A New Species of Fossil Mus (Muridae, Mammalia) from the Late Quaternary Deposits of Narmada Valley, Central India

A new species of fossil Mus (Muridae, Rodentia) is described from the Pleistocene fluviatile deposits of the Narmada valley (Central India). The species, Mus narmadaensis sp. Nov., has a comparatively smaller lower molar which is characterized by a narrow molar with well connected cusps, small anterior expansion of lingual anteroconid, protoconid and metaconid, reduced posterior cingulum in addition to hypoconid and entoconid nearly at the same level. The large M3 has centrally placed bulbous hypoconid. Among the extant species, the present one is closest to M. shortridgei in having similarly placed protoconid and metaconid in M1 and a well developed hypoconid in M3.


Introduction
Considered to be the most successful groups of living mammals, the murid rodents were originated in the Indian sub-continent about 14 ma ago.At present, they are found all over the world with ability to adapt themselves to varied environmental conditions and show marked species diversity.Today more than 70% of the murid species are found in the Indo-Australian region, whereas, 26% murid taxa are found in Africa [1].The oldest known fossil murid, Antemus chinjiensis was evolved from a cricetid Potwarmus primitivus and was recovered from the Chinji Formation (Siwalik sub-group) in the Potwar Plateau [2].The study of fossil murids in the Indian subcontinent was initiated by [3][4][5][6] and followed by [2] who made significant contribution to the study of Pakistan Siwalik by describing various murid taxa.Subsequently, a sizeable work on the Afghanistan murids was done by [7][8][9][10][11].As far as the Indian murids are concerned, a number of researches, e.g.[12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] have shown that the murids were widespread in the country from the Pliocene onwards.
We report here the lower molars of a new species of Mus from the Devakachar section of the Hirdepur Formation of the Narmada deposits.

Area of Study, Litho-Chronology and Fossil Material
Narmada, the largest river in the Central India, originates at the plateau of Amarkantak (22º40'N; 81º40'E) and after traversing across the middle of the Indian subcontinent, it joins the Gulf of Cambay near Baroda.The course of the river is controlled by the east-west lineament.
Between Bhedaghat (23º8'N; 79º48'E) and Hoshangabad (22º45'N; 77º45'E), the river forms a trough in which about 50m thick Quaternary fluviatile deposits are preserved.Though the deposits are much thicker in the south, the fossiliferous deposits are exposed in the northern fringe in the sections exposed along river Narmada and its tributaries.The Narmada deposits have been divided into seven lithostratigraphic Formations [36].The present study area forms a part of the flood plain facies of the Hirdepur Formation (Figure 1(a)), comprising greyish homoge-nous calcareous silt, interlayered with coarse sand, gravel and conglomerate with high degree of calcification.We studied a 17m thick profile at Devakachar (23º23'N; 79º 07'E), exposed by the Sher River (see Figures 1(a) and  (b)).It consists of sand, silt and cemented conglomerate including a fossil bearing horizon.The fossiliferous layer is 0.5 m in thickness and is composed of medium to coarse grained brownish coloured sand.It is about 9 m above the base of the profile (Figure 1(b)).The basin is very well known for a large number of vertebrate fossils including Elephas, Equus, Bos and several others [37,38].Lately, a discovery of the skullcap of Homo erectus [39] and additional Homo material [40] has made Narmada valley an important site for palaeontological studies.However, the microvertebrates have only been mentioned in a handful of reports, e.g.[21,41,42].The detailed magnetic stratigraphy of the Surajkund and Hirdepur Formations [36,43] and absolute date of the Toba volcanic ash found in the sediments [44] suggest that the boundary of both the formations lies at 74 ka BP and the top of the Narmada sequence is Holocene [40,42].Several lithics recovered from the Dhansi Formation (see Figure 1(c)) may represent the first unequivocal evidence for an early Pleistocene hominin presence in India [45].The Homo erectus horizon is only slightly older than the present fossil horizon.
We recovered a large number of microvertebrate remains, such as, murid rodents, lizards and fish from the Devakachar section.The murids are represented by lower molars and incisors.The lizards consist of dentaries, whereas, the cyprinid and channid fishes have teeth and spines.Here, we report only the murid material.

Horizon and age:
The horizon, a medium to coarse grained sand, is Middle to Upper Pleistocene in age.

ures 2(a)-(e)).
Etymology: The species has been named after the type area.

Differential Diagnosis
Smallest Mus ever reported, M 1 with highly reduced posterior cingulum, M 3 with a large second chevron; differing from Mus auctor [2] in having narrower M 1 and centrally placed hypoconid in M 3 ; from Mus sp.[2] in having a smaller M 1 and from Mus sp.[19] in having a reduced posterior cingulum in M 1 ; from M. flynni [19] in having a larger hypoconid in M 3 ; from M. jacobsi [22] in having poorly developed labial cingulum and lack of  accessory cusps in M 1 ; and differing from M. dhailai [23] in having a smaller M 1 and much larger hypoconid in M 3 .

Description
M 1 is a small and narrow cusp.The asymmetrical 'X' pattern is formed by the four anterior cusps.The labial cusps lie posterior to the lingual cusps in the first chevron.The labial anteroconid is smaller than the anteriorly displaced lingual anteroconid.The cusps are very strongly connected and the connection between the labial anteroconid and protoconid is stronger than between the lingual anteroconid and metaconid.The hypoconid and entoconid are more or less at the same level, the former being slightly bigger than the later.The posterior cingulum is small, oval, transversally flattened and highly reduced.M 1 has two roots.M 3 is roughly triangular in outline.The protoconid and metaconid are at the same level and are more or less of the same size in the anterior chevron.The hypoconid and entoconid are merged together to form a bulbous chevron which is centrally placed.The specimen has one complete root.

Comparisons
Mus narmadaensis sp.nov.can be differentiated from M. auctor [2], Mus sp.[19] and M. jacobsi [22] in having the following characters; smaller M 1 , marginal anterior displacement of lingual anteroconid relative to the labial anteroconid, protoconid and metaconid at the same level, poorly developed labial cingulum and highly reduced posterior cingulum in M 1 .However, M 3 of the present species is bigger than that of M. jacobsi and M. auctor.In the M 3 of the present species, the hypoconid is centrally placed, whereas, it is displaced lingually in M. auctor and labially in M. jacobsi.
The present M 1 s differ from M. flynni [19] in having a much smaller M 1 with lingual anteroconid showing small anterior displacement relative to labial anteroconid, hypoconid and entoconid occupying the same plane and a highly reduced posterior cingulum.M 3 of M. narmadaensis sp.nov. is larger than that of M. flynni and also has a larger hypoconid.The Narmada species is close to Mus sp.[2] in the relative position of cusps in the anterior chevron and a reduced posterior cingulum in M 1 but it has a much smaller size.Also, the connection of cusps is much stronger in the Narmada species.The present species is similar to M. dhailai [23,24] in the relative position of the labial and lingual anteroconid, protoconid, metaconid and in having a reduced posterior cingulum in M 1 and similarly placed hypoconid in M 3 but differs from it in having a much smaller M 1 and a bigger M 3 with a better developed hypoconid (Tables 2 and 3).A comparison of various species of Mus is shown in Figure 3.

Enamel Ultrastructure in Murid Incisor
The rodent enamel microstructure has the highest degree of complexity among mammals [46][47][48][49][50].In most rodents, the incisor enamel is made up of two layers, an inner portion known as Portio Interna (PI) with intersecting prisms which appear as Hunter-Schreger Bands (HSB) in the longitudinal section, and an outer portion known as Portio Externa (PE) with radial enamel in which the prisms are oriented parallel to each other.The presence of these two layers in the rodent incisor enamel is regarded as a characteristic feature which distinguishes it from lagomorphs where only Portio Interna with HSB is developed [51].Biomechanically, the HSBs serve as strengthening device inhibiting crack propagation [49,[52][53][54], whereas the radial enamel of the Portio Externa helps to maintain a sharp cutting edge because of its higher resistance to wear [55][56][57].The evolution of enamel of the rodent incisor is independent from that of the molar enamel [58].

Lower Incisor of Mus; NAR/I4 (Figures 4(a)-(b))
The longitudinal section reveals a PI with typical uniserial HSBs which are two prisms thick and a PE with the radial enamel.The HSBs are inclined at an angle of 60º to the Enamel-Dentine Junction (EDJ) (Figure 4(b)).As   the bands move towards the outer enamel, the angle of inclination gradually decreases from 60º to 30º and the prisms become parallel to the EDJ.The prisms of alternating bands intersect at an angle of 90º at the PE-PI junction and the crystallites of the IP run perpendicular to the long axis of the prisms.The outer and thick enamel is made up of horizontal interlocking prisms.The IP makes an angle of about 90º with the longitudinal prisms of HSB.The enamel thickness decreases towards the incisal

Discussion
The reduced posterior cingulum in M 1 in the present specimen points to its affinity with Pahari section of Mus [13].In India, Mus is represented by three subgenera, Mus with M. booduga and M. dunni; Pyromys with M. saxicola, M. shortridgei and M. platythrix; and Coelomys with M. mayori, M. pahari and M. crociduroides [14].
Coelomys section includes Asiatic species such as M. mayori, M. pahari, M. crociduroides and M. shortridgei [13].The Narmada Mus resembles M. pahari in general outline and the placement of cusps in the anterior chevron of M 1 .However, the second chevron in M 3 of M. pahari is weakly developed and shows a small lingually placed hypoconid.M. crociduroides has a weak second chevron in M 3 and is therefore different from the present species.M. mayori differs from M. narmadaensis sp.nov. in having a posteriorly displaced protoconid in M 1 and a lingually placed and weakly developed hypoconid in M 3 .Among the extant species, M. narmadaensis sp.nov.shows closest resemblance with M. shortridgei in having similarly placed protoconid and metaconid in M 1 and a well developed hypoconid in M 3 .
A very small size of the M. narmadaensis sp.nov.may be attributed to its getting isolated from the stock at the onset of glacial age during the Pleistocene period.Murids are very sensitive to the climatic changes and it is believed that the onset of cold climatic conditions wiped out several species of murids while some migrated to warmer regions [25].It may be postulated that M. narmadaensis sp.nov.was one of those species that migrated towards Central India from the Lesser Himalayan region at the onset of glaciation.It may have lived there in isolation for a considerable period due to which it could not evolve more progressively although its enamel shows some derived characters as much as in other Pleistocene species.

Figure 1 .
Figure 1.(a) Geological map of the Narmada Valley showing the study sites; modified after [36]; (b) Lithology of the Devakachar sections (present work) and Hirdepur Formation (stratotype section of the Hirdepur Formation is taken from [36]; 1(c) Chronology around Homo erectus locality in the Narmada valley after [26].

Figure 4 .
Figure 4. Longitudinal views of the enamel ultrastructure in the lower incisor in Mus (Bar represents 1 mm).direction(Figure4(a)).Near the tip of the incisor, the HSBs are more closely spaced just below the outer enamel and the IPM is dense below the PE.The PI is reduced towards the incisal end.At the tip of the incisor, only the radial enamel of the PE is present (Figure3(a)).The crystallites of the IPM are rectangular and serve to strengthen the enamel in a third dimension.This feature is generally seen in the derived species of murids.