Arvicola, Lacepede, 1799

Agadjanian, Alexander K. & Kondrashov, Peter, 2020, Late Pleistocene (Eemian) Mollusk And Small Mammal Fauna From Mikhailovka- 5 (Kursk Oblast, Central Russia), Fossil Imprint 76 (1), pp. 17-39 : 30-33

Arvicola ex gr. sapidus MILLER, 1908

M a t e r i a l. PIN 4970/117–137, 21 m1; PIN 4970/138– 158, 21 m2; PIN 4970/159–174, 16 m3; PIN 4970/175–199, 25 M1; PIN 4970/200–222, 23 M2; PIN 4970/223–240, 18 M3; PIN 4970/241–243, 3 calcanei; PIN 4970/ 244–246, 3 tali; PIN 4970/247–251, 5 ulnae; PIN 4970/252, 1 humerus.

D e s c r i p t i o n. Water vole teeth from Mikhailovka-5 are similar to the modern Arvicola terrestris in size. Crown cement is well developed on teeth of older individuals. Molars, especially m1 and M3, are variable in size and morphology ( Text-figs 7 , 8 ). On almost all teeth, the enamel has the same thickness on anterior and posterior enamel edges. It is thinner in younger teeth compared to those of older individuals ( Tab. 4).

Of the 21 m 1, only 1 belonged to a younger individual. The anterior part of the paraconid bears a fold resembling a “ Mimomys ” fold ( Text-fig. 7 ), which disappears when tooth crown wear exceeds 20 %. The same younger tooth has a dentine track on the labial side of the paraconid. This track reaches the occlusal surface on the worn teeth. The m2s are characterized by the fusion of the first pair of the opposing triangles. The m3s have both the first and second pairs of opposing triangles fused.

Both M1 and M2 have clearly separated triangles on the occlusal surface and are not different from the corresponding teeth of the other representatives of the genus. M3 has a unique structure. There is a significant variation in the complexity of the posterior part of the tooth. Some of the specimens have a simple, oval-shaped posterior lobe (metacone and posterior loop), while the others have a complex posterior lobe with convex outer walls and complex posterior loop. The teeth of younger individuals indicate that this differentiation is better expressed during the earlier stages of development. The more complex morphotypes have two external cement columns in the incoming angles on each side of the tooth, and the simpler types can have one on the lingual and two on the labial side. Few specimens show a tendency for the fusion of the triangles.

C o m p a r i s o n a n d c o m m e n t s. Molars of Late Pleistocene water voles were larger than those of Middle Pleistocene ones. Table 5 contains measurements of Pleistocene and modern species of Arvicola showing that A. mosbachensis from the Middle Pleistocene was the smallest in the genus. Arvicola from the first half of the Late Pleistocene including those of Mikhailovka-5 were much larger, but not as big as the modern A. terrestris . Although there are not enough specimens for a detailed statistical analysis, the trend can be established based on data from previous studies (Agadjanian 1983, Agadzhanyan 2012).

The taxonomic position of Arvicola from Mikhailovka-5 is determined by the occlusal pattern of its m1 and M3. Published data indicate that 20–25 % of the m1s of A. mosbachensis from Holsteinian have a “ Mimomys ” fold on the paraconid ( Radulesco and Samson 1977, Agadjanian 1983), but in modern Arvicola only 2–3 % show this feature. In Mikhailovka-5, the fold was present only on 1 m 1 out of 21 (~5 %).

M3 provides additional information about the identity of the Arvicola from Mikhailovka-5. The posterior part of this tooth in the Middle Pleistocene A. mosbachensis is significantly lobed, and the incoming angles between the metacone and posterior loop are deep, causing a separation of these prisms on some specimens. In modern Arvicola , the posterior triangle is fused with the metacone. Based on this feature, the Arvicola from Mikhailovka-5 is intermediate between the archaic and modern Arvicola , being different from both. It resembles more A. sapidus from southwestern Europe ( Agadjanian 2009). Based on previous studies ( Agadjanian and Erbaeva 1983), this species displays an array of primitive dental morphological features. Moreover, its number of chromosomes (2n = 40) is different from A. terrestris (2n = 36) ( Reichstein 1963), indicating it is a relic species that survived on the Iberian Peninsula. This species has been identified from the Late Pleistocene of northwestern Spain ( López-García et al. 2011) and southern France ( Montuire and Desclaux 1997). Eemian Arvicola is common in central and southern England ( Sutcliffe and Kowalski 1976), France ( Chaline 1972), Central Europe ( Jánossy 1985, van Kolfschoten 2000), and nearly all localities across the Russian Plain ( Markova 2000, Agadjanian 2009). Despite the similarity to A. sapidus , the water vole from Mikhailovka-5 represents a distinct taxon, here referred to as Arvicola ex gr. sapidus . It was likely widespread across Eastern Europe during the Mikulino interglacial. The taxonomy of Pleistocene Arvicola is complicated and Schmelzband-Differenzierungs- Quotient was shown to be unreliable in species identification ( Escudé et al. 2008a), so morphology is the only reliable key to identification (Agadjanian 1983, Agadzhanyan 2012). It has been even suggested that all Pleistocene Arvicola belonged to a single species, Arvicola cantiana ( Escudé et al. 2008a, b), which is an oversimplification. The evolution of Middle and Early Pleistocene Arvicola was complex and likely involved several transitional species ( Agadzhanyan 2012), including the vole present in the Mikhailovka-5 fauna.

Comparison of the Mikhailovka-5 fauna with other Eemian small mammal faunas

Late Pleistocene small mammal faunas have been known since the nineteenth century, mostly from archaeological excavations. These faunas were used to determine the geochronological position and the dynamics of environmental changes ( Lyell 1872). A similar approach was used by Nehring (1890), who introduced the concept of tundra-steppe for the landscapes of the Last glaciation of Central Europe. Since then, small mammal faunas have been widely used for stratigraphy and palaeogeographic reconstructions during the Pleistocene. Similarly, mollusk faunas also became an important source of data for such reconstructions ( Fejfar and Ložek 1957, Ložek 1964). Hundreds of small mammal faunas are known from the Late Pleistocene of Western, Central, and Eastern Europe, the Ural Mountains, Western and Eastern Siberia, and the Altai Mountains ( Agadjanian 2009). The majority of these faunas are associated with Middle and Late Palaeolithic localities, mostly correspond to the second half of the Late Pleistocene, and are limited to the Valdayian glacial. The faunas of the first half of the Late Pleistocene that belong to the Mikulinian (= Eemian = Ipswichian) interglacial are much less studied and rarely include data for both small mammals and mollusks, which makes the current study rather singular.

A review of the Eemian mammal faunas of Central Europe lists nine known localities of this age ( van Kolfschoten 2000); Markova mentioned 15 Mikulinian localities from the Russian Plain and the Crimea ( Markova 2000). There are few Riss-Würm (= Eemian) faunas, such as Santenay, Montignac, Combe-Grenal, and Baume Moula- Guercy known from France ( Chaline 1972, Defleur and Desclaux 2019). In Hungary, the upper strata of the Süttö locality can be referred to this age ( Jánossy 1985), indicating that the faunas of the last interglacial are uncommon in the fossil record. This lack in the fossil record is probably the result of a warm and humid climate that increases the rate of decomposition of fossil remains because it produces more favourable conditions for bacteria, fungi, and various invertebrates, including worms and insect larvae, than the dry and cold climate that is typical for glacial periods. Personal observations in the modern tundra indicate that decomposition is much slower than in Central Russia, where a dead mole becomes buried in the ground to a depth of 10 cm overnight by scavenging beetles. Further, the formation of extensive forests and dense grass cover in the open landscapes decrease the erosion of the ground, which slows the formation and thanatocenoses and taphocenoses, i.e., the rate of fossilization that accounts for the rarity of such faunas described for Mikhailovka-5 and increases their significance.

The fauna of Mikhailovka-5 is similar to that of other Mikulinian localities. These faunas are characterized by the abundance and diversity of insectivores, which often account for up to 10 % of small mammal remains. These similarities most likely reflect true faunal composition as fossil accumulation in the studied localities was not due to owl activity or selective deposition that you often observe in cave deposits ( Andrews 1990). Eemian faunas, such as those of Santenay ( France), Süttö ( Hungary), Schönfeld and Stuttgart-Untertürkheim ( Germany), and Borisova Gora ( Belarus) ( Chaline 1972, Jánossy 1985, Sanko and Motuzko 1991, van Kolfschoten 2000), typically contain shrews Sorex and Neomys and moles Talpa .

The bulk of the small mammal fossil assemblage at Mikhailovka-5 is formed of voles. Nearly all European small mammal faunas of this age are dominated by Microtus voles with M. arvalis and M. ex gr. agrestis being the most common ( Agadjanian 2009). Notably, these species are absent from the second half of the Middle Pleistocene of the Russian Plain or are represented by much more archaic morphotypes ( Agadjanian 2009). Another vole that is typical representative of the Mikhailovka-5 fauna is Microtus (Terricola) ex gr. subterraneus , which is morphologically quite different from the Middle Pleistocene Terricola and much closer to the modern species. Microtus (Terricola) subterraneus is known from the Borisova Gora locality in Belarus ( Sanko and Motuzko 1991), Gröbern 1 and Taubach in Germany ( van Kolfschoten 2000), and layers 2 and 3 at the Süttö 6 locality in Hungary ( Jánossy 1985). Microtus (Terricola) subterraneus has been reported from several Eemian localities in France, including Santenay (layers 5a, 5c, and 3), Regourdou (layers 2 and 7) and others ( Chaline 1972). This species was rather broadly distributed in Central and Southern regions of Western Europe in both Middle and Late Pleistocene ( Schreve 2001, Bogićević et al. 2012), but was only present in the central part of the Russian Plain during the Eemian ( Agadjanian 2009).

Clethrionomys voles are strong indicators of forested habitats and have been reported from 8 of the 15 Mikulian faunas of the Russian Plain ( Markova 2000). In the faunas of Borisova Gora and Konevich, they are the dominating species ( Kalinovski 1983, Markova 2000). Clethrionomys have been reported from 5 of the 9 Eemian localities in Germany ( van Kolfschoten 2000) and from several Eemian faunas in France, including Santenay (layers 5a, 5c and 3), and Regourdou (layers 8, 7, 5, and 2) ( Chaline 1972). Their abundance reaches 6 % at Mikhailovka-5.

Water vole Arvicola has been present in European small mammal faunas since the second half of the Middle Pleistocene until present. The evolution and morphology of this group of voles has been described in detail, including changes in the enamel molar structure that began during the Likhvinian (= Holsteinian = Hoxnian) interglacial and continue through the present time ( von Koenigswald 1973, Heinrich 1982, Agadjanian 1983, Desclaux et al. 2000, Agadjanian 2001, 2009, Agadzhanyan 2012). These changes included a transition from Mimomys - type of enamel differentiation to the Microtus - type, i.e., a transition from late Mimomys to the first Arvicola mosbachensis of the Likhvinian and to the molars that are typical for the modern Arvicola terrestris . Notably, the Mikulinian water voles have an enamel structure intermediate between A. mosbachensis and A. terrestris , which is seen among modern water voles only in the Arvicola sapidus from southern France and the Iberian peninsula ( Agadjanian 2009). It is likely that modern A. sapidus is a relic species from the Eemian with a more archaic morphology and karyotype ( Reichstein 1963). Interesting data have been reported from the Biśnik Cave locality in Poland. Arvicola is present in all strata from the late Middle to Upper Pleistocene, including the Weichselian (= Valdayian) glacial, but reaches the highest representation in layer 13, which was referred to MIS 5e and thus is Eemian in age ( Socha 2014). In this layer, the enamel differentiation (SDQ) index that characterizes the molar enamel structure is

1 – Mastyuzhenka 2 – Donskaya Negachevka SDQ 3 – Komintern 4 – Mikhailovka-5 200

180

160

140

120

100

80

60

40 1 2 3 4

very close to that of the Arvicola from Mikhailovka-5 (Textfig. 9). The SDQ index of molar enamel of the Arvicola amphibius (= terrestris ) from layers 14 and 15 of Biśnik Cave ( Socha 2014) indicates enamel of equal thickness across the molars, which is more typical for A. sapidus . Of the six Ipswichian localities from England, three represent a “transitional form” ( Sutcliffe and Kowalski 1976) between Arvicola cantiana (= A. mosbachensis ) and A. terrestris , which is very close to A. sapidus .

Small numbers of the wood mouse Apodemus ex gr. sylvaticus were recorded from Mikhailovka-5. This species is closely associated with broadleaf deciduous forests ( Gromov and Erbaeva 1995). It is present in dozens of Late Pleistocene localities of the British Isles, where they are found in all temperate interglacial fossil assemblages of the Late Pleistocene ( Sutcliffe and Kowalski 1976). Wood mice were very common in the Upper Pleistocene deposits of Northern Italy and Spain ( López-García et al. 2011, 2014) and of Central Europe ( Jánossy 1985, van Kolfschoten 2000, Döppes et al. 2008). Apodemus is much less common on the Russian Plain ( Markova 2000) but reaches significant numbers at the Borisova Gora in Belarus ( Sanko and Motuzko 1991).

Despite the overall similarities between Eemian faunas across Europe, there are a few differences. For example, the Mikhailovka-5 fauna does not contain any dormice ( Gliridae ), which is also true for most localities on the Russian Plain, except for the Borisova Gora site in Belarus. But even then, only 2 specimens out of 957 identified were referred to Glis sp. (0.2 % of the total) ( Markova 2000). Of the nine Central European Eemian localities, dormice ( Glis glis ) was only identified at Burgtonna ( van Kolfschoten 2000). The only hazel dormouse ( Muscardinus ) tooth in England was found in the stratotypic section of the Cromerian in the Cromer Forest bed of Norfolk ( Sutcliffe and Kowalski 1976). Muscardinus avellanarius are known from the Holocene cave deposits of England but are absent from the Late Pleistocene faunas. Dormice are common in the Eemian deposits of Spain, France, Italy, Hungary, and Poland, where at least three species, Eliomys quercinus , Muscardinus avellanarius , and Glis glis , have been identified ( Chaline 1972, Jánossy 1985, López-García et al. 2011, 2014, Socha 2014).

A distinctive feature of the Mikhailovka-5 fauna is the presence of the yellow steppe lemming Eolagurus . This rodent is typical for dry steppe habitats and is currently found in the Zaisan Depression of Kazakhstan, Western Mongolia, Northwestern China, and Inner Mongolia ( Gromov and Erbaeva 1995). The distribution of Eolagurus in the beginning of the Middle Pleistocene stretched from the lower reaches of the Volga and Don rivers to the lower reaches of the Dnieper and Dniester rivers ( Gromov and Baranova 1981). Eolagurus cf. luteus has been identified from 6 of the 15 Mikulinian localities on the Russian Plain and Crimea ( Markova 2000).

An unusual component of the Mikhailovka-5 fauna is the presence of the mole rat Spalax ex gr. microphtalmus. Besides Mikhailovka, this subterranean rodent has been identified from only one Eemian locality on the Russian Plain and Crimea, the El’tigen site in the Crimea ( Markova 2000). The mole rat is rare in corresponding faunas of Poland and Hungary and is completely absent from those of Central Europe, Great Britain, Spain, France, and Italy.

Another distinctive component of the Mikhailovka-5 fauna is the ground squirrel Spermophilus ex gr. suslicus. This rodent species does not occur in the Eemian faunas of Central Europe, Italy, Spain, or England; but one species of ground squirrel, Citellus citelloides , has been recorded from Süttö 6 in Hungary ( Jánossy 1985). Ground squirrels are known from Poland, and Citellus sp. of Eemian age has been reported from the Grotte Scladina in Belgium ( Döppes et al. 2008).

A distinctive feature of the cold epochs of the Late Pleistocene Eastern European faunas is the presence of jerboas; they were broadly distributed during that time and reached the upper Volga River, which is 400–500 km north of Moscow ( Agadjanian 2009). However, they are rare in the Mikulinian interglacial faunas and are known from the Shkurlat fauna from the Don River and the Zaskalnaya IX fauna from the Crimea ( Markova 1986, 2000). The Crimean peninsula marked the westernmost distribution of jerboas during the Late Pleistocene.

Lagomorphs in the Mikhailovka-5 fauna are represented by the steppe pika Ochotona cf. pusilla . Pikas were common in the Late Pleistocene faunas of Asia and Eastern Europe, but in small numbers and mostly during the cold epochs. They were much rarer during the Eemian interglacial. Of 15 Mikulinian localities on the Russian Plain and Crimea, Ochotona pusilla has been recorded in only five ( Markova 1986, 2000). Pikas have not been described from the Eemian faunas of Central and Western Europe.

Thus, overall the Mikhailovka-5 small mammal fauna is generally similar to the other Eemian faunas of the Russian Plain, but it differs in the presence of steppe species. The studied fauna also differs from the Eemian small mammal faunas of Central and Western Europe in records of ground squirrels, mole rats, and yellow steppe lemmings, but missing typical Mediterranean species, such as the porcupine and dormice.

Figures

Text-fig. 7. Lower molars of Arvicola ex gr. sapidus MILLER, 1908 from Mikhailovka-5. a–g: m1, h–j: m2, k–m: m3.

Text-fig. 8. Upper molars of Arvicola ex gr. sapidus MILLER, 1908 from Mikhailovka-5. a–d: M1, e–h: M2, i–q: M3.