastern Kazakhstan) after (Nikitin, 1972). (1) Quartz conglomerates; (2) quartz and quartz–feldspar sandstones; (3) siliceous–chlorite siltstones with interlayers of quartz–feldspar sandstone; (4) limestones; (5) clayey–chlorite siltstones with beds of feldspar sandstone; (6) gray feldspar sandstones with interlayers of clayey–chlorite siltstones; (7–10) Saryshok Formation (Lower Ordovician); (7) feldspar sandstones with beds of conglomerates, gravellites, and tuffs; (8) volcanomiсtic conglomerates and tuffs of moderate and felsic composition; (9) sandstones and siltstones; (10) tuffaceous sandstones with members of siltstones and conglomerates; (11) granodiorites of the Saryshok massif; (12) faults; (13) dip and strike; (14) localities of conodonts (а) and graptolites (b); (15) study area on the map of Kazakhstan. TS—Time Scale.

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The 15 m limestone member in the stratotype section of the Naiman Formation has the following structure (from bottom to top). The lower part is composed of light gray massive limestones with a large number of nonrounded fragments of dark-colored layered calcareous–siliceous siltstone (10–12 m thick). Higher in the section, the massive limestones pass gradually into platy light gray rock varieties, and siliceous siltstone fragments become smaller and disappear (thickness 2–3 m). The section is crowned by rhythmic alternation (1 m thick) of dark gray limestones and dark gray to black phtanites (spongolites) with individual interlayers from 1 to 10 cm thick (Fig. 2).

Fig. 2.
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Images of the carbonate member of the Naiman Formation in the Naiman Gully and its schematic cross section. (a) The upper part with alternation of limestones and phtanites; (b) the lower part with chert debris in carbonate matrix; (c) section. Legend: (1) siltstones; (2) coarse- and moderate-grained massive pale gray algal limestones; (3) fine-grained platy black limestones; (4) cherts; (5) chert debris; (6) sampling limestone interval for conodonts.

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The complete section of the carbonate member was found in a sole outcrop in the northern part of the Naiman Gully (48°54′56.66′′ N, 79°00′19.87′′ E) (Fig. 1). The rest of the territory is occupied by only the lower part of the member section.

The Middle Ordovician age of the Naiman Formation is proved by finds of graptolites of the Expansograptus hirundo Zone (Dapingian Stage, the lower part of the formation) and graptolites of the Paraglossograptus tentaculatus Zone (the lower part of the Darriwilian Stage, the upper part of the formation) (Nikitin, 1972; Orlova, 1993). Middle Ordovician trilobites and brachiopods were previously collected in limestones (Nikitin, 1972).

There have been frequent attempts to extract conodonts from limestones of the Naiman Formation. However, all collections were not representative and contained rare elements of species Periodon aculeatuszgierzensis Dzik, Drepanodus arcuatus Pander, and Panderodus mutatus (Branson et Mehl) (determinations of S.V. Dubinina) (Orlova, 1993; A.M. Zhilkaidarov, oral communication).

Extraction of conodonts from limestone that we have performed shows that conodonts are absent or rare in massive limestones of the lower part of the carbonate member, but they are abundant in limestones alternating with phtanites.

The bulk sample on conodonts weighing 22 kg was collected in the uppermost part of the section of limestones not exceeding 1 m. Following the standard procedure of processing carbonates with acetic acid, more 4000 conodont elements were extracted from the sample. Conodonts are well preserved, dark gray to black color corresponding to the Conodont Alteration Index (CAI) of about 5. The thermal alteration of phosphate material of the conodont elements does not allow us to distinguish elements composed of albide and hyaline tissue, which makes it difficult to recognize some taxa.

TAXONOMY OF CONODONT ASSEMBLAGE

In total, elements of 29 species of conodonts were found in the studied sample, 7 of which were identified based on open nomenclature (Plate I-IV). All platform taxa of the conodont assemblage occur as single specimens and are not identified to the rank of genus. One taxon is considered to be new, but its genus and species belonging is not identified.

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Plate I . Conodonts from limestones of the Naiman Formation. (1, 2) Histiodella kristinae Stouge: (1) Pa element, no. 13670/1, ×108; (2) Pa element, no. 13670/2, ×99; (3–9, 12) Histiodella altifrons Harris: (3) M element, no. 13670/3, ×128; (4) Pb element, no. 13670/4, ×84; (5) Pa element, no. 13670/5, ×87; (6) Pa element, no. 13670/6, ×89; (7) Sc element, no. 13670/7, ×125; (8) Sa element, no. 13670/8, posterior view, ×121; (9) Sb element, no. 13670/9, ×111; (12) Pb element, no. 13670/10, ×82; (10) Protopanderodus? nogamii (Lee), no. 13670/11, ×76; (11, 13–15) Ansella jemtlandica (Löfgren): (11) M element, no. 13670/12, ×91; (13) P element, no. 13670/13, ×95; (14) Sc element, no. 13670/14, ×104; (15) Sa element, no. 13670/15, ×88; (16, 17) Appalachignathus sp.: (16) Pa element, no. 13670/16, ×45; (17) Sb element, posterior view, no. 13670/17, ×95; (18, 19) Scolopodus? mufushanensis (An et Ding): (18) no. 13670/18, ×87; (19) posterior view, no. 13670/19, ×9; (20–22) Parapanderodus striatus (Graves et Ellison): (20) S element, no. 13670/20, ×118; (21) Sa element, no. 13670/21, ×115; (22) S element, no. 13670/22, ×78; (23–26) Drepanoistodus latus Pyle et Barnes: 23, S element, no. 13670/23, ×83; (24) M element, no. 13670/24, ×91; (25) S element, no. 13670/25, ×88; (26) S element, no. 13670/26, ×105; (27, 28) Naimanodus degtyarevi Tolmacheva: (27) typical specimen of the species, S element, no. 5/13218 (Tolmacheva, 2013); (28) same element, posterior view, ×115.

Plate II.
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Conodonts from limestones of the Naiman Formation. (1–4) Juanognathus jaanussoni Serpagli: (1) no. 13670/27, ×87; (2) no. 13670/28, ×74; (3) no. 13670/29, ×78; (4) no. 13670/30, ×82; (5, 6) Juanognathus sp.: (5) no. 13670/31, ×96; (6) no. 13670/32, ×89; (7–11) Anodontus longus Stouge et Bagnoli: (7) Sa element, no. 13670/33, ×98; (8) Sd element, no. 13670/34, ×87; (9) Sd element, no. 13670/35, ×87; (10) Sd? element, no. 13670/361, ×101; (11) Sc? element, no. 13670/37, ×95; (12) ?Triangulodus sp., no. 13670/38, ×57; (13–17) Costiconus ethingtoni (Fahraeus): (13) S element, no. 13670/39, ×70; (14) S element, dorsal view, no. 13670/40, ×77; (15) S element, no. 13670/41, ×76; (16) P element, no. 13670/42, ×77; (17) S element, no. 13670/43, ×75; (18) Drepanoistodus sp., no. 13670/44, ×78; (19) Drepanodus arcuatus Pander, no. 13670/45, ×87; (20) Drepanodus reclinatus (Lindstrom), no. 13670/46, ×69; (21, 22) Paroistodus horridus (Barnes et Poplawski): (21) M element, no. 13670/47, ×71; (22) S element, no. 13670/48, ×76; (23–25) Gen et sp. indet. 1: (23) no. 13670/491, ×87; (24) no. 13670/50, ×95; (25) no. 13670/51, ×89.

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Plate III.
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Conodonts from limestones of the Naiman Formation. (1, 2, 9, 11) Dzikodus sp.: (1) Pa element, no. 13670/52, ×71; (2) Pa element, no. 13670/53, ×67; (9) Pb element, no. 13670/54, ×72; (11) Pb element, no. 13670/55, ×5; (3, 4) Yangtzeplacognathus sp.: (3) Pa element, no. 13670/56, ×67; (4) Pa element, no. 13670/57, ×54; (5–7, 10, 13, 14) Baltoniodus sp.: (5) Sa element, no. 13670/58, ×82; (6) ?Pb element, no. 13670/59, ×84; (7) Sa element, no. 13670/60, ×84; (10) ?Pa element, no. 13670/61, ×65; (13) M element, no. 13670/62, ×99; (14) M element, no. 13670/63, ×86; (8, 12, 15) ? Yangtzeplacognathus sp.: (8) Sa element, no. 13670/64, ×67; (12) Sd element, no. 13670/65, ×120; (15) Pa element, no. 13670/66, ×71; (16) Spinodus spinatus (Hadding), Sb element, no. 13670/67, ×45; (17–21, 24) Periodon macrodentatus (Graves et Ellison): (17) M element, no. 13670/68, ×69; (18) Sa element, no. 13670/69, ×57; (19) Pa element, no. 13670/70, ×43; (20) Sb element, no. 13670/71, ×41; (21) Sc element, no. 13670/72, ×55; (24) Sb element, no. 13670/74, ×54; (22, 23) Erraticodon sp.: (22) М element, no. 13670/74, ×51; (23) Sa element, no. 13670/75, ×42.

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Plate IV.
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Conodonts from limestones of the Naiman Formation. (1–7) Protopanderodus calceatus Bagnoli et Stouge: (1) S element, no. 13670/76, ×82; (2) S element, no. 13670/77, ×84; (3) S element, no. 13670/78, ×93; (4) S element, no. 13670/79, ×130; (5) S element, no. 13670/80, ×105; (6) P element, no. 13670/81, ×91; (7) S element, no. 13670/82, ×64; (8–19) Venoistodus сf. V. balticus Löfgren: (8) M element, no. 13670/83, ×84; (9) M element, no. 13670/84, ×79; (10) P element, no. 13670/85, ×82; (11) S element, no. 13670/86, ×84; (12) S element, no. 13670/87, ×84; (13) S element, no. 13670/88, ×95; (14) S element, no. 13670/89, ×82; (15) S element, no. 13670/90, ×79; (16) S element, no. 13670/91, ×78; (17) S element, no. 13670/92, ×71; (18) S element, no. 13670/93, ×77; (19) S element, no. 13670/94, ×79.

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For conodont species which are of significance for biostratigraphy and biogeographic reconstructions, notes to its taxonomic identification, as well as the data on their distribution, are given. The samples represented in plates are stored in the Chernyshev Central Research Geological Museum (collection no. 13670).

Anodontus longus Stouge et Bagnoli, 1988 (Plate II, figs. 7–11) is represented by small conical elements, composed of albide tissue, with high base and an elongated cusp. Among elements are several transitive forms from symmetrical ones with pronounced lateral ribs, pointed posterior, and smoothed anterior side of a cusp to laterally compressed elements with keel on posterior and anterior sides of the base and a cusp. Ribs of the intermediate asymmetrical forms occupy anterolateral and posterolateral positions. Thin bedding is noted on the surface of elements.

A. longus occurs in the stratotype region (Western Newfoundland) in deposits from the upper part of the Tremadocian to lower part of the Floian Stage (Stouge and Bagnoli, 1988). The youngest conodonts A. longus are known in boundary deposits of the upper Floian–lower Dapingian substages in British Columbia (Northwestern Canada) (Pyle and Barnes, 2002). In Kazakhstan, apart from the Naiman Formation, they occur in relatively large amounts in the Uzunbulak Formation of the Chu-Ili Mountains (Southern Kazakhstan), which are ascribed to the lower part of the Darriwilian Stage.

Ansella jemtlandica (Löfgren, 1978) (Plate I, figs. 11, 13–15) is represented by non-denticulated P elements with smooth lateral sides and Sa and Sc elements, denticulated on the posterior side. Elements from the Naiman Formation are characterized by smaller and more irregular denticles than those of typical representatives of this species. A. jemtlandica is a widespread taxon, which is common in the lower and middle parts of the Darriwilian Stage of all paleocontinents.

In total, three elements of Appalachignathus sp. (Plate I, figs. 16, 17) were found: carminate Рb element and bipennate Sb and Sd elements. The Р elements are distinguished from those of the sole described species of this genus A. delicatulus Bergström, Carnes, Ethington, Votaw et Wigley, 1974 by the smooth and straight surface of the posterior cusp and base.

A. delicatulus is widespread species in the upper part of the Darriwilian Stage and the lower part of the Sandbian Stage (Pygodus serra Zone–lower part of the Amorphognathus tvaerensis Zone) of North America and Australia (Bergström et al., 1974; Zhen et al., 2001). The ancient Appalachignathus sp. (only S elements) was found in the upper part of the Zhongliangzi Formation of Northern China (Tangshanodus tangshanensis Zone, similar to H. sinuosa–H. holodentata zones) (An and Zheng, 1990).

The characteristic features of elements of Drepanoistodus latus Pyle et Barnes (Plate I, figs. 23–26), described in sediments of the Floian Stage of Canada (Pyle and Barnes, 2002, 2003), are a laterally wider, anteriorly inclined base, as well as a wide, laterally compressed and highly posteriorly inclined cusp. Within the Central Asian Fold Belt, D. latus occurs in the Kentash Formation of Northern Kyrgyzstan and the Karatal Formation of the Jalair-Naiman area of Southern Kazakhstan (Tolmacheva, 2014); both formations are ascribed to the Floian Stage (Lower Ordovician).

Erraticodon sp. (Plate III, figs. 16, 22, 23) is represented by large hyaline elements with smooth surface and elongated denticulated rods. Collection includes only Sa, Sc, Sd, and М elements. One or two median denticles on anterior cusps of Sc and Sd elements are elongated. The lateral short denticles of alate Sa elements have single large cusps. The dolaborate М element has a denticulated, relatively short posterior denticle and smooth, slightly elongated anterior edge. The complete composition of elements of the conodont apparatus of this seven-element (eight-element?) taxon is unknown in Kazakhstan.

Elements are similar in morphology, corresponding to elements E. hexianensis An et Ding, 1985 and E. gratus (Moskalenko, 1977), described in the Siberian Platform, and Erraticodon n. sp. A, described in Western Newfoundland (Stouge, 2012). In Kazakhstan, elements of this species occur in the lower Darriwilian deposits.

Elements of the genus Histiodella are dominated in the conodont assemblage of the sample studied. Two species of this genus were identified: Histiodella altifrons Harris, 1962 (Plate I, figs. 3–9, 12), Р elements of which have an upper smooth edge, and Histiodella kristinae Stouge, 1984 (Plate I, figs. 1, 2) with denticulated Р elements. Рa elements of H. altifrons are characterized by a high leaf-shaped and smooth upper edge. Pb elements have a pronounced reclinated cusp; the blade height decreases toward to the posterior part of an element. Apart from Р elements, the collection includes non-denticulated М, Sc, Sa, and Sb elements.

Fig. 3.
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Correlation of zonal subdivisions of Dapinian–lower Darriwillian stratigraphic intervals of different regions worldwide and integrated distribution interval of the conodont assemblage of the Naiman Formation.

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Fig. 4.
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Relative amount of elements of conodonts of different species from the Naiman Formation at the significant predominance of conodont elements of genus Periodon.

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Fig. 5.
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Distribution of taxa of the Naiman Formation in other regions worldwide. The following data have been used: Newfoundland (Stouge, 1984, 2012); Canadian Arctic (Pyle and Barnes, 2002, 2003); America (Bauer, 2010); Argentina (Mestre and Heredia, 2013; Serra et al., 2015); Baltoscandia (Bergström, 2007; Löfgren, 2003, 2004; Rasmussen, 2001); Tarim, Northern and Southern China (An et al., 1983; Wang et al., 2014; Wu et al., 2012, 2014; Zhang, 1998; Zhen et al., 2011; etc.); Australia, New Zealand (Percival et al., 2011; Zhen et al., 2009).

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Fig. 6.
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Paleogeographic reconstruction for the Middle Ordovician (after http://www.scotese.com/newpage1.htm, with amendments (Tolmacheva, 2014; Vandenbroucke et al., 2010, 2013; Zhen et al., 2015)) and conodont-based biogeographic zonation. (1) Thermophilic belt of the open-sea biogeographic realm; (2) cold-water belt of the shallow-water biogeographic realm; (3) North American biogeographic realm; (4) Siberian biogeographic realm; (5) Baltoscandian biogeographic realm; (6) Australasian biogeographic realm; (7) Kazakhstan biogeographic realm; (8) realms of undetermined biogeographic belonging; (9) localities of species J. jaanussoni, S.? mufushanensis, D. latus, and A. longus, recording the thermophilic belt of the open-sea biogeographic realm.

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The relatively elongated and small Pa elements of H. kristinae (Plate I, figs. 1, 2) have denticles on the posterior and anterior parts of the blade. Tips of the largest anterior cusps are situated above the tip of the main cusp.

H. altifrons elements were found only in Dapingian and lower Darriwilian sections of North America and Argentina (Bauer, 2010; Ethington and Clark, 1982; Lehnert, 1995). H. kristinae is a geographically widespread species, which has been found everywhere in fact, except for the platform part of the Central America (Bauer, 2010; Du et al., 2005; Mestre and Heredia, 2013; Stouge, 1984; Zhen et al., 2011).

Juanognathus jaanussoni Serpagli, 1974 (Plate II, figs. 1–4) is represented by conical elements with one or two lateral denticles, one of which is much better developed as usual than the others. Conodont elements from the Naiman Formation are distinguished from the typical ones from Argentina (Serpagli, 1974) by wider lateral denticles, as well as the occurrence of the second small denticle. Together with J. jaanussoni, the collection contains rare elements with two relatively small lateral denticles (keels), which do not reach the base (Plate II, figs. 5, 6). These elements are attributed to Juanognathus sp.

J. jaanussoni occurs in Argentina, North America, Newfoundland, Western Thailand, and Kazakhstan. The largest stratigraphic interval of this species distribution is characteristic of Argentina and Western Thailand, where it occurs from the upper Floian to the lower Darriwilian (Agematsu et al., 2008; Serra et al., 2015). In America and Newfoundland, this species was found in the interval of the Jumodontus gananda–Reutterodus andinus zones of the upper Floian–lower Dapingian substages (Ethington and Clark, 1982; Landing, 1976).

The stratotype section of the Naiman Formation is considered to be a reference one for recently described species Naimanodus degtyarevi Tolmacheva, 2013 (Plate I, figs. 27, 28). In addition, this species was found in the deposits of the lower part of the Darriwilian Stage of other localities of Kazakhstan, Kyrgyzstan, and Altai Mountains (Tolmacheva, 2013).

The species Parapanderodus striatus (Graves et Ellison, 1941) (Plate I, figs. 20–22) includes three types of small hyaline conical elements—without grooves, with one or two grooves on the posterior side. P. striatus is known in the Floian, Dapingian, and lower part of the Darriwilian stages of the Lower and Middle Ordovician of North America and Canada (Smith, 1991), Northern China, Tarim (Zhen et al., 2011), and Baltoscandia (Bagnoli and Stouge, 1997). In addition, this species was found in this stratigraphic interval of the Kazakhstan localities.

On the basis of the results of the recent revision of the early Darriwilian representatives of the genus Periodon (Stouge, 2012), Pa elements of Periodon macrodentatus (Graves et Ellison 1941) are characterized by three (three and a half) denticles on the anterior rod, Sa element with denticulated and lateral rods, and well-defined rim along the upper edge of denticles (Plate III, figs. 17, 18, 20, 24). The number of denticles on the anterior rod in the more ancient species P. hankensis Stouge, 2012 and younger species P. zgierzensis of Pa elements are in increasing and decreasing amounts, respectively.

Pa elements of Periodon zgierzensis Dzik, 1976 (Plate III, fig. 23) have four or more anterior denticles (Stouge, 2012). This species was found in the lower part of the Middle Ordovician and is considered as the youngest species, which replaced P. macrodentatus in the stratigraphic succession, correlated with the upper part of the H. kristinae (Dw2) Zone in Newfoundland (Stouge, 2012).

However, other elements of the genus Periodon presented in the collection are difficult to sort out between the above two species. As a result, they are considered together when estimating the biodiversity.

Elements of Protopanderodus? nogamii (Lee, 1975) (Plate I, fig. 10) have grooves on each of the lateral sides. This species is widespread in the stratigraphic interval from the lower part of the Floian Stage to the lower part of the Katian Stage in Eastern Australia, China, Southeast Asia, and Argentina (Zhen et al., 2009). In addition, Protopanderodus? nogamii occurs in all known carbonate sections of the Darriwilian Stage of Kazakhstan and Northern Kyrgyzstan (Tolmacheva, 2014).

Together with Protopanderodus varicostatus (Sweet et Bergström, 1962) the species Protopanderodus calceatus Bagnoli et Stouge, 1997 (Plate VI, figs. 1–7) is ascribed to the species with numerous ribs of the group Protopanderodus. On the interior side of М elements and the posterior side of S elements of P. calceatus are thin ribs; the basal edge of all elements is relatively straight. P. calceatus was found in the upper Floian–lower Sandbian of Baltoscandia (Bagnoli and Stouge, 1997; Löfgren, 1978; Rasmussen, 2001; Viira et al., 2001), China (Zhang, 1998), New Zealand (Zhen et al., 2009), and Tarim (Zhen et al., 2011).

Conical elements of the species Scolopodus? mufushanensis An et Ding, 1982 (Plate I, figs. 18, 19) have rounded section, a low base, highly inclined cusp, and a deep groove, passing along the length of the lateral side of a cusp.

A lateral groove occurs on the base, deeping to the median part of a cusp (Plate I, fig. 19). The basal edge of elements is straight, or its anterior side goes down a little bit below other parts of the basal edge.

The attribution of this species of the genus Scolopodus, proposed at the first description of the taxon, is rather conventional, because a series of elements belonging to this species corresponds incompletely to this genus. However, the deficiency of the conodont collections available does not allow one to ascribe this species to an independent species.

In Northern China, the species Scolopodus? mufushanensis occurs in the upper part of the Floian–lower part of the Dagpinian stages (An, 1987). In British Columbia, elements of this probable taxon were identified as Coniform indet. L from the Jumudontus gananda Zone of the uppermost part of the Floian Stage (Pyle and Barnes, 2002).

Venoistodus сf. V. balticus Löfgren, 2006 is represented by small albide elements with a rib on both lateral sides of geniculate and non-geniculate elements and with thin bedding on lateral surfaces. М elements have elongated posterior and anterior edges of the base. They are different from the species V. balticus, identified by А. Löfgren in Baltoscandia, in the occurrence of more defined lateral ribs on S elements. The collection includes rare S elements with several ribs (Plate VI, figs. 15, 18, 19), which are also ascribed to this species. Similar forms with ribbed elongated geniculate elements occur on all continents in a wide stratigraphic interval from the Lower to Upper Ordovician.

It should be noted that neither genus nor species belonging was revealed for 15 similar elements. Conical non-geniculate elements gen et sp. indet 1 (Plate II, figs. 23–25) have a undefined base and keeled anterior and posterior edges. On the lateral sides of the base, close to its anterior side, are denticles, which are defined in different elements to a varying extent. It is hardly possible to recognize these denticles on laterally compressed elements, while elements with the rounded section of a basal edge have rather recognizable lateral denticles (Plate II, fig. 25). The base of elements and the anterolateral sides have insignificant bedding.

AGE AND TAXONOMIC DIVERSITY OF CONODONTA OF THE NAIMAN FORMATION

The age of conodonts from the limestone sample of the Naiman Formation is still a debatable issue, since the taxonomic composition of the conodont assemblage includes species with different stratigraphic intervals of distribution.

This is most clearly seen in two species of the genus Histiodella found in this sample. The most ancient species is H. altifrons, the distribution of which in the lower Middle Ordovician deposits of Laurentia does not extend beyond the Dapingian Stage–the lowermost part of the Darriwilian Stage (Bauer, 2010; Lehnert, 1995). The biozonal scale of the North American Platform in which the H. altifrons Zone is correlated with the upper part of the Dapingian Stage is based on the stratigraphic distribution of species of the genus Histiodella, (Fig. 3) (Bauer, 2010; The Geological…, 2012; Zhang et al., 2005).

The H. holodentata species crowns the biozonal sequence of this genus in sections of the central part of North America (Fig. 3). The first occurrence of this species is noted stratigraphically above the last occurrence of H. altifrons. In Newfoundland, H. holodentata is replaced upsection by H. kristinae (Stouge, 1984); the distribution interval of this species corresponds to the upper part of the Kunda Horizon and the Azeri Horizon of Baltoscandia.

Thus, the biostratigraphic distribution intervals of H. altifrons and H. kristinae do not overlap. Accordingly, their joint occurrence in a sample indicates either a strong condensation of the upper part of limestone bed and, accordingly, the sampling of a significant stratigraphic interval or redeposition of the conodont assemblage (Fig. 3).

The mixing of different-aged conodonts in the assemblage from limestones of the Naiman Formation is also confirmed by the occurrence of other taxa. Thus, the first occurrence of such species as P. macrodentatus, A. jemtlandica, and P. horridus is noted beginning from the early Darriwilian (Albanesi and Barnes, 2000; Löfgren, 2003, 2004; Stouge, 2012). In Baltoscandia, H. kristinae appears in the uppermost part of the Kunda Horizon (Eoplacognathus pseudoplanus Zone) along with C. ethingtoni (Löfgren, 2004). The first occurrence of P. zgierensis, mostly well described in the Newfoundland sections, is noted preliminarily around that time (Stouge, 2012). At the same time, the assemblage contains S.? mufushanensis and D. latus, which have been found so far only in Floian and Dapingian deposits (An, 1987; Pyle and Barnes, 2002, 2003).

On the basis of finds of the youngest conodonts, the age of the upper part of the Naiman limestone cannot be older than the middle part of the Kopal Horizon in Kazakhstan, which is correlated with the upper part of the Kunda Horizon (E. pseudoplanus Zone) of Baltoscandia (Fig. 3). Finds of Paraglossograptus tentaculatus graptolites in beds lying stratigraphically higher than limestone do not contradict this age estimate.

In total, 26 taxa of conodonts were identified in the sample studied, 19 of which were determined at the species level; the rest were determined by using the open nomenclature. Elements of the genus Periodon dominate in the sample, accounting for 83% of all fragments of conodonts found in the sample (Fig. 4). D. cf. D. costatus, H. cf. H. altifrons, and P. calceatus amount to 11, 6, and 5% of the assemblage, respectively; other species are 3% or less.

The taxonomic diversity of the conodont assemblage from the Naiman limestone is uniquely high for one sample. Thus, the alpha diversity (diversity at a single sampling site) of Ordovician conodonts in the most well-studied regions, such as Baltoscandia and Southern China, does not exceed 20–22 species (Goldman et al., 2013; Männik and Viira, 2012). The general diversity (gamma diversity) of conodonts in all studied different-facies sections of Southern China and Baltoscandia, summarized over the informal substages of the lower Middle Ordovician, are 23 and 39 species at most, respectively (Goldman et al., 2013; Wu et al., 2012). In the Baltoscandian sections, the high gamma diversity of conodonts can be explained by a wide range of facies in which conodonts have been studied, from shallow-water sediments of Estonia to the open-sea strata of Norway.

A uniquely high taxonomic alpha diversity in the limestone from the Naiman Formation is most likely associated with the mixing of species from a fairly wide stratigraphic interval from the P. macrodentatus Zone of the upper part of the Dapingian Stage to the P. aculeatus Zone of the middle of the Darriwilian Stage (Middle Ordovician).

BIOGEOGRAPHIC FEATURES OF THE COMPLEX

It is considered that the early Darriwilian was a period of maximum provinciality and high biodiversity of Ordovician conodonts (Goldman et al., 2013). The occurrence of a contrast fauna on different paleocontinents and many rare species occurring as individual specimens is still an obstacle to the development of conodont-based biogeographic paleoreconstructions. These paleoreconstructions have been developed for now only for the Early and Late Ordovician (Bergström, 1990; Sweet and Bergström, 1984; Zhen and Nicoll, 2009; Zhen and Percival, 2003).

Apart from the location of paleocontinents, the biogeographic zoning of conodonts, like other pelagic organisms, is determined by the following parameters related to climate and oceanic circulation: surface water temperature, water salinity, illumination, and nutrient flux (Tolmacheva, 2014; Vandenbroucke et al., 2009, 2013; Zhen and Percival, 2003).

The modern biogeographic zoning represents the existence of an open-sea biogeographic realm, which is not related to the location of paleocontinents, and the shallow-water sea region, which is subdivided into climatic zones and realms, characterizing different epicontinental basins. Conodont species found in the siliceous deposits of Kazakhstan, the Urals, Scotland, and Australia (Tolmacheva, 2014; Tolmacheva et al., 2004) and in those of the deepwater/relatively deep-water carbonate facies of other regions worldwide outline the open-sea biogeographic realm (Tolmacheva, 2014; Zhen and Percival, 2003). Conodonts found only in shallow-water sediments are formally ascribed to species of the shallow-water biogeographic realm.

The Lower Ordovician individual biogeographical realms have been distinguished in almost all paleocontinents and smaller continental blocks: Laurentia, Siberia, Australia, Northern China, Southern China, Argentina, Baltoscandia, Britain, and others (Bergström, 1990; Pohler and Barnes, 1990; Sweet and Bergström, 1984; Zhen and Percival, 2003). The continental blocks of Eastern Gondwana are united into the Australasian biogeographic realm (Zhen et al., 2015).

By now, there are quite a lot of data available which allow one to correlate Early Darriwilian conodonts of the Naiman Formation of Kazakhstan with conodonts from other regions. Conodonts of this age have been studied in all major Ordovician paleobasins: Newfoundland (Stouge, 2012), Canada (Pyle and Barnes, 2002, 2003), America (Bauer, 2010), Argentina (Feltes et al., 2016; Mestre and Heredia, 2013; Serra et al., 2015), Baltoscandia (Bergström, 2007; Löfgren, 2003, 2004; Rasmussen, 2001), Northern and Southern China (An et al., 1983; Zhang, 1998; Wang et al., 2014; Wu et al., 2012, 2014; etc.), and Australia (Percival et al., 2011).

In the Naiman sample, open-sea conodont species widespread on many paleocontinents (A. jemtlandica, D. arcuatus, P. horridus, P. macrodentatus, P. zgierzensis, S. spinatus, C. ethingtoni, and others (Fig. 5)) compose a smaller part of the complex (38% of taxa), whereas they greatly exceed the shallow-water taxa in the number of specimens (70%). Four taxa defined only at the genus level are excluded from these statistics.

It should be noted that the numerical predominance of elements of the genus Periodon found in the Naiman Formation is typical of the siliceous deposits of Kazakhstan and the Urals (Dubinina and Ryazantsev, 2008; Tolmacheva, 2014) and is noted in Dapingian and early Darriwilian carbonate sediments of different paleocontinents (Pohler and Barnes, 1990; Stouge, 2012). This may indicate deep-sea or open-sea environments of formation of carbonates, limestone of the Naiman Formation in our case.

Some of the shallow-water species in the studied complex, in particular, P. striatus and P. calceatus, are found on many paleocontinents, including Baltoscandia, North America, Australia, Tarim, and Northern China. Their wide distribution is evidence that these species inhabited an oceanic environment. One can expect that they will be found in cherts.

Four species (J. jaanussoni, S.? mufushanensis, D. latus, and A. longus) from the Naiman Formation are known only from the Australasian and/or Laurentia realms (Fig. 6). Thus, J. jaanussoni was found in North America, Argentina, Northern and Southern China, and West Thailand, while S.? mufushanensis was found in Northern China and the Canadian Arctic, and D. latus and A. longus were found in the Canadian Arctic (Pyle and Barnes, 2002). Considering the finds of these species in Kazakhstan, located relative to Canada, according to the existing paleogeographic reconstructions, i.e., on the opposite hemisphere of the Earth, the distribution of these species was quite wide. Nevertheless, our knowledge on the taxonomic compositions of conodont faunas is insufficient to prove their occurrence on other continents. The known localities of these species are limited only to the paleobasins at equatorial latitudes (Fig. 6). There are no data on these species in the taxonomic composition of the sufficiently well-studied conodont faunas from the Baltoscandian paleobasin, which was located at relatively high latitudes in the early part of the Middle Ordovician (Cocks and Torsvik, 2005). This indicates a possible biogeographic zoning by climatic zones, characteristic of pelagic biota, which was developed according to the biogeographic distribution of graptolites at the end of the Late Ordovician (Vandenbroucke et al., 2009, 2013).

The only species in the Naiman Formation occurring just in the Australasian biogeographic realm is P.? nogamii. This species is widespread in the platform blocks of Gondwana, including its eastern (Southern and Northern China, Tarim, Australia, Sibumasu, Malaysia) and western (Cis-Cordillera Argentina) parts (Zhen and Nicoll, 2009). The wide distribution of this species in many sections of Kazakhstan (Tolmacheva, 2014) shows the belonging of this region to the Australasian biogeographic realm and its relative geographical proximity to the eastern margin of Gondwana.

Conodonts of the Naiman Formation include five endemic forms (Fig. 5), all of which, except for gen. et sp. indet. 1, were also described in other sections of Kazakhstan (Tolmacheva, 2014). N. degtyarevi, which was found in Altai and in Northern Kyrgyzstan, can be considered as the regional endemic species for Kazakhstan, albeit with a wider area of distribution (Tolmacheva, 2013). The presence of a significant number of endemic forms enables us to characterize the Kazakhstan fauna as an independent biogeographic zoning unit. In addition, this confirms the relative remoteness of the water basins of Kazakhstan both from the eastern margin of Gondwana and from other paleocontinents in the Ordovician time.

CONCLUSIONS

This work presents the first systemized data on the taxonomic composition of Middle Ordovician conodonts from carbonate rocks of Kazakhstan. A high taxonomic diversity of the conodont assemblage from the upper part of the limestone member of the Naiman Formation is explained by the mixing in the sample of conodonts from a wide stratigraphic interval from the upper part of the Dapingian Stage to the middle of the Darriwilian Stage. On the basis of the youngest conodonts, the age of the Naiman Formation, the terrigenous interval of which is poorly characterized faunistically, was refined.

In general, the taxonomic composition of the conodont assemblage of the Naiman Formation of Kazakhstan shows that the distribution of conodonts is controlled by the laws of biogeographic zoning of pelagic faunas; the distribution of graptolites (another pelagic group in the Ordovician) follows the same laws. The open-sea representatives of the conodont assemblage are geographically widespread, probably, in accordance with climatic belts or vast oceanic circulation zones. In general, the conodont faunas of Kazakhstan are ascribed to the Australasian biogeographic realm, covering Eastern Gondwana. This paleogeographic conclusion is confirmed by the study results of groups of benthic fauna—trilobites and brachiopods (Fortey and Cocks, 2003; Nikitin et al., 2003; Popov et al., 2009). At the same time, a large number of regional endemic species were identified in Kazakhstan sections that enable us to consider this region as an independent biogeographic unit within the Australasian biogeographic realm. Taking into account the fact that the latter includes several large and quite detached regions in terms of biogeography (Southern China, Northern China, and Australia), the Australasian biogeographic realm can be regarded as a biogeographic zoning unit of a higher rank than the biogeographic realm.