Palaeobiogeography and palaeogeographical implications of Permian marine bivalve faunas in Northeast Asia (Kolyma–Omolon and Verkhoyansk–Okhotsk regions, northeastern Russia)

Palaeogeography, Palaeoclimatology, Palaeoecology 298 (2010) 42–53

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Palaeobiogeography and palaeogeographical implications of Permian marine bivalve faunas in Northeast Asia (Kolyma–Omolon and Verkhoyansk–Okhotsk regions, northeastern Russia) Alexander S. Biakov a,⁎, G.R. Shi b a b

North-East Interdisciplinary Science Research Institute of the Russian Academy of Sciences, Far East Branch, 16 Portovaya, Magadan 685000, Russia School of Life and Environment Sciences, Deakin University, Melbourne Campus at Burwood, 221 Burwood Highway, Burwood, Victoria 3125, Australia

a r t i c l e

i n f o

Article history: Received 29 October 2009 Received in revised form 5 April 2010 Accepted 14 April 2010 Available online 24 April 2010 Keywords: Permian Bivalve faunas Kolyma–Omolon province Verkhoyan–Okhotsk province Palaeobiogeography Northeast Asia

a b s t r a c t The paper considers the biogeography and palaeogeographic implications of the Permian marine bivalve faunas of Northeast Asia, with a focus on the dynamic relationships between biotic similarities and palaeogeographic distance through an interval of ca. 50 million years. A stage-by-stage time series analysis of the biotic similarities between two previously recognized biochores in Northeast Asia, the Kolyma–Omolon and Verkhoyan–Okhotsk provinces, has been carried out using both the Jaccard and Dice similarity indices based on the spatio-temporal distributions of 355 Permian marine bivalve species in Northeast Asia. The outcome of this analysis, combined with other empirical data and previously published tectonic, sedimentological and palaeontological information, suggests that (1) the bivalve faunas from these two provinces were distinctive from one another as two separate biochores throughout all but the earliest (Asselian) Permian stages and (2) the biotic similarities between the Verkhoyan–Okhotsk and Kolyma– Omolon provinces remained consistently low since Sakmarian, all falling well below the minimum threshold of the Jaccard index of 0.42 required for distinguishing marine biotic provinces. We interpret these belowthreshold Jaccard biotic similarities as an indication of significant palaeogeographic separation between the Verkhoyan-Okhotsk and Kolyma–Omolon provinces, which is in turn considered to indicate rifting and seafloor spreading of the Omolon microcontinent and associated terranes and island arcs away from the North Asian craton, at least from the Sakmarian to the beginning of the Late Permian. Palaeo-distance separation appears to be the primary and most significant biogeographic determinant in accounting for the differences in the spatial distribution of most Permian bivalve species in Northeast Asia. Several other variables also appear to have played a significant role, including regional climate conditions, ocean currents and merged island chains as geographic barriers. In particular, the relatively high biotic similarity between the Verkhoyan–Okhotsk and Kolyma–Omolon provinces during the Late Wuchiapingian and Changhsingian may have been related to the shallowing of the deep-water basins (Oimyakon, AyanYuryakh, Balygychan and Sugoi basins) that had previously separated the two provinces and the flooding (submergence) of the Okhotsk–Taigonos volcanic arc system, thus allowing the invasion of lower latitude warm-water Palaeotethyan and even Gondwanan species into Northeast Asia. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Biogeographic analysis of fossil biotas offers an important tool to evaluate the degree of palaeogeographic separation of tectonic blocks in geological time (e.g. Fallaw, 1983; Blodgett et al., 2002). However, until recently this technique has not been applied to the Late Palaeozoic marine faunas of Northeast Asia, despite the region's complex palaeogeography, tectonism and regional high diversity of marine faunas. In this paper, we attempt to reconstruct the regional

⁎ Corresponding author. Fax: + 7 4132630051. E-mail addresses: [email protected] (A.S. Biakov), [email protected] (G.R. Shi). 0031-0182/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2010.04.019

Permian marine palaeobiogeography and palaeogeography of Northeast Asia based on a detailed quantitative analysis of faunal similarities (mainly marine bivalve faunas) among various tectonic units in this vast region through different stages of the Permian. The quantitative biogeographic analysis was carried out using two binary similarity indices (Jaccard index and Dice index) in the hope that temporally varied biotic similarities between two adjacent palaeobiotic provinces (Verkhoyan–Okhtosk and Kolyma–Omolon provinces) in Northeast Asia through the Permian would shed some lights on the dynamics of palaeogeographic separation between the two provinces and the changing regional palaeogeographic configuration in Northeast Pangea over an interval of some 50 million years before the close of the Palaeozoic Era. The working scenario we took in

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Fig. 1. Modern tectonic map showing major tectonic elements in Northeast Asia (modified from Shpikerman, 1998; Biakov, 2006).

this approach is that under normal biogeographic conditions the “law of geographic or distance decay” would prevail as a key biogeographic determinant (Soininen et al., 2007). This means that overall similarities between two biotic provinces would decrease with their mutual increasing geographic separation, all other factors being equal. It is thus conceivable that biotic similarities through geologic time between areas or biotic provinces, to some extent at least, would reflect the changing regional palaeogeography and palaeoenvironmental conditions. In particular, for this paper we are interested in finding out how far the Verkhoyan–Okhotsk biogeographic province was palaeogeographically separated from the Kolyma–Omolon biogeographic province, and how this spatial configuration between the two biotic provinces evolved through the Permian. Northeast Asia, as used in this paper as a geologic entity, refers to the territory in northeastern Russia, comprising from west to east: the Verkhoyansk fold belt, the Okhotsk microcontinent and the Omolon microcontinent, as well as a number of other smaller-scale fold belts and tectonostratigraphic terranes (Fig. 1). To the west of this region lies Siberia, which occupies much of central Russia and is geologically characterized by the North Asian (or Siberian) craton. The Verkhoyansk fold belt may be further divided into the Northern Verkhoyansk, Western Verkhoyansk, and Southern Verkhoyansk sectors (Fig. 1). Both the Okhotsk and Omolon microcontinents are surrounded by other tectonic elements interpreted either as fold belts framing these microcontinents, or as terranes of various geodynamic nature and histories (e.g., continental margin basins, island-arcs, accretionary prisms, and deep-ocean basins) (Fig. 1).

To date, several geodynamic models, based on various tectonic ideas, have been developed for the Northeast Asian region (Bogdanov and Til'man, 1992; Zonenshain et al., 1990; Parfenov, 1984; Parfeonov et al., 1993; Parfeonov, 1995; Parfenov and Kuz'min, 2001; Nokleberg et al., 1994; Chekhov, 2000). In most of these works, models of tectonic evolution and palaeogeographic reconstruction for the Permian are mainly based on generalized materials of regional geological studies; few has been able to use reliable palaeomagnetic data because of severe structural deformation, tectonic overprinting and remagnetization. On the other hand, Permian marine faunas are widespread in Northeast Asia although there exist significant local differences in species composition, diversity and stratigraphic distributions. These faunas, especially bivalves and brachiopods, have been long and well studied taxonomically and well integrated biostratigraphically and calibrated in recent years with the Permian international chronostratigraphical timescale (Fig. 2). Therefore, naturally the biogeography of these Permian marine faunas would provide important insights into how the various tectonic units were dispersed in space during the Permian and how they interacted with one another in a changing and evolving regional palaeogeography through the Permian. Likewise, aspects of Permian marine biogeography of Northeast Asia have been considered in several previous publications (e.g., Ustritsky, 1970; Ganelin and Kotlyar, 1984; Grunt, 1995). Of significant note among these earlier studies is the recognition by (Ganelin and Kotlyar, 1984) of two distinctive Permian marine biogeographic provinces in Northeast Asia based on brachiopod

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faunas. In their view, the former province occupied the passive continental margins on the eastern (present-day orientation) side of the North Asian craton (Fig. 1) and as such its Permian bivalve faunas are characterized mostly by shallow-water marine communities. The Omolon–Kolyma province, on the other hand, was much more dispersed in space and was represented, during the Permian, by both shallow- and deep-water bivalve communities which inhabited a wide range of geological structures and depositional environments including shelf waters surrounding micro-continents (or massifs), continental slopes, ocean basins, trenches and volcanic arcs (Biakov, 2006). It should be noted, however, all of these earlier studies concerning the Permian marine biogeography of Northeast Asia suffered from lacking a comprehensive up-to-date taxonomic database at the species level in an updated and refined Permian biostratigraphic framework. Moreover, none of the previous works attempted to relate biogeographic similarities between provinces and areas (tectonic units) with palaeogeographic distance in a quantitative sense. In recent years, extensive new fossil collections have been made and documented (e.g., Biakov, 2006, 2008), significantly adding to the space–time distribution database of Permian bivalve faunas in Northeast Asia. With this now much-expanded new taxonomic database and a refined Permian biostratigraphy (Fig. 2), we consider it timely now to conduct a detailed quantitative biogeographic analysis of these bivalve faunas through the Permian. Some of these materials have been briefly referred to by the senior author in several recent notes (Biakov, 2003, 2007b; Biakov et al., 2007) as pilot studies to test various geodynamic models, but none of these had the details and extent of data coverage, synthesis and interpretations offered in this study.

2. Material and methods Permian marine successions containing abundant and diverse marine invertebrate faunas are common in Northeast Asia, in particular within the Verkhoyansk fold belt, the Okhotsk microcontinent and its flanking fold belts, Ayan-Yuryakh anticlinorium, Prikolyma terrane, Omulevka terrane, Balygychan terrane, Omolon microcontinent and its framing fold belts including the Gizhiga fold zone, Oloi-Yarakvaam terrane and the Taigonos fold zone (Fig. 1). Throughout these tectonic units, the Permian successions are represented by a wide range of sedimentary facies, thickness and faunal assemblages suggesting the existence of a diverse range of geodynamic conditions, geographic separation and local depositional environments (Biakov et al., 2005). For the purpose of this study, we have constructed, for the first time, a detailed and most comprehensive ‘space–time’ distribution database of all known Permian bivalve species in Northeast Asia (Appendix A), based on both the senior author's own collections and also information obtained from the literature and senior author's own studies of other colleagues' collections kept in various museums in Russia. This database comprises the stratigraphic and temporal occurrences of some 355 bivalve species from seventeen bivalve zones throughout the Permian (Fig. 2, Appendix A). In the database, the species noted with ‘aff.’ were considered by us as new species, and those identified with ‘ex gr.’ were treated as the same species, and all species were treated equally regardless their mode of occurrence (rare or common). Although the database is the most updated, detailed and hence the best currently available, it has several limitations apart from the intrinsic nature of the incompleteness associated with the fossil record. First, the stratigraphic intervals (time bins) sampled and compared (see Table 1) are uneven in duration, necessitated by the uneven nature in the

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Table 1 Bivalve species diversity data, Jaccard index and Dice index values for Northeast Asia through different stages of the Permian. Sko refers to the number of bivalve species for the Kolyma–Omolon province, Svo the number of species for the Verkhoyan–Okhotsk province, and C the number of species shared between the Kolyma–Omolon and Verkhoyan–Okhotsk provinces. Age

Total species richness

Sko Svo C

Jaccard index

Asselian Sakmarian Late Artinsk–Early Kung Middle Kungrian Early Roadian Late Roadian Early Wordian Late Wordian Early Capitanian Late Capitanian Early Wuchiap Late Wuchiap Changhsingian

53 41 32

52 37 24

23 18 17

22 0.42 14 0.34 10 0.32

0.59 0.51 0.49

72 42 43 25 51 23 20 14 40 43

41 26 26 13 19 18 10 3 18 22

51 31 30 20 41 12 14 14 37 34

20 14 14 8 9 7 4 3 15 15

0.43 0.49 0.50 0.48 0.30 0.47 0.33 0.35 0.55 0.54

0.28 0.33 0.33 0.32 0.18 0.30 0.20 0.21 0.38 0.37

Dice index

stratigraphic distribution of fossils. Second, these intervals have not been sampled equally in terms of sampling intensity and rock volume as both sampling properties are both invariably constrained by access and quality of outcrop sections. This inevitably has led to the reality that some intervals were better sampled than others — a point that we have considered very carefully when interpreting the successional changes in the biotic similarities. Third, although the samples from the Kolyma– Omolon and Verkhoyan regions were obtained from broadly comparable depositional environments (mostly shallow-water open shelf settings), it has not been possible to detail the exact depositional environment for specific collections because information of this type was not provided in the original, often systematics-oriented, literature; nor has it been possible to obtain the quantity and proportion of collections from different depositional settings. The Permian bivalve biostratigraphy of Northeast Asia and its regional and global correlations have been discussed in detail in a number of recent papers (e.g., Ganelin and Biakov, 2006; Klets et al., 2006; Biakov, 2008, 2009). The correlation of the bivalve-based biozones to the international Permian chronostratigraphic timescale of Jin et al. (1997) follows that of Biakov (2006), Biakov (2009) and Ganelin and Biakov (2006). It should be noted that no Permian conodonts are known from the study area. However, the correlation of the bivalve zones to the internationally recognized Permian stages is supported by associated Permian ammonoid faunas Kutygin (2006), and also through significant links to Permian mid-latitude transitional biotas of East Asia (Russian Far East, Mongolia, Japan and northeast China), where both fusulinoid and conodont faunas are present (see discussion in Shi, 2006). The geologic units initially chosen for constructing the database include most of the well-defined geological structures illustrated in Fig. 1. Then, depending on their biogeographic affinities, these geologic units are assigned to either of the two biogeographic provinces as originally recognized by Ganelin and Kotlyar (1984): the Verkhoyan-Okhotsk province and the Kolyma-Omolon province. The quantitative analysis of biogeographic affinities between the two biotic provinces was conducted using two popular biogeographic similarity coefficients, the Jaccard index (Jaccard, 1901) and the Dice index (Dice, 1945), mathematically determined as follows: Ij = C = ½ðS1 + S2 Þ–C ðJaccard indexÞ Id = 2C = ðS1 + S2 ÞðDice indexÞ

Fig. 2. Choronostratigraphy of Permian bivalve zones of Northeast Asia (from Biakov, 2008). Numeric numbers in the column next the bivalve zones correspond to the numbers for bivalve zones specified in Appendix A.

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Where S1 and S2 are respectively the number of species present in each of the two faunas (areas) compared, C the number of species shared between the two areas in question. As reviewed and evaluated by Shi (1993) and shown by many previous studies (e.g., Shen and Shi, 2004; Krasnov et al., 2009), the Jaccard index is the most robust and informative among a suite of many other similar metrics for measuring biogeographic similarities using binary (presence/absence) distribution data of taxa between areas through time. Nevertheless, like any other binary similarity measure used in a palaeontogical context, the Jaccard index is also sensitive to sample size and quality of samples (i.e. sampling efficiency). Therefore, it is necessary to check the robustness of the Jaccard values and their consistency with similarly calculated similarity values based on a different index. For this study, we used the Dice index as a cross-check against the Jaccard values for consistency and robustness. The Dice index was also considered by Shi (1993) to be one of the most reliable binary similarity measures next to the Jaccard index. As shown in Fig. 3, the values of these two indices are highly correlated to each other for every time interval, as shown by the overall regression strength indicator (R² = 0.9972). In this study, we have followed a simplistic approach in interpreting the geological significance of the Jaccard index-based biotic similarities, in that the Jaccard values are considered directly but inversely correlated to palaeogeographic distance; that is, the larger a Jaccard value between two palaeo-areas, the greater the palaeo-distance between them at a given geological time. This simplistic approach has roots in many of the modern-day biogeographic studies (where it is generally known as the ‘distance decay’ effect, e.g. Nekola and White, 1999; Nakaoka et al., 2006; Soininen et al., 2007), as well as in some palaeobiogeographical examples (e.g., Flessa et al., 1979; Fallaw, 1983). On the other hand, we recognize that biotic similarities between areas are also frequently influenced by many other critical factors other than distance. For example, in the marine realm ocean currents are known to play a critical and, in some cases, a primary role in causing significant fluctuations of biotic similarities between two areas over time without necessarily changing their mutual geographic distance. Although it is extremely difficult to quantify the influence of these other non-distance related factors on biotic similarities, for the purpose of this study we have given due consideration to these other factors in a qualitative sense, especially in regards to the possible role of ocean currents and the spatial distribution of terranes in northeast Asia during the Permian. Theoretically, it would have been desirable to test the statistical significance of the Jaccard (or Dice) values across consecutive stratigraphic intervals (time bins). Heltshe (1988) developed such a statistical procedure (known as the jack-knife approach) for the Jaccard index. However, Heltshe's technique assumes a quadrat sampling approach along an environmental continuum and either

equal quadrat size or sample size, and treats species differently depending on their frequency of distribution (i.e. unique and rare occurrences are differentiated). None of these assumptions were met by our data. This is because, as already indicated above, nether the quadrats (i.e. the tectonic blocks contained in each province, see Appendix A) or the two samples (i.e. the two provinces compared in our study) are equal in size, and all species in our database were treated equally regardless their frequency of distribution within each sample (province). Consequently, it was not feasible to test the statistical significance of the Jaccard values in the present study. Another related aspect that requires some comments concerns the equation of Jaccard-based biotic similarities to the recognition of biotic provinces. According to Jaccard (1912), if the degree of Jaccard similarity between two areas or biochores (provinces) is less than 0.50, these biochores could be considered biogeographically significantly different and a separation between them at the province level would be warranted. It should be noted that various threshold values of the Jaccard index have been used in differentiating and recognizing biochores. For example, Skarlato (1981), in his study of the presentday marine bivalve biogeography from middle latitudes of the western Pacific Ocean, considered that a Jaccard index value less than 0.50 would be required to differentiate bivalve-based marine provinces for the middle latitude western Pacific region. However, Semyonov (1982), on the basis of various groups of living marine invertebrates on the South American shelf waters, demonstrated that a Jaccard index value ranging from 0.42 to 0.58 was statistically sufficient for him to discriminate and characterize shallow-water marine biogeographic provinces. It is thus clear that there is no universally accepted values of the Jaccard index for differentiating provinces, as this may vary from region to region (thus environmentally controlled) and from taxon to taxon (hence taxonomically controlled). In this study, we have tentatively followed Semyonov (1982) in using 0.42 of the Jaccard index as the minimum value required to separate two distinctive biotic provinces. The various limitations intrinsic in our database and the constraints attached to the quantitative techniques we have used means that the successional changes of the Jaccard values need to be explained with caution in a palaeobiogeographical and palaeogeographical context. Thus, in our interpretations below we have not entirely relied on the Jaccard similarities and, where possible, we have also drawn supporting evidence from other empirical data as well as previously published tectonic, sedimentological and palaeontological information. 3. Biogeographic analysis and interpretations Our quantitative analysis of the biogeographic affinities between the Verkhoyan–Okhotsk and Kolyma–Omolon provinces was conducted using the finest possible biostratigraphic scale currently available (Fig. 2). The successional quantitative biogeographic similarities between the two provinces, expressed by Jaccard and Dice index values, are shown in Table 1 and plotted in Fig. 3. The overall species diversity changes of the two provinces as a whole through the Permian is shown in Fig. 4. A succession of four regional palaeogeographic reconstruction maps representing four different Permian time slices are given in Fig. 5, and the rational for these reconstructions in the context of Permian bivalve biogeography is provided below. 3.1. Asselian (Prothyris elongatus and Euchondria? mira Zones)

Fig. 3. A scatter plot showing the strong correlation between the Jaccard index and Dice index values of biotic similarities between the Verkhoyan–Okhotsk and Kolyma– Omolon provinces through the Permian (similarity values taken from Table 1).

The most diverse and best studied bivalve assemblages of the Asselian are known from the Omolon microcontinent (including its submerged south-eastern part). Bivalve faunas of this age are also present in the Southern and Western Verkhoyansk sectors, but there, they are less diverse and remain poorly studied. Generally, it can be

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to, Parallelodon bimodoliratus Dickins, Schizodus fitzroyensis Dickins, and Myophossa subarbitrata Dickins (see also list in Appendix A). Additionally, the Asselian bivalve assemblages from the Omolon microcontinent also contain some limited North American species such as Permophorus oblongus (Meek and Hayden) and Myalina permiana (Meek and Hayden). These affinities may suggest some biogeographic connections between Northeast Asia with northwestern Pangea in the Asselian, a biotic connection that appears to have also existed during the Middle and Late Carboniferous according to ammonoids (Bogoslovskaya, 1997). Of particular interest among the Asselian bivalve assemblages from Northeast Asia is the presence of some thermophilic forms (e.g. Pteronites) that are commonly present in the Palaeotethys and European part of Russia (including the Urals).

Fig. 4. A histogram showing the variation of the total bivalve species diversity of Northeast Asia (i.e. combining the Verkhoyan–Okhotsk and Kolyma–Omolon provinces) through the Permian.

concluded that Asselian bivalve faunas from both the Verkhoyan– Okhotsk and Kolyma–Omolon provinces, based on the taxonomic data currently available, were quite similar to each other with a relatively high Jaccard value of 0.42. We interpret this relative high biogeographic similarity as possibly suggesting a weak or narrow geographic separation between these two provinces, as depicted in Fig 5A. This observation is consistent with an earlier suggestion made by Ustritsky (1970), who noted that there was an overall weak geographical isolation among all the Asselian–Sakmarian marine faunas of the East Boreal basins in Northeast Asia. Ustritskiy's suggestion was later also confirmed by the study of Shi and Archbold (1993) based on a comparative biogeographic study of the brachiopods from Northeast Asia with coeval faunas from other regions of the circum-Pacific rim, and also by Astafyeva and Astafyeva-Urbajtis (1995) based on their study of bivalve faunas in Northeast Asia. Consequently, for the Asselian it is logical to consider the presence of a single biogeographic province occupying Northeast Asia, and here we adopt the name, Verkolyman province, first proposed by Shi and Archbold (1995); see also Grunt and Shi (1997), to represent the biochore in Northeast Asia for this time slice. It should be noted that practically all known bivalves of this time in the Verkolyman province are represented almost exclusively by shallow-water communities (Biakov, 2006). This would imply that during the Asselian there were no significant deep-water barriers to the migration and interchange of shallow-water-adapted bivalve species between the Verkhoyansk and Omolon regions. Theoretically, if all species are treated equally regardless their frequency of distribution (i.e. no differentiation between unique, rare and common species) the true biogeographic similarity between the two basins for the Asselian might have been much higher than the calculated Jaccard value (0.42, see Table 1), but this discrepancy could be in part due to the fact that the Asselian bivalve faunas in the Verkhoyansk-Okhotsk blocks have been under-studied and/or under-sampled. It is therefore quite possible that greater specific similarities may be found in the future between the Verkhoyan–Okhotsk and Kolyma–Omolon provinces as taxonomic studies continue. On the other hand, the 0.42 Jaccard value is close to the threshold Jaccard value of 0.5 used by Jaccard (1912) and Skarlato (1981) for distinguishing provinces, suggesting that some differences in species composition also existed between the Verkhoyansk–Okhotsk and Kolyma–Omolon regions during the Asselian. At this point it is interesting to note the shared presence of a number of comparable, if not entirely identical, Asselian bivalve species between the Verkolyman province of Northeast Asia with Gondwana, the latter primarily known from Western Australia (Dickins, 1963). These shared similarities include, but are not limited

3.2. Sakmarian to Early Artinskian (Merismopteria permiana, Cypricardinia eopermica, Cypricardinia borealica and Edmondia gigantea Zones) Some compositional differences in bivalve faunas between the Verkhoyan–Okhotsk and Kolyma–Omolon regions were emerging during the Sakmarian as indicated by a drop in the Jaccard similarity to 0.34 (Table 1). For example, both Verchojanogrammysia and Acanthopecten are endemic to Western Verkhoyansk, as are Omolonopecten, various Streblopteria, Schizodus, Oriocrassatella, Pyramus, Vacunella, Grammysiopsis, Undulomya and Cunavella to the Kolyma– Omolon region (see Appendix A). However, the differences in overall faunal aspects, especially at genus level, remained high. The increasing faunal difference at species level may be explained by the deepening of the Oimyakon basin during the Sakmarian (Fig. 5B), which may be related to a regional rifting event that became intensified rapidly separating the Kolyma–Omolon microcontinent and associated terranes away from the North Asian craton (Parfenov and Kuz'min, 2001; Tretyakov, 2004). Early Artinskian bivalve faunas of the Cypricardinia borealica and Edmondia gigantea Zones are poorly known, thus hindering a detailed quantitative analysis. Despite this, some preliminary observations are still possible. The bivalve communities from the C. borealica Zone appear to be closer to earlier assemblages, especially those from the Cypricardinia eopermica Zone, than overlying younger faunas. This may suggest that biotic interchanges between the Verkhoyan– Okhotsk and Kolyma–Omolon provinces were limited for early Artinskian. In comparison, some significant changes appear to have occurred from the C. borealica Zone to the E. gigantea Zone, in particular marked by increasing number of Gondwanan immigrants (e.g. Stutchburia and Pleurikodonta) and also invasion of a few warm temperate to warm-water Palaeotethyan elements (e.g. Pteronites and Myalina). 3.3. Late Artinskian–Early Kungurian (Aphanaia lima Zone) Compared to the preceding two zones, the bivalve assemblage of the Aphanaia lima Zone is better studied, though with a lower species diversity compared to Asselian–Sakmarian times (Fig. 4). Most of the best studied bivalve faunas of this zone come from the Omolon microcontinent, Gizhiga Zone and the Western Verkhoyansk sector, although bivalve assemblages assignable to this zone but understudied are also known from the Prikolyma and Oloi–Yarakvaam terranes, Okhotsk microcontinent, and Northern and Southern Verkhoyansk sectors. The bivalve communities of the Kolyma–Omolon province comprise 24 species, including 9 endemics. This province is also characterized by the last surviving Omolonopecten, in conjunction with some endemic species of Aphanaia, Schizodus, Permophorus, Vacunella, Myonia, Wilkingia, the presence of Palaeolima ex gr. kasanensis (Netschajew) and some others. In comparison, the Verkhoyan–Okhotsk province contains 17 species, including 4

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Fig. 5. Schematic palaeogeographic reconstruction maps of Northeast Asia at four different time slices through the Permian, also showing various migration pathways into and out of Northeast Asia (see text for more explanation).

endemics. There were 10 species in common with the Kolyma– Omolon Province. The relative low Jaccard index (0.32, see Table 1) for this time could suggest that the geographic separation between the Verkhoyan–Okhotsk and Kolyma–Omolon regions has now become significant. 3.4. Middle Kungurian (Aphanaia andrianovi Zone) The bivalve assemblages from this time interval are among the best studied, well dated and correlated, and the most diverse of all the Permian bivalve faunas in Northeast Asia (Fig. 4). Abundant information about the character and species composition of this

zone is known from the Omolon microcontinent, Oloi–Yarakvaam terrane, Okhotsk microcontinent and its framing, Western and Southern Verkhoyansk sectors. Less but significant data are also available from the Gizhiga fold zone, Prikolyma terrane, and Northern Verkhoyansk. In the Kolyma–Omolon province there are 41 species including 17 endemics, in comparison with 51 species from the Verkhoyan– Okhotsk province including 20 endemics. There are 20 species common to two provinces, giving a Jaccard index valued at 0.28 (Table 1). The Okhotsk Basin is characterized by 7 species, including 4 also found elsewhere only in the Verkhoyansk Basin, suggesting a continuing biogeographic connection between the two. This close

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biotic connections thus refutes the assumption made by some previous researchers (e.g. Natapov and Surmilova, 1995) that there existed significant geographic separation between the Okhotsk microcontinent from the North Asian craton margin. In our view, the Okhotsk basin was biogeographically united with the Verkhoyansk basin, forming a single Verkhoyan–Okhotsk province (Fig. 4B). This province, in the mid-Kungurian, was well distinguished by numerous endemic species of Inoceramus-like bivalve genera, notably including Costatoaphanaia, Aphanaia, Praekolymia and others. At the species level, Euchondria sp. and many species of Schizodus, Permophorus and Myonia are also endemic to this province. In contrast, the Kolyma–Omolon province is characterized by abundant pectinides and some long-surviving species from Schizodus, Astartella and Wilkingia. One genus, Kolymopecten, appears to have flourished in both provinces. The Late Kungurian Aphania korkodonica Zone in Northeast Asia has not yet been sufficiently studied to provide useful data for a detailed biogeographic analysis and comparison with other faunas. Nevertheless, it is clear that its low species diversity, as is currently known, may suggest a major mass extinction event for the Permian bivalve faunas in Northeast Asia (Biakov et al., 2006a) although the details of this extinction are yet to be fully documented and subject to further detailed taxonomy of its constituent species. 3.5. Early Roadian (Aphania dilatata Zone) This zone is best known from the Omolon microcontinent, Western Verkhoyansk, Okhotsk microcontinent and its flanking areas. Bivalve assemblages assignable to this zone also exist in other areas, albeit less studied. The Kolyma–Omolon province was inhabited by 26 species of bivalves, including 6 endemics. The most characteristic species for this zone are: Septimyalina? karavaevae Biakov, “Heteropecten” orientalis Fredericks, “Heteropecten” sp. nov., Vnigripecten sp. nov. (a species close to Vnigripecten phosphaticus (Girty) from the Phosphoria Formation of western USA), another new, yet unnamed, pectinoid genus, and Cyrtorostra sp. For the Verkhoyan– Okhotsk province, there are 31 bivalve species, 14 of which are endemic, and they are mostly from Kolymiidae genera (e.g., endemic genus of Cyrtokolymia Astafieva, Aphanaia judomensis Astafieva, Kolymia yurii Astafieva, K. pergamenti Muromzeva, K. anaticula Astafieva and K. churavtsovi Biakov), and representatives of Pseudomonotis, Stutchburia and Wilkingia. There are 14 species shared between the Verkhoyan–Okhotsk Province and Kolyma–Omolon provinces, giving a Jaccard value of 0.33. This low value is comparable with preceding Sakmarian–Artinskian times, suggesting the persistence of significant biogeographic distance between the two provinces into the Early Roadian. 3.6. Late Roadian (Kolymia inoceramiformis Zone) In the Kolyma–Omolon province, this zone is characterized by 26 bivalve species, including 8 endemics. The most typical are Kolymia taskanica Biakov, “Heteropecten” sp. nov. and Praeundulomya ex gr. gijigensis (Maslennikow). The Verkhoyan–Okhotsk province was inhabited by 30 species, including 13 endemics, the most characteristic of which are Aphanaia judomensis Astafieva, Kolymia yurii Astafieva, K. pergamenti Muromzeva, K. anaticula Astafieva and K. simkiniformis Biakov. There are 14 species common to both provinces, resulting in a Jaccard index valued at 0.33. As far as the present known Permian bivalve fossil record is concerned, representatives of the kolymiid bivalve community appear to have invaded the Balygychan terrane for the first time. Overall, the bivalve assemblage from this terrane demonstrates higher similarity in species composition to the Verkhoyansk basin than to the Omolon microcontinent, though Cyrtokolymia, the most diagnostic genus for the Verkhoyansk communities of this zone, is absent. The close biogeographic

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connection between the Balygychan terrane and the Verkhoyan– Okhotsk basins may be explained by the presence of the Okhotsk– Taigonos Arc, a possible biogeographic migration corridor postulated to have provided continuous shallow-water connection between the shallow shelf waters of the Verkhoyan–Okhotsk province to the Balygychan terrane located within the Ayan-Yuryakh basin (Fig. 5C). 3.7. Early Wordian (Kolymia plicata Zone) The species richness of this zone is relatively low compared to bounding bivalve zones above and below, with a total of 25 species (Fig. 4). Among the 13 species found from the Kolyma–Omolon province, 3 are endemic, and most of other species come from the Kolymiidae family, with a few others representing Streblopteria and Kolymopecten. The low species diversity in this province at this time is of particular note as the strata encompassing this bivalve biozone are, in most places in the Kolyma–Omolon province, dominated by the so-called “kolymic limestone facies”, a Kolymia-dominated bioclastic limestone in which usually 3 to 5 species of Kolymia occur in great abundance (Ganelin, 1997; Biakov et al., 2006b). In comparison to the Kolyma-Omolon province, the Verkhoyan–Okhotsk province attained a slightly higher species diversity with 20 species, 11 of which are endemic. Here, Kolymiidae genera and species also dominate, like in the Kolyma– Omolon province. There are 8 species shared between the Verkhoyan– Okhotsk and Kolyma–Omolon provinces, generating a Jaccard value of 0.32, nearly complete comparable with that of the Late Roadian (Table 1). 3.8. Late Wordian (Kolymia multiformis Zone) This zone is well known and well studied and occurs widely in Northeast Asia, with extensive materials having been collected from the Omolon microcontinent, Okhotsk microcontinent and its surrounding fold belts, Western, Southern, and Northern Verkhoyansk sectors. In the Kolyma–Omolon province, the Kolymia multiformis Zone is represented by an assemblage of 19 bivalve species, 6 of them endemic. Characteristic species include some peculiar pectinoids: Fasciculiconcha sp. nov., Neptunopecten maslennikovi Biakov, “Crittendenia” propria (Biakov), Cypricardinia maslennikovi (Lutkevich and Lobanova), as well as some species similar to North American forms (“Heteropecten” cf. girtyi Newell, “H.” cf. gryphus Newell). On the other hand, the Verkhoyan–Okhotsk Province at this time is marked by a highly diverse bivalve fauna, with 41 species, including 27 endemics (Table 1). Specific most characteristic for the province are coarseribbed Kolymiid genus Okhotodesma, large and flattened Taimyrokolymia ustritskyi Biakov, Kolymia verchojanica Lutkevich and Lobanova, some endemic species of Kolymiidae and Myonia. Remarkable for Northern Verkhoyansk is the presence of Atomodesma sp. nov., a form close to the peri-Gondwanan species A. exaratum Beyrich originally described from Timor. Despite its high species richness, there are only 9 species shared between the Verkhoyan–Okhotsk province with the Kolyma–Omolon province, giving a Jaccard similarity coefficient of 0.18, the lowest for the entire Permian (Table 1). This extremely low Jaccard similarity may suggest a substantially steepened biogeoraphic distance between the two provinces during the K. multiformis Zone. This biotic disconnection may have been due to an episode of significant deepening of the Oimyakon basin that separated the two provinces (Fig. 5C). To produce a Jaccard similarity value as low as 0.18, the Oimyakon basin (ocean) would have been wide enough to serve as an effective migration barrier to the biotic interchange between the two provinces, although at this stage we could not give a precise figure as to the physical distance of this separation. Another point of note in this context is that the deepening (or expansion) of the Oimyakon basin, which was connected to the Okhotsk and AyanYuryakh basins to the south (Fig. 5C), was probably associated with, or initiated by, another phase of accelerated rifting and associated seafloor spreading of terranes, volcanic arcs and microcontinents

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attached to the Kolyma–Omolon province (Fig. 5C). This suggestion of accelerated rifting based on biogeographic evidence is also corroborated by the discovery of basaltic lavas in the Okhotsk microcontinent and the Ayan-Yuryakh anticlinorium, where they are interbedded with the fossiliferous strata belonging to the K. multiformis Zone (Biakov and Vedernikov, 1990; Biakov, 2007a).

Maitaia belliformis Biakov. The Verkhoyan–Okhotsk province was inhabited by 14 species, most characteristic of which are M. tenkensis Biakov, Australomya sulcatiformis (Astafieva–Urbajtis), Cunavella etheridgeiformis Astafieva–Urbajtis, as well as several endemic species from the genera Phestia, Maitaia and Megadesmus. Very characteristic for Northern Verkhoyansk is the presence of Atomodesma variabile Wanner, a species elsewhere also known from Timor (Wanner, 1922).

3.9. Early Capitanian (Maitaia bella Zone) Globally the Capitanian is of great significance not only because it marks an interval of a mass extinction of global scale involving many marine invertebrate groups including bivalves, it is also characterized by enhanced global migrations of several well known marine invertebrate genera such as the brachiopod genera Cancrinelloides, Terrakea and Tomiopsis; bivalve genera Maitaia, Glyptoleda, Australomya and Trabeculatia; and the ammonoid genus Timorites, leading to the pronounced formation of both bi-temperate and bipolar distributions of marine invertebrate taxa (Shi et al. 1995; Shi and Grunt, 2000). In this context, the Maitaia bella Zone is significant as it records one of the lowest number of bivalve species among all the Permian bivalve zones, with only 31 species, 18 of which are known from the Kolyma–Omolon province and the remainder from the Verkhoyan– Okhotsk province. Eight species are shared between the two provinces, giving a Jaccard value of 0.30. 3.10. Late Capitanian (Maitaia belliformis Zone) The Maitaia belliformis Zone in the Kolyma–Omolon province is characterized by an assemblage of 10 species, including 5 endemics. Among the endemics, the family Pectinidae is dominant, including such characteristic species as Fasciculiconcha sp. nov. and Saturnopecten? sp. nov. The Verkhoyan–Okhotsk province has a slightly higher species diversity (Table 1), with 20 species, 7 of them endemic to the province, mostly represented by species from Modiolus, Maitaia, Wilkingia and Australomya. Among the endemics, Glyptoleda is worth noting. This genus is elsewhere known only from Novaya Zemlya, New Zealand and Australia. The Jaccard similarity between the Kolyma–Omolon and Verkhoyan–Okhotsk provinces for this time zone is low (0.2) as there are only four species shared between the two provinces among the total of 20 species recorded from the Maitaia belliformis Zone (Table 1, Fig. 3). The low biotic similarity between the Kolyma–Omolon and Verkhoyan–Okhotsk provinces may suggest the return of a significant biogeographic barrier between the two provinces during the Maitaia belliformis Zone. This inference is also corroborated by lithofacies data of this age. At this time, widespread and substantial deep-water mudstone facies, often of turbiditic origin, accumulated in basins that separated the Omolon microcontinent and the North Asian Craton (Fig. 5C) and contained few fossils. In the field, these deep-water facies are characterized by an extensive range of debris-flow deposits or debrites, including olistostromes, mixtites (Grinenko et al., 1997) and diamictites (Biakov and Vedernikov, 1990; Biakov et al., 2005). The extensive occurrence of these deep-water argillaceous–siliceous deposits, locally sharply replacing shallow-water carbonate rocks in the Omolon microcontinent, has been linked to a significant sea level rise estimated to be no less than 100 m (Kashik et al., 1990). 3.11. Early Wuchiapingian (Maitaia tenkensis Zone) This bivalve zone has the lowest bivalve species diversity of all Permian bivalve faunas in Northeast Asia (Table 1), with only 14 species for both provinces, and 3 of them are shared between the two provinces, giving a Jaccard similarity value as low as for the preceding late Capitanian (Table 1). In the Kolyma–Omolon province, bivalves are practically unknown, except for the rare occurrence of three species including Maitaia cf. tenkensis Biakov and a species close to

3.12. Late Wuchiapingian (Maitaia hurenensis and Intomodesma costatum Subzones) These two bivalve subzones are among the best documented and correlated Permian biostratigraphic levels in Northeast Asia. In terms of both species diversity and the Jaccard index (Table 1, Fig. 4), it is clear that significant changes had occurred across the Early to Late Wuchiapingian boundary for the bivalve faunas in Northeast Asia, characterized by a marked increase of species richness and biotic similarity between the Kolyma–Omolon and Verkhoyan–Okhotsk provinces. The Kolyma–Omolon province was inhabited by 18 bivalve species, only 3 of which were endemic. The Verkhoyan–Okhotsk province, on the other hand, attained a richness of 40 species, including 20 endemics. The most notable of these endemic specific include large Phestia magna (Popow), Intomodesma bicarinatum (Muromzeva), Maitaia hurenensis Biakov, Streblopteria rotunda (Lutkevich and Lobanova), “Fasciculiconcha” tompo (Muromzeva), Pachymyonia bicarinata (Astafieva–Urbajtis), as well as some endemic representatives of the genera Intomodesma, Maitaia, Megadesmus and also possibly Solenomorpha. There are 15 species common between the two provinces, deriving a Jaccard coefficient valued at 0.38, an 81% increase over the preceding Early Wuchiapingian interval. As noted below, this reversal to relative high Jaccard similarities comparable to those of the earliest Permian (Asselian–Sakmarian) was also apparently maintained for the Changhsingian (Fig. 3; Table 1), and its implications are further discussed below and also in Section 4 of this paper. 3.13. Changhsingian (Intomodesma evenicum and Intomodesma postevenicum Subzones) The general trend of relative high species diversity of the Late Wuchiapingian persisted into the Changhsingian. In the Kolyma– Omolon province, 22 bivalve species are known from the two Changhsingian bivalve subzones, including 7 endemics. Most of the endemic species are from Modiolus, Promytilus, Maitaia, Wilkingia, Etheripecten, Cyrtorostra and also possibly Guizhoupecten. The Verkhoyan–Okhotsk province is characterized by 33 species, including 17 endemics. Most characteristic are Phestia magna (Popow), Intomodesma bicarinatum (Muromzeva), Maitaia regularicostata (Muromzeva), Saturnopecten kusnezovi (Muromzeva), Pachymyonia bicarinata (Astafieva–Urbajtis), Australomya sulcatiformis (Astafieva–Urbajtis), and some endemic species of Intomodesma, Obliquipecten, Cunavella, Wilkingia and Pachymyonia. There are 16 common species common to both the Verkhoyan–Okhotsk and Kolyma–Omolon provinces during the Changhsingian, giving a Jaccard coefficient of 0.37, which is consistent with the relatively high biotic similarity already established during the Late Wuchiapingian (Table 1). Of particular note is the first occurrence of Guizhoupecten and Claraioides in the Changhsingian of Northeast Asia. Both genera are characteristic for coeval bivalve faunas of the warm-water eastern Palaeotethyan regions including South China, suggesting invasion of lower latitude warmwater species into higher latitudes in the latest Permian. 4. Discussion When the temporal trend of the biogeographic similarities between the Verkhoyan–Okhotsk and Kolyma–Omolon provinces, is

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examined as a whole through the Permian, several interesting observations can be made with respect to the dynamic relationships between palaeogeography (palaeo-distance) and biotic similarities. First, except for the intervals of Middle Kungurian and Late Wordian, there appears to be an overall decreasing trend in biotic similarity between the two provinces from the Sakmarian through to the end of early Wuchiapingian, followed by a return to relatively high biotic similarity from late Wuchiapingian to Changhsingian (Table 1). This decreasing trend of biotic similarities with time could be explained by increasing palaeo-distance separation between the two provinces, and this interpretation is consistent with independent sedimentary facies data that suggest a gradual deepening and widening of the Oimyakon Basin through the Early to Middle Permian (Fig. 5). The two intervals (Middle Kungurian and Late Wordian) that do not fit into this overall trend are both of relatively high bivalve species diversity (Fig. 4), suggesting good sampling control. However, both intervals are characterized by low Jaccard values compared to bounding intervals (Table 1), and as such they may signify times of rapid deepening (but not necessarily widening) of the Oimyakon Basin, hindering biotic interchange between the Verkhoyan–Okhotsk and Kolyma–Omolon provinces. The return of relative high biotic similarity between the two provinces for the Late Wuchiapingian and Changhsingian intervals, both marked by relatively high species diversity (Table 1), are also inconsistent with the overall decreasing similarity trend mentioned above and hence cannot be satisfactorily explained by the widening and deepening of the Oimyakon Basin. In our view, the most plausible explanation for these relatively high biotic similarities attained during the late Lopingian (Late Wuchiapingian and Changhsingian) may be given by the possible effects of other biogeographic determinants in addition to palaeo-distance. These other possible biogeographic factors, in this case, may have included local facies control on species richness and composition, ocean currents and the invasion of alien species from and into Northeast Asia at opportune climatic times and when migration corridors became available. For example, at the beginning of the Late Permian the Jaccard biotic similarity between the Verkhoyan–Okhotsk and Kolyma–Omolon provinces increased to 0.38, from a very low value of 0.21 at the end of Middle Permian (Table 1). Interestingly, the timing of this biotic similarity increase coincided with an influx of latest Permian–earliest Triassic warmwater Palaeotethyan bivalves, especially those from South China(e.g., Claraioides and Guizhoupecten) and Early Triassic ammonoids (Otoceras) and conodonts [Hindeodus typicalis (Sweet) and Clarkina cf. changxingensis (Wang and Wang)] into the southern Verkhoyansk, Balygychan, Okhotsk, and Omolon basins (Muromtseva and Gus'kov, 1984; Biakov, 2001, 2007a; Zakharov, 2002; Kozur et al., 1995). The timing of invasion is also well correlated with the disappearance of deep-water lithofacies, replaced with widespread occurrence of shallow-water kolymic limestone. The former would suggest a climatic amelioration in Northeast Asia during the Late Permian allowing the range expansion and hence invasion of lower-latitude warm-water adapted species into higher latitudes. The occurrence of widespread of shallow water bioclastic carbonates has been linked to the shallowing, but not necessarily narrowing, of the Oimyakon and Ayan-Yuryakh basins (Fig. 5), therefore facilitating the biotic interchange between the two provinces (Biakov, 2004). 5. Conclusions A similarity index-based semi-quantitative analysis of the spatiotemporal distributions of 355 Permian marine bivalve species in the Verkhoyan–Okhtosk and Kolyma–Omolon provinces of Northeast Asia has revealed the following: 1. The bivalve faunas from these two provinces were very similar to each other in species composition during the Asselian (earliest

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Permian), with a Jaccard index valued at 0.42. This relative high biotic similarity implies that these two biotic ‘provinces’ are best treated as a single province for the Asselian. Further, we suggest that the existence of a single biotic province in Northeast Asia during the Asselian indicates a close geographic proximity of the Omolon microcontinent to the North Asian craton (Fig. 5A). 2. From Sakmarian till the end of Permian, the Jaccard-based biotic similarities between the Verkhoyan–Okhotsk and Kolyma–Omolon provinces remained consistently low, ranging from 0.18 to 0.38, all falling well below the minimum threshold of the Jaccard index of 0.42 considered by (Semyonov, 1982) to be required for distinguishing marine biotic provinces. We interpret these belowthreshold Jaccard biotic similarities as an indication of significant palaeogeographic separation between the Verkhoyan–Okhotsk and Kolyma–Omolon provinces, which is in turn interpreted to indicate rifting and sustained sea-floor spreading of the Omolon microcontinent and associated terranes away from the North Asian craton (Fig. 5), at least from the Sakmarian to the beginning of the Late Permian. For the Middle Kungurian and Late Wordian intervals, the small ocean basin (Oimyakon Basin) that separated the Verkhoyan–Okhotsk and Kolyma–Omolon provinces may have deepened rapidly, leading to temporarily heightened biotic dissimilarity between the two provinces. 3. Two major marine biogeographic barriers were apparent in Northeast Asia during the Permian. The first was a complex system of deep-water basins (Oimyakon, Ayan-Yuryakh, Balygychan and Sugoi basins) that were created by rifting initiated in the Early Carboniferous, followed by sea-floor spreading during the Early Permian. These deep-water basins, at least partially or locally oceanic, separated the Northeast Asian craton from the Omolon microcontinent and its associated terranes (Fig. 5); they appear to have acted as significant barriers preventing free faunal exchanges between the Kolyma–Omolon and Verkhoyan– Okhotsk provinces throughout most of the Permian. Another significant biogeographic barrier also existed between Northeast Asia and the Palaeotethyan basins (Fig. 5), and this barrier has been identified as the Okhotsk–Taigonos volcanic arc (Biakov, 2003, 2007b, 2005), which was most active during the Early Capitanian. This volcanic arc was merged for most of the Permian, thus forming an effective biogeographic barrier for marine faunas between Northeast Asia and the Palaeotethys (Fig. 5). However, the arc system appears to have been at least partially submerged a few times during the Permian, allowing the influx of Palaeotethyan and even Gondwanan species into the Kolyma– Omolon and Verkhoyan–Okhotsk basins. By the end-Permian and into Early Triassic when globally the climate ameliorated, this barrier to a great extent may have been largely eliminated, enabling influx of a large number of warm-water Palaeotethyan (especially Cathaysian) bivalves such Claraioides, Guizhoupecten and Early Triassic ammonoids (Otoceras), conodonts (Hindeodus typicalis (Sweet) and Clarkina cf. changxingensis (Wang and Wang) occurring in the southern Verkhoyansk, Balygychan, Okhotsk, and Omolon basins. Apart from palaeo-distance separation as a key biogeographic determinant in accounting for the differences in the spatial distribution of Permian bivalve species in Northeast Asia, regional climate conditions and ocean currents also appeared to each have played a significant role. Thus, the reversal to a relative high biotic similarity between the Verkhoyan-Okhotsk and Kolyma–Omolon provinces in the latest Permian (from Late Wuchiapingian to Changhsingian) may be interpreted to indicate both shallowing of the previously deepwater marine basins and a climatic amelioration in northern high latitudes; both events would have facilitated the invasion of lower latitude warm-water Palaeotethyan species into Northeast Asia (Fig. 5D).

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4. The consistently high proportion of shared bivalve species between the Verkhoyansk and Okhotsk regions throughout the Permian is interpreted to signal a single uniform biogeographic entity uniting these regions (albeit different at provincial level since the Asselian). This interpretation contradicts the idea of some previous authors (e.g., (Natapov and Surmilova, 1995)) which interpreted the Okhotsk microcontinent as an allochthonous long-traveled terrane far removed from the North Asian craton during the Permian. Acknowledgements This study has been financially supported by the Russian Foundation for Basic Research, Projects (08-05-00100, 08-05-00155, and 09-05-98518) to A.S. Biakov and an Australian Research Council grant (DP07772161) to G.R. Shi. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.palaeo.2010.04.019. References Astafyeva, M.M., Astafyeva-Urbajtis, K.A., 1995. About Permian borealic bivalve mollusks. In: Kruchinina, N.V., Modzalievskaya, T.L. (Eds.), Palaeobiogeography, Centres of Origin, and Migration of Organisms. Tesisy of 41st Session of the Russian Paleontological Society, St. Petersburg, pp. 4–5. in Russian. Biakov, A.S., 2001. Complete succession of the Permian in Northeast Asia: paleontological evidence for the presence of Changhsingian analogues. Doklady Akademii Nauk 378 (4), 399–401. Biakov, A.S., 2003. On Permian geodynamics and paleogeography of Northeast Asia (from sedimentologic and biogeographic data). In: Goncharov, V.I. (Ed.), Geodynamics, Magmatism and Minerageny of Continental Margins in North Pacific: Magadan, SVKNII DVO RAN, vol. 1, pp. 131–134. in Russian. Biakov, A.S., 2004. Permian rock sequences of the Balygychan Uplift (Northeastern Asia). SVKNII DVO RAN, Magadan. . 87 pp. (in Russian with English summary). Biakov, A.S., 2006. Permian bivalve mollusks of Northeast Asia. Journal of Asian Earth Sciences 26 (3–4), 235–242. Biakov, A.S., 2007a. Permian biostratigraphy of the Northern Okhotsk Region (Northeast Asia). Stratigraphy and Geological Correlation 15 (2), 161–184. Biakov, A.S., 2007b. Permian biogeography of terranes of the Verkhoyansk–Kolyma folded region based on bivalves. In: Khanchuk, A.I. (Ed.), Tectonics and Metallogeny of the Circum-North Pacific and Eastern Asia: Proceedings of the Leonid Parfenov Memorial Conference, Khabarovsk, June 11–16. Institute of Tectonics and Geophysics. Far East Branch of the Russian Academy of Sciences, Khabarovsk, pp. 74–77. Biakov, A.S., 2008. Permian bivalve mollusks of Northeast Asia: zonal stratigraphy, event correlation, palaeogeography. Doctor of Geology and Mineralogy Science Thesis. VSEGEI, St.-Petersburg, 42 pp. (in Russian) (unpublished). Biakov, A.S., 2009. New regional stratigraphical scheme of the Permian of NE Asia based on bivalve mollusks. In: Kruchinina, N.V., Modzalievskaya, T.L. (Eds.), Palaeontology and Perfection of Stratigraphical Base of Geological Mapping. Materially of 55th Session of the Russian Paleontological Society, St. Petersburg, pp. 34–36. in Russian. Biakov, A.S., Vedernikov, I.L., 1990. Stratigraphy of Permian deposits of northeast framework of Okhotsk Massif, central and southeast parts of Ayan-Yuryakh antiklinorium. North-East Interdisciplinary Scientific Research Institute. Far East Branch of the Russian Academy of Sciences, Magadan. 69 pp. (in Russian). Biakov, A.S., Prokopiev, A.V., Kutygin, R.V., Vedernikov, I.V., Budnikov, I.V., 2005. Geodynamic formation environments of Permian sedimentary basins in Verkhoyansk-Kolyma folded area. Otechestvennaya geologiya 5, 81–85 (in Russian). Biakov, A.S., Ganelin, V.G., Kutygin, R.V., 2006a. Biodiversity changes of main groups of Permian biota in the Verkhoyansk and Kolyma–Omolon basins during the end Early to the beginning of the Middle Permian. In: Kruchinina, N.V., Modzalievskaya, T.L. (Eds.), Modern Palaeontology: Classic and Nontraditional. Tesisy of 52nd Session of the Russian Paleontological Society, St. Petersburg, pp. 35–37 (in Russian). Biakov, A.S., Vedernikov, I.V., Kolesov, E.V., 2006b. Preliminary results of studying of Permian deposits of southern part of the Omulevka block (Northeastern Asia). In: Chereshnev, I.A. (Ed.), Geology, Geography, and Biodiversity of Northeast Russia. Materially of Far East Regional Conference Dedicated A.P. Vas`kovskyi. North-East Scientific Center. Far East Branch of the Russian Academy of Sciences, Magadan, pp. 71–75. in Russian. Biakov, A.S., Prokopiev, A.V., Kutygin, R.V., Vedernikov, I.L., Budnikov, I.V., 2007. Geodynamic environments of Permian sedimentary basins in the Verkhoyansk– Kolyma folded region. In: Khanchuk, A.I., Khanchuk, A.I. (Eds.), Tectonics and Metallogeny of the Circum-North Pacific and Eastern Asia: Proc. of the Leonid Parfenov Memorial Conference, June 11–16, Khabarovsk. Institute of Tectonics and Geophysics. Far East Branch of the Russian Academy of Sciences, Khabarovsk, pp. 79–81.

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