Due to continental climate and the consequent lack of extensive glaciation, Beringia was a refugium for arctic biota during Pleistocene warm stages and center of dispersal during cold stages, when cold-adapted taxa spread over much of Eurasia far into the middle latitudes. Also, the intercontinental faunal exchange between Eurasia and the New World took place via the Beringian landmass during phases of global sea level lowering. As result of ice sheet formation in Northern Europe and North America and the fixation of tremendous water masses, the huge and shallow Laptev, East Siberian, Chukchi and Bering shelves in the North of Yakutia and in the East of Chukotka became, at such times, fully exposed and together formed the Beringian landmass.

Apart from being a refugium during the late Middle and Late Pleistocene, the western part of Beringia is furthermore regarded as center of development or center of origin of preadapted late Pleistocene mammoth faunal complex components, like Ovibos (resp. Praeovibos), Rangifer, Gulo, Alopex and advanced forms of Mammuthus, which evolved there during the late Early to the early Middle Pleistocene giving rise to define the Northeast Siberian land mammal age Olyorian (Sher, 1986; Kahlke, 1999).

The Olyorian comprises a long period of time for the evolution of the mammoth faunal complex spanning from the late Early to the early Middle Pleistocene (early Eopleistocene to early Neopleistocene according to the Russian nomenclature), i.e., 1.4–0.5 Ma (Sher, 1971; Sher, 1986), though without exactly defined lower and upper boundaries. The type locality of the Olyorian is situated at the Chukochya River in the Kolyma lowlands about 150 km NW of Cherskiy (Figure 1; Sher, 1986). Classical Olyorian sites in NE Siberia are furthermore located at the Krestovka River (Figure 1; Sher et al., 1979) as well as at the Adycha River (Kaplina et al., 1983) and in the surroundings of Cherskiy, where important Olyorian sites are known from Tretiy Ruchey between Zeleniy Mys and the town of Cherskiy (Sher et al., 2011) as well as the Cherskiy Ovrag (Davydov, 2007) from where plant remains are described in the present paper.

As part of the regional Northeast Siberian (West Beringian) Quaternary sequence, the Olyorian mammal age can be further separated into two faunal units on the base of the evolutionary succession of collared lemmings (Sher, 1997). The lower Olyorian (Chukochyan) is characterized by the presence of the predecessor of modern collared lemmings Predicrostonyx compitalis and the likewise archaic vole Allophaiomys pliocaenicus and is correlated with the late part of the reverse polarity Matuyama Chron in the palaeomagnetic record including the inserted magnetically normal Jaramillo event. The upper Olyorian (Akanian) is defined by the occurrence of the more advanced collared lemming Dicrostonyx renidens and corresponds with the earliest part of the normal polarity Brunhes Chron although also including the Matuyama/Brunhes reversal and the uppermost Matuyama Chron.

The Olyorian is characterized by a unique assemblage of large mammals, which appeared for the first time in this region and, in the case of Gulo, Rangifer and Mammuthus primigenius, for the first time worldwide. Remains of an “archaic northern elephant” in Chukochyan deposits and of a more advanced form in Akanian sediments (Sher, 1986) represent a distinct mammoth lineage. They are actually morphologically very advanced pre-empting an evolutionary level of mammoths that occurred outside Beringia only 300–400 ka later (Sher, 1986; Kahlke, 1999). Genomic analyses based on ancient DNA with an age of more than 1 million years prove the affiliation of lower Olyorian mammoth remains at the Krestovka and Adycha sites to the steppe-mammoth lineage (van der Valk et al., 2021). Genomic data from another, only 650,000-year-old, thus Akanian, mammoth found at the Chukochya site reveal that it was an early representative of the woolly mammoth (M. primigenius), which thus evolved in Western Beringia and descended from the Adycha lineage of the steppe-mammoth (van der Valk et al., 2021).

Numerous bones of a very large horse assigned to Equus (Plesippus) verae are characteristic of both units, the Chukochyan and the Akanian. According to recent cladistic analyses of morphometrical measurements of cranial and postcranial elements of 30 extinct and extant horse taxa, Plesippus must be placed within the genus Equus (Cirilli et al., 2021). Equus verae belongs to the stenonine clade, which comprises modern zebras and asses and descends from E. stenonis and, as all Old-World Equids, from its North American progenitor E. simplicidens. Equus suessenbornensis, occurring in Central Europe later during the Middle Pleistocene, coincides morphometrically with E. verae and is regarded successor of an Asian parent form (Forsten, 1986 referred to in; Kahlke, 1999). In addition to the large E. verae, the sympatric occurrence of a second, smaller, equid in Olyorian deposits is reported (Sher et al., 1979). From the upper (Akanian) unit, also caballine horses are described (Sher, 1986).

Remains of a giant moose (Cervalces sp.) and of Bison sp. were regularly found in both units. Both taxa were, at this time, widespread over the Eurasian middle and high latitudes as is documented by abundant findings, e.g., at the Epivillafranchian site of Untermassfeld in Germany (Kahlke et al., 2011). Also cave bears had an extensive distribution in middle latitudes at this time, but the discovery of a mandible determined as Ursus savini ssp. nordostensis near Cherskiy represents the northernmost and easternmost find of spelaeoid bears worldwide (Sher et al., 2011).

The Olyorian findings of Rangifer ex. gr. tarandus are, together with fossils from Cape Deceit, Kotzebue Sound, Western Alaska, the oldest known reindeer fossils worldwide suggesting the origin of reindeer in Beringia (Sher, 1986; Kahlke, 1999). An archaic form of another typically arctic faunal element is represented in Olyorian deposits as well: the oldest finds of Praeovibus beringiensis indicate the origin of musk ox in NE Siberia (Sher, 1971). Praeovibos can be considered as early morphotype of the extant Ovibos moschatus as is suggested by ancient DNA studies (Campos et al., 2010). Apart from Praeovibos, postcranial remains of another, smaller Ovibovine were also detected in Olyorian deposits and might possibly represent Soergelia (Sher et al., 1979), which is proven for the upper unit Akanian (Sher, 1986). Soergelia was widely distributed in NE Siberia (Sher, 1971; Boeskorov, 2019) and probably originated in the NE Siberian Arctic (Kahlke, 1999). Thus, it is regarded a Beringian bovid as well.

Even Saiga, undoubtedly an immigrant from the zonal steppe belt, was already present in West Beringia during Olyorian times. The Olyorian finds of Saiga are the worldwide oldest known fossils of that genus so far (Kahlke, 1999). As saiga is, more than any other Beringian faunal element, indicative of zonal steppe grasslands, it can be assumed that it evolved in the inner continental steppe belt.

Some large mammals expanded into the region from the inner continental steppe belt, as is the case of Saiga, Equus and Cervalces. Several of the listed taxa evolved, however, in Beringia during the Early Pleistocene developing adaptations to environmental conditions making them fit for huge range expansions across the Holarctic and far into the mid latitudes during later cold climate phases, which became, in the course of the Early-Mid Pleistocene Transition, progressively longer and colder. The Olyorian faunal complex is therefore regarded as an important predecessor of the late-Pleistocene Mammuthus-Coelodonta faunal complex (Kahlke, 1999).

Only limited data are available on the habitat conditions for large herbivores in Beringia during the Olyorian period. Palaeobotanical and entomological data from the Krestovka and the Chukochya sites suggest that climate and vegetation changed only slightly within the deposition period of the sequence (Giterman et al., 1982). According to palynological results from the Krestovka sequence, non-arboreal pollen with Poaceae, Artemisia, Chenopodiaceae and Caryophyllaceae were most abundant throughout the studied sequence indicating open grassland vegetation and aridity. The existence of such environments is confirmed by entomological data revealing the prevalence of grassland dwellers in Chukochyan deposits and the existence of few taiga species suggesting that groves of trees and shrubs persisted. The percentage of arboreal taxa in the pollen spectra, mainly Alnus (alnobetula ssp.) fruticosa, Betula, sp., Betula Sect. Nanae and Pinus pumila with rare Larix, declined from up to 40% at the base of the sequence to nearly 5% in the middle part and then slowly increased towards the top of the sequence. Available plant macrofossil data represent only azonal, aquatic vegetation presumably from small ponds (Giterman et al., 1982). Due to the limitations of pollen data in northern environments, like overrepresentation of long-distance-transported pollen in conjunction with low local pollen production and the lack of taxonomic differentiation (Birks and Birks, 2000), plant macrofossil data are, however, especially valuable for the reconstruction of vegetation and environmental conditions in particular when they represent zonal vegetation.

In the present paper, we use plant macrofossils sampled from Early Olyorian (Chukochyan) deposits in a gully near Cherskiy, to reconstruct zonal vegetation and environmental conditions existing at the life-time of the cold pre-adapted Beringian faunal complex.

The sampled outcrop “Cherskiy Ovrag” is situated near the North-East Science Station of the Pacific Geographical Institute and about 3 km east of Cherskiy at the right bank of the Panteleikha River, a tributary of the Kolyma River (Figure 1). The study site is located at the northeastern edge of Yakutia at the junction to the Anyui Upland macroslope. Starting from the left bank of the Panteleikha River, the Kolyma lowland extends over hundreds of km to the west and merges with the Yana-Indigirka lowland, together forming the vast Northeast Siberian coastal lowlands.

We recovered 750 plant macrofossils from altogether 33 plant taxa. The abundance of individual taxa was not considered in detail because the number of preserved seeds and fruits does not necessarily reflect the actual percentage of plant taxa in the former vegetation but depends on taphonomic circumstances and other coincidences. The studied assemblage represents plants from a wide spectrum of habitats co-occurring in distances of few dozens to hundreds of meters probably during a period of few years. As the plant remains are partly tiny and are well preserved, a redeposition over long distances can be excluded. Plant species only occur under environmental conditions that meet their requirements and together with other plants with similar requirements. Their find thus indicates the presence of these conditions, habitats and communities even when the original set of plant species is incomplete and fragmentary. The recovered taxa were assigned to discrete plant communities according to the phytosociological classification following the ecological requirements of their modern representatives. They are listed in Table 1 in the order of their probable occurrence in certain plant communities.

The composition of the studied Olyorian plant macrofossil assemblage of the Cherskiy Ovrag reflects predominant grassland vegetation consisting of meadow steppes and tundra steppes interspersed with wooded patches composed of cold-resistant deciduous trees and shrubs (Larix gmelinii s.l. and Betula). The results give the impression of a mosaic-like open landscape with sparse larch groves and are consistent with pollen and entomological data from the Krestovka site, about 160 km to the southwest of Cherskiy (Figure 1; Giterman et al., 1982), for the Olyorian mammal age. Like the Cherskiy Ovrag, the Krestovka site is situated at the junction of the Kolyma lowland and an upland, in this case the Yukagir Plateau, and the Chukochyan deposits at the Krestovka site display a similar lithology mainly comprising alluvial sand interspersed with sandy silt beds. In the pollen data, low levels of shrub taxa and larch are consistent with our data. Also, the abundance of Poaceae, Artemisia, Chenopodiaceae and Caryophyllaceae in the pollen spectra confirm steppe-like vegetation as reconstructed with our macrofossil data from the Cherskiy Ovrag. As described in the introduction, plant macrofossils from the Krestovka site mainly originate from aquatic plants such as Sparganium, Menyanthes and several Potamogeton species (Sher et al., 1979). The few terrestrial plant remains stem from Larix and Rubus; both taxa are preserved also in the studied deposits of the Cherskiy Ovrag.

In our results, the zonal vegetation, i.e., the vegetation that correlates with the macroclimate and that attains dominance under this climate, is represented by dry grassland, i.e., tundra steppe and meadow steppe. The majority of identified plant remains in both abundance and diversity belong to dry grassland taxa, which cannot tolerate shading, as occurring in closed woodlands, or surplus moisture in the active layer, as in tundra wetlands.

Among the mammals preserved in Olyorian deposits, there are several grassland taxa, which indicate an open landscape and steppe-like vegetation as well. Saiga, an immigrant from the zonal steppe belt, indicates dry climate, a thin snow cover, a firm ground or, respectively, a dry active layer and widespread grassland vegetation. Chenopodiaceae, detected in both the pollen and macrofossil data (Chenopodium prostratum), are, besides other steppe forbs, considered the basic food of saiga (Kahlke, 1999; Abaturov et al., 2005). The presence of the giant moose Cervalces with its iconic huge antlers suggests vegetation was open rather than consisting of closed taiga forests. Remains of steppe mammoth (M. trongontheri), and bones of archaic giant horses (E. verae) are also clue for open grassland and steppe like vegetation existing during the time of deposition. Steppe mammoths had already evolved abrasion-resistant teeth as an effective adaptation to relatively hard grass-rich diet and are thus indicators of steppe-like vegetation.

In the presented Early Olyorian (Early Pleistocene) plant macrofossil assemblage, the presence of trees and shrubs indicates interglacial-like temperature conditions but the dominance of dry grassland species and the occurrence of halotolerant plants suggests aridity, which is actually rather characteristic of cold stages. The palaeontological data suggest that the described grass and forb dominated open forest steppe persisted over a long period of time, i.e., during the entire Olyorian age as is indicated by pollen data from both the Chukochyan and the Akanian units at the Krestovka site continuously dominated by herb pollen, mainly from Poaceae, Artemisia and Chenopodiaceae and with only a small amount of larch and birch over all of the sequence (Giterman et al., 1982).

The Olyorian mammal age comprised a long period of time spanning from the late Early to the early Middle Pleistocene, i.e., 1.4–0.5 Ma (Sher, 1986) and, thus, included the Early-Middle Pleistocene transition (EMPT), which occurred about 1.25–0.7 Ma ago (between MIS 37, and MIS 18). The EMPT is described as a shift from relatively low-amplitude 41 kyr lasting climate cycles with a more or less symmetrical curve of warming and cooling to high-amplitude 100 kyr cycles with a more sawtooth-like pattern as result of abrupt warming followed by a slow descent of temperature (Pisias and Moore, 1981; Clark et al., 2006; Cohen and Gibbard, 2019). Prior to the EMPT, the amplitude of climate fluctuations was smaller and the duration of climate cycles was shorter, which possibly resulted, in West Beringia, in relatively dry, cool but more moderate climate that became progressively colder in the course of the EMPT but remained more stable than in other parts of the Holarctic. The palynological record from the composite sediment core of Lake El’gygytgyn, ICPD Site 5,011-1, about 467 km SE of the Cherskiy Ovrag provides valuable information on the regional history of vegetation and climate in West Beringia over the EMPT interval, i.e., between 1,091 and 715 kyr (Zhao et al., 2018). The pollen data indicate shrub tundra and cold steppe communities alternatingly dominating over the major part of the studied Early Olyorian period until MIS 24 (from 1,091 until 930 kyr) with Poaceae, Cyperaceae and Artemisia pollen and high contents of Selaginella rupestris spores prevailing during cold phases and increased amounts of Betula and Alnus (shrub birch and shrub alder) with sporadic occurrences of Larix during warm stages. The data suggest a gradual opening of the vegetation cover during this time period with a continuous but decreasing presence of larch in scattered woodland stands. The (supra-) regional pollen data thus indicate that the local vegetation we reconstructed for the Cherskiy Ovrag, consisting of grasslands in a mosaic with scattered woodlands, was widespread and existed over a long period of time. A hiatus in the pollen record comprised MIS 23.

MIS 22 around 890 kyr ago was a turning point in the West Beringian vegetation history as recorded in the El’gygytgyn core. Cold steppe biome scores in the pollen record significantly increased from this point in time onwards indicating accelerated aridification. The long-term cooling and aridification trend observed in the El’gygytgyn record corresponds with global ice-sheet expansion, sea-level lowering, coast-line shifts, shelf exposure and an intensified Siberian High and is considered additionally strengthened by the Tibetan Plateau uplift (Zhao et al., 2018).

The boundary between the lower and upper Olyorian, i.e., between the Chukochyan and Akanian was not yet exactly defined and is not preserved in a continuous geological record. But MIS 22 is a good candidate for this boundary as it represents the first high-amplitude glaciation that followed the 100 kyr periodicity. The switch from 41 to 100 kyr periodicity entailed long-lasting environmental changes resulting in the strengthening of aridification and the expansion of cold steppe vegetation and drove the evolution of cold and drought-resistant herbivores. The Chukochyan period therefore likely corresponds to the time prior to MIS 22. This phase was, according to the El’gygytgyn pollen record, characterized by a gradual expansion of grassland vegetation and the persistence of more or less scattered larch and shrub birch woodlands reflecting rather slow cooling and relative climatic stability. This relative stability of cold and dry climate over such a long period of time possibly resulted in the evolution of cold-adapted grazers and, eventually, of the mammoth steppe ecosystem.

The openness of vegetation is consequently not necessarily exclusively a function of climate in West Beringia but was probably also the result of megafaunal influence. The impact of the large herbivores on vegetation is documented in the macrofossil assemblage by plant taxa that are considered ruderal in modern vegetation, i.e., that occur at severely disturbed sites on nutrient-rich, compacted ground. Such taxa as Carex duriuscula, Chenopodium prostratum, Sibbaldianthe bifurca and Polygonum aviculare are resistant to overgrazing and trampling and are hardly competitive without regular disturbances of the plant cover, e.g., by herbivores. Steppe vegetation is fostered by the removal of woody plants due to decreasing shading and increasing insolation and evaporation. Steppes therefore replace woodlands in extremely continental areas at the middle courses of the Yana and Indigirka Rivers in Yakutia after wildfires (Yurtsev, 1982). In the Olyorian assemblage, the absence of any charcoal precludes that wildfire was the reason for the low percentage of trees and shrubs. Instead, the presence of nitrophilous pioneer plants such as Sibbaldianthe bifurca and Urtica dioica is rather an indication for zoogenic impact. Megaherbivores, especially proboscideans, are considered ecosystem engineers, which maintain vegetation openness, and in wooded landscapes, create mosaics of different structural types of vegetation with high habitat and species diversity (Johnson, 2009). Due to consumption of woody plants in winter and increased nutrient turnover, grasses and forbs are fostered by the presence of megaherbivores. The continuous existence of grassland vegetation and megaherbivores such as the steppe mammoth and, later, the evolution of the woolly mammoth in the study region demonstrates the importance of the unglaciated Arctic of Beringia as a cold laboratory for the evolution of the mammoth steppe biome – the ice-age ecosystem that, during the Middle and Late Pleistocene, spread over large proportions of the Northern hemisphere. Therefore, West Beringia can be regarded as the cradle of the mammoth steppe biome.

• A plant macrofossil assemblage from the Cherskiy Ovrag reveals the predominance of grassland vegetation composed of tundra steppes, meadow steppes and saline meadows in West Beringia during the Early Olyorian.