late ordovician to early silurian conodont faunas from the ... - BioOne

J. Paleont., 81(3), 2007, pp. 490–512 Copyright 2007, The Paleontological Society 0022-3360/07/0081-490$03.00

LATE ORDOVICIAN TO EARLY SILURIAN CONODONT FAUNAS FROM THE KOLYMA TERRANE, OMULEV MOUNTAINS, NORTHEAST RUSSIA, AND THEIR PALEOBIOGEOGRAPHIC AFFINITY SHUNXIN ZHANG

AND

CHRISTOPHER R. BARNES

School of Earth and Ocean Sciences, University of Victoria, P.O. Box 1700 STN CSC, Victoria, British Columbia, Canada V8W 2Y2, ⬍[email protected]⬎, ⬍[email protected]⬎ ABSTRACT—The Late Ordovician and Early Silurian conodont faunas of northeast Russia are described, based on 39 conodont-bearing samples collected from Kanyon, Padun, Tirekhtyakh, Maut, and lower Sandugan Formations at Mirny Creek and Ina River sections, Omulev Mountains. The faunas are represented by a total of 776 identifiable conodont specimens assigned to 35 species representing 25 genera, among which a new species, Periodon mirnyensis, is established. Based on graptolite zones at the same locality and other conodont faunas worldwide, the faunas are assigned to the Caradocian, early Ashgillian, late Ashigillian, early Llandovery, and late Llandovery. The Ordovician faunas belong to outer platform to upper slope facies with a North Atlantic Realm affinity; the Silurian faunas are from slope and basin facies. The conodont faunas are the first to be described from the Kolyma Terrane, which has an enigmatic paleogeographic position during the Early Paleozoic.

INTRODUCTION

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OMULEV Mountains area in northeast Russia hosted the Omulev Uplift in the upper Kolyma Basin during the Late Ordovician to Early Silurian time interval (Fig. 1). Representative strata are well exposed along the Mirny Creek and Ina River sections (Figs. 2–6). The Ordovician and Silurian deposits along Mirny Creek and Ina River were first studied by A. A. Nikolaev and M. M. Oradovskaya during their 1958 fieldwork in the Inaniya and Ina basins (Oradovskaya, 1963). Later, the sections were studied by Oradovskaya (1963) and Sobolevskaya (1970). The sections at both Mirny Creek (Figs. 3–5) and Ina River (Figs. 3, 6) represent a diversity of facies, from deep-water graptolite-bearing shale to brachiopod-coral-graptolite-bearing carbonates with some bioherms. The sections attracted great interest during early efforts to formally define the Ordovician-Silurian boundary. The Pacific Science Congress XIV (1979) organized a special field excursion to the sections specifically to examine the unusually complete Ordovician-Silurian boundary interval strata. Despite the fact that their remote location makes them difficult to access, the sections continue to be among the most informative exposures for studies of the mass extinction and paleoclimatic and paleoceanographic changes associated with the Ordovician-Silurian boundary interval. Diverse faunas have been reported, particularly the graptolites (Obut et al., 1967; Obut and Sennikov, 1979; Koren, 1991; Koren and Sobolevskaya, 1979; Koren et al., 1979; Koren et al., 1983; Sobolevskaya, 1974; Sennikov, 1998). The non-graptolite faunas that have been reported include those of brachiopods (Nikolaev et al., 1977; Kulkov and Vladimirskaya, 1979; Koren et al., 1979; Koren et al., 1983), corals (Preobrazhensky, 1966; Khaisnikova and Latypov, 1979), acritarchs (Sheshegova, 1979), trilobites (Koren et al., 1979; Koren et al., 1983), chitinozoans (Zaslavskaya, 1979), and conodonts (Dumoulin et al., 2002). However, the conodonts have not been described and studied in detail. Conodont samples were collected by one of the authors (C. R. Barnes) during a 1979 field excursion to the sections. Forty-nine 2 kg samples were collected from both sections through the Upper Ordovician to Lower Silurian interval, but particularly across the Ordovician-Silurian boundary. Thirty-nine out of 49 samples were productive. A total of 776 conodont specimens are assigned to 37 HE

species representing 27 genera with 10 elements left in open nomenclature (Appendices 1 and 2). These include 17 species representing 15 genera from the Caradocian and Ashgillian (Upper Ordovician) and 23 species representing 15 genera from the Llandovery (Lower Silurian), among which only two species representing two genera are common both in Ordovician and Silurian samples. One new species, Periodon mirnyensis, is described; most of the species illustrated by this paper are well known and only require brief taxonomic remarks. Importantly, the conodont biostratigraphic distribution through this interval can be tied directly to several graptolite zones. The faunas provide additional data for understanding the area’s enigmatic paleogeographic position and paleobiogeographic affinities during the Early Paleozoic. The discovery of the conodont fauna fills a gap in our understanding and allows a comparison of the fauna with those from other paleocontinents. STRATIGRAPHIC FRAMEWORK

In the northeast and Far East of Russia, the Ordovician strata have been assigned to the following tectonic-stratigraphic megazones: I) Verkhoyansk, II) Omulev-Selennyakh, III) Okhotsk-Omolon, IV) Chukchi, V) Penzhina, and VI) Allakh-Yun (Fig. 1). The Omulev-Selennyakh megazone (II) is subdivided into Selennyakh (II1), Yasachnaya (II2), Urultun (II3), Eriekhe (II4), and Rassokha (II5) zones. Based on field observation and the Russian literature cited above, the stratigraphy is summarized as follows. Within the Omulev-Selennyakh megazone, the Ordovician is mainly represented by miogeoclinal strata, including carbonates and siliciclastics, of variable thickness. Multiple facies are represented, including outer shelf, slope, and deep-water basin. The Ordovician includes some local volcaniclastic facies in the marginal zones. The megazone represented the east margin of the Verhojansk-Kolyma depression during the Ordovician (Oradovskaya, 1988a, 1988b). The study area, including the Mirny Creek and Ina River sections within the Omulev Mountains area, is located in the Yasachnaya zone (II2), Omulev-Selennyakh megazone (II). The Upper Ordovician-Lower Silurian stratigraphic succession is divided into the Kanyon, Padun, and Tirekhtyakh (Upper Ordovician), and Chalmak, Anika, and Sandugan (Lower Silurian) horizons (Oradovskaya and Sobolevskaya, 1979; Koren et al., 1983). Later, the horizons were formally named as formations with the same

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FIGURE 1—Distribution and structural-facies zonation of the Ordovician deposits in northeast Russia (modified from Oradovskaya, 1988a, fig. 1). I, Verhoyansk megazone; II, Omulev-Selennyakh megazone with zones: II1 (Selennyakh), II2 (Yasachnaya), II3 (Urultun), II4 (Eriekhe), II5 (Rassokha); III, Okhotsk-Omolon megazones with zones: III1 (Okotsk), III2 (Molandzha), III3 (Kedon); IV, Chukchi megazone; V, Penzhina megazone; VI, Allakh-Yun megazone; VII, Dzhagdin-Shevli megazone with zones: VII1 (Shevli), VII2 (Dzhagdin). The inset map (modified from Koren et al., 1983, fig. 2) shows the detailed stratigraphy and the sections along the Mirny Creek and Ina River. O2ss: Middle Ordovician Son Formation; O2kn: Middle Ordovician Kanyon Formation; O3pd: Upper Ordovician Padun Formation; O3tr: Tirekhtyakh Formation; S1mt: Lower Silurian Maut Formation; S1sd: Lower Silurian Sandugan Formation; S2bz: Upper Silurian Bizon Formation.

names, with the exception that the Chalmak and Anika Formations were combined into the Maut Formation. The horizons and formations have been alternatively used by some authors (e.g., Oradovskaya, 1988a; Koren et al., 1988). This paper uniformly uses the term formation. The entire sequence is further divided into 21 units named A–U (Oradovskaya and Sobolevskaya, 1979). Units A–S are well exposed along Mirny Creek (Figs. 4–5), but

unit S is in tectonic contact with unit T strata at Mirny Creek. The base of the Silurian is drawn at the base of unit R. Llandovery beds including units T and U outcrop along the Ina River (Figs. 3, 6). The Kanyon Formation is about 412 m thick at Mirny Creek and is divided into units A–E. Units A–C are characterized by dark gray limestone alternating with calcareous shale and siltstone

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FIGURE 2—Stratigraphic subdivision of the Upper Ordovician and Lower Silurian in northeast Russia (modified from Oradovskaya, 1988a and Koren, personal commun.).

bearing abundant graptolites of the Climacograptus peltifer and lower Diplograptus ingens wellingtonensis–Dicranograptus clingani zones. Units D–E are predominantly massive and thick bedded dark gray and bluish gray calcareous siltstone with thin layers of gray coarse calcareous limestone, which contain rare graptolites indicating most of the Dp. ingens wellingtonensis–Dc. clingani Zone. The Kanyon Formation is Caradocian (Late Ordovician) in age (Figs. 2, 4). The Padun Formation is about 500 m thick at Mirny Creek, and contains units F–M (lower part), which are represented by medium bedded limestone. In the lowermost and uppermost parts of the formation, the limestone alternates with mudstone and carbonaceous shale, respectively. Bioherms occur in the uppermost part of the formation. The formation spans the entire Orthograptus quadrimucronatus Zone and is early Ashgillian (Late Ordovician) in age (Figs. 2, 4). The Tirekhtyakh Formation is from 255 to 320 m thick locally and contains units M (upper part)–Q. The dominant lithology is limestone, especially bioclastic limestone, with large algal and coral bioherms and thin interbeds of silty carbonaceous shale. The formation includes distinctive brachiopods that define the Tcherskidium unicum and Hirnantia beds; the former is subdivided into units M (upper part)–P, which is correlated to the Appendispinograptus supernus Zone, and the latter coincides with unit Q, which corresponds to the Normalograptus extraordinarius and Persculptograptus persculptus zones. The formation is late Ashgillian (Late Ordovician) in age (Figs. 2, 5). The Maut Formation is about 220 m thick and is divided into units R, S, T, and U. Units R, S, and T belong to the Chalmak horizon and Unit U to the Anika horizon (Oradovskaya and Sobolevskaya, 1979; Koren et al., 1983). Unit R is about 45.5 m

thick and is characterized by dark gray bedded siliceous limestone, siliceous lenticular limestone, and black calcareous graptolitic shale that indicate the Akidograptus ascensus and Parakidograptus acuminatus zones (Figs. 2, 3, 5). Unit S is about 75 m thick. The lower part of unit S is composed of massive conglomeratic limestone and medium to thin-bedded silty limestones, with bands of graptolitic shale that affirm the lower Rhuddanian Cystograptus vesiculosus Zone; the upper part of unit S consists of gray brachiopod-bearing finely conglomeratic limestone and flaggy siliceous limestone interbedded with black graptolite-bearing shale that is assigned to the upper Rhuddanian Coronograptus cyphus Zone. Unit T is about 125 m thick and is composed of dark-gray calcareous siltstone near the base, and at a higher level limestone interbedded with graptolitic shale indicating the Aeronian Demirastrites triangulatus and D. convolutus zones (Figs. 2, 3, 6). Unit U, about 30 m thick, is dominated by calcareous shale, mudstone, and limestone, with graptolites of the Telychian Spirograptus turriculatus Zone (Figs. 2, 3, 6). The Sandugan Formation, about 100–120 m thick locally, consists of ribboned argillaceous limestone and brecciated limestone interbedded with shale. It spans the Llandovery-Wenlock boundary (Figs. 2, 3, 6) and Wenlock Series. The lowermost part of the formation yields abundant graptolites of the Oktavites spiralis Zone. Rare graptolites of the Monograptus riccartonensis–M. flexilis zones are found in the middle part, and the Monograptus testis–Cyrtograptus lundgreni zones are documented in the upper part. CONODONT FAUNAS AND THEIR PALEOBIOGEOGRAPHIC AFFINITIES

Many of the conodonts recovered from the rocks at the Mirny Creek and Ina River sections are broken, deformed, or recrystallized, with a black color indicating a CAI value of 5. However,


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FIGURE 3—Correlation of stratigraphic sections of the Ordovician-Silurian boundary deposits in the Mirny Creek and Ina River basin (modified from Oradovskaya and Sobolevskaya, 1979).

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ZHANG AND BARNES—LATE ORDOVICIAN–EARLY SILURIAN CONODONTS, NORTHEAST RUSSIA some specimens are well preserved (Figs. 7–10) and allow age assignments of the faunas. Figures 4–6 provide the occurrence data of each species through the sections. The faunas are discussed below in stratigraphic order. Caradocian fauna.⎯The conodont faunas from units A and B, which are correlated with the Climacograptus peltifer Zone, lower Caradocian (Fig. 4), are characterized by Belodina compressa (Branson and Mehl, 1933), B. cf. B. confluens Sweet, 1979, Drepanoistodus suberectus (Branson and Mehl, 1933), Drepanodus cf. D. santacrucensis Dzik, 1994, Panderodus unicostatus (Branson and Mehl, 1933), Paroistodus? mutatus (Branson and Mehl, 1933), Periodon mirnyensis, n. sp., Protopanderodus liripipus Kennedy, Barnes and Uyeno, 1979, P. insculptus (Branson and Mehl, 1933), and Walliserodus curvatus (Branson and Branson, 1947). Among these species, Belodina compressa is the nominate species of the B. compressa Zone, Midcontinent Realm. It was correlated to the uppermost Climacograptus peltifer and lowest C. wilsoni graptolite zones, lower Caradocian by Harris et al. (1995). At the Mirny Creek section, Belodina compressa first occurs about 10 m above the base of unit A within the lowest C. peltifer Zone and extends to the base of unit I within the middle Orthograptus quadrimucronatus Zone, lower Ashgillian (Fig. 4). The lowest occurrence of B. compressa at Mirny Creek appears to be the oldest known appearance, because its lowest occurrence can be correlated to the Midcontinent Plectodina aculeata Zone linked by the graptolite Climacograptus peltifer. The more interesting species is B. cf. B. confluens, whose first appearance is also within the C. peltifer Zone, lower Caradocian. The diagnostic character of B. confluens (see Systematic Paleontology) was developed in northeast Russia earlier than in the B. confluens Zone in the Midcontinent Realm, where it occurs in the upper Dicranograptus clingani and lower Pleurograptus linearis graptolite zones, upper Caradocian (Harris et al., 1995). Periodon Hadding, 1913 is a typical pandemic, deep-water representative that is common in the North Atlantic Realm (Fortey and Barnes, 1977; Pohler, 1994; Rasmussen, 1998; Johnston and Barnes, 1999). The new species, Periodon mirnyensis, has some transitional characters between late Early Ordovician P. aculeatus Hadding, 1913 and Late Ordovician P. grandis (Ethington, 1959) (see Systematic Paleontology). The stratigraphical occurrence and the morphology of the new species indicate that it probably represents an evolutionary transition between P. aculeatus and P. grandis. Overall, this fauna is of early Caradocian in age and shows a mixture of Midcontinent and North Atlantic Realm affinities. However, taking the most abundant and diverse sample (SIB7915) (Fig. 4) as an example to assess affinity, this fauna shows a closer relationship to the North Atlantic Realm than the Midcontinent Realm, with Periodon mirnyensis and Protopanderodus liripipus totaling 53% of the fauna and Belodina compressa comprising only 3%. The upper Caradocian includes units C, D, and E that are equivalent to the Diplograptus ingens wellingtonensis–Dicranograptus clingani zones, within which the dominant lithology is siltstone. Few limestone samples could be collected from this interval and only a few conodonts were recovered. Early Ashgillian fauna.⎯Units F to O are correlated to the graptolite Orthograptus quadrimucronatus Zone and most of the

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Appendispinograptus supernus Zone. This part of the section (Padun and lower Tirekhtyakh Formations) was sampled at widely spaced intervals, and few conodonts were recovered. One sample (SIB79-22) (Fig. 4) from lower unit I, Padun Formation at Mirny Creek section, which is correlated to the middle O. quadrimucronatus Zone, contains the highest occurrence of Belodina compressa. The last appearance of B. compressa in northeast Russia is obviously later than in the Midcontinent Realm, which is in the upper ‘‘Plectodina’’ tenuis Zone, mid-Caradocian (Sweet, 1988, chart 1). Late Ashgillian fauna.⎯Although unit Q, which is correlated with the Normalograptus extraordinarius and Persculptograptus persculptus zones, was sampled closely, few conodonts were recovered, although the zonal species Amorphognathus ordovicicus Branson and Mehl, 1933 in the lower unit Q (Fig. 5). The more abundant Ashgillian conodonts occur in the uppermost unit O and unit P, which correlate with the uppermost Appendispinograptus supernus Zone. This fauna includes the following species: Amorphognathus ordovicicus, Aphelognathus sp., Belodina cf. B. confluens, Paroistodus? mutatus, Protopanderodus insculptus, P. liripipus, Panderodus unicostatus, Walliserodus curvatus, Scabbardella altipes (Henningsmoen, 1948), Eocarniodus gracilis (Rhodes, 1955), Gamachignathus ensifer McCracken, Nowlan and Barnes, 1980, Hamarodus europaeus (Serpagli, 1967), and Icriodella cf. I. prominens Orchard, 1980. Except for a few cosmopolitan species, such as Panderodus unicostatus and Walliserodus curvatus, the other species within this fauna show a mixture of North Atlantic and Midcontinent Realm affinities. Among these species, A. ordovicicus and B. confluens are used as zonal species for the early Ashgillian of North Atlantic Realm and late Caradocian of North American Midcontinent Realm, respectively (Harris et al., 1995). The youngest zonal species is Gamachignathus ensifer. Gamachignathus was first recognized from the Upper Ordovician Vaure´al and Ellis Bay Formations of Anticosti Island, Quebec, where it appeared first in the late Richmondian and flourished in the Gamachian (McCracken et al., 1980). At the Mirny Creek section, sample (SIB79-39) (Fig. 5) contains G. ensifer and is 8.5 m above the base of unit P and 4.5 m below the base of unit Q (or the base of the Normalograptus extraordinarius Zone), so this fauna can be correlated with the upper Appendispinograptus supernus Zone, uppermost Richmondian. Hamarodus europaeus occurs in Rawtheyan and Hirnantian strata in England and Wales (Orchard, 1980), representing the North Atlantic Realm. It is present in the A. ordovicicus Zone, Rawtheyan, in northern Italy (Serpagli, 1967; Ferretti and Serpagli, 1999) and Germany (Ferretti and Barnes, 1997), but it is also common in the upper half of the A. supernus Zone, Caradocian elsewhere in Europe (Bergstro¨m, 1971). H. cf. H. europaeus appeared in Gamachian strata in eastern Canada (Nowlan, 1983). H. europaeus comprises 48% of the total fauna within sample SIB79-39 that contains the most abundant and diverse conodonts in the upper Ashgillian interval (Fig. 5). Eocarniodus gracilis was reported from Rawtheyan and Hirnantian strata, England and Wales (Orchard, 1980), and is also known from Caradocian and Ashgillian strata in Spain, Bohemia, France, Norway, Sardinia, the Carnic Alps and Thuringia, as well as the Lower Silurian of the Carnic Alps (Ferretti and Barnes, 1997); it is restricted to the North Atlantic Realm. This species

← FIGURE 4—Ordovician conodont distribution at Mirny Creek section. Stratigraphic subdivision is modified from Oradovskaya and Sobolevskaya (1979).

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FIGURE 5—Silurian conodont distribution at Mirny Creek section. Stratigraphic subdivision is modified from Oradovskaya and Sobolevskaya (1979).

comprises 20% of the total fauna within sample SIB79-39. Species of Protopanderodus are common in the Late Ordovician in North Atlantic Realm (Bergstro¨m, 1973). Only Aphelognathus sp. and Belodina cf. B. confluens are of Midcontinent affinity, representing only 2% of the fauna within sample SIB79-39. Overall, these data, together with graptolite data (Koren, 1991;

Koren and Sobolevskaya, 1979; Koren et al., 1983; Koren et al., 1988), indicate that the fauna of this interval is of Rawtheyan– Hirnantian, late Ashgillian age. Although it contains minor representatives of the Midcontinent Realm, it shows a strong affinity to the North Atlantic Realm. Typical representatives of North Atlantic Realm are Hamarodus Viira, 1974, Icriodella Rhodes,


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FIGURE 6—Silurian conodont distribution at Ina River section. Stratigraphic subdivision is modified from Oradovskaya and Sobolevskaya (1979).

1953, Periodon, and Scabbardella Orchard, 1980 (Nowlan et al., 1997), which are common within the collection. The Ordovician conodont collection made from the KolymaOmolon superterrane by Dumoulin et al. (2002) contained abundant (78%) cosmopolitan and tropical cosmopolitan species because of preferential sampling from deeper-water strata. The Ordovician megafossils from both the North Alaska carbonate platform and the Farewell Terrane exhibited some similarity to the coeval forms from the Kolyma-Omolon superterrane (Blodgett, 1998; Blodgett et al., 2002). However, the origin of the Kolyma-Omolon superterrane has remained unclear. Some conodonts reported in the present study, such as Hamarodus and Eocarniodus Orchard, 1980, which were not reported by Dumoulin et al. (2002), are characteristic of the conodont fauna from parts of Europe (Serpagli, 1967; Orchard, 1980; Ferretti and Barnes, 1997; Ferretti and Serpagli, 1999), which the latter authors referred to

the Mediterranean Province, suggesting a high temperate location for the Kolyma Terrane during the Late Ordovician. Early Llandovery fauna.⎯Unit R is correlated to the graptolite Akidograptus ascensus and Parakidograptus acuminatus Zone. This interval was sampled closely on both banks of Mirny Creek, and a relatively abundant and diverse fauna was recovered from samples SIB79-46, 47, 32–36 (Fig. 5). Except for Panderodus unicostatus and Walliserodus curvatus, which extend from Upper Ordovician strata, the other species have their first occurrence elsewhere in the Lower Silurian. Species that occur in this interval include Dapsilodus obliquicostatus (Branson and Mehl, 1933), Decoriconus fragilis (Branson and Mehl, 1933), Distomodus kentuckyensis Branson and Branson, 1947, Kockellela? manitoulinensis (Pollock, Rexroad and Nicoll, 1970), Oulodus panuarensis Bischoff, 1986, Rexroadus cf. R. kentuckyensis (Branson and Branson, 1947), Pseudooneotodus beckmanni (Bischoff and Sannemann, 1958), Ozarkodina cf. Oz. australensis Bischoff, 1986,

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Oulodus cf. Ou. planus (Nicoll and Rexroad, 1969), Ou. sp. 1 and Ou. sp. 2. This fauna occurs through the entire P. acuminatus Zone, except for the lowest 10 m. Sample SIB79-46 (Fig. 5) is from 10 m above the base of unit R or the base of A. ascensus Zone, and it contains most of the species mentioned above. The base of the Dapsilodus obliquicostatus Zone in Lower Silurian basin facies was recognized by the first appearance of the nominate species and correlated with the graptolite Coronograptus gregarius Zone in the Canadian Cordillera, northern Yukon Territory (McCracken, 1991a). This zone may correlate with the lowermost Llandovery Oulodus? nathani (⫽Rexroadus nathani of Zhang and Barnes, 2002) Zone of the carbonate platform facies (McCracken, 1991b). The lowermost occurrence of the zone was confirmed by the co-occurrence of D. obliquicostatus and R. nathani at the base of the A. acuminatus Zone in the Cape Phillips Formation, Canadian Arctic (Jowett and Barnes, personal commun.). The fauna recovered from unit R and lowest unit S is probably correlated to the Dapsilodus obliquicostatus–Decoriconus fragilis interval Zone, whose base is characterized by the last appearance of R. nathani, and whose upper boundary is near Rhuddanian-Aeronian boundary (Jowett and Barnes, personal commun.), but at Mirny Creek R. nathani was not found and the Rhuddanian-Aeronian boundary cannot be located precisely. The most abundant sample (SIB79-46) (Fig. 5) from this interval contains 31% Dapsilodus obliquicostatus and Decoriconus fragilis, which together exhibit a strong affinity to the Early Silurian fauna from slope or basin environments (e.g., Aldridge and Mabillard, 1981; Aldridge and Jeppsson, 1984; Armstrong, 1990; McCracken, 1991a, 1991b; Jowett and Barnes, personal commun.). Middle-Late Llandovery fauna.⎯Unit S is correlated with the graptolite Cystograptus vesiculosus and C. cyphus zones, unit T with the Demirastrites triangulatus, D. convolutus and Stimulograptus sedgwickii zones, unit U with the Spirograptus turriculatus–Globosograptus crispus Zone, and the strata above unit U, the lower part of the Sandugan Formation, to the Oktavites spiralis Zone. SIB79-48 (Fig. 5) is the only productive sample from middle unit S at the Mirny Creek section, occurring in the lower C. cyphus Zone. Most species extend from the lower part of the Silurian section, with the only additional species being Pterospathodus amorphognathoides amorphognathoides Walliser, 1964. At the Ina River section, samples SIB79-7 and SIB79-8 (Fig. 6) contain Polygnathoides aff. P. siluricus; in the former sample, this species occurs with P. a. amorphognathoides. The Pa and Pb elements of P. aff. P. siluricus (Figs. 10.25, 10.26, and 10.28) found in the Ina River section bear some resemblance to those of P. siluricus previously reported from the Late Silurian (e.g., Jeppsson, 1983; Klapper and Murphy, 1975; Uyeno, 1981, 1990). The appearance of P. aff. P. siluricus together with P. amorphognathoides in unit T, which correlates to the Demirastrites triangulatus and D. convolutus zones, suggests that P. siluricus probably evolved in the late Early Silurian. SUMMARY

• The conodont faunas reported in this study include a total of 776 specimens assigned to 34 species representing 25 genera, with 10 elements in open nomenclature from 39 conodontbearing samples out of 49 collected from Mirny Creek and Ina River sections, Kolyma Terrane, Omulev Mountains, northeast Russia. • This is the first detailed report of conodonts from this region, and fills a biostratigraphic and paleobiogeographic gap in the geological record of northeast Russia. • The faunas span an interval of Caradocian and Ashgillian

(Late Ordovician) through Llandovery (Early Silurian); the conodont biostratigraphy is correlated to the graptolite zonation that is well established in these sections. • The Late Ordovician conodont fauna shows a close paleobiogeographic affinity to the North Atlantic Realm; in particular, the Ashgillian fauna is most similar to the Mediterranean Province, suggesting a high temperate location for the Kolyma Terrane during this interval. The Silurian fauna is similar to those from coeval deep-water deposits and is less diagnostic of paleolatitude. SYSTEMATIC PALEONTOLOGY

The specimens illustrated in Figures 7–10 are deposited in the National Type Collection of Invertebrate and Plant Fossils at the Geological Survey of Canada (GSC), Ottawa, Ontario. The terminology used for multielement taxonomy follows the letter designation of Sweet (1988) for the elements within wellknown multielement apparatuses (S, M, and P notation). For a few apparatuses, the letter designations proposed by the original authors, or morphological terms, are employed. Appendices 1 and 2 record the conodont species and their abundance for each of the productive samples, which are accessible via the Journal of Paleontology’s ‘‘Supplemental Database’’ (www.journalofpaleontology.org). The taxa are assigned below into Ordovician taxa and Silurian taxa based on their stratigraphic occurrence in the Mirny Creek and Ina River sections. The species are organized in alphabetical order under these two subheadings. The following species are not taxonomically treated in detail in this study: Ordovician species.—Belodina compressa (Branson and Mehl, 1933) (Fig. 7.11–7.14); and Gamachignathus ensifer McCracken, Nowlan and Barnes, 1980 (Fig. 7.24–7.25). Silurian species.—Belodella sp. (Fig. 9.16); Oulodus panuarensis Bischoff, 1986, emended Zhang and Barnes, 2002 (Fig. 9.30–9.34); Pseudooneotodus beckmanni (Bischoff and Sannemann, 1958) (Fig. 10.29); and Pterospathodus pennatus procerus (Walliser, 1964) (Fig. 10.36–10.39). Species common to both the Ordovician and Silurian.—Panderodus unicostatus (Branson and Mehl, 1933) (Fig. 8.21–8.28); and Walliserodus curvatus (Branson and Branson, 1947, emended Zhang and Barnes, 2002) (Figs. 8.34–8.38, 9.4–9.9). Ordovician taxa.—This section includes 14 species belonging to 12 genera. Genus AMORPHOGNATHUS Branson and Mehl, 1933 Type species.—Amorphognathus ordovicicus Branson and Mehl, 1933. AMORPHOGNATHUS

Branson and Mehl, 1933 Figure 7.1–7.7

ORDOVICICUS

Multielement Amorphognathus ordovicicus BRANSON AND MEHL, 1933. FERRETTI AND BARNES, 1997, p. 26, 28, pl. 1, figs. 1–15 (includes synonymy through 1994); FERRETTI AND SERPAGLI, 1999, pl. 1, figs. 12–14; DZIK, 1999, p. 248; pl. 2, figs. 13–37, 38–47.

Types.—Figured specimens, GSC 127463–127469. Material examined.—Pa element—12; Pb element—1; Sa element—1; Sb-Sc element—4. Occurrence.—Tirekhtyakh Formation, Mirny Creek section. Discussion.—No M and Sd elements are found, but a few Pa and S elements allow a specific identification. Among the Pa specimens, both sinistral and dextral forms and both blade type and

ZHANG AND BARNES—LATE ORDOVICIAN–EARLY SILURIAN CONODONTS, NORTHEAST RUSSIA non-blade type of anterior process (Bergstro¨m, 1964) occur. Probably, the Pa specimens with an anterior blade process represent a juvenile growth stage. Genus APHELOGNATHUS Branson, Mehl and Branson, 1951 Type species.—Aphelognathus grandis Branson, Mehl and Branson, 1951. APHELOGNATHUS? sp. Figure 7.8 Types.—Figured specimen, GSC 127470. Material examined.—Pa element—1. Occurrence.—Tirekhtyakh Formation, Mirny Creek section. Discussion.—The generic assignment is questioned because only one incomplete specimen of Pa element was recovered; however, the Pa element has angular shape, denticles confluent along most of length, and basal cavity flared laterally, which are all characters of the Pa element of some Aphelognathus species. Genus BELODINA Ethington, 1959 Type species.—Belodina grandis Stauffer, 1935. BELODINA cf. B. CONFLUENS Sweet, 1979 Figure 7.9–7.10 cf. Belodina confluens SWEET, 1979, p. 59–60, figs. 5.10, 5.17, 6.9 (includes synonymy through 1968); NOWLAN, 1983, p. 662, 664, pl. 3, figs. 3, 4 (includes synonymy through 1981); MCCRACKEN, 1987, pl. 2, figs. 1–4; NOWLAN, MCCRACKEN AND CHATTERTON, 1988, pl. 2, figs. 16–21; MCCRACKEN AND NOWLAN, 1989, pl. 1, figs. 19–21; pl. 2, figs. 1, 2; UYENO, 1990, p. 71–72, pl. 1, figs. 8, 9; NOWLAN, MCCRACKEN AND MCLEOD, 1997, pl. 1, figs. 1, 2; pl. 3, figs. 5, 6, 9, 10. cf. Belodina compressa (BRANSON AND MEHL). NOWLAN AND BARNES, 1981, p. 12, pl. 8, figs. 1–4; MCCRACKEN AND BARNES, 1981, p. 75, pl. 3, figs. 10–12, 16–18; LENZ AND MCCRACKEN, 1982, pl. 2, figs. 3–5.

Types.—Figured specimens, GSC 127471–127472. Material examined.—compressiform element—12; grandiform element—16; eobelodiniform element—3. Occurrence.—Kanyon, Padun (?) and Tirekhtyakh Formations, Mirny Creek section, and Tirekhtyakh, Ina River section. Discussion.—Sweet (1979, p. 59) noted the difference between Belodina confluens and B. compressa with the former ‘‘having a smoothly arcuate anterior margin, which is not interrupted by a short, straight segment just anterior to the anterobasal corner that is typical of such elements in the apparatus of B. compressa.’’ The specimens of the compressiform element with a smoothly arcuate anterior margin (Fig. 7.9) occur in several samples in the Kanyon and Tirekhtyakh Formations at the Mirny Creek section. Compared with the type specimen of the compressiform element in B. confluens (Sweet, 1979, figs. 6–9), these specimens are similar in having the anterior margin regularly curved throughout its length, but are different in 1) having a straight basal margin, whereas the posterior and anterior parts of the basal margin form an angle of 140⬚ in the type specimen, and 2) the anterior margin is much more curved, so that the distal cusp is parallel to the basal margin, compared to the upward directed distal portion of the cusp in the type specimen. Genus DREPANODUS Pander, 1856 Type species.—Drepanodus arcuatus Miller, 1889. DREPANODUS cf. D. SANTACRUCENSIS Dzik, 1994 Figure 7.26–7.27 cf. Drepanodus santacrucensis DZIK, 1994, p. 70–71, pl. 16, figs. 8–13, text-fig. 9a.

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Types.—Figured specimens, GSC 127488–127489. Material examined.—ne element—1; ke element—1. Occurrence.—Kanyon Formation, Mirny Creek section. Discussion.—D. santacrucensis contains ne, hi, ke, pl, oz, and sp elements, but in the collection from northeast Russia, only ne, and ke elements were recovered. As noted by Dzik (1994, p. 70– 71), ‘‘the most characteristic feature for the species is very flat ne element.’’ Both ne and ke elements recognized by this study are fragile and thin cristate elements, but the upper margin of the base is longer than in the type specimens. Genus EOCARNIODUS Orchard, 1980 Type species.—Prioniodus gracilis Rhodes, 1955. EOCARNIODUS GRACILIS (Rhodes, 1955) Figure 7.19–7.23 ‘carniodiform’ element, FERRETTI AND BARNES, 1997, p. 38, pl. 3, figs. 20–22 (includes synonymy through 1982).

Types.—Figured specimens, GSC 1127481–127485. Material examined.—M element—5; Sa element—5; Sb1 (unbowed) element—7; Sb2 (bowed) element—8; Sc element—8. Occurrence.—Tirekhtyakh Formation, Mirny Creek section. Discussion.—The apparatus of E. gracilis was established by Orchard (1980), taking Prioniodus gracilis Rhodes, 1955 as the type species. P. gracilis is a form species that actually is a dolabrate ramiform element (Rhodes, 1955, p. 136, pl. VIII, figs. 5, 6). Among all elements assigned to the multielement species E. gracilis by Orchard (1980, p. 20, pl. 2, figs. ?13, 14, ?15, 18–21, 24–26, ?27, 28, 30, ?31, 33, 34, ?36, 38), none can be identified as the form species P. gracilis. Furthermore, there is no dolabrate ramiform element that is similar to the form species P. gracilis from either the carniodiform elements recognized by Ferretti and Barnes (1997) from Thuringia, Germany, or the E. gracilis Orchard elements recognized by the present authors from northeast Russia. No Pa? and Pb? elements of Orchard (1980, pl. 2, figs. 13–15, 20, 21, 28) are recognized; the Sa element has an isosceles triangle profile with two processes in which the proximal is higher than the distal; the Sb element can be differentiated into two types—unbowed (Sb1) and bowed (Sb2). The symmetry of the unbowed Sb element is similar to that of the Sa element, but the proximal and distal ends of the lateral processes are about the same height, and all the denticles are of similar size. Genus HAMARODUS Viira, 1974 Type species.—Distomodus europaeus Serpagli, 1967. HAMARODUS

EUROPAEUS (Serpagli, 1967) Figure 7.31–7.37

Hamarodus europaeus (SERPAGLI, 1967). FERRETTI AND BARNES, 1997, p. 22–23, pl. 3, figs. 1–14 (includes synonymy through 1994).

Types.—Figured specimens, GSC 127493–127499. Material examined.—Pa element—7; Pb element—7; M element—36; Sa element—8; Sb-Sc element—23. Occurrence.—Tirekhtyakh Formation, Mirny Creek section. Discussion.—Ferretti and Barnes (1997) noted that the M element is the most common component of the apparatus, and the ratio of P to M elements is about 0.7 and 0.5 in the German and Carnic Alps fauna, respectively; however, it is 0.4 in the fauna from northeast Russia. The apparatus reconstruction of Ferretti and Barnes (1997) contains more elements than those of Dzik (1976) and Orchard (1980), including Pa, Pb, M, Sa, Sb, Sc, and Sd elements. In the

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ZHANG AND BARNES—LATE ORDOVICIAN–EARLY SILURIAN CONODONTS, NORTHEAST RUSSIA Russian collection, no Sd element is recognized; Sb and Sc elements are difficult to differentiate, due to the incomplete preservation of the posterior process; the Pb (?) element appears different from that in some of the earlier reconstructions (e.g., Dzik, 1976; Orchard, 1980; Ferretti and Barnes, 1997). The Pb element illustrated by Dzik (1976, fig. 36b) from the Carnic Alps is deeply excavated, thin-walled, and has rather slender processes which are frequently broken, so Orchard (1980) argued that Dzik’s (1976, fig. 36b) Pb element has an oral edge that is atypical of the species. However, the Pb element illustrated by Orchard (1980, pl. 4, fig. 29) does not really show the typical character of the species due to poor preservation. Ferretti and Barnes (1997) illustrated better preserved specimens, but the difference between Pa and Pb elements was still not clear in their complete apparatus. In the Russian material, the elements with a deep, wide basal cavity and sharp anterior, upper, and posterior margins can be divided into two groups: 1) Pb1 with anterior and posterior edges of the base with a small number of short denticles, and with rather thin walls and lighter color (Fig. 7.33); 2) Pb2 with anterior and posterior edges of the base and the latter shorter than the former, and the posterior edge with ill-defined short denticles, and with thick walls and dark color (Fig. 7.37). We provisionally assign the latter to the Pb element of H. europaeus, but need more material to prove this relationship or assign it to a different species from H. europaeus. Another element (Fig. 7.36) with a bifurcating anterior edge near the base and ill-defined short denticles on the lower posterior edge is questionably included in this species. Genus ICRIODELLA Rhodes, 1953 Type species.—Icriodella superba Rhodes, 1953. ICRIODELLA cf. I. PROMINENS Orchard, 1980 Figure 7.28–7.30

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Discussion.—Only three specimens representing Pa, Sa, and Sb elements are present in the Russian fauna. The Pa element resembles that of I. prominens; the upper surface of the posterior process bears five mostly discrete denticles on each lateral edge, but the height of the cusp is uncertain because of incomplete preservation. Genus PAROISTODUS Lindstro¨m, 1971 Type species.—Oistodus parallelus Pander, 1856. PAROISTODUS?

(Branson and Mehl, 1933) Figure 7.46–7.48

MUTATUS

Paroistodus? mutatus (BRANSON AND MEHL). MCCRACKEN AND BARNES, 1981, p. 88, pl. 3, figs. 7–9 (includes synonymy to 1981).

Types.—Figured specimens, GSC 127508–127510. Material examined.—oistodiform element—16; acodontiform element—7; distacodontiform element—8. Occurrence.—Kanyon Formation, Mirny Creek section. Discussion.—Both McCracken and Barnes (1981) and Nowlan and Barnes (1981) described P.? mutatus from the Upper Ordovician of Anticosti Island, Quebec, which includes oistodiform, acodontiform and distacodontiform elements. Nowlan and Barnes (1981) included the low-based elements both having a costa on each side and having a costa on the inner side in the distacodontiform, and separated the acodontiform and disacodontiform elements into low-based and high-based forms. All three kinds of elements can be recognized in the Russian fauna, but it is difficult to divide the acodontiform and distacodontiform elements into low-based and high-based forms. Genus PERIODON Hadding, 1913 Type species.—Periodon aculeatus Hadding, 1913. PERIODON MIRNYENSIS new species Figures 7.38–7.45, 8.1–8.6

cf. Icriodella prominens ORCHARD, 1980, p. 22, pl. 1, figs. 19–21, ?22, 25, ?27, 28; NOWLAN, 1983, p. 666, pl. 1, figs. 8–10.

?Periodon sp. A. NOWLAN, 1983, p. 666–667, pl. 1, figs. 15–19.

Types.—Figured specimens, GSC 127490–127492. Material examined.—Pa element—1; Sa element—1; Sb element—1. Occurrence.—Tirekhtyakh Formation, Mirny Creek section.

Diagnosis.—A species of Periodon in which the falodiform M element is an advanced aculeatus-type with more than three denticles on anterior margin of cusp and a sinuous basal margin, and with S elements that have hindeodelloid denticulation.

← FIGURE 7—1–7, Amorphognathus ordovicicus (⫻60). From SIB79-39, Tirekhtyakh Formation; 1–3, upper views of Pa element, GSC 127463, 127464, 127465; 4, anterior-lateral view of Pb element, GSC 127466; 5, 7, lateral views of Sb (or Sc) element, GSC 127467, 127468; 6, lateral view of Sa element, GSC 127469. 8, Aphelognathus? sp. (⫻60). From SIB79-40, Tirekhtyakh Formation; lateral view of Pa element, GSC 127470. 9–10, Belodina cf. B. confluens (⫻60). From SIB79-17, Kanyon Formation; 9, lateral view of compressiform element, GSC 127471; 10, lateral view of eobelodiniform element, GSC 127472. 11–14, Belodina compressa (⫻60). 11–12 from SIB79-22, Padun Formation; 11, lateral view of compressiform element, GSC 127473; 12, lateral view of grandiform element, GSC 127474; 13, 14 from SIB79-1, Tirekhtyakh Formation; 13, lateral view of grandiform element, GSC 127475; 14, lateral view of eobelodiniform element, GSC 127476. 15–18, Drepanoistodus suberectus. 15 from SIB7918, Kanyon Formation; lateral view of oistodiform, GSC 127477 (⫻35); 16 from SIB79-3, Tirekhtyakh Formation; lateral view of suberectiform, GSC 127478 (⫻25); 17, 18 from SIB79-18, Kanyon Formation (both ⫻60); 17, anterior-lateral view of curvatiform element, GSC 127479; 18, lateral view of honocurvatiform element, GSC 127480. 19–23, Eocarniodus gracilis (⫻100). From SIB79-39, Tirekhtyakh Formation; 19, posterior view of M element, GSC 127481; 20, posterior view of Sa element, GSC 127482; 21, posterior view of Sb2 (unbowed) element, GSC 127483; 22, posterior view of Sb2 (bowed) element, GSC 127484; 23, outer-lateral view of Sc element, GSC 127485. 24, 25, Gamachignathus ensifer (⫻60). From SIB79-39, Tirekhtyakh Formation; 24, outer-lateral view of Pa element, GSC 127486; 25, outer-lateral view of Pb element, GSC 127487. 26, 27, Drepanodus cf. D. santacrucensis (⫻60). From SIB79-17, Kanyon Formation; 26, lateral view of ke element, GSC 127488; 27, lateral view of ne element, GSC 127489. 28–30, Icriodella cf. I. prominens (⫻50). From SIB79-39, Tirekhtyakh Formation; 28, upper view of Pa element, GSC 127490; 29, posterior view of Sa element, GSC 127491; 30, posterior-lateral view of Sb (or Sc) element, GSC 127492. 31–37, Hamarodus europaeus (⫻60). From SIB79-39, Tirekhtyakh Formation; 31, inner-lateral view of M element, GSC 127493; 32, lateral view of Sa element, GSC 127494; 33, inner lateral view of Pb1 element, GSC 127495; 34, inner lateral views of Pa element, GSC 127496; 35, anterior-lateral view of Sb (or Sc) element, GSC 127497; 36, anterior-lateral view of Pb (?) element, GSC 127498; 37, inner-lateral view of Pb2 element, GSC 127499. 38–45, Periodon mirnyensis (⫻60). From SIB79-17, Kanyon Formation; 38, inner-lateral view of Pa element, GSC 127500; 39, innerlateral view of Pb element, GSC 127501; 40, lateral view of Sa element, GSC 127502; 41, outer-lateral view of M element, GSC 127503; 42, 43, fragments of S element showing the hindeodelloid denticulation, GSC 127504, 127505; 44, 45, inner-lateral views of Sb (or Sc) element, GSC 127506, 127507. 46–48, Paroistodus? mutatus (⫻80). From SIB79-15, Kanyon Formation; 46, outer-lateral view of oistodiform element, GAC 127508; 47, lateral view of distacodiform element, GSC 127509; 48, lateral view of acodiform element, GSC 127510.

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ZHANG AND BARNES—LATE ORDOVICIAN–EARLY SILURIAN CONODONTS, NORTHEAST RUSSIA Description.—M element is falodiform—geniculate, coniform, with anteriorly denticulate cusp and posteriorly extended base; number of denticles on anterior margin range from four to six; base is large and open with a sinuous margin; angle between base and cusp is about 30⬚–45⬚. S elements form a transition series from dolabrate through bipennate and tertiopedate to alate elements. All S elements have long denticulate posterior process with hindeodelloid denticulation—one longer and more robust denticle alternating with one or two shorter and slender denticles, with the former about twice height and width of the latter. Other fragments of the posterior processes appear sinuous. Both M and S elements show an inverted basal cavity. Pa and Pb elements are similar to those of P. aculeatus and P. grandis. Etymology.—Named after the type locality of Mirny Creek. Types.—Holotype, GSC 127516; paratypes, GSC 127500– 127507, 127511–127515. Material examined.—Pa element—6; Pb element—3; M element—16; Sa element—3; Sb element—5; Sc element—8. Occurrence.—Kanyon Formation, Mirny Creek section. Discussion.—Bergstro¨m and Sweet (1966, p. 365) noted that the M element of P. grandis is ‘‘its most characteristic and diagnostic structure’’ and that this structure differs from the corresponding element of P. aculeatus in ‘‘having a large, subtriangular base, an essentially straight basal margin, and anterior denticles appressed to the anterior margin of the cusp rather than being developed on a more or less conspicuous anterior process.’’ Kennedy et al. (1979) noted another difference between the two species—the development of a rudimentary hindeodelloid denticulation in P. grandis but not in P. aculeatus. In the Russian collection, among 16 specimens of the M element, no grandis-type M element was recognized; the well-preserved specimens of S elements show a hindeodelloid denticulation. Therefore, this new species seems to be a distinct transitional form between P. aculeatus and P. grandis. The persistent denticulation of the anterior margin of M elements first occurred in P. aculeatus as noted by Lo¨fgren (1978). The denticles on the anterior margin of Early Ordovician P. aculeatus are restricted to two or three in number (Johnston and Barnes, 2000). However, these numbers range from four to six in the northeast Russian specimens. Among the specimens assigned to Periodon cf. P. aculeatus by Kennedy et al. (1979) from the Middle Ordovician Tetagouche

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Group, New Brunswick, the M elements included both aculeatustype and grandis-type, and the S elements have a non-hindeodelloid denticulation. Kennedy et al. (1979) noted that both P. grandis and P. aculeatus co-occurred in the late Middle Ordovician. The former could have separated from the latter in the Middle Ordovician, with both continuing as separate species to the late Middle Ordovician, with M elements of P. grandis evolving more rapidly than the non-M elements. P. aculeatus appears to have become extinct late in the Middle Ordovician, whereas P. grandis became extinct in the Late Ordovician. Periodon sp. A. described by Nowlan (1983) from Late Ordovician Grog Brook Group, northwestern New Brunswick, has a similar M element to the new species, but the only specimen that is a possible S element (Nowlan, 1983, p. 666–667, pl. 1, fig. 17) has no preserved denticles on the posterior process. It is apparent that in the evolution of P. aculeatus to P. grandis, the number of denticles on the anterior margin of the M element increased; the M element was modified from aculeatus-type to the grandis-type; and the S elements were modified from regular denticulation to the hindeodelloid denticulation. However, the Russian material does not support the hypothesis that the M element of P. grandis evolved more rapidly than the non-M elements (Kennedy et al., 1979), because the Russian and New Brunswick material are not sufficient to determine whether the grandis-type M element or the hindeodelloid denticulation in S elements appeared earlier. Genus PROTOPANDERODUS Lindstro¨m, 1971 Type species.—Acontiodus rectus Lindstro¨m, 1955. PROTOPANDERODUS INSCULPTUS (Branson and Mehl, 1933) Figure 8.11–8.15 Protopanderodus insculptus (BRANSON AND MEHL, 1933). MCCRACKEN, 1989, p. 16–18, pl. 3, figs. 9–14, 17, 19; text-fig. 3k–p (includes synonymy to 1982).

Types.—Figured specimens, GSC 127521–127525. Material examined.—asymmetric acontiodontiform (a/b)—6, denticulate symmetric acontiodontiform (c)—5, adenticulate symmetric acontiodontiform (c)—2, and scandodontiform (e)—2. Occurrence.—Kanyon, Padun (?) and Tirekhtyakh Formations, Mirny Creek section; Tirekhtyakh Formation, Ina River Formation. Discussion.—McCracken (1989) emended the generic diagnosis

← FIGURE 8—1–6, Periodon mirnyensis (⫻60). From SIB79-15, Kanyon Formation; 1, 2, inner-lateral views of Pb element, GSC 127511, 127512; 3, outer-lateral view of Sb element, GSC 127513; 4, inner-lateral view of Sc (?) element, GSC 127514; 5, 6, outer-lateral views of M element, GSC 127515, 127516 (holotype). 7–10, Protopanderodus liripipus (⫻60). 7, 8 from SIB79-37, Tirekhtyakh Formation; 7, outer-lateral view of asymmetric acontiodontiform (a/b) element, GSC 127517; 8, lateral view of symmetric acontiodontiform (c) element, GSC 127518; 9, 10 from SIB79-15, Kanyon Formation; 9, inner-lateral view of asymmetric acontiodontiform (a/b) element, GSC 127519; 10, lateral view of scandodontiform (e) element, GSC 127520. 11–15, Protopanderodus insculptus (⫻60). 11, from SIB79-4, Tirekhtyakh Formation, lateral view of symmetric denticulate acontiodontiform (c) element, GSC 127521; 12, 13 from SIB79-3, Tirekhtyakh Formation; 12, symmetric adenticulate acontiodontiform (c) element, GSC 127522; 13, symmetric denticulate acontiodontiform (c) element, GSC 127523; 14, 15 from SIB79-29, Tirekhtyakh Formation; 14, asymmetric denticulate acontiodontiform (a/b) element, GSC 127524; 15, asymmetric adenticulate acontiodontiform (a/b) element, GSC 127525. 16–20, Scabbardella altipes (⫻70). From SIB79-39, Tirekhtyakh Formation; 16, 17, lateral views of distacodiform (Sa) element, GSC 127526, 127527; 18, 19, lateral views of drepanodiform (M) element, GSC 127528, 127529; 20, lateral view of acodiform (Sb/Sc) element, GSC 127530. 21–28, Panderodus unicostatus (⫻70). 21–23 from SIB79-15, Kanyon Formation; 21, inner-lateral view of truncatiform element, GSC 127531; 22, lateral view of aequaliform element, GSC 127532; 23, inner-lateral view of compressiform element, GSC 127533; 24–28 from SIB79-46, Maut Formation; 24, inner-lateral view of compressiform element, GSC 127534; 25, inner-lateral view of acuatiform element, GSC 127535; 26, inner-lateral view of tortiform element, GSC 127536; 27, inner-lateral view of asymmetric graciliform element, GSC 127537; 28, lateral view of subsymmetric graciliform element, GSC 127538. 29–33, Panderodus recurvatus (⫻70). From SIB79-33, Maut Formation; 29, 33, inner-lateral views of asymmetric graciliform element, GSC 127539, 127540; 30, inner lateral view of tortiform element, GSC 127541; 31, inner-lateral view of compressiform element, GSC 127542; 32, inner-lateral view of acuatiform element, GSC 127543. 34–38, Walliserodus curvatus (⫻70). From SIB79-47, Maut Formation; 34, outer-lateral view of multicostatiform element, GSC 127544; 35, 36, inner-lateral views of curvatiform elements, GSC 127545, 127546; 37, outer-lateral view of unicostatiform element, GSC 127547; 38, posterior view of dyscritiform element, GSC 127548.

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ZHANG AND BARNES—LATE ORDOVICIAN–EARLY SILURIAN CONODONTS, NORTHEAST RUSSIA to include asymmetric acontiodontiform (a/b), symmetric acontiodontiform (c), and scandodontiform (e) elements, which is followed by the present study. In the P. insculptus apparatus reconstructed by McCracken (1989) based on northern Yukon material, both asymmetric acontiodontiform (a/b) and symmetric acontiodontiform (c) elements have the denticulate forms, but no denticulate asymmetric acontiodontiform (a/b) elements have been found in the collection from northeast Russia. PROTOPANDERODUS LIRIPIPUS Kennedy, Barnes, and Uyeno, 1979 Figure 8.7–8.10 Protopanderodus liripipus KENNEDY, BARNES, AND UYENO. MCCRACKEN, 1989, p. 18–20, pl. 3, figs. 15, 16, 18, 20–25; text-fig. 3g–j (includes synonymy to 1985).

Types.—Figured specimens, GSC 127517–127520. Material examined.—asymmetric acontiodontiform (a/b)— 10, symmetric acontiodontiform (c)—10, and scandodontiform (e)—8. Occurrence.—Kanyon, Padun (?) and Tirekhtyakh Formations, Mirny Creek section. Discussion.—Both McCracken (1989) and Dzik (1994) redescribed this species; the latter did not include the former’s work in synonymy, although the former reported the distribution of the species. Protopanderodus liripipus illustrated by Dzik (1994, p. 74–76, pl. 14; figs. 6, 7; text-fig. 11c) included only symmetric and asymmetric acontiodontiform elements, and both elements have a long and prominent anticusp, which is different from the holotype and paratypes of the species (Kennedy et al., 1979, pl. 1, figs. 9–19). Genus SCABBARDELLA Orchard, 1980 Type species.—Scabbardella altipes (Henningsmoen, 1948). SCABBARDELLA

ALTIPES (Henningsmoen, 1948) Figure 8.16–8.20

Scabbardella altipes (HENNINGSMOEN, 1948). FERRETTI AND BARNES, 1997, p. 34, pl. 1, figs. 17–22 (includes synonymy to 1994).

Types.—Figured specimens, GSC 127526–127530. Material examined.—drepanodiform (M)—5, distacodiform (Sa)—8, acodiform (Sb-Sc)—7.

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Occurrence.—Tirekhtyakh Formation, Mirny Creek section. Discussion.—Orchard (1980) established the genus Scabbardella, and Dzik (1994) questioned whether it is necessary to separate it from the genus Dapsilodus Cooper, 1976, because of their close similarity. Three different morphotypes were named as drepanodiform, acodiform and distacodiform (Orchard, 1980, p. 25– 26, pl. 5, figs. 2–5, 7, 8, 12, 14, 18, 20, 23, 24, 28, 30, 33, 35); meanwhile they were illustrated as M, Sc-Sb, and Sa (Orchard, 1980, p. 15, text-fig. 4c). The former terminology is followed in the present study. The Russian distacodiform element shows the following variations: the basal margin from straight (Fig. 8.17) to sinuous (Fig. 8.16); the upper margin of the base from short (Fig. 8.17) to long (Fig. 8.16); the angle of postero-basal corner from about 60⬚ (Fig. 8.17) to about 30⬚ (Fig. 8.16). Silurian taxa.—This section includes 18 species belonging to 11 genera. Genus DECORICONUS Cooper, 1975 Type species.—Paltodus costulatus Rexroad, 1967. DECORICONUS

(Branson and Mehl, 1933) Figure 9.10–9.15

FRAGILIS

Decoriconus fragilis (BRANSON AND MEHL). ZHANG AND BARNES, 2002, p. 11–12, fig. 15.25–15.31 (includes synonymy through 1988).

Types.—Figured specimens, GSC 127558–127563. Material examined.—acontiodontiform (Sa) element—6; paltodontiform (Sb) element—15; drepanodontiform (Sc) element— 10. Occurrence.—Maut Formation, Mirny Creek section. Discussion.—The species includes three morphotypes: acontiodontiform (Sa), paltodontiform (Sb) and drepanodontiform (Sc) elements. Zhang and Barnes (2002) considered that the anticusplike lower anterior margin of form species Drepanodus aduncus illustrated by Nicoll and Rexroad (1969) is a variable form of drepanodontiform (Sc) in D. fragilis, which is further confirmed by the Russian material. The different specimens of the drepanodontiform (Sc) element with shorter or longer anticusp-like lower anterior margin can be found in the same sample (SIB79-46) (Fig. 9.10, 9.11).

← FIGURE 9—1–3, Dapsilodus obliquicostatus (⫻80). From SIB79-46, Maut Formation; 1, lateral view of acodiform (Sb/Sc) element, GSC 127549; 2, 3, lateral views of distacodiform (Sa) element, GSC 127550, 127551. 4–9, Walliserodus curvatus (⫻70). 4, 6 from SIB79-39, Tirekhtyakh Formation; 4, outer-lateral view of unicostatiform element, GSC 127552; 6 inner-lateral view of curvatiform element, GSC 127553; 5 from SIB7915, Kanyon Formation; lateral view of deboltiform element, GSC 127554; 7 from SIB79-34, Maut Formation; posterior view of dyscritiform element, GSC 127555; 8 from SIB79-35, Maut Formation; lateral view of deboltiform element, GSC 127556; 9 from SIB79-33, Maut Formation; outer-lateral view of multicostatiform element, GSC 127557. 10–15, Decoriconus fragilis (⫻150). 10–12 from SIB79-46, Maut Formation; 10, 11, inner-lateral views of drepanodiform element, GSC 127558, 127559; 12, inner-lateral view of paltodontiform element, GSC 127560; 13, 15 from SIB79-34, Maut Formation; 13, inner-lateral view of acontiodontiform element, GSC 127561; 15, inner-lateral views of drepanodiform element, GSC 127562; 14 from SIB79-33, Maut Formation; inner-lateral view of paltodontiform element, GSC 127563. 16, Belodella sp. (⫻150). From SIB79-49, Maut Formation; lateral view of M (?) element, GSC 127564. 17–21, Distomodus kentuckyensis (⫻50). From SIB79-47, Maut Formation; 17, lateral view of M element, GSC 127565; 18, lateral view of Pb element, GSC 127566; 19, inner-lateral view of Sb-Sc element, GSC 127567; 20, upper view of Pa element, GSC 127568; 21, posterior view of Sa element, GSC 127569. 22, ?Johnognathus sp. (⫻50). From SIB79-10, Maut Formation; upper view of a fragment of Pa element, GSC 127570. 23–25, Kockelella? manitoulinensis (⫻80). 23, 24 from SIB79-33, Maut Formation; outer-upper and upper views of Pa element, GSC 127571; 25, from SIB79-46, Maut Formation; upper-lateral view of Pa element, GSC 127572. 26, 27, Distomodus staurognathoides (⫻50). From SIB79-7, Maut Formation; 26, upper view of Pa element, GSC 127573; 27, innerlateral view of M element, GSC 127574. 28, 29, Oulodus cf. O. planus (⫻60). From SIB79-10, Maut Formation; 28, inner lateral view of M element, GSC 127575; 29, inner-lateral view of Sc element, GSC 127576. 30–34, Oulodus panuarensis (⫻60). From SIB79-46, Maut Formation; 30, posterior view of Pa element, GSC 127577; 31, posterior view of Pb element, GSC 127578; 32, posterior view of Sb element, GSC 127579; 33, posterior view of M element, GSC 127580; 34, inner-lateral view of Sc element, GSC 127581. 35–40, Rexroadus cf. R. kentuckyensis (⫻60). From SIB79-47, Maut Formation; 35, inner-lateral view of Pa element, GSC 127582; 36, posterior view of M element, GSC 127583; 37, posterior view of Pb element, GSC 127584; 38, posterior view of Sa element, GSC 127585; 39, posterior view of Sb element, GSC 127586; 40, innerlateral view of Sc element, GSC 127587.

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ZHANG AND BARNES—LATE ORDOVICIAN–EARLY SILURIAN CONODONTS, NORTHEAST RUSSIA Genus DISTOMODUS Branson and Branson, 1947, emended Cooper, 1975 Type species.—Distomodus kentuckyensis Branson and Branson, 1947. DISTOMODUS

Branson and Branson, 1947 Figure 9.17–9.21

KENTUCKYENSIS

Distomodus kentuckyensis BRANSON AND BRANSON, 1947. ZHANG AND BARNES, 2002, p. 13, fig. 14.18–14.26 (includes synonymy through 1987).

Types.—Figured specimens, GSC 127565–127569. Material examined.—Pa element—8; Pb element—1; M element—3; Sa element—3; Sb-Sc element—8. Occurrence.—Maut Formation, Mirny Creek section. Discussion.—Only two samples (SIB79-46 and SIB79-47) contain the Pa element that is the most characteristic element of the apparatus. One specimen (Fig. 9.20) resembles the form species Icriodina irregularis Branson and Branson, 1947 and the Pa element of D. kentuckyensis as illustrated and described by Rexroad (1967) and Cooper (1975), respectively. However, the M element illustrated here differs from the holotype of the form species D. kentuckyensis (Branson and Branson, 1947, pl. 81, figs. 40, 41), which has an extremely straight anterior margin and a long anticusp, but the former is similar to the same component in the multielement apparatus of D. kentuckyensis interpreted by Cooper (1975, pl. 2, fig. 14) which has a reclined cusp with an anteriorly convex anterior margin and a short anticusp. DISTOMODUS

STRAUROGNATHOIDES

(Walliser, 1964)

Figure 9.26–9.27 Distomodus staurognathoides (WALLISER, 1964). ZHANG AND BARNES, 2002, p. 13–14, fig. 14.1–14.17 (includes synonymy through 1999).

Types.—Figured specimens, GSC 127573–127574. Material examined.—Pa element—2; M element—1; Sb-Sc element—1. Occurrence.—Maut Formations, Ina River section. Discussion.—Among the few Russian specimens, one Pa element (Fig. 9.26) is almost perfectly preserved, which resembles both the paratypes of the form species Hadrognathus staurognathoides (Walliser, 1964, pl. 13, figs. 6, 15) and the holotype of multielement species Distomodus tridens (Bischoff, 1986, pl. 7, figs. 1a, 1b).

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Genus JOHNOGNATHUS Mashkova, 1977 Type species.—Johnognathus huddlei Mashkova, 1977. ?JOHNOGNATHUS sp. Figure 9.22 Type.—Figured specimen, GSC 127570. Material examined.—Pa (?) element—1. Occurrence.—Maut Formation, Ina River section. Discussion.—Only one fragment was recovered, which may represent the anterior part of the possible Pa element. The upper surface of the specimen is worn, but the specimen displays a rectangular outline with straight lateral sides, and the ornamentation on the lateral sides is similar to that of ?‘‘Johnognathus’’ huddlei Mashkova, 1977 described by Uyeno and Barnes (1983, pl. 8, fig. 25) from Anticosti Island. Genus KOCKELELLA Walliser, 1957 Type species.—Kockelella variabilis Walliser, 1957. KOCKELELLA? MANITOULINENSIS (Pollock, Rexroad, and Nicoll, 1970) Figure 9.23–9.25 Kockelella? manitoulinensis (POLLOCK, REXROAD, AND NICOLL, 1970). ZHANG AND BARNES, 2002, p. 19, 21, fig. 13.1–13.6 (includes synonymy through 1990).

Types.—Figured specimens, GSC 127571–127572. Material examined.—Pa element—3; Pb element—1; M element—1; Sb element—2; Sc element—4. Occurrence.—Tirekhtyakh and Maut Formations, Mirny Creek section. Discussion.—Zhang and Barnes (2002) discussed the confusion in the generic assignment of a blade Pa element with a deflected posterior process. The Pa element of K.? manitoulinensis is deflected in its posterior quarter, with short and fused denticles and widely flared basal cavity. The flared basal cavity differs in different specimens, some wider (Fig. 9.23, 9.24) than the others (Fig. 9.25). Genus OULODUS Branson and Mehl, 1933 Type species.—Cordylodus serratus Stauffer, 1930. OULODUS cf. O. PLANUS (Walliser, 1964) Figure 9.28–9.29 cf. Ozarkodina plana (WALLISER, 1964). SWEET AND SCHO¨NLAUB, 1975, p. 52–53, pl. 1, figs. 1–6 (includes synonymy to 1972); Oulodus planus

← FIGURE 10—1–4, Oulodus sp. 1 (⫻60). From SIB79-33, Maut Formation; 1, posterior view of Pb element, GSC 127588; 2, inner-lateral view of M element, GSC 127589; 3, posterior view of Sb element, GSC 127590; 4, inner-lateral view of Sc element, GSC 127591. 5–8, Oulodus sp. 2 (⫻60). From SIB79-34, Maut Formation; 5, posterior view of Pa element, GSC 127592; 6, inner-lateral view of Sc element, GSC 127593; 7, posterior view of Sb element, GSC 127594; 8, posterior view of Sa element, GSC 127595. 9–15, Ozarkodina cf. O. masurensis (⫻60). 9–13 from SIB79-8, Maut Formation; 9, outer-lateral view of Pa element, GSC 127596; 10, inner-lateral view of Pb element, GSC 127597; 11, 12, innerlateral views of Sc element, GSC 127598, 127599; 13, posterior view of Sb element, GSC 127600; 14, 15 from SIB79-48, Maut Formation; 14, outer-lateral view of Pa element, GSC 127601; 15, outer-lateral view of Pb element, GSC 127602. 16–18, Ozarkodina sp. (⫻70). From SIB79-33, Maut Formation; inner-lateral, upper and inner-lateral-upper views of Pa element, GSC 127603. 19–24, Ozarkodina cf. O. n. sp. B of Simpson and Talent (1995) (⫻60). 19, 20 from SIB79-35, Maut Formation; 19, posterior view of Sa element, GSC 127604; 20, outer-lateral view of Pa element, GSC 127605; 21–24 from SIB79-47, Maut Formation; 21, outer-lateral view of Pa element, GSC 127606; 22, inner-lateral view of Sc element, GSC 127607; 23, posterior view of M element, GSC 127608; 24, posterior view of Sb element, GSC 127609. 25–28, Polygnathoides aff. P. siluricus (⫻50). 25, 27, 28 from SIB79-8, Maut Formation; 25, upper view of Pa element, GSC 127610; 27, posterior view of Sb element, GSC 127611; 28, inner-lateral view of Pb element, GSC 127612; 26 from SIB79-7, Maut Formation; upper view of Pa element, GSC 127613. 29, Pseudooneotodus beckmanni (⫻90). From SIB79-47, Maut Formation; upper view, GSC 127614. 30–35, Pterospathodus amorphognathoides amorphognathoides (⫻60). 30 from IB79-7, Maut Formation; outer-lateral view of Pb1 element, GSC 127615; 31–35 from SIB79-48, Maut Formation; 31, upper view of Pa element, GSC 127616; 32, outer lateral view of asymmetric carnuliform element, GSC 127617; 33, symmetric carnuliform element, GSC 127618; 34, outer-lateral view of Pb2 element, GSC 127619; 35, upper view of a fragment of Pa element, GSC 127620. 36–39, Pterospathodus pennatus procerus (⫻60). From SIB79-12, Sandugan Formation; 36, upper view of Pa element, GSC 127621; 37, outer lateral view of Pb1 element, GSC 127622; 38, outer-lateral view of Pb2 element, GSC 127623; 39, outer lateral view of carnuliform element, GSC 127624. 40–43, ichthyoliths (⫻90). From SIB 79-10, Maut Formation; upper views, GSC 127625, 127626, 127627, 127628.

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planus (WALLISER, 1964). BISCHOFF, 1986, p. 80–84, pl. 19, figs. 39– 41; pl. 20, figs. 1–7, 17–44; pl. 21, figs. 1–12.

Types.—Figured specimens, GSC 127575–127576. Material examined.—M element—7; Sc element—2. Occurrence.—Maut Formation, Ina River section. Discussion.—The apparatus of Ozarkodina plana (Walliser) reconstructed by Sweet and Scho¨nlaub (1975) has a carminate Pa element (⫽Pb element of original reconstruction); however, the apparatus of Oulodus planus planus reconstructed by Bischoff (1986) has a modified digyrate Pa element. There is no evidence from the Russian material to support either of the reconstructions, because of the limited material. The species assigned to Oulodus cf. O. planus in this study is largely based on the M element, which is similar to the M element of both reconstructions. OULODUS sp. 1 Figure 10.1–10.4 Types.—Figured specimens, GSC 127588–127591. Material examined.—Pb (Pa?) element—1; M element—2; Sb element—1; Sc element—1. Occurrence.—Maut Formation, Mirny Creek section. Discussion.—There is insufficient material to reconstruct the apparatus because only Pb (Pa?), M, Sb, and Sc are found in the Russian collection. All these elements have compressed denticulation; the profile of the Pb (Pa?) element is similar that of Rexroadus kentuckyensis (Zhang and Barnes, 2002, fig. 10.42, 10.47). The outline of the M element resembles that of Aspelundia fluegeli (Walliser, 1964) and A. expansa n. sp. of Armstrong (1990, pl. 3, figs. 3, 15) and Oulodus? expansus (Armstrong, 1990) of Zhang and Barnes (2002, fig. 10.10). OULODUS sp. 2 Figure 10.5–10.8 Types.—Figured specimens, GSC 127592–127595. Material examined.—Pa element—1; Pb element—1; Sb element—3; Sc element—5. Occurrence.—Maut Formation, Mirny Creek section. Discussion.—The specimens that are assigned to this species are different from Oulodus sp. 1, in having robust, discrete peglike denticulation. The Pa element is similar to that of some Ordovician Oulodus species, such as Oulodus oregonia (Branson, Mehl and Branson, 1951), O. robustus (Branson, Mehl and Branson, 1951), and Oulodus subundulatus (Sweet, Turco, Warner and Wilkie, 1959) (Sweet and Scho¨nlaub, 1975, pl. 2, figs. 2, 8, 14) in having one of the lateral processes bent downward and bowed posteriorly. Genus OZARKODINA Branson and Mehl, 1933 Type species.—Ozarkodina typica Branson and Mehl, 1933 (⫽junior synonym of Hindeodella confluens Branson and Mehl, 1933). OZARKODINA cf. O. MASURENENSIS Bischoff, 1986 Figure 10.9–10.15 cf. Ozarkodina masurenensis BISCHOFF, 1986, p. 141–144, pl. 22, figs. 22–40; pl. 23, figs. 1–20.

Types.—Figured specimens, GSC 127596–127602. Material examined.—Pa element—6; Pb element—1; ?M element—1; Sa element—1; Sb element—1; Sc element—2. Occurrence.—Maut Formation, Ina River section. Discussion.—The Pa element is similar to that of O. masurenensis in having 1) carminate form with lower edges of processes forming an angle of 200⬚–210⬚; 2) anterior process bearing long, almost erect denticles, decreasing in size towards cusp; 3) denticles on posterior process posteriorly inclined, highest in central

part of process; 4) longitudinal ridge developing beneath base of denticles on both sides of the element; however, it is different from that of O. masurenensis in lacking a small denticle flanking the cusp on the posterior process. Pb element resembles the type specimens of Pb element of the species. The possible M element is makellate (⫽dolabrate of original description), without preserved denticles, which can be compared with the type specimens of M element of the species. Sb element is different from that of the species in lacking a large denticle that is almost same size as cusp, or even larger on one of the lateral processes. Sa element is similar to that of O. masurenensis in having a moderately large, triangular basal cavity, but different in having the two processes bent both posteriorly and downwards. OZARKODINA cf. O. n. sp. B Simpson and Talent, 1995 Figure 10.19–10.24 cf. Ozarkodina n. sp. B SIMPSON AND TALENT, 1995, p. 160–161, pl. 10, figs. 19–22.

Types.—Figured specimens, GSC 127604–127609. Material examined.—Pa element—7; M element—1; Sb element—3; Sc element—2. Occurrence.—Maut Formation, Mirny Creek section. Discussion.—The overall shape and denticulation of Pa element resembles the most complete specimen of Ozarkodina n. sp. B from Australia (Simpson and Talent, 1995, pl. 10, fig. 20). An incomplete angulate specimen was illustrated as a possible Pb element of the species (Simpson and Talent, 1995, pl. 10, fig. 22); however, the Pb element was not recovered. The species was established without M and S elements (Simpson and Talent, 1995, p. 160–161). Several specimens found in the same sample (SIB79-47) with the Pa element may represent M, Sb, and Sc elements. M element has one lateral process that bends downwards and slightly posteriorly; cusp is broken, but probably recurved posteriorly; the denticles on the lateral process are closely packed together, but the number of the denticles is unknown, as the process is broken. Sb element is digyrate, with one process bent downwards and another process bent both downwards and posteriorly, forming an angle of about 90⬚, and five to seven denticles on each process, which are slightly anteriorly-posteriorly compressed; the cusp is erect or slightly recurved posteriorly; the basal cavity is triangular in shape and restricted beneath the cusp. Sc element is bipennate, the anterior process bent downwards and slightly stretched anteriorly. OZARKODINA sp. Figure 10.16–10.18 ?Ozarkodina steinhornensis eosteinhornensis (WALLISER, 1964). JEPPSSON, 1989, pl. 2, fig. 1a–1c (only).

Description.—Pa element is carminate; posterior process weakly bowed, and is probably shorter than anterior process based on one incomplete specimen. Upper side of element is composed of a sharp, straight ridge, which becomes gradually lower from anterior to posterior; it is unknown whether the element is denticulate, since the anterior process is broken. Basal cavity is large, widely flared, sub-square in shape, and outer side is slightly wider than inner side; on upper side of basal cavity, there is a narrow offset basal flare on inside, whereas offset basal flare on outside is wider and tends to bifurcate. Types.—Figured specimen, GSC 127603. Material examined.—Pa element—1. Occurrence.—Maut Formation, Mirny Creek section.

ZHANG AND BARNES—LATE ORDOVICIAN–EARLY SILURIAN CONODONTS, NORTHEAST RUSSIA Discussion.—One incomplete specimen represents a Pa element of Ozarkodina species. This specimen has no cusp; a sharp, straight ridge extends along the two processes of the specimen; the basal cavity is wide and forms an offset basal flare on both inner and outer sides. These characters resemble those of one specimen of O. steinhornensis eosteinhornensis illustrated by Jeppsson (1989, pl. 2, fig. 1a–1c) from Klonk, Czech Republic, but the offset basal flare of the Russian specimen does not tend to form a lateral process compared to the Czech specimen. It is questionable whether the specimen illustrated as O. s. eosteinhornensis by Jeppsson (1989, pl. 2, fig. 1a–1c) should be assigned to this species, because while the type specimen illustrated by Walliser (1964, pl. 20, fig. 21) and other types (Walliser, 1964, pl. 20, figs. 7–16, 19, 20, 22–25) have denticles above the basal cavity, none of them show a sharp, straight ridge above the basal cavity. The Russian specimen probably represents a transitional form between O. oldhamensis (Rexroad, 1967), which has a ridge above the basal cavity and denticles near the end of two processes, and the specimen illustrated as O. s. eosteinhornensis by Jeppsson (1989, pl. 2, fig. 1a–1c). Genus PANDERODUS Ethington, 1959 Type species.—Paltodus unicostatus Branson and Mehl, 1933. PANDERODUS RECURVATUS (Rhodes, 1953), emended Zhang and Barnes, 2002 Figure 8.29–8.33 Panderodus recurvatus (RHODES, 1953). ZHANG AND BARNES, 2002, p. 31–32, fig. 16.1–16.27 (includes synonymy to 1995).

Types.—Figured specimens, GSC 127539–127543. Material examined.—tortiform element—4; asymmetrical graciliform—18; acuatiform—3; compressiform—7. Occurrence.—Maut Formation, Mirny Creek section. Discussion.—Zhang and Barnes (2002) recognized that P. recurvatus had the same apparatus plan as P. unicostatus with aequaliform, truncatiform, tortiform, asymmetrical graciliform, subsymmetrical graciliform, acuatiform and compressiform elements. However, among the Russian conodonts, aequaliform, truncatiform, subsymmetrical graciliform and acuatiform elements are not recognized, most likely because of the limited material. Genus POLYGNATHOIDES Branson and Mehl, 1933 Type species.—Polygnathoides siluricus Branson and Mehl, 1933. POLYGNATHOIDES aff. P. SILURICUS Branson and Mehl, 1933 Figure 10.25–10.28 Pa element aff. Polygnathoides siluricus BRANSON AND MEHL, 1933. KLAPPER AND MURPHY, 1975, p. 56, pl. 8, figs. 16–21 (includes synonymy to 1972); UYENO, 1981, p. 47, pl. 9, figs. 22, 23, 26–29 (includes synonymy to 1977). CRAIG, ETHINGTON AND REPETSKI, 1986, pl. 2, fig. 7. Pb element aff. Polygnathoides emarginatus BRANSON AND MEHL. KLAPPER AND MURPHY, 1975, p. 56, pl. 8, figs. 22–25 (includes synonymy to 1972); UYENO, 1990, p. 68, pl. 15, figs. 4, 5, 9 (includes synonymy to 1981). Multielement synonymy aff. ?Polygnathoides siluricus BRANSON AND MEHL, 1933. JEPPSSON, 1983, fig. 1a–e.

Types.—Figured specimens, GSC 127610–127613. Material examined.—Pa element—2; Pb element—1; ?Sb element—1. Occurrence.—Maut Formation, Ina River section. Discussion.—Klapper and Murphy (1975) noted that the form species P. siluricus and P. emarginatus may be the P (⫽Pa) and

509

O (⫽Pb) elements of a single apparatus, because they are commonly similar in morphology. Further, Uyeno (1981, 1990) indicated that the two morphotypes are commonly found associated and may represent the Pa and Pb elements of an apparatus. The Russian material supports these observations; however, the form species ‘‘Neoprioniodus’’ latidentatus Walliser, 1964 illustrated by Uyeno (1981, pl. 9, figs. 18, 19) was considered as the possible M element of the apparatus (Uyeno, 1981, p. 47), but this interpretation is not followed by the present study, because the denticulation of ‘‘N.’’ latidentatus is similar to that of Oulodus sp. 2 (Uyeno, 1981, pl. 9, figs. 14–17, 20, 21) from the same collection. Jeppsson (1983, fig. 1a–e) illustrated the apparatus of P. siluricus with Pa, Pb, M, Sa, and Sc elements, which is questioned by this study, because he provided neither synonymy nor description about the reconstruction. Only two Pa, one Pb and one possible Sb specimens were recovered. One Pa specimen is broken (Fig. 10.25), but resembles the four cotypes originally defined by Branson and Mehl (1933, pl. 3, figs. 39–42). Another Pa specimen (Fig. 10.26) is perfectly preserved and has a node on the outer side next to the central node; this is a variable character of the Pa element that has been found in Austria (Walliser, 1964, pl. 17, fig. 2) and central Nevada, USA (Klapper and Murphy, 1975, pl. 8, fig. 20). The possible Sb element is found with Pa and Pb elements in the same sample (SIB79-8); it has a similar denticulation to that of the Pb element: denticles fused with free tip. The possible Sb element is digyrate having both long and short processes with nine and four denticles, respectively; both processes are bent both posteriorly and downwards; the short process has a large denticle near the extremity of the process. This study suggests that P. siluricus was probably not restricted to the Late Silurian, with the Russian specimens from the upper Llandovery representing the earliest known evolutionary stage. Genus PTEROSPATHODUS Walliser, 1964 Type species.—Pterospathodus amorphognathoides Walliser, 1964. PTEROSPATHODUS AMORPHOGNATHOIDES AMORPHOGNATHOIDES Walliser, 1964 Figure 10.30–10.35 Pterospathodus amorphognathoides amorphognathoides WALLISER, 1964. MA¨NNIK, 1998, p. 1021–1035, pl. 4, figs. 1–20, 22–27; pl. 5, figs. 1–40; text-figs. 11–15 (includes synonymy to 1998).

Types.—Figured specimens, GSC 127615–127620. Material examined.—Pa element—3; Pb1 element—1; Pb2 element—1; symmetric carnuliform element—1; asymmetric carnuliform element—1. Occurrence.—Maut Formation, Mirny Creek and Ina River sections. Discussion.—A new reconstruction for Pterospathodus was made by Ma¨nnik (1998) who combined the genus Pterospathodus Walliser and Carniodus Walliser into Pterospathodus, which includes Pa, Pb1, Pb2, Pc, M1 (⫹ M2 in P. pennatus procerus), Sc1, Sc2, Sc3, Sb1, Sb2, Sa, carnuliform with five morphs, carniciform, and curved element with three morphs; this interpretation is followed in this study. Among the Russian collection, P. amorphognathoides amorphognathoides is not represented by a full apparatus as reconstructed by Ma¨nnik (1998), because of the limited specimens. The Pa element of P. a. amorphognathoides recognized herein is questionable; however, the Pb1 and Pb2 elements confirm the occurrence of the species, and they resemble those of Population 1 of Ma¨nnik (1998, pl. 4, figs. 7, 9).

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Genus REXROADUS Zhang and Barnes, 2002 Type species.—Oulodus? nathani McCracken and Barnes, 1981. REXROADUS cf. R. KENTUCKYENSIS (Branson and Branson, 1947) Figure 9.35–9.40 cf. Rexroadus cf. R. kentuckyensis (Branson and Branson, 1947). ZHANG AND BARNES, 2002, p. 35–36, fig. 10.37–10.48 (includes synonymy to 1988).

Types.—Figured specimens, GSC 127582–127587. Material examined.—Pa element—2; Pb element—2; M element—3; Sa element—1; Sb element—2; Sc element—5. Occurrence.—Maut Formation, Mirny Creek section. Discussion.—Except for the Pa element, all other elements are similar to those of Oulodus? kentuckyensis recognized by McCracken and Barnes (1981, pl. 6, figs. 1–18) and R. kentuckyensis by Zhang and Barnes (2002, fig. 10.37–10.40, 10.42–10.44, 10.46–10.48). However, the Pa element is different from that of the apparatus illustrated by McCracken and Barnes (1981, pl. 6, figs. 19, 20) and Zhang and Barnes (2002, fig. 10.41, 10.45) in lacking partially fused denticles on the straight anterior process. ACKNOWLEDGMENTS

Financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) is gratefully acknowledged. T. N. Koren, M. M. Oradovskaya and R. F. Sobolevskaya kindly provided guidance in the field to collect samples. In particular, we acknowledge the help of T. N. Koren in reviewing an early draft of the manuscript and discussing the more recent Russian research in the region. We also appreciate the helpful comments of the reviewers of the manuscript: G. S. Nowlan and D. J. Over. REFERENCES

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SWEET, W. C., AND H. P. SCHO¨NLAUB. 1975. Conodonts of the Genus Oulodus Branson and Mehl, 1933. Geologica et Palaeontologica, 9:41– 59. UYENO, T. T. 1981. Stratigraphy and conodonts of Upper Silurian and Lower Devonian rocks in the environs of the Boothia Uplift, Canadian Arctic Archipelago, Pt. II, Systematic study of conodonts. Geological Survey of Canada Bulletin, 292:39–75. UYENO, T. T., AND C. R. BARNES. 1983. Conodonts of the Jupiter and Chicotte Formations (Lower Silurian), Anticosti Island, Que´bec. Geological Survey of Canada, Bulletin 355, 49 p. UYENO, T. T. 1990. Biostratigraphy and conodont faunas of Upper Ordovician through Middle Devonian rocks, eastern Arctic Archipelago. Geological Survey of Canada Bulletin, 401, 211 p. UYENO, T. T., AND C. R. BARNES. 1983. Conodonts of the Jupiter and Chicotte Formations (Lower Silurian), Anticosti Island, Que´bec. Geological Survey of Canada, Bulletin 355, 49 p. VIIRA, V. 1974. Ordovician conodonts of the Baltic Region. Tallin, USSR, 142 p. (In Russian) WALLISER, O. H. 1957. Conodonten aus dem oberen Gotlandium Deutschlands und der Karnischen Alpen. Notizblatt des Hessischen Landesamtes fu¨r Bodenforschung zu Wiesbaden, 85:28–52. WALLISER, O. H. 1964. Conodonten des Silurs. Abhandlungen des Hessische Landesamtes fu¨r Bodenforschung, 41, 106 p. ZASLAVSKAYA, N. M. 1979. Upper Ordovician and Early Silurian chitinozoan assemblages from the Pacific paleobiogeographic province in Siberia and neighbouring regions, p. 90. In M. M. Oradovskaya and R. F. Sobolevskaya (eds.), Guidebook to Field Excursion to the Omulev Mountains. USSR Pacific Science Congress XIV. ZHANG, S., AND C. R. BARNES. 2002. A new Llandovery (Early Silurian) conodont biozonation and conodonts from the Becscie, Merrimack and Gun River Formations, Anticosti Island, Que´bec. Paleontological Society Memoir, 57 (Journal of Paleontology, Volume 76, no. 2, supp.), 46 p. ACCEPTED 27 JANUARY 2006 APPENDIX 1 Late Ordovician conodont distribution at Ina River and Mirny Creek. APPENDIX 2 Early Silurian conodont distribution at Ina River and Mirny Creek.

APPENDIX 1—Ordovician taxa.

Section & Sample Species Amorphognathus ordovicicus Pa Pb M Sa Sb-Sc Aphelognathus sp. Pa ? Belodina cf. B. confluens compressiform el. grandiform el. eobelodiniform el. Belodina compressa compressiform el. grandiform el. eobelodiniform el. Drepanodus santacrucensis ne ke Drepanoistodus suberectus oistodiform (M) suberectiform (Sb) homocurvatilorm (Pa?) curvatiform (Sc?) drepanoistodiform element Eocarniodus gracilis M Sa Sb1 (unbowed) Sb2 (bowed) Sc Gamachignathus ensifer modified prioniodiform (Pa) Pb Hamarodus europaeus oistodiform (M) cordylodiform Sb-Sc) hibbardelliform (Sa) Pa Pb? Icriodella cf. l. prominens Pa Sa Sb Panderodus unicostatus aequaliform truncatiform tortiform asymmetrical graciliform subsymmetrical graciliform acuatiform compressiform Paroistodus? mutatus oistodiform acodontiform disacodontiform Periodon mirnyensis Pa Pb M Sa Sb Sc Protopanderodus insculptus a/b (asymmetrical) adenticulate c denticulate c e

Ina River

Northwest Bank, Mirny Creek

SIB-79–1 SIB-79–3 SIB-79–4 SIB-79–5

SIB-79–14 SIB-79–15 SIB-79–17 SlB-79–18 SIB-79–20 SIB-79–22

2 3 2

1

1 1 1

1

1? 1

3 5 1

2 1 1 1 1 1

1 1

2 4 2

2

1

1 1 1 8

1 2 3

2

1 1

3

10 2 1

1

1 1 11 1 2 4

5 2 5 2 3 4

2

1 2 2

1

APPENDIX 1—Extended. Northwest Bank, Mirny Creek SIB-79–24

SIB-79–26

SIB-79–27

SIB-79–28

Southeast Bank, Mirny Creek SIB-79–29

SIB-79–37

SIB-79–38

1

SIB-79–39

SIB-79–40

10 1

SIB-79–41

SIB-79–42

1?

1 4 1 1 1?

3 5 2

1 2

2

2 2

1?

1 1

1 5 5 7 8 8 2 1 36 23 8 7 7 1 1 1

1

1 1 4

1 3

4

6

1

1

1

4 2

1 1 1 1 3

4 3 2

5 1 1

2

APPENDIX 1—Continued.

Section & Sample Species Protopanderodus liripipus a/b (asymmetrical) c (symmetrical) e (scandontiform) Scabbardella altipes M dreponodiform Sa distacodiform Sb-Sc acodiform Walliserodus curvatus unicostatiform curvatiform deboltiform dyscritiform multicostatiform Unknown

Ina River


SIB-79–1 SIB-79–3 SIB-79–4 SIB-79–5

SIB-79–14 SIB-79–15 SIB-79–17 SlB-79–18 SIB-79–20 SIB-79–22 8 6 4

1

?1 2 2

1

1

3

APPENDIX 1—Continued, Extended. Northwest Bank, Mirny Creek SIB-79–24

SIB-79–26

SIB-79–27

SIB-79–28

Southeast Bank, Mirny Creek SIB-79–29

SIB-79–37

SIB-79–38

2 3 1

1 2

SIB-79–39

SIB-79–40

1 4 4 7

?1

2?

2 2

1 1

4

4

2

SIB-79–41

SIB-79–42

5

Belodella sp. M Dapsilodus obliquicostatus M Sa Sb-Sc Decoriconus fragilis drepanodontiform acontiodontiform paltodontiform Distomodus kentuckyensis Pa Pb M Sa Sb-Sc Distomodus staurognathoides Pa Pb M Sb-Sc ?Johnognathus sp. Kockelella ? manitoulinensis Pa Pb M Sb So Oulodus panuarensis Pa Pb M Sa Sb So Oulodus cf. O. planus M Sc Oulodus sp. 1 Pa Pb M Sb Sc Oulodus sp. 2 Pa Pb Sb Sc

Section & Sample Species

APPENDIX 2—Silurian taxa. Ina River


Southeast Bank, Mirny Creek

1

1

1

1

2 1

1

1

3 1

1 1

2?

3 1

1 1 1

1 1 1 ?1 1

1

3

5 1 2 1 2

3 1 5

1 1 3

7 4 8

1 1 3

1

1

1

1

1 3

1

1

1

1 2

1 4

1 1 2 4

1 1 1

1

1 1

1

1

1

SIB-79–7 SIB-79–8 SIB-79–9 SIB-79–10 SIB-79–11 SIB-79–12 SIB-79–46 SIB-79–47 SIB-79–48 SlB-79–49 SIB-79–32 SIB-79–33 SlB-79–34 SIB-79–35 SIB-79–36

6

Ina River



1 1 1

3 1 3 2

2

1? 1 1 2

2

?1 ?1

1

1 1 1 1

2

3

1

2 10 1 3 5

1 2

3

5 4

1 7

1

2 2

6

2 1

5

4 6

1

8

2 8

3

3 2

7 1

4 1

2 5

1 1 1 7

3

3 1

2 2

5

1

1


Ozarkodina cf. O. n.sp. B Simpson & Talent Pa M Sa Sb Sc Ozarkodina sp. Pa Panderodus recurvatus aequaliform truncatiform tortiform asymmetrical graciliform subsymmetrical graciliform acuatiform compressiform Panderodus unicostatus aequaliform truncatiform tortiform asymmetrical graciliform 6 subsymmetrical graciliform acuatiform compressiform Polygnathoides aff. P. siluricus Pa 1 Pb Sb? Pseudooneotodus beckmanni Pterospathodus pennatus procerus Pa Pb1 ?Pb2 ? carnuliform Pterospathodus amorphognathoides Pa Pb1 1 Pb2 carnutiform

Ozarkodina cf. O. masurenensis Pa Pb M Sa Sb Sc


APPENDIX 2—Continued.

7

Rexroadus cf. R. kentuckyensis Pa Pb M Sa Sb Sc Walliserodus curvatus unicostatiform curvatiform deboltiform dyscritiform multicostatiform homocurvatiform element unknown ichthyolith


APPENDIX 2—Continued. Ina River



1 10

1

1 7 1 2

5 4 3 2

2 2 3 1 2 5

1

1 1

3 1

2 1 2 1

3 1? 1 2

1
