Introduction

Lake Baikal is the largest intracontinental freshwater basin of Central Asia and the deepest on Earth (Fig. 1). It is characterised by three basins, the older Southern and Central basins and the younger Northern basin, that have successively developed in the Baikal Depression since the Late Cretaceous (Logachev 1974; Mats et al. 2001; Mats et al. 2011). The Baikal region was characterised by the ancient peneplain of Paleozoic rocks similar to the Siberian mainland. The original relief was leveled by tectonic and geomorphologic processes during the Late Cretaceous and the Paleogene, leaving valleys and shallow basins.

Fig. 1
figure 1

Digital relief of the Baikal region. Data from SRTM v.4 were used in the design of the diagram with a resolution of 90 m. The position of Olkhon Island is indicated by a box (see also Fig. 2a). (Daxner-Höck et al. 2022a, this issue)

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The geologic history of the Baikal Depression occurred in three stages (Mats et al. 2011):

  1. 1.

    The Archeo-Baikal stage (Late Cretaceous–early Oligocene, 70–30 Ma),

  2. 2.

    The Proto-Baikal stage (late Oligocene–Early Pliocene, 30–3.5 Ma) and

  3. 3.

    The Palaeo-Baikal stage (Late Pliocene–Quaternary, 3.5 Ma–today).

The Southern and Central basins of Lake Baikal developed since the Late Cretaceous (1. Archeo-Baikal stage). Initially being small shallow lakes (2–6 meters deep), they became united to a single lake during the late Oligocene and the Miocene (2. Proto-Baikal stage), reaching depths of a few hundred meters. At that time, also the smaller Northern basin started to develop to a lake of several hundred meters depth. Simultaneously small, shallow lakes and wetlands existed in the western part of the Baikal Depression.

During this stage collision of the Indian and Eurasian plates caused tectonic activities, that were accompanied by uplift of landscapes surrounding the Baikal Depression and deepening of the lake (up to 400–500 m). Olkhon Island started to separate from the western hinterland during the Late Miocene and Early Pliocene, and the northern basin fused with the middle and southern basin of Lake Baikal (Mats et al. 2011; Fig. 3–d).

During the Pleistocene (3. Palaeo-Baikal stage) Lake Baikal reached depths of more than 1000 meters in the course of extended tectonic processes along the Baikal rift system. The tectonic block of Olkhon Island, bounded by listric faults, began to sink, dipping towards the west, where parts of the island subsided into Lake Baikal at today’s Maloe More Strait (Little Sea; Fig. 2), (Mats 1993; Ufimtsev 1993).

Fig. 2
figure 2

Tagay Bay location on the digital elevation model of the Middle Baikal region (a), and typical landscapes of Olkhon Island with a general view from the south-west to the Tagay locality (b). The lower photo (c) shows the location of the excavation against the general background of the Tagay transsect (Tagay-1 section), view from the north. Data from SRTM v.4 were used in the design of diagram A. (Daxner-Höck et al. 2022a, this issue). Photos by the authors

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In the course of the Miocene, Olkhon Island was still part of the Siberian mainland west of the Baikal Lake (the Proto–Baikal Stage). Similar geological and geomorphological features characterise the landscape, such as denudated plateaus, where streams had deeply cut into the crystalline basement. During the Miocene the valleys and basins were filled by reworked terrigeneous sediments. The partly boggy lakes accumulated gypsum-bearing carbonate, montmorillonitic clays, sands, silts and lignite of the Tagay Formation (Mats et al. 2011; p. 414, Fig. 3b). The lakes and coastal wetlands were densely populated by a diverse fauna and flora (Logachev et al. 1964; Vislobokova 1990). One of these Miocene lake areas, the Tagay area, is located at the Tagay Bay of Lake Baikal, at the north-western part of the Olkhon Island. There, sediments of the Tagay Formation (Mats et al. 2011) are exposed along the shoreline of the extant Lake Baikal (Fig. 2c).

Fig. 3
figure 3

Fossil evidence from sediment layers 11, 10, 9, 8, 7, 6, 5 and 3 along the Tagay-1 section. Coordinates 53° 9'34.74"N, 107°12'43.12"E. Photo from the authors

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The present contribution aims at a synthesis of fauna and flora and at a reconstruction of various palaeoenvironments of the Baikal region, specifically of the Tagay region at Olkhon Island, during the depositional time of the Tagay sequence.The geochemical pattern of sediments along the Tagay-1 section informs about the palaeoclimate (Ivanova et al. in prep.), and a third topic is the age dating of the Tagay fauna by correlation of biochronologic and magnetostratigraphic data (Kazansky et al. 2022, this issue) and the Geologic Time Scale (GTS 2000).

Material and methods

The palaeontological method of wet-screening of test samples from section Tagay-1 was performed in order to optimise collecting of small mammal fossils (Daxner-Höck et al. 2022b, this issue). We collected 17 test samples from bottom to top of the 12-13 meters long section. Each sample was approximately 20 kg. The lower part of the section was free of fossils, only from eight layers with promising fossil record (upper part of the section) were larger samples processed. The total amount of investigated sediment was ~ 2000 kg. The number of identifiable small mammal fossils is 111 (tooth fragments and isolated incisors are not counted here). Additionally, numerous shell fragments of gastropods and badly preserved remains of lower vertebrates were collected, but they are not suitable for species identification.

The methods applied for identification of geochemical sediment-compositions, for definition of the rare and trace elements, and the statistical methods are described in detail by Ivanova et al. (in prep.). For geochemical analyses 59 samples were collected. A total of 37 samples for palaeomagnetic and rock magnetic studies were taken at a 20-50 cm vertical interval over the entire Tagay-1 section of 13 meters. Each sample represents one sampling level and comprises 8 cm3 internal volume. The subsequent laboratory studies were carried out in the laboratories of Geodynamics and Paleomagnetism of the Trofimuk Institute of Petroleum Geology and Geophysics of the Siberian Branch of RAS, Novosibirsk (Kazansky et al. 2022, this issue).

Abbreviations

MN:

European Neogene Mammal Zone (Steininger 1999)

LMS/A:

Chinese Land Mammal Stage/Age (Qiu et al. 2013)

Coll. Kossler:

fossils collected by Kossler in the 1990s

Ma:

million years

Kyr:

100.000 years

Miocene fauna and flora of the Tagay locality

The fossil site Tagay was discovered by Kitaynik (1958). However, in the 1960s, for the first time a very diverse fauna and flora from the Tagay section at the Tagay Bay was published by Logachev et al. (1964: 41–42). The faunal list of these collections comprises mainly unspecified genera of Pisces (Silurus, Rutilus, Esox, Perca and Lucioperca), Amphibia (Bufo and Rana), Testudinata (Baikalemys gracilis) and Serpentes (Coluber), various Aves (Ardea, Crex, Porzana, Anser, Anas, Branta, Nyroca, Striges) and Mammalia (Proscalops, Talpa, Soricidae, Sciuridae, Monosaulax, Myoxidae, Cricetodon cf. sansaniensis, Mustelidae, Felidae, Anchitherium (?), Metaschizotherium (?), Dicerorhinus, Palaeomeryx, Bovidae) (Logachev et al. 1964: 41).

Later, parts of the fauna were revised: the fish fauna by Filippov et al. (2000), Testudinata by Ivanjev and Khosatzky (1970), Khosatzky and and Chkhikvadse (1993) and Danilov et al. (2012), among Serpentes five taxa were identified by Rage and Danilov (2008), one Boidae, three Colubridae and one Viperidae (all unspecified). The Artiodactyla, revised by Vislobokova (1994, 2004) are: Amphitragulus boulangeri, Lagomeryx parvulus, Stephanocemas sp., Palaeomeryx cf. kaupi, Brachyodus intermedius and Orygotherium tagaiensis Vislobokova, 2004. Logachev’s collection of the three-toed horse Anchitherium (?) sp. was completed in later years and published as Anchitherium aurelianense by Klementiev and Sizov (2015).

The section studied by Kossler in the 1990s is ~ 4 meters shorter, and the „units“ described by Kossler (2003) cannot be exactly correlated with individual sediment layers of Logachev’s and our Tagay-1 section from 2014. However, the general lithological characterisation and the fossil-bearing part of Kossler’s section are more-or-less in agreement with Logachev (1964) and our Tagay-1 section (studied 2014). The fossils (Coll. Kossler) comprise Gastropoda (Gastropcopta and Vallonia), Pisces (Palaeocarassius, Palaeotinca, Leuciscinae and Esox), Anura (Rana and Pelophylax), Chelonia (? Baicalemys gracilis Khosatzky and Chkikvadse, 1993), Squamata (? Chalcides nov. spec. and Texasophis), Insectivora (Desmaninae and Erinaceinae) and Rodentia (Sciurinae gen. et spec. indet., Miodyromys sp., Keramidomys aff. mohleri Engesser, 1972 vel K. aff. fahlbuschi Qiu, 1996, Eomyops oppligeri Engesser, 1990 and Democricetodon sp.). These fossils are briefly listed and figured in Daxner-Höck et al. (2013: Pl. 22.1–22.2, p. 511). Recently, the Rodentia group was revised by Daxner-Höck et al. (2022a, 2022b, 2022c, all this issue) and Mörs et al. (2022, this issue) and the Eulipotyphla by Voyta et al. (2022, this issue).

In the course of the excavations of the Russian Academy of Sciences (principal investigators E.V. Syromyatnikova, A.S. Tesakov and A.M. Klementiev) in the years 2011–2012 and 2014–2016, rich fossil material was collected at the Tagay locality, laterally of our Tagay-1 section from 2014. The following fossils have been published since then, others remained so far unpublished. The published Amphibia are represented by Bufo mirus Syromyatnikova, 2015, and unspecified genera: Salamandrella, Bombina, Hyla, Pelophylax and Rana (Syromyatnikova 2014, 2015, 2016). Squamata are described as the new skink Calcides augei Čerňanský et al., 2020 and Lacertidae indet. (Čerňanský et al. 2020). Among Aves several species of waterbirds and arboreal birds were described, e.g. the duck Chenoanas sansaniensis (Milne-Edwards,1867), the new grebe Miobaptus huzhiricus Zelenkov, 2015 and a parrot (Zelenkov 2015, 2016a; Zelenkov et al. 2018; Volkova 2020). Additionally, manifold bird taxa of the families Phasanidae, Anatidae, Podicipedidae, Ardeidae, Gruidae, Rallidae, Charadriiformes, ?Pandionidae, Strigidae and Passeriformes are mentioned (Zelenkov and Martynovich 2013; Zelenkov et al. 2016b: Table 1). New Mammalia descriptions of this collection are: the Mylagaulidae species Lamugaulus olkhonensis Tesakov and Lopatin, 2015 and a small bear Ballusia zhegalloi Sotnikova et al., 2019.

Table 1 Composite small mammal fauna of section Tagay-1. In brackets the authors/references who studied the respective Eulipotyphla, Rodentia and Lagomorpha families of the Tagay collection from 2014
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Our field campaign of 2014 (principal investigator M.A. Erbajeva) focused on detailed studies of section Tagay-1, across the sequence of the Tagay Formation. For better comparison, the Tagay-1 section was placed approximately at the same part of the Tagay outcrop, where Logachev et al. (1964: fig. 12) had studied his Tagay section. 17 individual sediment layers were analysed from this new Tagay-1 section in order to provide information for multidisciplinary studies: on geochemistry (Ivanova et al. in prep.), palaeomagnetics (Kazansky et al. 2022, this issue), composition, structures and the included fossils of the sediment sequence. Fossil remains were only found in the upper part of the Tagay-1 section, totally 4 meters (at ~ meters 6–2), i.e. in sediment layers 11–3 (Daxner-Höck et al. 2022a, this issue: Fig. 3). All these layers yielded vertebrate fossils and fragmentary Gastropoda, except for layer 8, where only Gastropoda fragments were collected (Fig. 3). All Lagomorpha, Eulipotyphla and Rodentia of this fossil collection 2014 are listed below (Table 1) and published in the special issue (Daxner-Höck et al. 2022a, 2022b, 2022c; Erbajeva et al. 2022; Mörs et al. 2022; Voyta et al. 2022). Gastropoda (fragments) and lower vertebrates from these samples are not considered here, because of the poor preservation. Better and complete material would be needed for a definitive determination of individual taxa.

Palaeoenvironments and palaeoclimate

Palaeoenvironments

Palaeoenvironmental conditions of the Tagay region are reflected by abiotic factors, such as the palaeogeographic and geomorphologic development of the landscape and the palaeoclimate, and by biotic factors, such as fauna and flora. As outlined above, the landscape was formed by tectonic and geomorphologic processes. The original relief of the ancient peneplain, built up by Paleozoic rocks, was leveled during the Cretaceous and Paleogene, leaving basins and lakes, which were filled with Neogene sediments. One example of these former lake systems is represented by the Tagay Formation (Mats et al. 2011), exposed at the Tagay Bay of Olkhon Island, where we performed multidisciplinary studies along the Tagay-1 section (Fig. 2). The sediment sequence is characterised by debris flows of alluvial fans, floodplain accumulations and calcrete palaeosol horizons (Daxner-Höck et al. 2013). Biogenic remains suggest both deposition in place and transport by flowing water from adjacent places, as demonstrated by fossils and sediment compositions (Ivanova et al. in prep.) (Fig. 4).

Our present knowledge of fauna and flora from the Tagay locality (from the Tagay-1 section and its close vicinity) allow reconstruction of various palaeoenvironments, i.e. open water bodies, shallow lake areas and swamps, woodland environments, forests with dense undergrowth, and open landscapes. They are evidenced by the palynological record – that is: ferns, herbs, shrubs (Artemisia, Chenopodiaceae, Poaceae, Cyperaceae, Nymphaceae, Ranunculaceae, Lycopodiaceae) and many coniferous and deciduous trees (Picea, Abies, Pinus, Tsuga, Larix, Alnus, Betula, Quercus, Fagus, Carpinus, Castanea, Tilia, Celtis, Acer, Ulmus, Corylus, Carya, Juglans and Liquidamber) (Logachev et al. 1964; Demske and Kossler 2001; Daxner-Höck et al. 2013).

Open water habitats are evidenced by the fish fauna, by salamanders, toads, frogs and the pond turtle. The high diversity of waterbirds suggest extended wetland environments as far as Lake Baikal. There, diurnal and nocturnal birds of pray hunted for fishes, amphibians, snakes and small mammals. Floodplains and woodland environments along the rivers and lakes provided suitable habitats for large plant eaters (rhinocerotids, chalicotheriids, anthracotheriids) and semiaquatic living castorids, desmans and talpids (Logachev et al. 1964; Vislobokova 1994; Filippov et al. 2000; Zelenkov and Martynovich 2013; Syromyatnikova 2014, 2015, 2016; Zelenkov 2015, 2016a, 2016b, 2018; Mörs et al. 2022, this issue; Voyta et al. 2022, this issue).

Various ruminants and the three-toed horse fed on herbs and shrubs in floodplains and found shelter in the forested valleys. Forests with dense undergrowth were favoured by arboreal living birds and rodents (dormice, eomyids and chipmunks). However, more dry, open environments with sandy, stony patches are evidenced by ground dwelling small mammals (hamsters, aplodontids, lagomorphs, shrews, hedgehogs and subterranean digging groups such as moles), lizards and snakes (Vislobokova 1994, 2004; Klementiev and Sizov 2015; Sotnikova et al. 2019, Čerňanský et al. 2020; Rage and Danilov 2008; Daxner-Höck et al. 2013; Daxner-Höck et al. 2022a, 2022b, 2022c, all this issue; Erbajeva et al. 2022, this issue; Voyta et al. 2022, this issue).

Palaeoclimate

Geochemical investigations show that temperature and climate conditions during the formation of sediments of section Tagay-1 were fairly stabile, no abrupt climatic changes occurred. The palaeoclimate was temperate, however, it had a cyclic nature: wet and semiarid epochs of different intensity and duration alternated (Ivanova et al. in prep.-). The estimated palaeotemperatures in winter and in the annual mean were at least 4–8°C warmer than today (e.g. Irkutsk: MAT -1.2°C, mCMT -20.9°C, WMT 17.5°C) (Daxner-Höck et al. 2013: 515).

The basal part of the sediment sequence (Figs 3 and 7) is dominated by terrigeneous material, mainly by surface runoff, so the sedimentation was fed by erosion products of the weathering crust. During sedimentation of this lower part (layers 17–9), the palaeo-lake level and salinity stayed practically unchanged. The drawdown of the palaeo-lake and increasing salinity started in the higher middle part (layer 8), and reached the minimum water level and maximum salinity in layers 7–6. Sediments of the layers 8–6 accumulated in an arid climate. After accumulation of the layer 5, the water level began to rise, and during formation of layer 3 it fell again. The highest sedimentation rate was in layers 12–5, these are also the layers with the highest enrichment of biogenic elements (layers 12 and 7–5) and where active carbonate deposition took place (Ivanova et al. in prep.).

Stratigraphy and correlation

The Tagay-1 section, a sequence of seventeen sediment layers (totally 12-13 m) was tested for the fossil content, in order to find out whether the mammal communities of individual fossil layers differ from each other, and whether evolutionary trends of mammal lineages are visible. Seven layers (11, 10, 9, 7, 6, 5 and 3; i.e. ~ 4 m of the upper part of the section) yielded small mammal fossils: isolated teeth, maxilla- and mandibulary fragments and incisors of Eulipotyphla, Lagomorpha (Palaeolagidae) and Rodentia (Sciuridae, Aplodontidae, Mylagaulidae, Gliridae, Castoridae, Eomyidae and Cricetodontinae). Bone fragments of lower vertebrates and Gastropoda were also found (Table 1 and Fig. 3).

The small mammal compositions of these subsequent fossil layers do not differ substantially, nor show any trend of species evolution occurring in lower, middle and upper fossil layers (Daxner-Höck et al. 2022a, 2022b, 2022c; Mörs et al. 2022; Erbajeva et al. 2022; Voyta et al. 2022, all this issue). This homogenous faunal composition – it was also recognised by Sizov and Klementiev (2015) and Čerňanský (2020) – and the lack of any visible evolution suggest a rather short time of deposition, that likely does not exceed a few Kyr.

Biostratigraphy / Biochronology

The faunal list of Tagay-1 (Table 1) comprises mammal genera, well known to range all over Eurasia from the Early to the Middle Miocene, typical Late Miocene (Tortonian and Messinian) genera are not present.

However, on species level there are significant differences between the east and west of Eurasia. The biochronological concept of the Europen Neogene, the “European Miocene Mammal Zones“ (MN1–MN13), is based on European mammal evolution, on single species and/or their evolution, and – if available – on correlation with magnetostratigraphic and/or radiometric data (Steininger 1999). The “Chinese Land Mammal Stages/Ages (LMS/A)“ (Qiu et al. 2013) are mainly based on Chinese taxa, that are rarely found in other parts of Asia or in Europe. Hence, reliable biostratigraphic/biochronologic correlation of the Tagay fauna with either the European “MN-Zones“ or the “LMS/A“ is difficult, since they do not have the characteristic species in common.

Hitherto, the published age estimations of the Tagay fauna range from the Early to the Middle Miocene (Logatchev et al.1964; Vislobokova 1994; Pokatilov 2004; Daxner-Höck et al. 2013; Sizov and Klementiev 2015; Tesakov and Lopatin 2015; Syromatnikova 2016; Zelenkov 2018; Sotnikova et al. 2019; Čerňanský et al. 2020; Volkova 2020 and references herein). The majority of authors vote for an Early Miocene age. The Middle Miocene age was favoured by Daxner-Höck et al. (2013: Fig. 22.2), because the first available Eomyidae fossils from Tagay (Coll. Kossler) suggested correlation with the characteristic Eomyidae from Anwil (Europe; Middle Miocene, Serravallian, MN7/8). However, all these former datings/age estimations, respectively, are not confirmed by magnetostratigraphic or radiometric data.

Our multidisciplinary studies of the Tagay-1 section (2014) provide both biostratigraphic/biochronologic data of small mammals (Table 1) and palaeomagnetic data (Kazansky et al. 2022, this issue), that for the first time allow correlation of the palaeontologic and palaeomagnetic record with the Geologic Time Scale (2020). The biostratigraphic/biochronologic relevant species are the rodents Spermophilinus debruijni Daxner-Höck et al., 2022a, this issue, Ansomys sp., Lamugaulus olkhonensis Tesakov and Lopatin, 2015, Euroxenomys minutus (von Meyer, 1838), Leptodontomys cf. gansus Zheng and Li, 1982, Keramidomys sibiricus Daxner-Höck et al., 2022b, this issue, Democricetodon cf. lindsayi Qiu, 1996, Gobicricetodon filippovi Sen and Erbajeva, 2011 and the lagomorph Amphilagus plicadentis Erbajeva, 2013.

Spermophilinus debruijni Daxner-Höck et al., 2022a: The strongly simplified tooth pattern (hypocone, entoconid and conules/conulids are absent) sets the new species apart from all Spermophilinus species known from Europe, Asia Minor and China. The tooth sizes are within the range of Spermophilinus besana Cuenca Bescós, 1988, known to be the smallest and oldest Spermophilinus species of Europe (range: MN3–MN5) and Asia Minor (range: MN2–MN9). All Spermophilinus species younger than Early Miocene are larger. The low tooth crowns and small size of S. debruijni Daxner-Höck et al., 2022a, combined with relatively primitive dental pattern suggest a late Early Miocene (late Burdigalian) rather than a Middle Miocene or younger age (Daxner-Höck et al. 2022a, this issue).

Ansomys sp.: The genus is distributed over the Northern Hemisphere with a total stratigraphic range from the Oligocene to the Late Miocene. In Eurasia it is a rare species, however, best known from the Early to Late Miocene (Shanwangian, Tunggurian and Bahean) assemblages of Nei Mongol in China. Only three teeth, identified as Ansomys sp., are available from the Tagay locality. The Tagay specimens are most similar in size with the type species Ansomys orientalis Qiu, 1987 from Sihong (Early Miocene; Shanwangian) in eastern China, nevertheless, the Tagay specimens differ from the type specimens by slightly more advanced molar features. Ansomys species of the Middle and Late Miocene of China have higher tooth crowns and even more complex molar pattern, i.e. accessory crests of lower molars. Ansomys sp. from Tagay suggests a late Burdigalian age (Daxner-Höck et al. 2022a, this issue).

Lamugaulus olkhonensis Tesakov and Lopatin, 2015 belongs to the subfamily Promylagaulinae of the Mylagaulidae family. The subfamily is known to range in North America from the late Oligocene to the Middle Miocene, and likely dispersed to Asia via Beringia in the course of the Early Miocene. Lamugaulus olkhonensis from the Baikal region was dated at Early Miocene (Tesakov and Lopatin 2015).

Euroxenomys minutus (von Meyer, 1838) from Tagay-1 is the first record of this small trogontheriine castorid species in Asia. Representatives of Euroxenomys are rare elements in Northern Hemisphere Miocene faunas, and in Asia so far only reported from Japan (Mörs and Tomida 2018). E. minutus has been exclusively recorded from Europe (including Turkey), with a stratigraphic range from the Early Miocene (Burdigalian, MN3) to the Late Miocene (Messinian, MN13) and during this long time span, there is a clear trend to larger cheek teeth. The size of the Tagay specimens is intermediate to teeth from the Early Miocene of Europe (MN3) and the Middle Miocene (Sansan, MN6), suggesting an Early Miocene age (late Burdigalian) rather than a Middle Miocene or younger age (Mörs et al. 2022, this issue).

Leptodontomys cf. gansus Zheng and Li, 1982 from Tagay, was before described as Eomyops oppligeri Engesser, 1990 (Daxner-Höck et al. 2013) and later corrected to L. cf. gansus, since the diagnostic relevant mandible with incisor was collected from the Tagay locality in 2014 (Daxner-Höck et al. 2022b, this issue). L. gansus is a long-lived species, ranging from the Early to the Late Miocene of China, thus the stratigraphic value is limited.

Keramidomys sibiricus Daxner-Höck et al. 2022 from Tagay, before described as Keramidomys aff. mohleri Engesser, 1972 vel Keramidomys aff. fahlbuschi Qiu, 1996 by Daxner-Höck et al. (2013), turned out to be a new species. Some primordial tooth characteristics, reminiscent of Asianeomys fahlbuschi (Wu et al. 2006) and Keramidomys fahlbuschi (Qiu and Li 2016) from the earliest to Early Miocene (Xiejian – Shanwangian LMS/A) of China, but rarely known from Middle to Late Miocene occurrences of K. fahlbuschi, suggest an early evolutionary stage of the Asian Keramidomys lineage, and likely a late Early Miocene (late Burdigalian) age (Daxner-Höck et al. 2022b, this issue).

Democricetodon cf. lindsayi Qiu, 1996 is a small to medium sized Chinese species of Cricetodontinae with a stratigraphic range from the Early to the Late Miocene. Within this long time of evolution some molar characteristics changed gradually. The primitive characters (specifically of M1-2) of the Tagay specimens indicate an early rather than an advanced stage of evolution. The suggested age is around the Early/Middle Miocene transition (Daxner-Höck et al. 2022c, this issue).

Gobicricetodon filippovi Sen and Erbajeva, 2011 from Tagay-1 provides the oldest record of the genus and species. Gobicricetodon likely descended from a Cricetodon-like ancestral stock (Sen and Erbajeva 2011). So far six species are known to range from the Middle to the Late Miocene of China, Kazakhstan and Aya Cave at Olkhon Island (Siberia). The species Gobicricetodon filippovi was first described from Aya Cave. The Aya Cave site is located on the Olkhon plateau of the western Baikal coast, south-west from the Tagay locality. The Aya Cave transect shows basal shallow lake sediments which resemble the Tagay Fm. (without fossils). In the middle part of the section fluvial sediments follow, that contain the characteristic fossils G. filippovi and Amphilagus tomidai Erbajeva et al., 2016 (Middle Miocene). Sediments of the upper part yield fragments of Arvicolidae fossils (Plio-Pleistocene) (Filippov et al., 1995; Erbajeva and Filippov 1997, Fig. 2). The new findings from Tagay show, that G. filippovi from Tagay has even more Cricetodon-like tooth characters and less advanced Gobicricetodon-characters than the specimens from Aya Cave. Moreover, the lower stratigraphic position suggest an older age of the Tagay fossils than the fossils from Aya Cave, likely a late Burdigalian age (Daxner-Höck et al. 2022c, this issue).

Amphilagus plicadentis Erbajeva, 2013 from Tagay-1 shares the main tooth pattern with the Aquitanian type (from the locality Unkheltseg in Mongolia; Aquitanian, Mongolian biozone D), though the P3 is smaller and the teeth have slightly modified dental characteristics. From A. tomidai of the neighbouring, younger locality Aya Cave (Middle Miocene) it differs by smaller and lower teeth and less cement filling of reentrant folds. A. plicadentis from Tagay supports an Early Miocene age (Burdigalian; Erbajeva et al. 2022, this issue).

Fig. 4
figure 4

The modern Lake Baikal and landscapes of the Baikal region (Siberia). a Sunset at Lake Baikal. b–c Coastal zone of Olkhon Island showing cliffs of the crystalline basement. d–f Wetland and wooded environments of the Baikal region (2013-2014). Photos of the authors

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Magnetic polarity stratigraphy and correlation

According to the results of palaeomagnetic studies in the Tagay-1 secton (Kazansky et al. 2022, this issue) a large magnetozone of reversed polarity R1 was identified. It is bounded by fragments of normal polarity zones (N1 and N2). The R1 zone in turn is complicated by two subzones of normal polarity of a lower rank (R1n1 and R1n2) (Fig. 5). A direct comparison of the R1 magnetozone with the magnetic polarity scale was impossible so far because of the absence of reliable biostratigraphic benchmarks (Kazansky et al. 2022, this issue). However, based on comparison with sedimentation rates of lacustrine sediments of Lake Baikal and the Valley of Lakes in Mongolia the time of sedimentation of the Tagay-1 sequence could be calculated (Kazansky et al. 2022, this issue). These calculations allow correlation of the R1 magnetozone with one of the long intervals of prevailing reversed polarity during the Miocene, in accordance with the Geologic Time Scale (GTS2020; Fig. 6).

Fig. 5
figure 5

Lithology column, rock magnetic and palaeomagnetic data for the Tagay-1 section. MS magnetic susceptibility; NRM natural remanent magnetization; Qn Königsberger ratio; MDF – median destructive field; D –declination; I – inclination (both in stratigraphic coordinate system); K – precision parameter, a95 –confidence angle. Red lines mark zero values of D and I. Black infilling – normal polarity, white – reversed polarity (Kazansky et al. 2022, this issue)

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Fig. 6
figure 6

Possible versions of correlation of the Tagay polarity pattern with the magnetic polarity time scale (Geological Time Scale 2020). Red – “Serravallian”, yellow – “Langhian”, green – “Burdigalian” version, respectively (Kazansky et al. 2022, this issue)

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Hitherto, the age of the Tagay fossils has been estimated on the basis of previous palaeontological studies, ranging from the Early Miocene (Vislobokova 1990) to the end of the Middle Miocene (Daxner-Höck et al. 2013). Such a wide time span (see GTS2020) includes at least three relatively long intervals of predominant reversed polarity with minor normal polarity events. Thus, three different correlations of the Tagay-1 magnetic polarity pattern with GTS2020 would be possible (Kazansky et al. 2022, this issue):

  1. 1)

    Late Burdigalian – chrons from C5Dn to C5Cn.1n (~17.5 - ~16.0 Ma).

  2. 2)

    Langhian - chrons from C5Cn.1n to C5ADn (~16.0 - ~14.5 Ma).

  3. 3)

    Serravallian to lowermost Tortonian - chrons C5ABn – C5n.2n (~13.5 - ~11.0 Ma).

The upper Burdigalian (1. version) allows to make the following direct correlations of the Tagay-1 section (of 12-13 m thickness): the upper boundary of magnetozone N1 corresponds to the upper boundary of chron C5Dn; magnetozone R1 corresponds to chron C5Cr and subcrons C5Cn.2r and C5Cn.1r; subzones R1n1 and R1n2 correspond to subchrons C5Cn.3n and C5Cn.2n, respectively, while magnetozone N2 correlates with the lower part of subchron C5Cn.1n (Fig. 7). The according age estimations of deposition of the Tagay-1 sequence ranges from ~ 17.2 to ~ 16.2 Ma. This ~ 1Ma depositional time correlates well with the estimated sedimentation rates calculated before.

Fig. 7
figure 7

Correlation of the magnetic polarity pattern of the Tagay-1 section with the magnetic polarity time scale (Geological Time Scale 2020) (modified from Kazansky et al. 2022, this issue)

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The new fossil record from coll. 2014 (Fig. 3 and Table 1) supports the upper Burdigalian (1. version) of correlation (Fig. 6, green part). The main argument for the 1. version is the archaic dental pattern of rodents and lagomorphs of Tagay-1 (S. debruijni, Ansomys sp., E. minutus, K. sibiricus, D. cf. lindsayi, G. filippovi , A. plicadentis), which resembles that of of their Early Miocene ancestors. A second argument concerns the stratigraphic sediment sequence of Aya Cave, a locality in the close vicinity of Tagay. There, lacustrine sediments of the Tagay Formation are overlain by fluvial sediments which contain a more advanced, a Middle Miocene fauna (Filippov et al., 1995; Erbajeva and Filippov 1997, Fig. 2; Sen and Erbajeva 2011).

The Langhian (2. version; Fig. 6, yellow part) cannot be excluded but is unlikely, because of the primitive dental characteristics of the Tagay species. Other than that, the Middle and Late Miocene species of the same genera develop significantly more advanced dental pattern.

The Serravallian to Tortonian (3. version; Fig. 6, red part) has to be excluded, because no characteristic rodent taxa (e.g. Plesiodipus, Prosiphneus, Murinae, Cricetinae, Alactaginae) are represented in Tagay-1.

The palaeontological and the palaeomagnetic records are in agreement to correlate the magnetozone R1 (covering the entire Tagay-1 section) with the upper Burdigalian, i.e. from the upper boundary of C5Dn to the lower boundary of C5Cn.1n, which correlates with a depositional time of ~ 1 Ma (Figs. 5, 6 and 7; see also Kazanzsky et al. 2022, this issue). Fossils were only recovered from the upper part of the section, from layers 11–3 (Fig. 7). The homogenous faunal compositions and the fact that no evolution could be recognised, suggests a rather short depositional time of the fossil layers 11 to 3. We estimate no more than a few Kyr.

According to the upper Burdigalian version the fossil layers (11–3) correlate with subchrons C5Cn.2r – C5Cn.1r, and with the depositional time of ~16.5 to ~ 16.3 Ma of the Geologic Time Scale (GTS2000) (Fig. 7). Consequently, the suggested age of the Tagay-1 fauna is ~ 16.5 to ~ 16.3 Ma.

Conclusions

A brief introduction into the geologic history shows, that the development of Lake Baikal was driven by tectonic processes since the Late Cretaceous. The Olkhon Island, the largest island of Lake Baikal, was originally part of the Siberian mainland. It started to separate from the western hinterland during the Miocene and Early Pliocene, and completely lost its land-connection during the Pleistocene. Similar geologic and geomorphological features characterise the Miocene Olkhon Island and Siberian mainland: denudated plateaus, where streams deeply cut into the crystalline basement and filled the basins and valleys with reworked terrigeneous sediments. Consequently, lakes and wetlands developed in the basins. One of these Miocene lake deposits is represented by the Tagay sequence, which was studied along the Tagay-1 section in summer 2014.

Lithologic and geochemical analyses of sediment samples and the fossil record along the Tagay-1 section clearly indicate various palaeoenvironments, open water bodies, shallow lake areas and surrounding swamps, woodlands along rivers, floodplains, forests with dense undergrowth, and open landscapes in higher position.

All these palaeoenvironments are evidenced by the Miocene fauna of Tagay, comprising aquatic living animals (fishes, amphibians, waterbirds, pond turtle), semiaquatic animals (beaver, desman), forest dwellers and inhabitants of floodplains (manifold large and small mammals and birds of prey) and ground dwellers (hares, some rodents and lipotyphlans, lizards and snakes) inhabiting more dry areas. These palaeoenvironments are also represented by the palynological record.

The palaeoclimate can be described as temperate, however, it was warmer than today and had a cyclic nature: wet and semiarid epochs of different intensity and duration alternated.

With the exception of the American immigrant Lamugaulus and the Asian genus Gobicricetodon, the Tagay fauna comprises mammal genera that are well known to range all over the Holarctic region from the Early Miocene (Burdigalian) to the Middle Miocene (Langhian and Serravallian). Typical Late Miocene (Tortonian and Messinian) genera are not represented. However, The archaic dental structures of small mammal species support the late Burdigalian correlation (1. Version) with the palaeomagnetic pattern (Kazansky et al. 2022, this issue). The palaeontological record and the magnetic polarity pattern of fossil layers 11–3 correlate with the subchrons C5Cn.2r – C5Cn.1r of Chron C5C and the upper Burdigalian stage of the Geologic Time Scale (GTS2000). The corresponding age range of the Tagay fauna is ~16.5 to ~16.3 Ma.