On the origin of the Middle Pleistocene larger voles

Quaternary International, Vol. 19, pp. 47-50, 1993. Printed in Great Britain. All rights reserved.

1040~182/93 $24.00 © 1993 INQUA/PergamonPress Ltd

ON THE ORIGIN OF THE MIDDLE PLEISTOCENE LARGER VOLES Thijs van Kolfschoten Institute of Prehistory, P.O. Box 9515, 2300 RA Leiden, The Netherlands

The Quaternary smaller mammal faunas are often dominated by voles. The evolutionary stages of these voles have appeared to be very useful for biostratigraphical correlations. Of particular importance are the larger voles. Although they have been studied extensively, there is n o c o n s e n s u s o n the relationship between the Early and Middle Pleistocene larger voles, which hampers the establishment of detailed biostratigraphical correlations. Four different models published during the past decade are presented and briefly discussed in this paper. The broadly accepted relationship between M. ostramosensis and the Biharian larger voles, referred to M. savini by many authors, is treated more extensively and is questioned. The other models, such as a proposed relationship between Cromeromys s.l. and the Biharian larger voles are not investigated well enough, nor are they well established. It is therefore concluded that the Early Pleistocene larger voles should be re-investigated in detail in order to shed more light on the origin of the Middle Pleistocene larger voles.

water vole Arvicola terrestris and of the fossil 'Mimomys savini' by presenting different models.

INTRODUCTION

Voles are dominant in most Quaternary smaller mammal faunas and their morphological characteristics, stratigraphical distribution and phyletic evolution have been studied by many authors over a long period of time (e.g. Hinton, 1926; Schreuder, 1943; Kretzoi, 1965; Chaline, 1972; Meulen, 1973; Koenigswald, 1973, 1980; Heinrich, 1978, 1987; Fejfar and Heinrich, 1981; Rabeder, 1981; Mayhew and Stuart, 1986; Kolfschoten, 1990a, 1990b). The fossil record has indicated that some species, such as Pliomys episcopalis, hardly evolved at all, whereas the evolution of other voles appears to have been rather rapid (Chaline, 1993). Evolution is chiefly manifested as an increase in hypsodonty, which resulted in an increase in the height of the enamel-free areas and which can result in the disappearance of the Mimomys islet (the mimomyan enamel pit) and the disappearance of root formation. In addition, an increase in size may occur, as well as changes in the morphology of the occlusal surface of M 3 and M 1. The evolutionary stages of certain species are often used for biostratigraphical correlations. Of particular importance is the evolution which took place within the group of larger voles to which belong Mimomys occitanus, M. polonicus, M. pliocaenicus, M. ostramosensis, M. savini and the living Arvicola terrestris andArvicola sapidus. This group has been studied extensively; however, there is no general model which explains the relationship between these species. The lineage Mimomys occitanus - M. polonicus - M. pliocaenicus - M. ostramosensis seems to be broadly accepted and the gradual evolution of the Mimomys occitanusostramosensis lineage is corroborated by image analysis of occlusal surfaces (Viriot et al., 1990). However, because of a lack of data, there is no consensus regarding the origin of the Biharian larger voles, which are often referred to as Mimomys savini, nor on the origin and evolution of the living Arvicola terrestris. This lack of knowledge hampers the establishment of detailed biostratigraphical correlations. The aim of this paper is to discuss the origin of the living 47

THE ORIGIN OF ARVICOLA T E R R E S T R I S

The first appearance of Arvicola terrestris cantiana, the most primitive representative of the species Arvicola terrestris (Kolfschoten, 1990a; Koenigswald and Kolfschoten, in press), during the second half of the 'Cromerian Complex', seems to be well-established. Transitional populations of larger voles, some with rooted molars as well as a high percentage of unrooted ones, are known from several localities (Isernia, Italy; Prezletice, Czechoslovakia; Kusnezovka, Russia). The origin of Arvicola sapidus and its relationship to Arvicola terrestris as well as to the Biharian larger vole Mimomys savini is still unclear. Different models have been published. Rekovets (1990) states that "the phyletic line which led to the development ofA. sapidus is likely to branch from the arvicolid common trunk at the initial stage of the A. mosbachensis (= Arvicola terrestris) line and evolves parallel to it" whereas Rabeder (1981) postulates two lineages: one which leads to Arvicola sapidus and one which leads to A. terrestris. In Rabeder's opinion, the two lineages become separated during the beginning of the Csarnotian, long before the beginning of the Pleistocene. Arvicola sapidus still inhabits Portugal, Spain and southern France. A detailed study of the fossil record of that area could contribute to our knowledge about the origin and evolution ofA. sapidus and its relationship to A. terrestris.

THE ORIGIN OF THE LATE BIHARIAN LARGER VOLES

The Single Lineage Model Many authors (e.g. Kretzoi, 1965; Koenigswald, 1973; Meulen, 1973; Stuart, 1975, 1981; Chaline and Sevilla, 1990; Lister, 1992) refer to the larger voles with rooted molars from

48

T. van Kolfschoten

late Biharian faunas, such as West Runton Freshwater Bed (Great Britain) and Voigtstedt (Germany), as a single species, M i m o m y s savini. T h e origin of M i m o m y s savini is, judging from the literature (e.g. Chaline and Sevilla, 1990), rather well-established and the lineage M i m o m y s occitanus -

Two Lineages

Instead of the single lineage (Model I) described above, there are, according to Rabeder's (1981) hypothesis, two separate lineages: the M i m o m y s (Mimomys) lineage with e.g.

stehlini - polonicus - pliocaenicus - ostramosensis - savini

(Fig. 1: Model I) seems to be accepted by many authors. The relationship between M i m o m y s ostramosensis and M i m o m y s savini was suggested by Janossy and Meulen (1975), when they published the original description of M i m o m y s ostramosensis. They state: "In conclusion it may be suggested that M i m o m y s ostramosensis is closely related to both M. pliocaenicus and M. savini, because it is intermediate between these species in those morphological features which are thought to show evolution within the M i m o m y s lineages most clearly". However, M i m o m y s ostramosensis differs from M. savini in a number of aspects: - - M 1 of M. savini is smaller in size, ----~e enamel islet of M~ ofM. savini can only be observed in juvenile teeth, - - M 1 of M. savini has two roots whereas M [ of M. ostramosensis usually has three roots, of which the two anterior ones are not always completely separated, ---the enamel free areas (tracks) of the upper and the lower molars of M. savini are much higher, - - t h e morphology o f M 3 ofM. savini is more complex due to the deep second lingual re-entrant angle. If the lineage M i m o m y s occitanus - stehlini - polonicus pliocaenicus - ostramosensis - savini is correct, it is remarkable that there is an increase in size in the transition from M. pliocaenicus to M. ostramosensis and a decrease during the next step, the evolution to M. savini. Such a fluctuation in size cannot be observed in the larger voles from the Zuurland borehole which has yielded a sequence of early Pleistocene mammal faunas. The larger voles from these faunas hardly differ in size at all and never reach the dimensions of M. ostramosensis. This aspect, combined with the differences listed above, gives occasion to question the direct relationship between M. ostramosensis and M. savini. MODEL I

A.tet, restris

I I I I

M.savini

I I I I

M.ostramosenais

I I I I M.pliocaenicu$

I I I I

M.polonicus

I I I

M. polonicus, M. pliocaenicus, M. ostramosensis, M. savini and 'Arvicola' sapidus and the M i m o m y s (Microtomys) lineage with M. kretzoii, M. hintoni, M. reidi, M. coelodus, M. milleri, Arvicola cantiana andA. terrestris. T h e species of

these two lineages (Fig. 1: Model II) show morphological similarities and the evolutionary trend in the two lineages is more or less identical. However, the representatives of the M. (Microtomys) are slightly smaller and the rate of evolution within this lineage is higher. This model is based on the assumption that two rather similar larger voles, which differ slightly in size and stage of evolution, coexisted in Early, Middle and Late Pleistocene faunas, e.g. the one from West Runton Freshwater Bed, where, according to Rabeder (1981) two species, M. savini and M. 'intermedius' (= M. milleri) occur. Hinton (1926) referred the larger voles from West Runton Freshwater Bed to 3 different species: M. intermedius, M. majori and M. savini. Kretzoi (1965), however, indicated that the larger voles from West Runton Freshwater Bed and from Voigtstedt should be regarded as a single species, M. savini, an opinion which has been accepted by many authors (e.g. Koenigswald, 1970; Chaline, 1972; Stuart, 1975, 1981). Kretzoi's statement undermines the model with two parallel lineages which coexisted from before the Pleistocene period until recent times. The assumption that the late Biharian larger voles from e.g. West Runton Freshwater Bed and Voigtstedt belong to a single species, leads to to a different model (Fig. 1: Model III) with two different lineages presented by Horacek (1990); one with, e . g . M , polonicus and M. pliocaenicus, which ends during the early Biharian and another lineage with M. savini and the genus Arvicola, which branched off from M. coelodus during the Villanyian. The relationship between M. coelodus and M. savini has not yet been thoroughly investigated and the lineage should therefore not be regarded as well-established. M O D E L III

M O D E L II

M O D E L IV

A.sapidus

A.terrestris

A.tetrestris

A.terrestris

I I I I

I I I I

I I I I

I I I I

M.savini

M.milleri

M.savini

Crorneromys intermedius

I

I

I I I I

t I I I

J I

I I I

I I

I I

M.ostramosensis

Cromeromys s.I.

I I I I

I J

I I I I

M.ostramosensts

M.coelodus

M.ostramosensis

I I I I

I I I I

M.coelodus

r

M.phknzaenicus

M.reidi

M.pliocaenicus

M.pliocenicus

I I I I

I I f I

L t I

I I I I

M.polonicus

M.Io~zoii

M.polonicus

M.potonicus

I I I

I I I

f I I

I I I

FIG. I. Four differentmodelsto explainthe relationshipbetweenthe late Plioceneand Pleistocenelargervoles.ModelI is accordingto Chalineand Sevilla (1990), Model II according to Rabeder (1981), Model III according to Hor~cek(1990) and Model IV is based on the hypothesis of Zazhigin (1980).

Middle PleistoceneLarger Voles What about Cromeromys ? In the opinion of some Russian authors, such as Zazhigin (1980), there is no close relationship at all between either the Mimomys occitanus -pliocaenicus - ostramosensis lineage or M. coelodus and the larger late Biharian voles. They refer to the late Biharian larger voles with a 'Mimomys savini' morphology, as Cromeromys intermedius, whereas other authors (e.g. Smirnov et al., 1986; Yakovlev, 1987) use the name Mimomys ( Cromeromys ) intermedius. Zazhigin (1980) defined the genus Cromeromys, which according to him, differs from the genus Mimomys in its complicated upper third molars and the absence of the hollow enamel column, or Mimomys islet, in the anterior loop of M I. The type species is Cromeromys irtyshensis (Zazhigin, 1980); dating from the Late Pliocene and found in the Kochkovsky suite deposits of Southwestern Siberia, Podpusk, Durnoi Log (Kamen-na-Obi). The diagnosis (kindly translated by A. Tesakov) is: cement is present in the reentrant angles. Simplification of the paraconid (= anteroconid) of M l takes place by simple broadening of its elements' confluence corresponding to individual age. The hollow enamel column (enamel islet) in the anterior loop of the M 1 is absent. There are no simplified parts in the M 3. The enamel pattern of the M 3 crown is close to that of Dolomys milleri Nehring. Three species have so far been referred to the genus: Cromeromys irtyshensis, C. intermedius (Newton, 1881) and C. newtoni (F. Major, 1902). They represent two groups or lineages which differ in size; the first two species have larger dimensions (occlusal length Ml: 2.7-3.5 mm) than the last one (occlusal length Mm: 2.3-2.9 mm). Cromeromys is known from Late Pliocene deposits from Western Siberia and Transbaikalia and from Early and Middle Pleistocene deposits from Eurasia. The northeasternmost location is the Kolyma lowland. The best diagnostic feature to distinguish Cromeromys from Mimomys is the complex morphology of M 3. Mimomys molars usually have a simple morphology and an enamel islet, while Cromeromys molars have a more complex morphology with a second lingual reentrant angle and without an enamel islet. The larger voles in the fauna from the West Runton Freshwater Bed, often assigned to M. savini, have an M 3 with a Cromeromys morphotype and are therefore considered to belong to the genus Cromeromys. They have been referred to C. intermedius by Zazhigin (1980). The Biharian 'M. savini', the ancestor of Arvicola terrestris, is according to Zazhigin's model (Fig. 1: Model IV) part of the Cromeromys lineage and not related to any one of the Mimomys lineages mentioned above. To summarize it can be stated that there are at least four different hypotheses concerning the origin of Arvicola terrestris. In the first hypothesis, there is only one lineage with, e.g.M, pliocaenicus, M. ostramosensis and M. savini. Model II presents the idea that there are two different lineages, which have been separated from the Ruscinian to the present day. The lineage with Mimomys voles of smaller dimensions (e.g.M. reidi, M. coelodus and M. milleri) leads, according to this pliocaenicus, M. ostramosensis and M. savini leads to A. sapidus. The third option is also based on two different lineages; one with M. polonicus and M. pliocaenicus, which ends during the early Biharian and the other with M. savini

49

and the genus Arvicola, which branched off from M. coelodus during the Villanyian. The fourth model postulates no relationship at all between A. terrestris and the lineages mentioned above. A. terrestris is, according to this model, the result of evolution within one of the Cromeromys lineages.

DISCUSSION Judging from the different hypotheses discussed above, there seems to be consensus regarding the existence of the lineage Mimomys polonicus - M. pliocaenicus - M. ostramosensis, but a disagreement about the relationship between this lineage and the late Biharian larger voles. Another point of discussion is the number of species to which the late Biharian larger voles from Northwestern and Central Europe should be referred. Are we dealing with two different, but rather similar species, as suggested in Model II, or with only one species as assumed by the other models? The localities West Runton Freshwater Bed and Voigtstedt have yielded a large collection of molars from a larger vole all of which are assigned to M. savini. The morphology of Ml and M 3 from these collections show a wide range of variation, the same variation that can also be observed in a huge sample of early Arvicola terrestris molars from Miesenheim I, dating from the late Cromerian (Kolfschoten, in preparation). In spite of this variation, there are no indications of the presence of two different species. The morphological variation seems to conform to that existing within one species (Kretzoi, 1965). This implies that Model II should be rejected or at least called in question. If we accept the existence of only one larger vole in the late Biharian fossil record of Northwestern and Central Europe, we have to consider its ancestor. Is the species related to the M. polonicus - M. pliocaenicus - M. ostramosensis lineage (Model I) or related to and branched off from M. coelodus, as suggested in Model III (and partly in Model II)? There are, as stated above, arguments to doubt the direct relationship between M. ostramosensis and the late Biharian larger voles. The relationship with M. coelodus is also not well established. The holotype ofM. coelodus comes from the fauna of Kislang (Hungary) and has been described by Kretzoi (1954). The species is characterized by the absence o f a Mimomys ridge and the presence of a well developed enamel islet. According to Kretzoi (1954), the species is closely related to M. intermedius. Rabeder (1981) referred part of the larger vole molars from Deutsch-Altenburg 2 and 4 to M. coelodus. The molars are high-crowned; the presence of an enamel islet is only be indicated in M, and the morphology of this tooth is dominated by the 'intermedius' morphotype. The relationship between M. coelodus and the late Biharian larger voles has not been investigated properly up to now and the taxonomic status ofM. coelodus is so far unclear; it might be the descendant ofM. reidi, as suggested by Rabeder (1981). The fourth hypothesis (Model IV) assumes that the larger voles from, e.g. West Runton Freshwater Bed and Vorgtstedt form a link in the Cromeromys lineage. If this model is correct it is interesting to know when the first representatives of Cromeromys s.1. migrated to Central and Northwestern Europe. The larger voles from Tegelen (The Netherlands) are

50

T. van Kolfschoten

referred to as M. pliocaenicus (Freudenthal et al., 1976; Kolfschoten and Meulen, 1986). Within this group there is, however, at least one very high-crowned M 1 without an enamel islet and the M 3 specimens can be divided into two groups on the basis of the height of the enamel-free areas (Kolfschoten, 1990b). A subdivision into two groups, based on the height of the enamel-free areas and the presence of the enamel islet of M 3 and M l, is also indicated in the record of larger voles from the Zuurland borehole Fauna 9 with a Late Tiglian or Early Eburonian age (Kolfschoten, 1988; 1990b). The occurrence of these high-crowned specimens, together with the earliest Allophaiomys, might be indicative of an early migration to Central and Western Europe. However, the morphology of the larger M 3 from Zuurland 9 shows no Cromeromys features; they occur first in the Zuurland sequence, in Fauna 7, which is of Eburonian or Early Waalian age. Whether the high-crowned variants without enamel islets in the fauna from Tegelen and Zuurland should be regarded as abnormal representatives of the normal larger vole of the M. pliocaenicus lineage or as indications of the presence of a second species, related to either M. coelodus or to Cromeromys s.l., is not clear. A re-investigation of the larger voles from Tegelen and of the old and recently collected samples from the Zuurland borehole is essential to increase our knowledge concerning the origin of the Middle Pleistocene larger voles.

ACKNOWLEDGEMENTS The research of the author has been made possible by a fellowship from the Royal Netherlands Academy of Arts and Sciences, for which I am grateful. Furthermore I would like to thank Mr. L. W. Hordijk (Brielle) for his permission to study the vertebrate fossils from the Zuurland boreholes and Dr. A.S. Tesakov (Moscow) for our inspiring discussions and for translating relevant parts of some Russian papers.

REFERENCES Chaline, J. (1972). Les Rongeurs du Pi6istoc~ne moyen et supEfieur de France (Syst6matique, biostratigraphie, pal6oclimatologie). Cahiers de Pal~ontologie, Ed. C. N. R. S., 410 pp. Chaline, J, Laurin, B., Brunet-Lecomte, P. and Viriot, L. (1993). Morphological trends and rates of evolution in Arvicolids (Arvicolidae, Rodentia) at species level: towards a Puntuated EquilibriafDisequilibda Model. In: Chaline, J. and Werdelin, L. (Eds), Modes and Tempo of Evolution in the Quaternary. Quaternary International (this volume). Chaline, J. and Lanrin, B. (1984). Le role du climat dans l'6volution graduelle de la lign6e Mimomys occitanus-ostramosensis (Arvicolidae, Rodentia) au plioc~ne sup~rieur. Geobios, M~moire Special, 8, 323-331. Chaline, J. and Sevilla, P. (1990). Phyletic gradualism and developmental heterochronies in a European Plio/Pleistocene Mimomys lineage (Arvicolidae, Rodentia). International Symposium on the Evolution, Phylogeny and Biostratigraphy of Arvicolids, pp. 85-98. Fejfar, O. and Heinrich, W. D. (1981). Zur biostratigraphischen Untergliederung des kontinentalen Quart~s in Europa anhand von Arvicoliden (Rodentia, Mammalia). Eclogae geologicae Helvetiae, 74, 997-1006. Freudenthal, M., Meijer, T. and Meulen, A. J. van der (1976). Preliminary report on a field campaign in the continental Pleistocene of Tegelen (The Netherlands). Scripta Geologica, 3, 1-27. Heinrich, W.-D. (1978). Zur biometrischen Effassung eines Evolutionstrends bei Arvicola (Rodentia, Mammalia) aus dem Pleistoz~ Thiidngens. Siiugetierkudliche Information, 2, 3-21 Heinrich, W.-D. (1987). Neue Ergebnisse zur Evolution und Biostratigraphie yon Arvicola (Rodentia, Mammalia) im Quartiqr Europas. Zeitschrift fiir geologische Wissenschafien, 15, 389-406.

Hinton, M. A. C. (1926). Monograph of the Voles and Lemmings (Microtinae) Living and Extinct, Vol. 1, 1-488. British Museum (Natural History), London. Hor~lcek, I. (1990). On the context of Quaternary arvicolid evolution: changes in community development. International Symposium on the Evolution, Phylogeny and Biostratigraphy of Arvicolids, pp. 201-222. J~nossy, D. and Meulen, A. J. van der (1975). On Mimomys (Rodentia) from Osztramos-3, North Hungary. Proceedings of the Koninklijke Nederlandse Akademie van Wetenschappen, B 78, 381-391. Koenigswald, W. yon (1970). Mittelpleistoz~ine kleins~mger aus tier Spaltenfiillung Petersbuch bei Eichsth'tt. Mitteilungen der Bayerische. Staatssammlung fiir Paliiontologie und historische Geologie, 10, 407-432. Kocnigswald, W. yon (1973). Ver~ndernngen in der Kleins~iugerfauna yon Mitteleuropa zwischen Cromer und Eem (Pleistoz~in). Eiszeitalter und Gegenwart, 23/24, 159-167. Koenigswald, W. yon (1980). Schmelzstruktur und Morphologie in den Molaren der Arvicoliden (Rodentia). Abhandlungen der Senckenbergsche Naturforschende Gesellschaft, 539, 1-94. Koenigswald, W. von and Kolfschoten, T. van (in press). The Mimomys-Arvicola boundary and the enamel thickness quotient (SDQ) of Arvicola as stratigraphic markers in the Middle Pleistocene. In: Turner, C. (ed.), Proceedings of the Cromer-Symposium, Norwich (1990). Kolfschoten, T. van (1988). The Pleistocene Mammalian Faunas from the Zuurland borehole. Mededelingen Werkgr. Tert. Kwart. Geol, 25(1), 73-86. Kolfschoten, T. van (1990a). The evolution of the mammal fauna in the Netherlands and the middle Rhine Area (Western Germany) during the late Middle Pleistocene. Mededelingen Rijks Geologische Dienst, 43, 3, 1-69. Kolfschoten, T. van (1990b). Review of the Pleistocene Arvicolid Faunas from The Netherlands. International Symposium on the Evolution, Phylogeny and Biostratigraphy of Arvicolids, pp. 255-274. Kolfschoten. T. van and Meulen, A.J. van der (1986). Villanyian and Biharian mammal faunas from The Netherlands. Memorie della Societa Geologica ltaliana, 31, 191-200. Kretzoi, M. (1954). Bericht fiber die calabdsche (villafrankische) Fauna von Kislang Fejer. Jahresbericht der Ungarischen Geologische Anstellung, 1953/1, 212-264. Kretzoi, M. (1965). Die Nager und Lagomorphen von Voigtstedt in Thfifingen and ihre chronologische Aussage. Paliiontologische Abhandlungen, 2, 3, 587-660. Lister, A. M. (1992). Mammalian fossils and quaternary biostratigraphy. Quaternary Science Reviews, 11, 329-344. Mayhew, D. F. and Stuart, A. J. (1986). Stratigraphic and taxonomic revision of the fossil vole remains (Rodentia, Microtinae) from the Lower Pleistocene deposits of Eastern England. Philosophical Transactions of the Royal Society, London, B 312, 431-485. Meulen, A. J. van der (1973). Middle Pleistocene smaller mammals from the Monte Peglia (Orvieto, Italy) with special reference to the phylogeny of Microtus (Arvicolidae, Rodentia). Quaternaria, XVII, 1-144. Rabeder, G. (1981). Die Arvicoliden (Rodentia, Mammalia) ans dem Plioz[in und dem ~teren Pleistoz~in von Nieder-Osterreich. Beitriige Paliiontologie Osterreichs, 8, 1-373. Rekovets, L. I. (1990). Principal developmental stages of the water vole genus Arvicola (Rodentia, Mammalia) from the Eastern European Pleistocene. International Symposium on the Evolution, Phylogeny and Biostratigraphy of Arvicolids, pp. 369-384. Schreuder, A. (1943). Fossil Voles and other Mammals (Desmana, Talpa, Equus, etc.) out of well-borings in the Netherlands. Verhandelingen Geologie en Mijnbouw Gen., XIII, 399-434. Smirnov, N. G., Bolshakov, V. N. and Borodin, A. V. (1986). Pleistocene rodents of the norihern West-Siberia. Nauka, Moscow. 143 pp. (in Russian). Smart, A. J. (1975) The vertebrate fauna of the type Cromerian. Boreas, 4, 63-76. Stuart, A. J., (1981). A Comparison of the Middle Pleistocene Mammal Faunas of Voigtstedt (Thuringia, GDR) and West Runton (Norfolk, England). Quartiirpaliiontologie, 4, 155-163. Meulen,'A. J. van der (1973). Middle pleistocene smaller mammals from the Monte Peglia, (Orvieto, Italy) with special reference to the phylogeny of Microtus (Arvicolidae, Rodentia). Quaternaria, XVII, 144. Viriot, L., Chaline, J. and Schaaf, A. (1990). Quantification du gradualisme phyi6tique de Mimomys occitanus ~t Mimomys ostramosensis (Arvicolidae, Rodentia) it l'aide de l'analyse d'images. Comptes Rendus de l'Acadmie des Sciences, Paris, t. 310, S~rie II, 1755-1760. Yakolov, A. G. (1987). The reference elements of small mammalian fauna of the Pleistocene of the B ashkir Urals. In: The Pleistocene of Urals, Nauka, Moscow, 112 pp. (in Russian) Zazhigin, V. S. (1980). Rodents of late Pliocene and Anthropogene of South of Western Siberia. Nauka, Moscow. 156 pp. (in Russian).