Amphilagus plicadentis (Lagomorpha, Mammalia) from the Tagay locality (Olkhon Island, Baikal region, Eastern Siberia).

New excavations in the Tagay locality have revealed the presence of Amphilagus plicadentis Erbajeva, 2013, which belongs to the palaeolagine Lagomorpha rather than leporids, previously referred to Procaprolagus sp. (Logachev et al., 1964). Herein, we report a comprehensive morphological description of this species and compare it with the other known species of the genus Amphilagus. The Tagay lagomorph is conspecific with the Early Miocene species Amphilagus plicadentis found at locality Unkheltseg (UNCH-A) (biozone D; Early Miocene) in the Valley of Lakes, Central Mongolia. The Tagay specimens suggest a more advanced evolutionary stage of A. plicadentis and an age around Early/Middle Miocene transition. Supplementary Information: The online version contains supplementary material available at 10.1007/s12549-022-00554-y.

Geology and lithology of the Tagay-1 section at Olkhon Island (Lake Baikal, Eastern Siberia), and description of Aplodontidae, Mylagaulidae and Sciuridae (Rodentia, Mammalia).

Excavations along the Tagay-1 section shed light into the composition of small mammal assemblages of the Tagay site. The present paper focuses on the geology and geomorphology of Olkhon Island, the lithology and fossil evidence along the Tagay-1 section and descriptions of the aplodontid, mylagaulid and sciurid rodents. The described fossils are isolated teeth of four taxa, Ansomys sp. (Aplodontidae), Lamugaulus olkhonensis Tesakov and Lopatin, 2015 (Mylagaulidae), Sciuridae indet. and Spermophilinus debruijni nov. spec. (Sciuridae). The archaic tooth pattern of these rodents suggests an age around the Early/Middle Miocene transition.

Gliridae and Eomyidae (Rodentia) of the Miocene Tagay fauna (Olkhon Island, Lake Baikal, Eastern Siberia).

The small mammals Myomiminae indet. (Gliridae), Leptodontomys cf. gansus Zheng and Li, 1982 (Eomyidae) and the new species Keramidomys sibiricus nov. spec. (Eomyidae) are described. They were collected from six layers of the middle to upper part of the Tagay-1 section on Olkhon Island. The glirid Myomiminae indet. is represented by only a few isolated teeth, the small eomyid Leptodontomys cf. gansus by a mandible with two teeth, and the second small eomyid Keramidomys sibiricus nov. spec. by several isolated teeth and a mandible. The ancestral tooth characteristics of Keramidomys sibiricus nov. spec. indicate an early evolutionary stage of Keramidomys in Asia. The suggested age of the assemblage is Early/Middle Miocene transition.

The northernmost Eurasian Miocene beavers: Euroxenomys (Castoridae, Mammalia) from Olkhon Island, Lake Baikal (Eastern Siberia).

The castorid dental material described in this paper derives from Miocene, fossiliferous deposits of the Baikal rift valley, exposed at Tagay Bay on Olkhon Island in the Lake Baikal, in eastern Siberia. It consists of maxillary fragments and isolated upper and lower teeth of the small trogontheriine beaver Euroxenomys minutus (von Meyer, 1838). It is the first record of the species in Asia and at the same time the northernmost occurrence of Eurasian Miocene beavers. The magnetostratigraphic correlation of the Tagay -1 section, indicates a late Burdigalian, Early/early Middle Miocene age of ~16.5 to ~16.3 Ma that corresponds to the Mammalian Neogene zone MN4/5. The presence of E. minutus in Tagay is an indicator for an Orleanian European-Siberian bioprovince during the Mid-Miocene Climate Optimum, and for a continuous belt of humid, warm-temperate to subtropical forests, stretching from Europe to Siberia, and probably further to East and South-Eastern Asia. In Eurasia, beaver remains are an indicator of permanent water bodies, which is in agreement with the palaeoenvironment of the Tagay locality.

Occurrences of Sinolagomys (Lagomorpha) from the Valley of Lakes (Mongolia).

The genus Sinolagomys is an early representative of the family Ochotonidae, appearing first in the late early Oligocene of Central Asia. It is known in China from Shargaltein Tal, Taben Buluk, Ulan Tatal, and northern Junggaria, and a few specimens from Tatal Gol, Mongolia have been published. For most occurrences, the genus is not represented by many specimens. Extensive studies in the Valley of Lakes, Central Mongolia, have produced a large number of sinolagomyin fossils spanning at least 10 million years and belonging to five species: Sinolagomys kansuensis, Sinolagomys major, Sinolagomys gracilis, Sinolagomys ulungurensis, and Sinolagomys badamae sp. nov. Descriptions of these are given, as well as definition of the new species. Sinolagomyins flourished during the late Oligocene and early Miocene and came to occupy vast territories from China through Mongolia and Kazakhstan. The evolution of this ochotonid group is characterized by increasing taxonomic diversity and progressive development of rootless cheek teeth.

The record of Aplodontidae (Rodentia, Mammalia) in the Oligocene and Miocene of the Valley of Lakes (Central Mongolia) with some comments on the morphologic variability.

The present publication reports new discoveries of Oligocene and early Miocene aplodontid rodents from the Taatsiin Gol area (Valley of Lakes) in Mongolia. The fossil aplodontids recovered in this area are mainly composed of dental remains, some fragmentary jaws plus one partially preserved skull. Aplodontid rodents have been found from the early Oligocene (local biozone A) to the early middle Miocene (local biozone D). Altogether, eight taxa belonging to five genera have been identified in the investigated deposits: Ninamys arboraptus, Ninamys kazimierzi, Promeniscomys cf. sinensis, Prosciurus? mongoliensis and Prosciurus? sp. nov. in the early Oligocene; N. arboraptus, Proansomys badamae sp. nov. and Ansomyinae indet. in the late Oligocene; and Ansomys sp.1 in the early Miocene. In addition, although outside of the topic of the present special issue, one additional taxon, Ansomys sp.2, is reported from the ?middle Miocene. The material of aplodontids is usually relatively scarce in Asian localities. For the first time, with a sample size of 81 specimens, the material from Central Mongolia (mainly from the Oligocene) now allows a more accurate description of the morphological and size variability and resolves some systematic problems. The study of these aplodontids reveals that they are more abundant and diverse in the early Oligocene and that the diversity decreases during the late Oligocene and Miocene. One hypothesis, to explain the opposite diversity trend observed previously for sciurids in the same region, is that both Sciuromorpha families might have competed for the same resources from the early Oligocene to the middle Miocene in Central Mongolia.

Cricetidae (Rodentia, Mammalia) from the Oligocene of the Valley of Lakes (Mongolia): the genera Aralocricetodon, Eocricetodon, Bagacricetodon, Witenia and Paracricetodon.

We describe the remains of Aralocricetodon Bendukidze, 1993; Bagacricetodon Gomes Rodrigues et al., 2012; Eocricetodon Wang, 2007; Witenia de Bruijn et al., 2003 and Paracricetodon Schaub, 1925 (Cricetidae, Rodentia, Mammalia) from the Taatsiin Gol and Taatsiin Tsagaan Nuur areas (Mongolia). The studied material (comprising 128 teeth) stems from 9 localities and 23 fossil layers spanning a time interval from ∼33 to ∼24 Ma (early to late Oligocene) and covering the biozones from A to C1. The general dental pattern between the species from the early and late Oligocene differed. The occlusal pattern of the molars was more complicated and the crowns were lower during the early versus late Oligocene. This indicates a change in diet towards more abrasive plants. Several of the studied species were common in both Europe and Asia Minor during the early Oligocene. The species collected from the late Oligocene have also been recorded in Kazakhstan and China. This indicates an interesting biogeographical pattern that merits future study.

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

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.

Oligocene and early Miocene mammal biostratigraphy of the Valley of Lakes in Mongolia.

The Taatsiin Gol Basin in Mongolia is a key area for understanding the evolution and dispersal of Central Asian mammal faunas during the Oligocene and early Miocene. After two decades of intense fieldwork, the area is extraordinarily well sampled and taxonomically well studied, yielding a large dataset of 19,042 specimens from 60 samples. The specimens represent 176 species-level and 99 genus-level taxa comprising 135 small mammal species and 47 large mammals. A detailed lithostratigraphy and new magnetostratigraphic and radiometric datings provide an excellent frame for these biotic data. Therefore, we test and evaluate the informal biozonation scheme that has been traditionally used for biostratigraphic correlations within the basin. Based on the analysis of the huge dataset, a formalised biostratigraphic scheme is proposed. It comprises the Cricetops dormitor Taxon Range Zone (Rupelian), subdivided into the Allosminthus khandae Taxon Range Subzone and the Huangomys frequens Abundance Subzone, the Amphechinus taatsiingolensis Abundance Zone (early Chattian), the Amphechinus major Taxon Range Zone (late Chattian), subdivided into the Yindirtemys deflexus Abundance Subzone and the Upper Amphechinus major T. R. Z., and the Tachyoryctoides kokonorensis Taxon Range Zone (Aquitanian). In statistical analyses, samples attributed to these biozones form distinct clusters, indicating that each biozone was also characterised by a distinct faunal type.

Geochemistry and mineralogy of the Oligo-Miocene sediments of the Valley of Lakes, Mongolia.

The Valley of Lakes is approximately a 500-km elongate depression in Central Mongolia, where Eocene to Miocene continental sediments are long known for their outstanding fossil richness. The palaeontological record of this region is an exceptional witness for the evolution of mammalian communities during the Cenozoic global cooling and regional aridification. In order to precisely elucidate the climatic evolution of the region, we studied the mostly siliciclastic sediments with several levels of paleosols for their sedimentology, mineralogy, major and trace element composition and δ13C and δ18O composition. The obtained results show that temperate hydrothermal fluids induced a strong illitization of the fluvial and lacustrine sediments. This finding contradicts the current conceptual view that the fine fraction of the sediments is of aeolian origin. Moreover, the diagenetic growth of illite resulted in a strong overprinting of the sediments and, subsequently, largely disturbed the pristine mineralogical and geochemical composition of the sediments that could have carried any palaeo-climatic information. An exception is the δ13C (and δ18O) isotope values of authigenic carbonate found in calcrete horizons that still record the ambient climatic conditions prevailing during paleosol formation. Our novel δ13C and δ18O record suggests an early Oligocene aridification in Central Asia at ∼31 Ma, whereas the Oligocene glacial maximum shows no increase in aridification. A second, regional-scale aridification occurs at ~25 Ma and corresponds to a late Oligocene marked mammalian turnover in the Valley of Lakes sediments.

Diversification rates in Ctenodactylidae (Rodentia, Mammalia) from Mongolia.

Gundis, or comb rats, are rodents of the family Ctenodactylidae. Extant gundis are restricted to Africa and represent a vestige of the diversity that the ctenodactylids attained at both palaeoecological and palaeobiogeographical levels. Here, we present an updated review of the Ctenodactylidae from the Valley of Lakes, Mongolia, based on the study of large collections now available. We have recognised 13 valid species of ctenodactylids grouped into five genera: Karakoromys, Huangomys, Tataromys, Yindirtemys, and Prodistylomys. The ctenodactylids show an initial burst in diversification in the early Oligocene followed by a sequential generic extinction of Karakoromys, Huangomys, and Tataromys. A maximum richness peak at the late Oligocene was followed by a profound diversity crisis. Yindirtemys, the only surviving genus, persisted into the Miocene, joining three Prodistylomys species. These last representatives of the group disappeared coinciding with the late Xiejian faunal reorganisation (Mongolian biozone D).

Evolution of the genus Eucricetodon (Rodentia, Mammalia) from the Valley of Lakes (Mongolia): a taxonomical description and update on the stratigraphical distribution.

The Oligocene fossil deposits from Valley of Lakes in Central Mongolia have provided a wealth of rodent fossils. Among these, cricetids are a very important part. To date, only the Miocene genera have been described in detail. Here, we focus on the Oligocene genus Eucricetodon from this region. Eucricetodontinae are the most abundant fossils in the Oligocene Valley of Lakes faunas. The present study consists of the description of five species of cricetid rodents from 43 localities ranging in age from the early Oligocene to the early-late Oligocene. In addition to Eucricetodon asiaticus described in Mongolia in 1923, we have found Eucricetodon bagus and Eucricetodon jilantaiensis that were described from Nei Mongol and Eucricetodon occidentalis discovered in Kazakhstan. This taxonomical study provides new information regarding the evolution of the Cricetidae in Central and Eastern Asia during the Oligocene and, more particularly, regarding their phylogenetic relationships and the evolutionary trends.

A new species of Argyromys (Rodentia, Mammalia) from the Oligocene of the Valley of Lakes (Mongolia): Its importance for palaeobiogeographical homogeneity across Mongolia, China and Kazakhstan.

We describe a new species of Rodentia (Mammalia), Argyromys cicigei sp. nov. from Toglorhoi (fossil bed TGW-A/2a) in Mongolia and Ulantatal (fossil beds UTL 1 and UTL 7) in China. Its tooth morphology differs from the type species Argyromys aralensis from Akespe in Kazakhstan by smaller size and simpler structures. Argyromys has been assigned in different families of Muroidea, such as Tachyoryctoididae and Spalacidae. However, the presence of common characters indicates a closer relationship of Argyromys with the genera of Cricetidae s.l. (subfamilies Eucricetodontinae; Cricetopinae; Cricetodontinae and Gobicricetodontinae among others) from Asia than with the earliest representatives of Spalacidae or the endemic Tachyoryctoididae. Argyromys cicigei sp. nov. possesses a simple anterocone and anteroconid in the upper and lower first molars, respectively, which is characteristic for Cricetidae s.l. It has a flat occlusal surface in worn specimens; weakly-developed posterolophs; an oblique protolophule and metaloph on the upper molars and it lacks a labial anterolophid on the m1. These traits are also typical of the Oligocene genera Aralocricetodon and Plesiodipus, included in the subfamilies Cricetodontinae and Gobicricetodontinae respectively. The cladistic analysis performed here supports this hypothesis. The clade formed by Argyromys species is grouped with other cricetid taxa (s.l). Spalacids, however, form a different clade, as do the tachyoryctoids. Previous authors state that the Aral Formation (Kazakhstan) should be dated to the Oligocene instead of the Miocene, based on the presence of several taxa. The finds of Argyromys in both regions supports the statement that they are closer in age than previously thought. The occurrence of Argyromys in Kazakhstan, Mongolia and China evidences the biogeographic unity of the Central Asian bioprovince during the Oligocene.

Stepwise onset of the Icehouse world and its impact on Oligo-Miocene Central Asian mammals.

Central Asia is a key area to study the impact of Cenozoic climate cooling on continental ecosystems. One of the best places to search for rather continuous paleontological records is the Valley of Lakes in Mongolia with its outstandingly fossil-rich Oligocene and Miocene terrestrial sediments. Here, we investigate the response by mammal communities during the early stage of Earth's icehouse climate in Central Asia. Based on statistical analyses of occurrence and abundance data of 18608 specimens representing 175 mammal species and geochemical (carbon isotopes) and geophysical (magnetic susceptibility) data we link shifts in diversities with major climatic variations. Our data document for the first time that the post-Eocene aridification of Central Asia happened in several steps, was interrupted by short episodes of increased precipitation, and was not a gradual process. We show that the timing of the major turnovers in Oligocene mammal communities is tightly linked with global climate events rather than slow tectonics processes. The most severe decline of up 48% of total diversity is related to aridification during the maximum of the Late Oligocene Warming at 25 Ma. Its magnitude was distinctly larger than the community turnover linked to the mid-Oligocene Glacial Maximum.