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Palaeobiodiversity and Palaeoenvironments
 
Paleobiodivers Paleoenviron. 2017; 97(1): 111–218.
Published online 2017 March 7. doi:  10.1007/s12549-016-0257-9
PMCID: PMC5367740

Oligocene stratigraphy across the Eocene and Miocene boundaries in the Valley of Lakes (Mongolia)

Abstract

Cenozoic sediments of the Taatsiin Gol and TaatsiinTsagaan Nuur area are rich in fossils that provide unique evidence of mammal evolution in Mongolia. The strata are intercalated with basalt flows. 40Ar/39Ar data of the basalts frame the time of sediment deposition and mammal evolution and enable a composite age chronology for the studied area. We investigated 20 geological sections and 6 fossil localities of Oligocene and early Miocene deposits from this region. Seventy fossil beds yielded more than 19,000 mammal fossils. This huge collection encompasses 175 mammal species: 50% Rodentia, 13% Eulipotyphla and Didelphomorphia, and 12% Lagomorpha. The remaining 25% of species are distributed among herbivorous and carnivorous large mammals. The representation of lower vertebrates and gastropods is comparatively poor. Several hundred SEM images illustrate the diversity of Marsupialia, Eulipotyphla, and Rodentia dentition and give insight into small mammal evolution in Mongolia during the Oligocene and early Miocene. This dataset, the radiometric ages of basalt I (~31.5 Ma) and basalt II (~27 Ma), and the magnetostratigraphic data provide ages of mammal assemblages and time ranges of the Mongolian biozones: letter zone A ranges from ~33 to ~31.5 Ma, letter zone B from ~31.5 to ~28 Ma, letter zone C from ~28 to 25.6 Ma, letter zone C1 from 25.6 to 24 Ma, letter zone C1-D from 24 to ~23 Ma, and letter zone D from ~23 to ~21 Ma.

Electronic supplementary material

The online version of this article (doi:10.1007/s12549-016-0257-9) contains supplementary material, which is available to authorized users.

Keywords: Mongolia, Oligocene, Miocene, Correlation, Stratigraphy, Mammals

Introduction

The Valley of Lakes is an intermontane depression with a NW–SE longitudinal axis. It is bounded by the Khangai Mountains in the north and the Gobi Altai Mountains in the south. Our working area, the Taatsiin Gol region and Taatsiin Tsagaan Nuur region, ranging from 100° 55′ to 102° 05′ longitude and 45° 11′ to 45° 45′ latitude, is part of the Valley of Lakes (Fig. 1).

Fig. 1
Location of the working area in the Taatsiin Gol and Taatsiin Tsagaan Nuur region, which is part of the Valley of Lakes in Central Mongolia. “Taatsiin Gol” means River Taatsiin (gol = river), but also the area around the ...

This region has been intensively explored during the past two decades. In a collaboration between the Mongolian Academy of Sciences and the Natural History Museum Vienna, fieldwork was conducted during eight field seasons between 1995 and 2012.

The present study focuses on the stratigraphically lower part, the Oligocene and lower Miocene sediment sequences of the Hsanda Gol and Loh Formations. Here, 20 geological sections and 6 fossil sites are described and illustrated for the first time. Along these sections, fossils were collected from 70 fossil horizons. They contain the richest small mammal assemblages ever found in Mongolia and outline Cenozoic mammalian evolution (Daxner-Höck 2000, 2001; Daxner-Höck and Wu 2003; Erbajeva 2007; Schmidt-Kittler et al. 2007; Ziegler et al. 2007; Erbajeva 2013; Erbajeva and Daxner-Höck 2014; Wessels et al. 2014; Daxner-Höck et al. 2014, 2015; Maridet et al. 2014a, b, 2015; Erbajeva et al. 2017, this issue; López-Guerrero et al. 2017a, b, this issue; Maridet et al. 2017, this issue; Oliver et al. 2017, this issue; Harzhauser et al. 2016). In contrast, the record of large mammals (Vislobokova and Daxner-Höck 2002; Morlo and Nagel 2002, 2006, 2007; Nagel and Morlo 2003; Heissig 2007), lower vertebrates (Böhme 2007), and gastropods (Stworzewicz 2007; Neubauer et al. 2013) is comparatively scarce. It has to be noted that fossils which were collected before from the studied area (by American-Mongolian, Soviet-Mongolian, and Polish-Mongolian expeditions) are not included in the present dataset because their precise stratigraphic position remains questionable. Consequently, descriptions of these fossils were not considered in the present study.

The Cenozoic strata are intercalated with basalt flows, and 40Ar/39Ar data provide a timeframe for sediment deposition and the included fossils. Thus, basalt ages and Mongolian letter zones yield a composite age chronology for the studied area (Daxner-Höck et al. 1997; Höck et al. 1999; Daxner-Höck et al. 2010; Daxner-Höck and Badamgarav 2007; Harzhauser et al. 2017, this issue). Additional basalt data were provided by Devjatkin et al. (2002). Finally, magnetostratigraphic measurements (Kraatz and Geisler 2010; Sun and Windley 2015) were performed along the Taatsiin West plateau (sections—TGR below basalt I and TGR-C; Figs. 13, ,14,14, and and15)15) and in Tatal Gol (see Kraatz and Geisler 2010).

Fig. 13
The sections TGR-C and TGR-C′ are located at the southeast rim of the Taatsiin plateau (western plateau) and are exposed toward south. The lower part shows the Hsanda Gol Fm., the upper part is built up by units of the Loh and Tuyn Gol Fms. and ...
Fig. 14
The eastern rim of Taatsiin plateau exposes the sections TGR-B+B′, TGR-AB, TGR-A, and basalt I (~31.5 Ma) along of the orographic right side of Taatsiin Gol. From bottom to top, the sections display strata of the Tsagan Ovo, Hsanda ...
Fig. 15
The section TGR-B and B′ is shown in detail. It displays the light-coloured sediments of the Tsagan Ovo Fm., the lower red beds of the Hsanda Gol Fm., which are overlain by basalt I. Above basalt I, the upper Hsanda Gol Fm. is locally visible ...

Materials and methods

Fieldwork comprised geological mapping and studying geological sections based on lithology, structures, tectonics, and the fossil content. Basalt samples were dated by the 40Ar/39Ar method at the University of Vienna. Magnetic susceptibility and Gamma log measurements of sediments were carried out along five key sections, and sediment samples were taken for geochemical analyses and to determine the δ18O and δ13C patterns (Richoz et al. 2017, this issue). Along the geological sections, more than 100 palaeontological test samples and 60 bulk-samples of one to several tons of sediment were taken for wet screening in the field laboratory at the Taatsiin Gol camp. Sieves with 0.5, 2.5, and 5.0 mm mesh sizes were used.

In the field camp, the teeth, jaws, and bones were picked out from the dry residual using head lenses and field microscopes. The subsequent process of cleaning, identifying, and arranging the fossils took place at the NHMW (Natural History Museum Vienna). SEM images of small mammal teeth were taken using a Philips XL 20 scanning electron microscope at the Biocenter, University of Vienna. The fossils are stored in the collections of the NHMW and the MPC (Institute of Paleontology and Geology, Academy of Sciences of Mongolia).

Geological setting and stratigraphy

The Taatsiin Tsagaan Nuur Basin belongs to the Valley of Lakes, which is one of the Pre-Altai depressions in Mongolia, between the Gobi Altai mountains in the south and the Khangai mountains in the north. Here, above a Precambrian to Permian basement, the basin is filled by continental Jurassic, Cretaceous, and Cenozoic sediments. The basin tectonics is complex and beyond the scope of this study. Note, however, that several fault systems were observed in the course of geological mapping here (Höck et al. 1999). A prominent fault close to the northern margin of the basin, the Del fault, strikes NW–SE to W–E and was mapped from the Dzun Hsir in the east along the southern escarpment of the Uskok range (= Ushgoeg range) to the northwest close to Unzing Churum. As already described by Berkey and Morris (1927), the movement along the fault is a dip-slip towards the south (southwest) with an offset of at least 20 to 30 m. The fault plane varies from south dipping to vertical. Along the Del fault, sediments of the Tsagan Ovo Fm. and the Hsanda Gol Fm. including basalt I are inclined. In contrast, horizontally bedded sediments of the Loh Fm. on top of the Eocene–Oligocene strata date the Del fault as late Oligocene or earliest Miocene. Two younger fault systems striking NE–SW and E–W are overlain by the middle Miocene basalt III (Höck et al. 1999). The recent seismic activity south of the Valley of Lakes along the northern rim of the Gobi Altai, i.e. along the Gobi Altai or Ikh Bogd fault, has a sinistral sense of movement but also a dip-slip component towards the N. There, the last major earthquake took place in 1957 with a magnitude 8/9 (Baljinyam et al. 1993; Kurushin et al. 1997; Schlupp 1996). The recent Petro Matad’s exploration program of seismic, gravity, and stratigraphic core drilling demonstrates up to 4 km of folded and faulted basin fill, Mesozoic to Paleogene episodes of extension forming a half graben, and Neogene to recent episodes of compression (Fig. 2). The latter caused the ongoing uplift of the Gobi Altai range (Bag Bogd Massif).

Fig. 2
Unpublished geologic cross-section of the Taatsiin Gol and Guchin-Us basins (Mongolia) that was made available to us for publication by Justin Tully, Petro Matad LLC

This area is one of the best places in Mongolia to study Paleogene and Neogene sediment-basalt associations. Four lithological units can be identified: the Tsagan Ovo, Hsanda Gol, Loh, and Tuyn Gol formations (Daxner-Höck et al. 1997; Höck et al. 1999). The rich fossil content allows an update of the letter zones A, B, C, C1, and C1-D (Oligocene) and D (lowermost Miocene). These informal letter zones were defined as Biozones according to the International Stratigraphic Guide (Harzhauser et al. 2017, this issue).

Lithological units

Tsagan Ovo Fm.

The basal unit, the Tsagan Ovo Fm., is dominated by alternating grey, green-grey, whitish gravels and partly cross-bedded sand layers. The hanging parts are generally finer clastic and frequently show trough and planar cross-bedding, channel fills, and ripples. Normal graded and inverse graded beds with rip up clasts in decimetre size occur. Normal graded sandy beds can pass into fine-grained ones, which show lamination and root traces. The Tsagan Ovo Fm. was interpreted as a braided fluvial fan with a palaeocurrent direction from N to S (Höck et al.1999: 92–95) and partly as lake deposits. The time of deposition was late Eocene based on magnetostratigraphic correlation (Kraatz and Geisler 2010; Sun and Windley 2015).

Hsanda Gol Fm.

In many outcrops of the study area, the Tsagan Ovo Fm. is topped by the Hsanda Gol Fm. The latter consists of the lower Hsanda Gol beds, basalt I, and the upper Hsanda Gol beds. The term Tatal Member was introduced for the Hsanda Gol beds below basalt I and Shand Member for Hsanda Gol beds above basalt I (Dashzeveg 1996). The lower Hsanda Gol beds are of early Oligocene age, including fossils of letter zone A. The upper Hsanda Gol beds, however, range from the early Oligocene (including fossils of letter zone B) to the late Oligocene (including fossils of letter zones C and C1) or even reach the Oligocene/Miocene transition (evidenced by fossils of letter zone C1-D in section TAT-E/32; Figs. 21 and and22).22). The sediments are poorly sorted clay and silty clay and are reddish brown, brick red, to dark brown. Rare sand lenses or layers can be imbedded locally, e.g. in the Hsanda Gol region (SHG-A/14 and SHG-D/12; Fig. 25). Within these sediments, caliche horizons with different features are present, including compact layers, nodules, caliche grading laterally into clay layers, or occurrences of calichized basalt (Höck et al. 1999: 95–97). The Hsanda Gol beds are well known for their fossil richness. Fossil concentrations were observed in, below, or/and above caliche layers, and partly articulated skeletons were found in fossil burrows. The caliche layers are interpreted as palaeosol horizons, but the origin of the fine-grained Hsanda Gol sediments is under discussion. The interpretations range from ephemeral lake deposits, and braided fluvial fan sediments of the Tsagan Ovo Fm. that were eroded and transported by wind and/or by ephemeral streams (Höck et al. 1999), to eolian loess transported by westerly winds (Sun and Windley 2015).

Fig. 21
The Tatal Gol section at the west side of the Tatal creek is composed of three parts, the lower part TAT-D/1-4, the middle part TAT-E/1-20, and the upper part TAT-E/21-35 at the “North Ridge.” From south to north, the section comprises ...
Fig. 22
Locality Tatal Gol. a Tatal Gol North Ridge shows sample points TAT-051/1-2, TAT-052/1-2, TAT-054, and the upper part of section TAT-E/21-35. b Tatal section Gol middle part shows section TAT-E/1-20 and fossil points TAT-055, TAT-043, TAT-044, and TAT-E/3. ...
Fig. 25
Sections SHG-A and SHG-D are located at the south-western part of the S  NE striking ridge connecting the Hsanda Gol and Loh localities. The southern part of the ridge is dominated by sediments of the Hsanda Gol Fm

Loh Fm.

Sediments of the Loh Fm. are most widespread in the study area. In many outcrops, the Hsanda Gol beds are covered by sediments of the Loh Fm., and in other places we found Loh sediments immediately on top of the Tsagan Ovo Fm. Loh sediments are predominantly trough cross-bedded, poorly sorted, polymict, matrix-supported gravels and sands of fluvial origin, with structures and colours similar to the Tsagan Ovo Fm. The two formations mainly differ in the gravel spectra: the Loh Fm. contains basalt, carbonate, and carbonate-tuff components due to erosion of basalts (I, II, and III) and Hsanda Gol sediments. Moreover, red to beige silty sand and sandy layers of several metres thickness can alternate with caliche and/or light-coloured sand and gravel layers (Höck et al. 1999: 97–100). These red-rose silts and caliche layers contain mammal fossils of late Oligocene to late Miocene age. The middle Miocene basalt III (13 Ma) is part of the Loh Fm.; it is frequently exposed on top of the plateaus east, north, and northwest of Taatsiin Gol.

Tuyn Gol Fm.

This formation crops out rarely and is restricted to the plateaus west and east of the Taatsiin Gol. The sediments are poorly sorted, grey-brown gravels of ~9 cm diametre. Quartz components with Fe2O3 coatings, along with basalt, siltstone, granite, quartzite, gneiss, rhyolite, sandstone, and pegmatite, dominate the gravel spectrum (Höck et al. 1999: 100).

Basalt ages

Basalts I

The basalts have been dated by the 40Ar/39Ar method, providing a stratigraphic framework in which the biostratigraphic data are fitted. Based on 31 dated basalt samples, three main groups of basalt occurrences were identified by Höck et al. (1999: 108–113; Fig. 18). These are the early Oligocene basalt I group around 31.5 Ma (32.2–30.4 Ma), the late Oligocene basalt II group around 28 Ma (29–27 Ma), and the middle Miocene basalt III around 13 Ma (13.2–12.2 Ma). The geochemistry and mineralogy of basalts I–III was described by Höck et al. (1999: 104–108: Table Table5,5, Figs. 12, ,13,13, ,14,14, ,15,15, ,16,16, and and17).17). Since then, additional basalt ages have become available, showing that the Oligocene basalt events (basalt I and basalt II groups) occurred more or less continuously (32.4–29.1 and 28.7–24.9 Ma, respectively). The middle Miocene volcanism (basalt III group), however, started after an interval of 10 million years (14.9–12.2 Ma) (Tables 1 and and22).

Fig. 63
a 40Ar/39Ar data of basalt I–III from the Taatsiin Gol and Taatsiin Tsaagan Nuur region (Valley of Lakes, Mongolia). Basalt I (20 samples), basalt II (14 samples), basalt III (14 samples)
Table 2
Localization and age dating of basalt samples from the Valley of Lakes are given in columns 1–5: 1 sample codes; 2 coordinates; 3 basalt group I, II, III; 4 40Ar/39Ar age of the basalt sample; 5 localization of the basalt sample/section or fossil ...
Table 5
Fossil list from Huch Teeg (section—RHN-A, samples—RHN-A/6-12, RHN-019-022) [the age of the assemblages is late Oligocene (letter zones C1 and C1-D) to early Miocene (letter zone D)]
Fig. 12
Locality Unkheltseg showing the sample points UNCH-A/3 and UNCH-A/4. a View from Unkheltseg toward southwest. Localization of section UNCH-A and samples UNCH-A/3 and UNCH-A/4. b, c Section UNCH-A: The contact zone between the Hsanda Gol Fm. with fossils ...
Fig. 16
Section TGL-A is located at the orographic left side of Taatsiin Gol (eastern plateau). The lower part of section TGL-A comprises the lower Hsanda Gol beds and basalt I (31.6 ± 0.5 Ma). Description of section TGL-A after ...
Fig. 17
Above the Oligocene basalt I, the upper Hsanda Gol beds (sample TGL-A/11) are located below light-coloured sand and gravel beds of the Loh Fm. On top, the middle Miocene basalt III (13.1 ± 0.2 Ma). Description of section ...
Fig. 18
Section TAR-A is located north-east of Taatsiin Gol. The section comprises sequences of the Loh Fm. including two basalt layers, basalt II and basalt III. Description of section TAR-A modified from Schmid (1999; Abb. A4)

In contrast, the regional distribution of basalt I and II differs significantly. Basalt I occurrences are concentrated in the southern and central part of the study area. They extend from the western as far as the easternmost investigated regions. The most prominent outcrops are visible at the plateau west of Taatsiin Gol (sections TGR-A, TGR-B, TGR-AB, TGW-A, HL-A; Figs. 8, ,14,14, and and15),15), the plateau east of Taatsiin Gol (section TGL-A; Fig. 16), along the Del fault (section DEL-B; Fig. 20) where the basalt I and tuff I are tilted, in Tatal Gol (section TAT-C; Fig. 23), and east of Tatal Gol (sections SHG-C and GRAB-II; Figs. 24 and and27).27). Basalt I is imbedded in red clay/silty clay of the Hsanda Gol Fm. In N–S direction, all basalt I occurrences are located south of basalt II. Basalt II is exposed in the northern parts of the study area in four main regions, the northwest region (section ABO-A; Fig. 6), the Unzing Churum region (section TAR-A; Figs. 18 and and19),19), the northern Tatal Gol region, and north of Ulan Tolgoi. Basalt II is bound to strata of the Loh Fm. For localization of sections, see Fig. 3.

Fig. 3
Geological map of the Taatsiin Gol and Taatsiin Tsagaan Nuur region in the Valley of Lakes (Höck et al. 1999). The letters az designate fossil places and investigated sections. a Luuny Yas (LUS), b Luugar Khudag (LOG-A), c Abzag Ovo (ABO-A), ...
Fig. 6
In Abzag Ovo, the samples ABO-A/3 and ABO-083 were taken from red silty claystone 1–2 m below the late Oligocene basalt II
Fig. 8
Khongil is located at the NW corner of the plateau at the orographic right side of Taatsiin Gol. Arrows mark two sample points HL-A/1 and HL-A/2 below the early Oligocene basalt I
Fig. 19
Section TAR-A from the locality Unzing Churum. Sample TAR-A/2 on top of basalt II yields fossils of letter zone C
Fig. 20
The Del section is located at the Del fault, close to the boundary between Mesozoic and Paleogene strata in the northern part of the Taatsiin Tsagaan Nuur basin. From the left to the right side of the picture, the Tsagan Ovo Fm. (in whitish-grey colours), ...
Fig. 23
Section TAT-C is located east side of the Tatal creek in the north-eastern part of the locality
Fig. 24
The Hsanda Gol section SHG-C is located below basalt I of a widespread basalt flow between the Tatal Gol and Hsanda Gol localities
Fig. 27
Talyn Churum is located in the eastern part of the studied area, southeast of the volcano Dzun Hsir. The sample GRAB-II was excavated below basalt I. Graves of the Bronze Age are visible in the background

Some basalt occurrences with fossil contact are chrono-stratigraphically important. In the Taatsiin Gol, Del, and Tatal Gol regions, basalt I is intercalated with strata of the Hsanda Gol Fm. Consequently, fossil beds below basalt I are older, and those above basalt I are younger than ~31.5 Ma. In the northern regions, for example, basalt II (sample M132/97) of section ABO-A (Fig. 6) is located immediately above fossil bed ABO-A/3 and dates the fossils older than 27.9 ± 0.9 Ma. In the Unzing Churum region (Figs. 18 and and19),19), basalt II (sample M132/97) is located immediately below fossil bed TAR-A/2 and dates the fossils younger than 27.4 ± 0.4 Ma (Tables 1 and and22).

Basalt III is part of the Loh Fm. and forms the top layer of several plateaus, i.e. the plateau to the left side (east) of Taatsiin Gol, the Unzing Churum plateau, and extended plateaus in the northwest region. In the latter, fossil-bearing strata are locally sandwiched between basalt II and basalt III.

Geological sections

The present study provides a detailed presentation and correlation of the elaborated sections comprising the Oligocene and lowermost Miocene strata. Middle and late Miocene sediment sequences have been excluded from this study because of ongoing investigations in this region.

We describe the sections/localities according to their location from west to east (Fig. 3a to z). A complete overview of all investigated sections/localities, fossil samples, the respective codes, coordinates, and letter zones are given in Table Table3.3. Some assemblage lists (e.g. TGR-C/1+2) are composite. They comprise fossils of two individual samples from subsequent, time-equivalent sediment layers of the same section. All these data are published here for the first time. For some published data of the figured sections, we give references in the figure captions.

Table 3
Overview of the figured sections/fossil sites, fossil samples, the respective codes, coordinates, and letter zones

Locality Luuny Yas

Samples: LUS-027, LUS-028, LUS-029 (=LUS-078)

Luuny Yas is the westernmost fossil point of the study area (Fig. 3a), first recognised during geological mapping in 1997. Later, in the field seasons 2006, 2011, and 2012, fossils were collected from the surface at three locations (LUS-027, LUS-028, and LUS-029; Fig. 4). So far, no geological section has been studied in detail. In Luuny Yas, the red-brown sandy silts of the Loh Fm. are topped by basalt III. At LUS-029, fossil concentrations are visible on top of a caliche layer. From this site, a test sample (sample LUS-078/~500 kg) was investigated. The lower Miocene and letter zone D are indicated by the small mammals (composite fossil list below).

Fig. 4
The locality name Luuny Yas means “dragon bone”. Arrows mark the fossil points LUS-027, LUS-028, and LUS-029 (= LUS-078)

Locality Luugar Khudag

Sample: LOG-A/1

Luugar Khudag is located in the northwestern part of the study area (Fig. 3b). The palaeontological sample LOG-A/1 (~500 kg brick-red sandy silt of the Loh Fm.) was taken close to a well in the dry river bed (Fig. 5). The lower Miocene is indicated by characteristic fossils of letter zone D.

Fig. 5
Sample point LOG-A/1 from the locality Luugar Khudag, Valley of Lakes

Locality Abzag Ovo

Section: ABO-A; samples: ABO-A/3,-083

Abzag Ovo and the section ABO-A are located ~30 km northwest of the Taatsiin Gol (Fig. 3c). At Abzag Ovo, the red silty claystone of the Hsanda Gol Fm. is up to 10 m thick. It is topped by a 1–5-m-thick basalt II, which was dated at 27.0 ± 0.9 Ma (40Ar/39Ar age). The palaeontological samples ABO-A/3 (~500 kg sampled 1997) and ABO-083 (~500 kg sampled 1997 and 2011) were taken 1–2 m below basalt II (Fig. 6). The two samples yield identical fossils. Basalt II and the small mammal assemblage indicate a late Oligocene age and letter zone C. Abzag Ovo is one of the rare assemblages yielding land gastropods.

Locality Toglorhoi

Section: TGW-A; samples: TGW-A/1-5

The locality Toglorhoi is located in the Khunug Valley, west of the Taatsiin Gol region (Fig. 3d). The section comprises ~7 m of red-brown sediments of the Hsanda Gol Fm. (Fig. 7). Fossil concentrations are mostly bound to caliche layers and caliche nodules. The colour of the silty clay grades from red-brown (TGW-A/1) to dark red-brown in its higher part (TGW-A/5). Bulk samples of several tons were investigated from all horizons with visible fossil content (TGW-A/1-5). Samples TGW-A/1, TGW-A/2a, and TGW-A/2b yield index fossils of letter zone C. The prevailing fossils of samples TGW-A/3, TGW-A/4, and TGW-A/5 from the higher part of the section are Tsaganomys and the large ctenodactylid Yindirtemys deflexus; the latter is an excellent marker of letter zone C1 (Table (Table4).4). The entire sequence is of late Oligocene age.

Fig. 7
Section TGW-A from the locality Toglorhoi is located in the Khunug Valley, west of the Taatsiin Gol region
Table 4
Fossil list from the locality Toglorhoi (section—TGW-A, samples—TGW-A/1, TGW-A/2a+2b, TGW-A/3+4, TGW-A/5) [the age of the assemblages is late Oligocene (letter zones C and C1)]

Locality Khongil

Samples: HL-A/1 and HL-A/2

Khongil is located at the NW corner of the Taatsiin plateau at the orographic right side of Taatsiin Gol (Fig. 3e). There, several metres of brick-red clay of the Hsanda Gol Fm. are exposed immediately below basalt I. The mammal fauna stems from two test samples HL-A/1 and HL-A/2 (for location, see Fig. 8). The early Oligocene age is indicated by basalt I and by respective fossils.

Locality Huch Teeg

Section: RHN-A; samples: RHN-A/6-12, RHN-019-023

Huch Teeg is located at the orographic right side of Taatsiin Gol, north of the western Taatsiin plateau (Fig. 3f). The direction of section RHN-A is N  S (Fig. 9). There, sediments of the Tsagan Ovo and Loh Fms. are exposed; the Hsanda Gol Fm. is missing. The present study does not consider the Tsagan Ovo Fm. from the northernmost part of the section. The fossil-bearing strata of the Loh Fm. (RHN-A/6-10) dip toward south. The southernmost part of the section (samples—RHN-A/11-12) is horizontally bedded and built up of light rose-brown to red-brown sandy silt.

Fig. 9
Huch Teeg is located at the orographic right side of Taatsiin Gol, north of the western Taatsiin plateau. Description of the section RHN-A modified from Schmid (1999; Abb. A5) and Daxner-Höck et al. (2013, Fig. 20.2)

The tilted northern part of the section (samples RHN-A/6-10, RHN-019, and RHN-023 of the Loh Fm.) yields fossils of letter zone C1, indicating the late Oligocene age. The horizontally bedded southern part (samples RHN-A/11 and RHN-021-22) starts with fossils of letter zone C1-D and ends with fossils of letter zone D (samples RHN-A/12 and RHN-020), indicating an early Miocene age (Table (Table5).5). There, concentrations of manganese precipitates and nodule are evident.

Locality Hotuliin Teeg

Sections HTE; samples: HTE-003-018 (Fig. 11b, c), HTE-057 (Figs. 10 and 11d); HTSE-009, HTSE-013 (Fig. 11f); HTS-056/1-3 (Fig. 11e)

Fig. 10
The Hotuliin Teeg section is located west of the Taatsiin Gol, in a wide NW-SE striking valley north of the main Taatsiin plateau (western plateau). The picture shows the lowermost, the late Oligocene part of the section
Fig. 11
Localities Hotuliin Teeg and Unkheltseg north of the western Taatsiin plateau. a View in southern direction from Hotuliin Teeg to Unkheltseg at the north rim of the plateau. b Sample place HTE-005 (= HTE-12/5) at the upper part of the HTE-section. c Sample ...

The Hotuliin Teeg section (HTE) and additional fossil points (HTSE and HTS) are located north of the western Taatsiin plateau (Fig. 3g). The area is flat and comprises no more than 23 m of sediment. The section HTE (Figs. 10 and 11a) was studied along of a dry creek. In the lower part, several layers of strongly weathered basalt alternate with silty-sandy claystone. On top of this first sequence (Fig. 11d, g), the late Oligocene is indicated by fossils of letter zone C1.

Upsection, alternating beds of calichized basalt and silty clay continue. The colour changes into light brown. The claystone ultimately grades into thick white chalky caliche, which is topped by badly sorted coarse sand and gravels (Fig. 11c). The boundary horizon between the caliche and gravely sand shows significant fossil concentrations (samples HTE-014-018 from the south bank of the dry creek, Figs. 10 and 11c). Upsection, similar fossil traps were found between sand-silt layers/lenses and caliche beds. The fossils indicate letter zone D and the lower Miocene. The top layer of the HTE-section is built up by 2–3 m of beige sand and gravel, which yield fossil bones of the rhinos cf. Hoploaceratherium gobiense and cf. Caementodon sp. (Fig. 11b; Table Table66).

Table 6
Fossils from Hotuliin Teeg (section—HTE, samples—HTE-003-012) [the age of the assemblages is early Miocene (letter zone D)]

Five samples from a neighbouring dry creek yield fossils of letter zone C1 and letter zone C1-D. These are the samples HTSE-009 and HTSE-013 (Fig. 11f) south-east, and the samples HTS-056/1-3 (Fig. 11e) south of the Hotuliin Teeg creek. Sample HTSE-009 consists of red clay above a white-green caliche layer. On top, basalt pebbles are exposed. Sample HTSE-013 was collected between rose and white caliche layers. Both samples yield fossils of letter zone C1 (Table (Table7),7), indicating the late Oligocene. Following the dry river westward leads to the fossil points HTS-056/1+2 and HTS-056/3, which yield fossils of letter zone C1-D and indicate the uppermost Oligocene.

Table 7
Fossils from Hotuliin Teeg (samples—HTE-057, HTSE-009, HTSE-013, HTS-056/1-3) [the age of the assemblages is late Oligocene (letter zones C1 and C1-D)]

Locality Unkheltseg

Samples: UNCH-A/3 and UNCH-A/4

Unkheltseg is located at the northern rim of the Taatsiin plateau, west of the Taatsiin Gol (Fig. 3h). In this area, basalt I is interrupted, and the brick-red clay of the Hsanda Gol Fm. is immediately overlain by a thin layer of rose silty sand and gravels of the Loh Fm. mixed with abundant caliche nodules and reworked basalt (section UNCH-A; Fig. 12b, c). Here, the Loh Fm. yields fossils of letter zone D; the Hsanda Gol clay yields fossils of letter zone B. When we investigated the first bulk samples UNCH-A/3 and UNCH-A/4, years ago, the formation boundary of section UNCH-A was not visible; thus, fossils of letter zone B and D were mixed in both samples. Later, the fossils could easily be separated into two parts, one of letter zone B and the second of letter zone D (Table (Table88).

Table 8
Fossils from the locality Unkheltseg (samples—UNCH-A/3B+4B and UNCH-A/3+4) [the ages of the assemblages are early Oligocene (letter zone B) and early Miocene (letter zone D)]

Locality Taatsiin Gol (south of the western plateau)

Sections: TGR-C and TGR-C′; samples: TGR-C/1, TGR-C/2, TGR-C/5+6, TGR-C/7 (Fig. 3i, Fig. 13)

The lower part of the sections consists of red-brown claystone alternating with red-rose caliche layers (layers—TGR-C/1-10). It is overlain by 55 cm of dark brown claystone, a thin layer of orange-pink caliche, and red claystone (sediment layers—TGR-C/11-13). The dark brown claystone and orange-pink caliche (TGR-C/11) mark the boundary between letter zones C and C1. The samples TGR-C/1+2 are very fossil-rich, which indicate letter zone C (Table (Table9).9). Upsection, olive-green claystone layers alternate with white chalky caliche and grade into red-brown caliche (layers—TGR-C/14-19 with poor fossil content).

Table 9
Fossils from the locality Tatsiin Gol south (composite samples—TGR-C/1+2 and TGR-C/5-7) [the age of the assemblages is late Oligocene (letter zone C)]

The boundary between the Hsanda Gol and Loh Fms. is marked by a second dark red-brown to chocolate-brown clay (sediment layer—TGR-C/20). The uppermost part of the section is dominated by light-coloured sand and gravel layers of the Loh Fm.; on top, gravels of the Tuyn Gol Fm. and/or Pleistocene gravels (TGR-C/21-26) are exposed.

According to magnetostratigraphic investigations (Sun and Windley 2015), the red clay-caliche sequences (layers—TGR-C/1-13) correspond with the palaeomagnetic polarity Chrons C9n–C8n.1n, with an age range of 27.4–25.2 Ma. The whitish clay and caliche sequence up to the chocolate brown clay (layers—TGR-C/14-20) below the sand-gravel sequence of the Loh Fm. correspond with Chrons C7Ar–C7n.2n (age 25.2–24.2 Ma). Our section TGR-C was described as section B by Sun and Windley (2015). The correlation of Mongolian letter zones and magnetostratigraphic data is discussed below. The data from section TGR-C confirm the hitherto estimated age range of ~28–25.6 Ma of letter zone C (Daxner-Höck et al. 2015).

Locality Taatsiin Gol (right side of the river Taatsiin; western plateau)

Sections: TGR-B, TGR-B′, TGR-AB, TGR-A; samples: TGR-B/1, TGR-AB/22, TGR-AB/21, TGR-A/13+14, TGR-ZO/1, and TGR-ZO/2

Along of the east rim of the Taatsiin plateau (orographic right side of the river Taatsiin), the sections TGR-A, TGR-B, and TGR-AB are exposed (Fig. 3k–m). There, four lithological units are visible: the Tsagan Ovo, Hsanda Gol, Loh, and Tuyn Gol Fms. In its lower part, the sections cover fluvio-lacustrine deposits of the Tsagan Ovo Fm. Upsection, and the brick-red clay of Hsanda Gol Fm. is topped by basalt I of early Oligocene age (40Ar/39Ar age, ~31.5 Ma). The fossil beds (TGR-A/13+14), located immediately below basalt I, comprise key fossils of letter zone A. Above basalt I, 7 m of upper Hsanda Gol beds follow. The samples TGR-B/1, TGR-AB/21, and TGR-AB/22 from above basalt I yield fossils of letter zone B. Upsection, light-coloured sand and gravels of the Loh Fm. follow; on top, brown gavels and boulders of the Tuyn Gol Fm. are exposed (Figs. 14 and and1515 and Höck et al. 1999; Fig. 6a).

Samples TGR-ZO/1 and TGR-ZO/2 were taken from red beds between two individual lava flows of basalt I at the east rim of the Taatsiin plateau (Fig. 3j). The mammal assemblages and basalt I indicate an early Oligocene age (Table (Table1010).

Table 10
Fossils from Taatsiin Gol right side (sections—TGR-A and TGR-B, samples—TGR-A/13+14, TGR-ZO/1+2, TGR-B/1, TGR-AB/21, TGR-AB/22) [the age of the assemblages is early Oligocene (letter zones A and B)]

Magnetostratigraphic investigations of the Tsagan Ovo Fm. and Hsanda Gol Fm., including basalt I, have been performed along a comparable section, which was named section A by Sun and Windley (2015). The strata above basalt I, containing the upper Hsanda Gol beds and the Loh Fm., were not considered in the magnetostratigraphic investigations.

From bottom to top, the sequences of the Tsagan Ovo Fm. correspond with Chrons C15r–C13r (>35–34 Ma/late Eocene). Thus, the boundary between the Tsagan Ovo Fm. and Hsanda Gol Fms. corresponds with the Eocene-Oligocene boundary. The lower Hsanda Gol strata and basalt I correspond with the palaeomagnetic polarity Chrons C13r–C12r, with an age range of ~34–31.2 Ma (Kraatz and Geisler 2010; Sun and Windley 2015), which is an early Oligocene age. These data agree with the 40Ar/39Ar ages measured from several samples of basalt I in the Taatsiin Gol region (Tables 1 and and22).

Locality Taatsiin Gol (left side of the river; eastern plateau)

Sections: TGL-A+A′; samples: TGL-A/1+2, TGL-A/11

Section TGL-A from the orographic left side of Taatsiin Gol (Fig. 3n) comprises the lower Hsanda Gol beds with fossils of letter zone A (samples TGL-A/1+2) and basalt I (31.6 Ma; Fig. 16). Above basalt I, section TGL-A′ displays the upper Hsanda Gol beds with fossils of letter zone B (sample TGL-A/11; Fig. 17) and a 25-m-thick sequence of the Loh Fm. The middle Miocene basalt III (13.1 Ma) forms the top of the plateau. Samples below basalt I (TGL-A/1+2) yield small mammal fossils and land gastropods (Table (Table11).11). The early Oligocene age and letter zone A are indicated by basalt I and the included fossils.

Table 11
Fossils from Taatsiin Gol left side (section—TGL-A, samples—TGL-A/1+2, TGL-A/11) [the age of the assemblages is early Oligocene (letter zones A and B)]

Locality Unzing Churum

Section: TAR-A; sample: TAR-A/2

Unzing Churum is located north-east of Taatsiin Gol (Fig. 3o). The lower part of section TAR-A consists of light-coloured fluvial sand and gravel deposits, which are overlain by basalt II. The 40Ar/39Ar age of basalt II is 27.4 ± 0.4 Ma (Höck et al. 1999 and Tables 1 and and2).2). Above basalt II, 8 m of brick-red sandy silt follow. Sample TAR-A/2 from the white-orange-red silty clay yields fossils of letter zone C (Figs. 18 and and19).19). Upsection, fluvial deposits follow. These include sand and silt layers and partly cross-bedded gravels. The section is topped by the middle Miocene basalt III, dated at 12.9 ± 0.1 Ma.

For palaeoenvironmental considerations, it is worth to note that pillow structures were formed locally where basalt II flowed into a shallow pond or lake (Höck et al. 1999; Fig. 10b).

Locality Del

Section: Del-B; samples: DEL-B/2, DEL-B/7+8, DEL-B/12

The locality Del is located in the northern part of the Taatsiin Tsaagan Nuur basin (Fig. 3p). The direction of section DEL-B is N  S. From north to south, strata of the Tsagan Ovo, Hsanda Gol, and Loh Fms. are affected by the Del fault and are tilted towards south. The Hsanda Gol beds are divided by a tuff layer several metres in thickness (tuff I; Fig. 20). The lower Hsanda Gol beds yield very rare fossils of letter zone A (sample DEL-B/2). Above tuff I, several caliche layers are imbedded in the upper Hsanda Gol beds. The abundant fossils from these caliche layers (samples DEL-B/7 and DEL-B/8) indicate letter zone B and the early Oligocene. The following grey-brown silt of the Loh Fm. lacks fossils. Upsection, sample DEL-B/12 from a red silt layer yields fossils of letter zone C1 (Yindirtemys deflexus), pointing to the late Oligocene (Table (Table1212).

Table 12
Fossils from the locality Del (section—DEL-B, samples—DEL-B/7+8 and DEL-B/12) [the age of assemblages DEL-B/7+8 is early Oligocene (letter zone B) and of assemblage DEL-B/12 is late Oligocene (letter zone C1)]

Locality Tatal Gol

For localization, see Fig. 3q, r. In Tatal Gol, two sections were studied, the composite section TAT-D+E (Figs. 21 and and22,22, Tables 13 and and14)14) and section TAT-C (Fig. 23, Table Table1616).

Table 13
Fossils from the locality Tatal Gol (section—TAT-D, sample—TAT-D/1) [the age of assemblage is early Oligocene (letter zone A)]
Table 14
Fossils from the locality Tatal Gol (section—TAT-E/1-32 and samples—Fig. 21a, b) [the ages of the assemblages are early Oligocene (letter zones A and B) to late Oligocene (letter zones C, C1, and C1-D)]
Table 16
Description of sections SHG-A and SHG-D from the Hsanda Gol region

Section TAT-D+E (Fig. 21) is located west of the dry creek, called Tatal Gol. The section is composed of three parts: the lower part TAT-D/1-4 (Fig. 22c), the middle part TAT-E/1-20 (Fig. 22b), and the upper part at the “North Ridge” is TAT-E/21-32 (Fig. 22a). In section TAT-C (Fig. 23, Table Table16),16), which is located east of the Tatal creek, the Hsanda Gol Fm. displays the lower Hsanda Gol beds, tuff and basalt I, and the upper Hsanda Gol beds.

Section TAT-D+E; samples: TAT-D/1, TAT-E/3, TAT-E/22, TAT-E/27, TAT-E/32; TAT-037, TAT-038-, TAT-054, TAT-055, TAT-043, TAT-044, TAT-051/1-2, TAT-052/1-2

In this section, the Hsanda Gol Fm. comprises fossils of letter zones A, B, C, C1, and C1-D, showing that the Hsanda Gol Fm. ranges from the early Oligocene to the Oligocene/Miocene transition. Although basalt I is missing in the western part of Tatal Gol, the lower and upper Hsanda Gol beds are easily recognisable by the included fossils.

The lower (= southern) part, section TAT-D, is composed of brick-red silty claystone, which yields abundant caliche concretions and a mammal assemblage that is very rich in fossils of letter zone A (sample TAT-D/1; Table Table13).13). A layer of coarse grey sand follows, which is topped by the brick-red clay of the Hsanda Gol Fm., yielding fossils of letter zone B. In the middle part (TAT-E/1-11), the upper Hsanda Gol Fm. comprises fossils of letter zone B (sample TAT-E/3; Table Table14).14). Upsection, fossils of letter zone C (sample TAT-055; Table Table14)14) were recovered from a reddish carbonatic claystone below a 3-m-thick red-rose caliche. On top, the colour of caliche changes to orange-red with dark brown clay clast inclusions. The fossils from the orange caliche and the overlying brick-red to dark-brown clay indicate letter zone C1. These fossil assemblages of letter zone C1 are characteristic of the higher part of the section (samples TAT-043 to TAT-E/27; Table Table14)14) and of sample points laterally of the main section. Finally, fossils of letter zone C1-D were found in the dark brown clay layers close to the top of the section at the North Ridge (samples TAT-052/2 and TAT-E/32) (Figs. 21 and and22,22, Table Table14),14), which are easily recognisable by the included fossils.

Section: TAT-C; samples: TAT-C/1-3, TAT-C/6-7

From bottom to top, the section displays lower Hsanda Gol beds below basalt and tuff I (samples—TAT-C/1-3) and upper Hsanda Gol beds (samples—TAT-C/6-7) above basalt I (Fig. 23; Table Table1515).

Table 15
Fossil list from section TAT-C in Tatal Gol (the samples TAT-C/1-3 below basalt I yield fossils of letter zone A, the fossils from samples TAT-C/6-7 above basalt I indicate letter zone B)

Locality Hsanda Gol

From the Hsanda Gol area, we investigated three sections: SHG-C (Fig. 3s), SHG-A, and SHG-D (Figs. 3t and u).

A basalt plateau of ~50 km2 extension is exposed between the Tatal Gol and Hsand Gol regions, and section SHG-C (Fig. 24) is located in its south-eastern corner. East of the basalt plateau, a SW  NE striking ridge consists of sequences of the Hsanda Gol Fm. The sections SHG-A and SHG-D are located at the southern part of this ridge. Following the ridge in NW direction, the Hsanda Gol Fm. is topped by strata of the Loh Fm. A small dry creek, the “Hsanda Gol,” east of the ridge is giving name to the entire region and to the Hsanda Gol Fm.

Section: SHG-C; samples: SHG-C/1-2

Below basalt I (31–32 Ma), 10 m of red-brown silty claystone yields fossils of letter zone A (samples—SHG-C/1 and SHG-C/2). The early Oligocene age is indicated by basalt I and by the fossils.

Sections: SHG-A and SHG-D; samples: SHG-A/1, SHG-A/9-10, SHG-A/12, SHG-A/15, SHG-A/17-20; SHG-AB/12, SHG-AB/15-20, SHG-AB/top

The Hsanda Gol sections SHG-A, SHG-D, and the SHG-AB samples have no contact to any basalt; however, letter zone B is indicated by the rich fossil content (Fig. 25; Tables 15 and and1616 and Table Table17).17). Here, the upper Hsanda Gol beds are composed of 35–40 m red-brown claystone alternating with caliche. This sequence is divided by 3 m of sandstone and gravels (layers—SHG-A/13+14 and SHG-D/12). A significant orange caliche layer (SHG-D/28-31) above dark brown clay-stone (SHG-D/27) terminates the lower Oligocene strata. On top of this sequence, fossils of letter zone C1 indicate the upper Oligocene.

Table 17
Fossils from the Hsanda Gol locality (sections SHG-C, SHG-A, and SHG-AB-samples yield fossils of the letter zones A, B, and C1)

Locality Loh

Sections: LOH-C and LOH-B; samples: LOH-C/1, LOH-B/3

The sections LOH-C and LOH-B (Fig. 3v and w) are located in the middle and north-eastern part of the SW  NE striking ridge, between the Hsanda Gol and Loh regions. From bottom to top, the sections display strata of the Hsanda Gol and Loh Fms. (Fig. 26). Above the orange caliche layer, characteristic fossils of letter zone C1 were found (LOH-C/1 and LOH-B/3). Upsection, light-coloured sand of the Loh Fm. alternates with red silty clay.

Fig. 26
Section LOH-B and fossil list of sections LOH-B and LOH-C

Locality Talyn Churum

Sample: GRAB-II

Talyn Churum is one of the eastern fossil points (Fig. 3x). The short section displays about 10 m of red-brown silty claystone of the Hsanda Gol Fm. below basalt I (31–32 Ma) (Fig. 27). The early Oligocene age of the included fauna (letter zone A) is indicated by basalt I.

Locality Ikh Argalatyn Nuruu

Sections: IKH-A and IKH-B; samples: IKH-A/1-5, IKH-B/2, IKH-B/5

From Ikh Argalatyn Nuruu, two sections were investigated, section IKH-A (Fig. 3y) and section IKH-B (Fig. 3z). The two sections are located in the easternmost part of the study area. Section IKH-A exposes red silty clay layers alternating with caliche of the upper Hsanda Gol Fm. Samples IKH-A/1-4 yield fossils of letter zone B. The top layer of orange caliche (yielding Y. deflexus) marks the lower boundary of letter zone C1 (Fig. 28,,29;29; Table Table1818).

Fig. 28
Ikh Argalatyn Nuruu is located in the eastern part of the studied area. Section IKH-A comprises sequences of the upper Hsanda Gol Fm. ranging from the early to the late Oligocene
Table 18
Fossils from the locality Ikh Argalatyn Nuruu [fossils of section IKH-A (samples IKH-A/1-4) indicate the letter zone B (early Oligocene), fossils from samples IKH-A/5 and IKH-B/5 indicate letter zone C1 (late Oligocene)]
Fig. 29
Section IKH-B is located in the easternmost part of the study area. The section comprises sequences of the Hsanda Gol and Loh Fms

Correlation of geological sections from the Valley of Lakes

Today, the combination of biostratigraphic and lithologic data from the Taatsiin Gol and Taatsiin Tsagaan Nuur regions, the 40Ar/39Ar ages of basalts (Tables 1 and and2),2), and magnetostratigraphic data (Sun and Windley 2015) allows correlation of sections and fossil horizons with the Geomagnetic Polarity Time Scale (GPTS) (Gradstein et al. 2012). This provides a composite age chronology for the entire sequence as demonstrated for selected key sections (Fig. (Fig.3030).

Fig. 30
Chronostratigraphic and geochronologic correlation of key sections from the Valley of Lakes. The stratigraphic chart includes the Geologic Time Scale and the GPTS (Gradstein et al. 2012; Ogg et al. 2004); the Chinese Mammal Ages Hsandagolian, Tabenbulukian, ...

Late Priabonian to early Rupelian (Hsandagolian/letter zone A)

As outlined above, the lower part of section TGR-AB (Figs. 14 and and30)30) comprises fluvio-lacustrine deposits of the Tsagan Ovo Fm. followed by brick-red clay of the Hsanda Gol Fm. (lower Hsanda Gol beds), which is topped by basalt I (40Ar/39Ar age ~31.5 Ma). The fossils of samples TGR-A/13+14 below basalt I (Table (Table11)11) evidence letter zone A and the early Hsandagolian Mammal age, respectively. These data allow correlation of magnetostratigraphic measurements along of the TGR section with the GPTS, showing that the lower Hsanda Gol beds and basalt I are to be correlated with Chrons C12r–C13r (section A in Sun and Windley 2015; Fig. 3) and the early Rupelian, respectively. The age range of the lower Hsanda Gol beds is ~34–31.5 Ma. The age range of the Tsagan Ovo sequence is >35 to ~34 Ma (late Piabonian). The Eocene and Oligocene boundary (EOB; Figs. 14 and and30)30) is located between the Tsagan Ovo and the Hsanda Gol Fms. at ~34 Ma (Kraatz and Geisler 2010; Sun and Windley 2015).

Sediment sequences of the early Rupelian (below basalt I or tuff I) are evidenced in the regions Taatsiin Gol (sections TGR-A, TGR-AB, TGR-B, HL-A, TGL-A), Del (section DEL-B), Tatal Gol (sections TAT-D and TAT-C), Hsanda Gol (section SHG-C), and Talyn Churum (GRAB-II) (see Figs. 30 and and3131).

Fig. 31
Chronostratigraphic correlation and calculation of geochronologic ages of mammal assemblages from the Valley of Lakes in Mongolia. The correlation chart includes the Geologic Time Scale and the Geomagnetic Polarity Time Scale (GPTS) (Gradstein et al ...

Late Rupelian (Hsandagolian/letter zone B)

From the upper Hsanda Gol beds with fossils of letter zone B, no magnetostratigraphic data are available. The lower boundary is basalt or tuff I (~31.5 Ma); the upper boundary is built by Hsanda Gol sequences, which include fossils of letter zone C, and which are located below basalt II (~27 Ma).

This lower part of upper Hsanda Gol beds is not only characterised by abundant fossils of letter zone B but also by increased number and thickness of caliche layers, alternating with brick-red clay/silty clay. In the Hsanda Gol region, the sequence is interrupted by a 2–3-m-thick sandstone layer (Fig. 25).

Sediment sequences of the late Rupelian are evidenced in the regions: Taatsiin Gol (sections TGR-AB, TGR-B, TGL-A), Unkheltseg (section UNCH-A), Del (section DEL-B), Tatal Gol (sections TAT-D, TAT-E, and TAT-C), Hsanda Gol (section SHG-A, SHG-AB, SHG-D), and Ikh Argalatyn Nuruu (sections IKH-A and IKH-B) (see Figs. 30 and and3131).

Early Chattian (Hsandagolian/letter zone C) to late Chattian (Tabenbulukian/letter zones C1 and C1-D)

In the Chattian, some sections consist of sediments of the Hsanda Gol Fm. (sections TGR-C, TGW-A, TAT-E) and others of the Loh Fm. (sections TAR-A, RHN-A). Thus, both formations occur in Chattian strata. Three sections are of special importance for correlation. The sections ABO-A (Fig. 6) and TAR-A (Figs. 18 and and19)19) provide biostratigraphic data and radiometric ages of basalt II. Magnetostratigraphic measurements of section TGR-C (Fig. 13) allow correlation with the GPTS. In section ABO-A, fossils of letter zone C (sample ABO-A/3) were recovered below basalt II (27.0 ± 0.9 Ma); in section TAR-A, fossils of letter zone C (sample TAR-A/2) occur above basalt II (27.4 ± 0.4 Ma) (Höck et al. 1999; Daxner-Höck et al. 2010). These geochronologic data are in agreement with section TGR-C. There, the upper Hsanda Gol beds contain rich mammal assemblages of letter zone C, and fossils of letter zone C1 were sporadically found from the uppermost part of the Hsanda Gol Fm. Magnetostratigraphic measurements from section TGR-C allow correlation of the Hsanda Gol beds with Chrons C9n–C7n.2n (total range 27.4–24.2 Ma). The boundary between the reddish-brown and olive-green claystone (TGR-C/13/14) was correlated with Chron C8n.1n at 25.2 Ma (Sun and Windley 2015; Fig. 3); it is 3 m above the dark-brown claystone (TGR-C/11) marking the boundary between letter zones C and C1 at 25.6 Ma. Thus, in section TGR-C, letter zone C ranges from 27.4 to 25.6 Ma, and the range of letter zone C1 is 25.6 to 24.2 Ma (Fig. 30).

In the locality Tatal Gol, a composite section (section TAT-D+E) displays the sequence ranging from the early Rupelian to the late Chattian. The sequence evidences the early Rupelian (sample TAT-D/1 with fossils of letter zone A), followed by the late Rupelian (sample TAT-E/3 with fossils of letter zone B), the early Chattian (sample TAT-055 with fossils of letter zone C), and the late Chattian/Tabenbulukian (samples—TAT-043, TAT-044, TAT-E/22, TAT-E/27, TAT-052/1 with fossils of letter zone C1); finally, the sequence is topped by dark-brown clay at the North Ridge (samples TAT-E/32 and TAT-052/2 with fossils of letter zone C1-D; Figs. 21 and and22;22; Figs. 30 and and3131).

In the Taatsiin Gol and Taatsiin Tsagaan Nuur region, characteristic Tabenbulukian fossils cannot be found earlier than 25.6 Ma (Chron C8n.2n). These fossils, Yindirtemys deflexus, Sinolagomys kansuensis, Bohlinosminthus parvulus, and Amphechinus major, mark the beginning of letter zone C1. Consequently, we follow Meng and McKenna (1998) and (Meng et al. 2008) and draw the Hsandagolian/Tabenbulukian boundary at 25.6 Ma (Figs. 30 and and31).31). We do not agree with Kraatz and Geisler (2010, Fig. 3) to shift the lower boundary of the Tabenbulukian Mammal age down to Chron C11r at 30.6 Ma. This opinion of Kraatz and Geisler (2010) contradicts our fossil data (elaborated above and illustrated in Figs. 32, ,33,33, ,34,34, ,35,35, ,36,36, ,37,37, ,38,38, ,39,39, ,40,40, ,41,41, ,42,42, ,43,43, ,44,44, ,45,45, ,46,46, ,47,47, ,48,48, ,49,49, ,50,50, ,51,51, ,52,52, ,53,53, ,54,54, ,55,55, ,56,56, ,57,57, ,58,58, ,59,59, ,60,60, ,61,61, and and62),62), and also contradicts the radiometric ages of basalt II (Höck et all 1999; and Tables 1 and and2)2) and the magnetostratigraphic correlation of section TGR-C (Sun and Windley 2015).

Fig. 32
Family Didelphidae Asiadelphis zaissanensis Gabunia, Shevyreva and Gabunia, 1990 from Taatsiin Gol (TGR-B/1, TGR-AB/21, TGR-AB/22) and Hsanda Gol (SHG-A/15+20), Valley of Lakes, Mongolia. Early Oligocene, letter zone B. Ziegler et al. (2007). a Left D3 ...
Fig. 33
Family Erinaceidae. Zaraalestes minutus (Matthew and Granger, 1924) from Tatal Gol (TAT-D/1; letter zone A) and Taatsiin Gol (TGR-B/1, TGR-AB/22; letter zone B), Valley of Lakes, Mongolia. Early Oligocene. Ziegler et al. (2007). a Left max. C2-P2 (NHMW ...
Fig. 34
Family Erinaceidae. Amphechinus taatsiingolensis Ziegler, Dahlmann and Storch, 2007 from Toglorhoi / Khunug Valley (TGW-A/2), Valley of Lakes, Mongolia. Late Oligocene, Biozone C. All figured specimens are the holotype (H) and paratypes. Ziegler et al. ...
Fig. 35
Family Erinaceidae. Amphechinus minutissimus Ziegler, Dahlmann and Storch, 2007 from Del (DEL-B/12), Valley of Lakes, Mongolia. Late Oligocene, letter zone C1. All figured specimens are the holotype (H) and paratypes. Ziegler et al. (2007). a Right p4 ...
Fig. 36
Family Erinaceidae. Palaeoscaptor gigas (Lopatin, 2002) from Taatsiin Gol (TGR-C/1+2) Valley of Lakes, Mongolia. Late Oligocene, letter zone C. Ziegler et al. (2007). a Right mand. p4-m2, labial view (NHMW 2005/0128/0001). b Right mand. p4-m2, occlusal ...
Fig. 37
Family Erinaceidae. Palaeoscaptor acridens (Matthew and Granger, 1924) from Tatal Gol (TAT-D/1; letter zone A), Khongil (HL-A/1+2; letter zone A) and Taatsiin Gol (TGR-AB/21, TGR-AB/22; letter zone B), Valley of Lakes, Mongolia. Early Oligocene. Ziegler ...
Fig. 38
Family Soricidae. Gobisorex kingae Sulimski, 1970 from Taatsiin Gol (TGR-B/1, TGR-AB/22), Ikh Argalatyn Nuruu (IKH-A/2), Hsanda Gol (SHG-AB/17-20), and Unkheltseg (UNCH-A/3B), Valley of Lakes, Mongolia. Early Oligocene, letter zone B. Ziegler et al. ( ...
Fig. 39
Family Soricidae. Taatsiinia hoeckorum Ziegler, Dahlmann and Storch, 2007 from Taatsiin Gol (TGR-B/1), Valley of Lakes, Mongolia. Early Oligocene, letter zone B. All figured specimens are the holotype (H) and paratypes. Ziegler et al. (2007). a Right ...
Fig. 40
Family Talpidae. Mongolopala tathue Ziegler, Dahlmann and Storch, 2007 from Tatal Gol (TAT-D/1) and Hsanda Gol (SHG-C/1), Valley of Lakes, Mongolia. Early Oligocene, letter zone A. Holotype (H), paratypes (P). Ziegler et al. (2007). ac Left mand. ...
Fig. 41
Family Aplodontidae. Ninamys kazimierzi Vianey-Liaud, Gomes Rodrigues and Marivaux, 2013 from Taatsiin Gol (TGR-B/1) and Ikh Argalatyn Nuruu (IKH-A/1+2), Valley of Lakes, Mongolia. Early Oligocene, letter zone B. Maridet et al. (2017, this issue). a Left ...
Fig. 42
Family Aplodontidae. Proansomys badamae sp. nov.Maridet, Daxner-Höck, López-Guerrero and Göhlich, 2017 from Taatsiin Gol (TGR-C/1, TGR-C/7), Valley of Lakes, Mongolia. Late Oligocene, letter zone C. Maridet et al. (2017, this issue). ...
Fig. 43
Fam. Cylindrodontidae. Anomoemys lohiculus (Matthew and Granger, 1923) from Del (DEL-B/7, letter zone B) and Taatsiin Gol (TGR-A/ 13, letter zone A and TGL-A/11, letter zone B), Valley of Lakes, Mongolia. Early Oligocene. Daxner-Höck et al. ( ...
Fig. 44
Family Ctenodactylidae. Tataromys minor longidens Schmidt-Kittler, Vianey-Liaud and Marivaux, 2007 from Taatsiin Gol (TGW-A/2b). Late Oligocene, letter zone C. Schmidt-Kittler et al. (2007). a Left P4 (NHMW 2006/0100/0013). b Left P4 (NHMW 2006/0100/0011). ...
Fig. 45
Fam. Ctenodactylidae. Karakoromys decessus Matthew and Granger, 1923 from Hsanda Gol (SHG-A/9, SHG-A*), letter zone B and Khongil (HL-A/1), letter zone A, Valley of Lakes, Mongolia. Early Oligocene. Schmidt-Kittler et al. (2007), Oliver et al. (2017, ...
Fig. 46
Fam. Ctenodactylidae. Yindirtemys birgeri Bendukidze, 1993 from Tatal Gol (TAT-051/2), Valley of Lakes, Mongolia. Late Oligocene, letter zone C1. Oliver and Daxner-Höck (2016). ad Left maxilla right and left P4-M3 (NHMW 2012/0060/0001). ...
Fig. 47
Fam. Ctenodactylidae. Yindirtemys shevyrevae Vianey-Liaud, Schmidt-Kittler and Marivaux, 2006 from Hsanda Gol (SHG-AB/17-18, SHG-AB/17-20) and Taatsiin Gol (TGR-AB/22, TGR-AB/21), Valley of Lakes, Mongolia. Early Oligocene, letter zone B. Schmidt-Kittler ...
Fig. 48
Fam. Eomyidae. Eomys aff. orientalis Wang and Emry, 1991 from Taatsiin Gol (TGR-AB/21) and Hsanda Gol (SHG-A/17-18), Valley of Lakes, Mongolia. Early Oligocene, letter zone B. Maridet et al. (2015). a Right mand. p4-m3 (NHMW 2013/0059/0001), TGR-AB/21. ...
Fig. 49
Fam. Dipodidae. Heosminthus chimidae Daxner-Höck, Badamgarav and Maridet, 2014 from Taatsiin Gol (TGR-B/1), Valley of Lakes, Mongolia. Early Oligocene, letter zone B. All figured specimens are the holotype (H) and paratypes. Daxner-Höck ...
Fig. 50
Fam. Dipodidae. Plesiosminthus asiaticus Daxner-Höck and Wu, 2003 from Huch Teeg (RHN-A/7), Valley of Lakes, Mongolia. Late Oligocene, letter zone C1. Daxner-Höck and Wu (2003), Daxner-Höck et al. (2014). a Left M1 (NHMW 2001/0064/0001/2). ...
Fig. 51
Fam. Dipodidae. Onjosminthus baindi Daxner-Höck, Badamgarav and Maridet, 2014 from Taatsiin Gol (TGR-A/13+14, TGL-A/2), Tatal Gol (TAT-D/1), and Hsanda Gol (SHG-C/1), Valley of Lakes. Early Oligocene, letter zone A. Paratype (P). Daxner-Höck ...
Fig. 52
Fam. Dipodidae. Parasminthus debruijni Lopatin, 1999 from Taatsiin Gol (TGR-C/1; letter zone C), Del (DEL-B/12; letter zone C1), and Tatal Gol (TAT-E/27; letter zone C1), Valley of Lakes. Late Oligocene. Daxner-Höck et al. (2014). a Right M1, ...
Fig. 53
Fam. Dipodidae. Litodonomys huangheensis Wang and Qiu, 2000 from Unkheltseg (UNCH-A/3), Valley of Lakes. Early Miocene, letter zone D. Daxner-Höck et al.( 2014). a Skull with right and left P4-M3 (NHMW 2013/0232/0001). b Right mand. m1-3 (NHMW ...
Fig. 54
Fam. Dipodidae. Allosmintus khandae (Daxner-Höck, 2001) from Talyn Churum (GRAB-II), Tatal Gol (TAT-D/1), and Taatsiin Gol (TGR-A/13), Valley of Lakes, Mongolia. Early Oligocene, letter zone A. Paratype (P). Daxner-Höck (2001), Daxner-Höck ...
Fig. 55
Fam. Cricetidae s.l. Cricetops dormitor Matthew and Granger, 1923 from Taatsiin Gol (TGR-AB/22) and Hsanda Gol (SHG* and SHG-AB/12), Valley of Lakes, Mongolia. Early Oligocene, letter zone B. Daxner-Höck et al. (2010), Maridet et al. (2014b). ...
Fig. 56
Fam. Cricetidae s.l. Eucricetodon asiaticus (Matthew and Granger, 1923) from Tatal Gol (TAT-C/3 and TAT-D/1; letter zone A), Hsanda Gol (SHG-A/20; letter zone B), Taatsiin Gol (TGR-B/1; letter zone B), and Ikh Argalatyn Nuruu (IKH-A/2; Biozone B), Valley ...
Fig. 57
Fam. Cricetidae s.l. Eucricetodon bagus Gomes Rodrigues, Marivaux and Vianey-Liaud, 2012 from Toglorhoi (TGW-A/2a and TGW-A/2b) and Taatsiin Gol (TGR-C/2), Valley of Lakes, Mongolia. Late Oligocene, letter zone C. Daxner-Höck et al. (2010), Maridet ...
Fig. 58
Fam. Cricetidae s.l. Eocricetodon meridionalis (Wang and Meng, 1986) from Unkheltseg (UNCH-A/3B; letter zone B), Del (DEL-B/7; letter zone B), Taatsiin Gol (TGR-B/1; letter zone B) (ac), and E. cf. meridionalis/E. meridionalis (Wang and Meng ...
Fig. 59
Fam. Cricetidae s.l. Aralocricetodon shokensis Bendukidze, 1993 from Taatsiin Gol (TGR-C/1, TGR-C/2; letter zone C), Toglorhoi (TGW-A/2a; letter zone C), Unzing Churum (TAR-A/2; letter zone C), and Del (DEL-B/12; letter zone C1), Valley of Lakes, Mongolia. ...
Fig. 60
Fam. Tachyoryctoididae. Tachyoryctoids bayarmae Daxner-Höck, Badamgarav and Maridet, 2015 from Taatsiin Gol (TGR-C/1+2; letter zone C) and Toglorhoi (TGW-A/3+4; letter zone C1), Valley of Lakes, Mongolia. Late Oligocene. Daxner-Höck et ...
Fig. 61
Fam. Tachyoryctoididae. Tachyoryctoides obrutschewi Bohlin, 1937 from Tatal Gol (TAT-051/2), Ikh Argalatyn Nuruu (IKH-B/5), and Hotuliin Teeg (HTE-057), Valley of Lakes, Mongolia. Late Oligocene, letter zone C1. Daxner-Höck et al. (2015). a Right ...
Fig. 62
Fam. Tachyoryctoididae. Tachyoryctoides kokonorensis Li and Qiu, 1980 from Hotuliin Teeg (HTE* and HTE-012), Valley of Lakes, Mongolia. Early Miocene, letter zone D. Daxner-Höck et al. (2015). a Left m1-2 (NHMW 2013/0456/0001), HTE*. b Left m3 ...

Sediment sequences of the early Chattian are evidenced in the regions Taatsiin Gol (section TGR-C), Toglorhoi (section TGW-A), Abzag Ovo (section ABO-A), Unzing Churum (section TAR-A), and Tatal Gol (section TAT-E).

Sediments of the late Chattian/Tabenbulukian are evidenced in the regions Hotuliin Teeg (section HTE and localities HTSE and HTS), Huch Teeg (section RHN-A), Toglorhoi (section TGW-A), Del (section DEL-B), Tatal Gol (section TAT-E), Hsanda Gol (section SHG-AB), Loh (sections LOH-B, LOH-C), and Ikh Argalatyn Nuruu (sections IKH-A, IKH-B).

Auitanian (Xiejian/letter zone D)

In the lower part of section HTE (Figs. 10 and and11),11), strata of the Loh Fm. contain fossils of letter zone C1. The main part of this section is built up by silt and silty claystone and caliche of the Loh Fm. Here, the fossil concentrations are mostly bound to sandy, gravely layers/lenses filling the relief of massive caliche (e.g. fossil layer HTE-007), or to thin layers of caliche nodules (fossil layers HTE-8). The fossils indicate letter zone D and allow correlation with the Xiejian mammal age and the lowermost Miocene.

Sediments of the Aquitanian/Xiejian are evidenced in the regions Hotuliin Teeg (section HTE), Unkheltseg (section UNCH-A), Huch Teeg (section RHN-A), Luuny Yas (locality LUS), and Luugar Khudag (section LOG-A).

Chronostratigraphic correlation of Mongolian letter zones and calculation of their age ranges

The initial characterisation of Mongolian letter zones was based on preliminarily determined rodents. It included integrated rodent lists, the first/last records, the most abundant/characteristic taxa, the lithostratigraphic position, and the relation to one of the basalt events (Daxner-Höck et al. 1997; Höck et al. 1999). Later, new taxonomic and field data enabled several updates of the original informal biozones (Daxner-Höck and Badamgarav 2007: 14, Tables 3 and and4;4; Daxner-Höck et al. 2010: 352, Tables 2, ,3,3, ,4,4, ,5,5, and and6;6; Daxner-Höck et al. 2014: 204–205; Daxner-Höck et al. 2015: 188–190). After finalising the taxonomy of almost all mammal groups, the huge dataset allowed to formalize the informal letter zones as biozones according to the International Stratigraphic Guide. Consequently, the letter zones A, B, C, C1, C1-D, and D, covering the Oligocene and lowermost part of the Miocene, were defined as Taxon Range Zones and Abundance Zones (Harzhauser et al. 2017, this issue). Moreover, the biostratigraphic, lithostratigraphic, radiometric, and magnetostratigraphic data from the study area enable correlations with the GPTS and help estimate the time ranges of the Mongolian biozones. All species and the respective stratigraphic ranges are listed in Table Table1919.

Table 19
The fossil list comprises all identified fossils from the Taatsiin Gol and Taatsiin Tsagaan Nuur region and their stratigraphic ranges (letter zones A–D)

Letter zone A

The lower Hsanda Gol Fm. correlates with Chron C12 r and the upper part of C13 (Kraatz and Geisler 2010; Sun and Windley 2015) and ranges from the Eocene/Oligocene boundary (EOB) at ~34 Ma to basalt I at ~31.5 Ma. Key fossils of letter zone A (Harzhauser et al. 2017, this issue) were recovered from the upper part of the lower Hsanda Gol beds, which correlate with Chron C12r.

Samples: TGR-A/13+14; TGL-A/1+2; HL-A/1+2; TAT-C/1-3; TAT-D/1; TAT-037; SHG-C/1+2; GRAB-II. The range is ~33 to ~31.5 Ma (early Oligocene/early Rupelian/early Hsandagolian) (Fig. 31).

Letter zone B

Fossils of letter zone B are present in upper Hsanda Gol beds above basalt I (~31.5 Ma). The upper boundary is built by sequences of the Hsanda Gol and Loh Fms., which include fossils of letter zone C.

Samples: TGR-B/1; TGR-AB/21, TGR-AB/22; TGL-A/11; UNCH-A/3+4B; DEL-B/7+8; TAT-054; TAT-E/3; TAT-038; TAT-C/6+7; SHG-A/6, SHG-A/9, SHG-A/12-15; SHG-A/15-20; SHG-AB/12; SHG-AB/17-20; IKH-A/1-4; IKH-B/2. The range is ~31.5 to ~28 Ma (early Oligocene/late Rupelian/late Hsandagolian).

Letter zone C

Fossils of letter zone C are present in sediments of the upper Hsanda Gol Fm. and/or lower Loh Fm., which correlate with Chron C9n–C8n.2n (section TGR-C; Sun and Windley 2015) and with radiometric ages of basalt II (27–28 Ma) from sections ABO-A and TAR-A.

Samples: TGR-C/1+2; TGR-C/5-7; ABO-A/3; ABO-083; TAR-A/2; TGW-A/1; TGW-A/2a+b; TAT-055.

The range is ~28 to 25.6 Ma (late Oligocene/early Chattian/latest Hsandagolian).

Letter Zone C1

Hsanda Gol or Loh sediments with fossils of letter zone C1 are correlated with Chrons C8n.2n–C7n.2n (section TGR-C, above sediment layer TGR-C/11; Sun and Windley 2015).

Samples: TGW-A/3+4; TGW-A/5; HTE-057; HTSE-009; HTSE-013; DEL-B/12; RHN-A/6; RHN-A/7; RHN-A/8-9; RHN-A/10; RHN-023; RHN-019; TAT-043; TAT-044; TAT-E/22; TAT-027; TAT-051/1-2; TAT-052/1; SHG-AB top.; LOH-C/1; LOH-B/3; IKH-A/5; IKH-B/5.

The range is 25.6 to 24 Ma (late Oligocene/late Chattian/Tabenbulukian).

Letter zone C1-D

Sediments of the upper Hsanda Gol Fm. or lower Loh Fm. comprising fossils of letter zone C1-D mark the uppermost Oligocene above letter zone C1 and below letter zone D.

Samples: HTS-056/1-3; RHN-021+022; RHN-A/11; TAT-E/32; TAT-052/2.

The estimated range of letter zone C1-D is 24 to ~23 Ma (late Oligocene/late Chattian/Tabenbulukian).

Letter zone D

The lower Loh Fm. with fossils of letter zone D is demonstrated as being early Miocene by the occurrence of Democricetodon sui Maridet et al. 2011, which has its first appearance (FAD) at 22.6 Ma (top of Chron C6Cn.1n) in the type locality S-II site XJ99005 of the Tieersihabahe section, Junggar Basin, China (Meng et al. 2006, 2008, 2013).

Samples: LUS-027-029; LOG-A/1; HTE-008; HTE-009; HTE-014-018; HTE-005; HTE-007; HTE-12/6; HTE-012/8; HTE*; HTE-012; HTE-12/7; UNCH-A/3+4; RHN-A/12; RHN-020.

The estimated range is ~23 to ~21 Ma (early Miocene/Aquitanian/Xiejian).

Fossil record and dental morphology of Marsupialia, Eulipotyphla, and Rodentia from the Oligocene and early Miocene of the studied area

Here, we introduce into the fossil record of the Oligocene and lowermost Miocene (Table (Table19);19); younger assemblages are not considered in this issue. The fossils were collected from 70 fossil horizons of 20 geological sections and 6 fossil points in the Valley of Lakes. The recovered fossils encompass Gastropoda (Stworzewicz 2007; Neubauer et al. 2013), Anura and Squamata (Böhme 2007), Creodonta, Carnivora and Leptictida (Morlo and Nagel 2002, 2007; Nagel and Morlo 2003), Perissodactyla (Heissig 2007), and Ruminantia (Vislobokova and Daxner-Höck 2002). The prevailing part of fossils—about 98% of more than 19,000 fossils—represents small mammals, of which 135 species-level taxa were counted. This small mammal dominance, however, results from wet screening of large samples.

Among small mammals, the order Rodentia dominates in genus, species, and specimen numbers, followed by Lagomorpa and Eulipotyphla and Marsupialia. Rodentia encompass the families Aplodontidae, Sciuridae, Eomyidae, Ctenodactylidae, Cylindrodontidae, Tsaganomyidae, Dipodidae, Cricetidae s. l., and Tachyoryctoididae. Together, they comprise 85 species-level taxa. Lagomorpha are represented by the families Leporidae, Palaeolagidae, and Ochotonidae, altogether with 23 species-level taxa. Eulipotyphla are represented by the families Erinaceidae, Soricidae, and Talpidae, together 25 species-level taxa. Additionally, two Marsupialia species of the family Didelphidae occur.

In this chapter, the richest small mammal collection ever found in Mongolia is illustrated by SEM images (Figs. 32, ,33,33, ,34,34, ,35,35, ,36,36, ,37,37, ,38,38, ,39,39, ,40,40, ,41,41, ,42,42, ,43,43, ,44,44, ,45,45, ,46,46, ,47,47, ,48,48, ,49,49, ,50,50, ,51,51, ,52,52, ,53,53, ,54,54, ,55,55, ,56,56, ,57,57, ,58,58, ,59,59, ,60,60, ,61,61, and and62).62). We give an overview of the diversity; show the manifold dental structures of marsupials, eulipotyphlans, and rodents; and provide a first impression of species, which are named in fossil lists or serve as index fossils for biostratigraphy. Fossils which indicate Taxon Range Zones and Abundance Subzones (Harzhauser et al. 2017, this issue) are written in bold letters (see list of figured species, below). Fossil descriptions are not included in this chapter; for more detailed information, we refer on the original descriptions and included references. Other fossil groups, such as gastropods, lower vertebrates, large mammals, lagomorphs, and the large-sized rodents Tsaganomyidae, are not figured in this paper. The figured teeth (SEM images) of marsupials, eulipotyphlans, and rodents are roughly arranged in systematic order. The figure captions include the taxon name, collection and inventory number, the locality, section, fossil layer, the age of the sample, respective letter zone, and the author who identified or described the fossils. For better comparison, all right-side fossils are mirrored (they are figured as if they were from the left side), and these numbers are underlined (e.g. Fig. 32b = right M1 of Asiadelphis zaissanensis). A scale bar shows the magnification of fossils.

The figured species are

Order Marsupialia

Family Didelphidae:

Asiadelphis zaissanensis (Fig. 32af)

Asiadelphis tjutkovae (Fig. 32g)

Order Eulipotyphla

Family Erinaceidae:

Exallerix pustulatus (Fig. 32h–j)

Zaraalestes minutus (Fig. 33al)

Zaraalestes sp. (Fig. 33m–p)

Amphechinus taatsiingolensis (Fig. 34a–n)

Amphechinus minutissimus (Fig. 35a–i)

Amphechinus major (Fig. 35j–q)

Palaeoscaptor gigas (Fig. 36ab)

Palaeoscaptor tenuis (Fig. 36c–n

Palaeoscaptor acridens (Fig. 37a–g)

Palaeoscaptor cf. rectus (Fig. 37hn)

Family Soricidae:

Gobisorex kingae (Fig. 38ai)

Taatsiinia hoeckorum (Fig. 39a–f)

Tavoonyia altaica (Fig. 39/g–n)

Family Talpidae:

Mongolopala tathue (Fig. 40af)

Order Rodentia

Family Aplodontidae:

Ninamys kazimierzi (Fig. 41ae)

Ninamys arboraptus (Fig. 41fh)

Prosciurus ? mongoliensis (Fig. i–k)

Promeniscomys cf. sinensis (Fig. 41l–r)

Proansomys badamae sp. nov. (Fig. 42ah)

Family Sciuridae:

Kherem shandgoliensis (Fig. 42i–j)

Pteromyini indet. (Fig. 42k–m)

Plesiosciurus aff. sinensis (Fig. 42n–p)

Family Cylindrodontidae:

Anomoemys lohiculus (Fig. 43af)

Ardynomys sp. (Fig. 43g–l)

Family Ctenodactylidae:

Tataromys minor longidens (Fig. 44a–j)

Tataromys sigmodon (Fig. 44k–s)

Tataromys plicidens (Fig. 44tzz)

Karakoromys decessus (Fig. 45a–i)

Huangomys frequens (Fig. 45jp)

Yindirtemys birgeri (Fig. 46a–d)

Yindirtemys deflexus (Fig. 46ek)

Yindirtemys suni (Fig. 46/l–q)

Yindirtemys shevyrevae (Fig. 47a–h)

Yindirtemys aff. ulantatalensis (Fig. 47i–j)

Prodistylomys nov. spec. 1 (in prep.) (Fig. 47k)

Prodistylomys nov. spec. 2 (in prep.) (Fig. 47lo)

Family Eomyidae:

Eomys aff. orientalis (Fig. 48ac)

Eomys cf. orientalis (Fig. 48d)

cf. Asianeomys bolligeri (Fig. 48ej)

Asianeomys dangheensis (Fig. 48k–s)

Family Dipodidae:

Heosminthus chimidae (Fig. 49ag)

Heosminthus borrae (Fig. 49h–q)

Plesiosminthus asiaticus (Fig. 50ac)

Plesiosminthus promyarion (Fig. 50d–h)

Plesiosminthus barsboldi (Fig. 50i–k)

Plesiosminthus olzi (Fig. 50l–s)

Onjosminthus baindi (Fig. 51a–g)

Bohlinosminthus parvulus (Fig. 51h–p)

Parasminthus debruijni (Fig. 52a–d)

Parasminthus cf. tangingoli (Fig. 52ej)

Parasminthus cf. asiaecentralis (Fig. 52k–l)

Litodonomys huangheensis (Fig. 53a–d)

Litodonomys jajeensis (Fig. 53e–h)

Allosmintus khandae (Fig. 54ae)

Allosminthus minutus (Fig. 54f–k)

Shamosminthus sodovis (Fig. 54lp)

Shamosminthus tongi (Fig. 54q)

Heterosminthus aff. nanaus (Fig. 54rs)

Heterosminthus firmus)

Heterosminthus cf. lanzhouensis (Fig. 54yzz)

Family Cricetidae s.l.:

Cricetops dormitor (Fig. 55ae)

Cricetops minor (Fig. 55f)

Selenomys mimicus (Fig. 55gh)

Eucricetodon asiaticus (Fig. 56a–f)

Eucricetodon caducus (Fig. 56gl)

Ulaancricetodon badamae (Fig. 56m–p)

Eucricetodon bagus (Fig. 57af)

Eucricetodon jilantaiensis (Fig. 57g–j)

Eucricetodon cf. occasionalis (Fig. 57k–n)

Paracricetodon sp./Witenia sp. (Fig. 57o–p)

Eocricetodon meridionalis (Fig. 58a–d)

Bagacricetodon tongi (Fig. 58eh)

Democricetodon sui (Fig. 58il)

Aralocricetodon shokensis (Fig. 59af)

Argyromys nov. spec. (Fig. 59g–j)

Family Tachyoryctoididae

Tachyoryctoids bayarmae (Fig. 60a–d)

Tachyoryctoides radnai (Fig. 60e–f)

Ayakozomys sp.(Fig. 60g–h)

Tachyoryctoides obrutschewi (Fig. 61a–d)

Tachyoryctoides tatalgolicus (Fig. 61e–h)

Tachyoryctoides kokonorensis (Fig. 62a–e)

Tachyoryctoides engesseri (Fig. 62f–h)

Conclusions

The Taatsiin Gol and Taatsiin Tsagaan Nuur region, part of the Valley of Lakes, yields Oligocene and Miocene sediment deposits. They are very important in several respects. First, the sequences of the Hsanda Gol and Loh Fms. contain a rich mammalian fauna and provide unique evidence of mammal evolution and climatic changes (Harzhauser et al. 2016). Second, the Cenozoic strata are intercalated with basalt flows, and the 40Ar/39Ar data of these basalts constrain the time of sediment deposition. Thus, basalt ages and Mongolian letter zones enable a composite age chronology for the studied area (Höck et al. 1999; Daxner-Höck et al. 2010).

From Luuny Yas in the northwest to Ihk Argalatyn Nuur in the east (~101–102° longitude), 20 sections and 6 fossil localities were investigated in detail (Table (Table3,3, Fig. 3). The description of sections are original, comprising lithology, sediment structures and thicknesses of sediment layers, illustrations of the localities/sections, the GPS positions, faunal lists of the fossil horizons, biozonation, radiometric ages of imbedded basalts, and magnetostratigraphic data (Figs. 4, ,5,5, ,6,6, ,7,7, ,8,8, ,9,9, ,10,10, ,11,11, ,12,12, ,13,13, ,14,14, ,15,15, ,16,16, ,17,17, ,18,18, ,19,19, ,20,20, ,21,21, ,22,22, ,23,23, ,24,24, ,25,25, ,26,26, ,27,27, ,28,28, and and2929).

The composite sequence includes four formations from bottom to top: The lowermost fluvio-lacustrine sequence is named Tsagan Ovo Fm. It is overlain by red clay and silt of the Hsanda Gol Fm., which itself is divided by basalt I (31.5 Ma) into the lower and upper Hsanda Gol beds. Upsection, fluvial deposits of the Loh Fm. follow, which are locally covered by pebbles of the Tuyn Gol Fm. Basalt II flows, dated at ~27 Ma, contact sediments of the Hsanda Gol and Loh Fms, as evidenced in sections ABO-A and TAR-A, respectively. Most basalt II occurrences with ages between ~25 and ~28 Ma do not have contact with fossil beds (Tables (Tables1and1and and2).2). The upper parts of several sections, which are built up by the Loh and Tuyn Gol Fms. and comprise fossils younger than lowermost Miocene, are not considered in this study.

Magnetostratigraphic measurements of the TGR sections show that the Tsagan Ovo Fm. corresponds with Chrons C15r–C13r, an age range of >35–34 Ma, which is late Eocene. The lower Hsanda Gol strata and basalt I correspond with the palaeomagnetic polarity chrons C13r–C12r, an age range of ~34–31.2 Ma (Kraatz and Geisler 2010; Sun and Windley 2015), which is early Oligocene. Thus, the boundary between the Tsagan Ovo and Hsanda Gol Fms. corresponds with the Eocene-Oligocene boundary (EOB). The boundary between the Hsanda Gol and Loh Fms. is heterochronous. Locally, Hsanda Gol sediments range to the latest Oligocene (e.g. section TAT-E; Fig. 21); in other regions, sedimentation of the Loh Fm. started in the early late Oligocene (e.g. section TAR-A; Fig. 18).

We sampled more than 19,000 mammal fossils from 70 individual fossil layers, yielding a total of 176 mammal species, mostly small mammals. The representation of large mammals, lower vertebrates, and gastropods is comparably poor.

This unique dataset enables evaluation and formalization of the Mongolian letter zones A, B, C, C1, C1-D, and D (Harzhauser et al. 2017, this issue). The biostratigraphic data from Oligocene and early Miocene sequences, the 40Ar/39 Ar ages of basalts I and II (Tables 1 and and22 and Höck et al. 1999), and magnetostratigraphic measurements (Kraatz and Geisler 2010; Sun and Windley 2015) help correlate sections and fossil sites with the Geomagnetic Polarity Time Scale GPTS (Gradstein et al. 2012) and assess the precise ages of mammal faunas and time ranges of Mongolian letter zones (Figs. 30 and and3131).

Importantly, the δ13C and δ18O isotope values of authigenic carbonate in calcrete horizons and analyses of mammal community structures reflect changes of the palaeoclimate during the Oligocene and early Miocene (Richoz et al. 2017, this issue; Harzhauser et al. 2016, accepted).

The manifold dental morphology is illustrated by SEM images of teeth from marsupials, insectivores, and rodents (Figs. 32, ,33,33, ,34,34, ,35,35, ,36,36, ,37,37, ,38,38, ,39,39, ,40,40, ,41,41, ,42,42, ,43,43, ,44,44, ,45,45, ,46,46, ,47,47, ,48,48, ,49,49, ,50,50, ,51,51, ,52,52, ,53,53, ,54,54, ,55,55, ,56,56, ,57,57, ,58,58, ,59,59, ,60,60, ,61,61, and and62),62), and Table Table1919 lists all investigated fossil taxa and the respective stratigraphic ranges.

Electronic supplementary material

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Acknowledgements

Open access funding provided by Austrian Science Fund (FWF). This research was supported by four projects of the Austrian Science Fund (FWF): P-10505-GEO, P-15724-N06, P-23061-N19 to G.D.-H. and a Lise Meitner grant M-1357-B17 to O.M. Travel expenses of G.D.-H. to China and Mogolia were partly covered by the Austrian Academy of Sciences. We thank our Mongolian and European team members for manifold support during several seasons of field campaigns and subsequent laboratory work in Ulaan Baatar, Vienna, and Graz. Special thanks to H.P. Schmid, T. Bolliger, E. Luginger-Öttl, G. Furtmüller, and O. Montag for unpublished field data; to Li Ping for identifying Cricetops minor from Tatal Gol; and to R. Quezada-Hinojosa for geochemical analyses and for drawing some geological sections. W. Frank provided so far unpublished 40Ar/39Ar data of basalts, and K. Constenius from Petro Matad LLC, Ulaan Baatar, made an unpublished geological cross-section available to us. M. Stachowitsch helped improve the English. We specifically thank the reviewers W. Wessels and an anonymous reviewer for careful comments and critical remarks, and the Editors P. Königshof and S. Weber for manifold information and help. All these persons and institutions are gratefully acknowledged for their support.

Compliance with ethical standards

Compliance with ethical standards

Conflict of interests

The authors declare that they have no competing interests.

Footnotes

This article is a contribution to the special issue “The Valley of Lakes in Mongolia, a key area of Cenozoic mammal evolution and stratigraphy”

Contributor Information

Gudrun Daxner-Höck, ta.ca.neiw-mhn@kceoh.nurdug, ta.gbs@kceoh.nurdug.

Rinchen Barsbold, moc.oohay@samoelap.

Baatarjav Bayarmaa, moc.oohay@58oib_aarayab.

Margarita Erbajeva, ur.tencsb.nig@avejabre.

Ursula Bettina Göhlich, ta.ca.neiw-mhn@hcilheog.alusru.

Mathias Harzhauser, ta.ca.neiw-mhn@resuahzrah.saihtam.

Eva Höck, ta.noa@hefom.

Volker Höck, ta.ca.gbs@kceoh.reklov.

Niiden Ichinnorov, moc.oohay@akhcii.

Yondon Khand, moc.oohay@samoelap.

Paloma López-Guerrero, moc.liamg@1891zepolamolap.

Olivier Maridet, hc.acissaruj@tediram.reivilo.

Thomas Neubauer, ta.ca.neiw-mhn@reuabuen.samoht.

Adriana Oliver, se.cisc.ncnm@anairda.

Werner Piller, ta.zarg-inu@rellip.renrew.

Khishigjav Tsogtbaatar, moc.liamg@nidnomtgost.

Reinhard Ziegler, ed.wb-muesumednukrutan@snms.relgeiz.

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