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NEW SPECIMENS OF MERYCOPOTAMUS (ARTIODACTYLA: ANTHRACOTHERIIDAE) FROM MIDDLE MIOCENE OF POTWAR PLATEAU, PAKISTAN.

Byline: M. Asim, K. Aftab, M. K. Nawaz, M. A. Khan, M. A. Babar, S. G. Abbas, M. Hussain, M. Akhtar and A. M. Qureshi

Keywords: Artiodactyls, Anthracotheriids, Merycopotamus, Chinji Formation, middle Miocene.

INTRODUCTION

The Potwar Plateau is the most fertile area in the Siwaliks that represents the extensive record of the vertebrate fossils (Pilbeam et al., 1979; Badgley et al., 2008; West et al., 2010). Anthracotheriidae was very diversified in this region during Neogene (Pilgrim, 1913; Forster-Cooper, 1924; Black, 1978; Gaziry, 1987; Pickford, 1987). The described specimens of Merycopotamus are collected from two different localities of the Chinji Formation: Chabbar Syedan and Dhok Bun Ameer Khatoon. Both the sites have yielded many mammalians remains that belong to the middle Miocene age. The newly found specimens document the poorly known anthracotheriid group from the Siwalik Group.

Chabbar Syedan: The studied outcrops (Lat. 33Adeg 00' 16.1' N; Long. 73Adeg 13' 29.0' E) are present nearby the Chabbar Syedan village, district Jhelum, Punjab, Pakistan (Fig. 1). The Formation is dominant in this area as the red claystone placed on subordinate hard grey sandstone with interbedding narrow beds of siliceous nodules (Aftab et al., 2015, 2019; Abbas et al., 2016). Chabbar Syedan denotes the middle stage of the Chinji Formation with approximate age of 13.2-12.2 Ma (Barry et al., 2002; Nanda, 2002, 2008). Dhok Bun Ameer Khatoon: The outcrops (Lat. 32Adeg 47' N; Long. 72Adeg 55' E) are located in the northeast of Choa Saidan Shah, Chakwal, Punjab, Pakistan (Fig. 1). The outcrops contain sandstones, siltstones and shales dumped in a riverine environment, mostly packed by uneroded igneous mineral sediments (Cheema, 2003; Khan et al., 2008, 2011). Dhok Bun Ameer Khatoon denotes the lower stage of Chinji Formation with age of almost 14.2-13.2 Ma (Barry et al., 2002; Nanda, 2002, 2008).

MATERIALS AND METHODS

The material was collected from the middle Miocene localities of the Potwar Plateau, Northern Pakistan (Fig. 1). The specimens, collected from the surface, were transported to the laboratory and prepared for the taxonomic study. The specimens were catalogued, and their length and width (in mm) measured at occlusal level by a digital caliper. The dental terminology and measurements follow Lihoreau (2003). The fossils are kept in Dr. Abu Bakr Fossil Display and Research Centre, Department of Zoology, University of the Punjab, Lahore, Pakistan.

Abbreviations: BMNH-British Museum of Natural History; PUPC-Punjab University Paleontological Collection, University of the Punjab, Lahore, Pakistan; l-left; r-right; p-lower premolar; m-lower molar; M-upper molar, Ma-million years ago.

Systematic Palaeontology: Family Anthracotheriidae Leidy, 1869

Genus MERYCOPOTAMUS Falconer and Cautley, 1847

Merycopotamus nanus Falconer, 1868

Holotype: BMNH, M40766, right M1 (Falconer, 1868).

Type locality: Khushalgarh, below Attock, the Punjab province, Pakistan (Falconer, 1868).

Diagnosis: As described by Lihoreau et al. (2007).

Stratigraphic level: Chinji Formation - middle Miocene (Falconer, 1868; Lihoreau et al., 2007).

New material: PUPC 17/232, rp3 (Dhok Bun Ameer Khatoon); PUPC 18/251, lp3 (Chabbar Syedan); PUPC 15/440, lm1 (Dhok Bun Ameer Khatoon); PUPC 16/209, lm3 (Chabbar Syedan).

Description and comparison: PUPC 17/232 is a rp3 having coarse enamel and is triangular in shape (Fig. 2(1)). The anterior valley is broad and open. The anterior lobe includes the protoconid and paraconid, whereas the hypoconid covers most of the tooth posteriorly. A large cingulid covers the base of the tooth.

PUPC 18/251 is a well-worn and fragile lp3 (Fig. 2(2)), that preserves the hypoconid and entoconid only partially. The base of the tooth is completely covered by a thick cingulum throughout its length. The metaconid is higher than all the other conids. The roots are well preserved and the enamel is highly wrinkled.

PUPC 15/440 is a well-worn lm1 (Fig. 2(3)), in which the protoconid, hypoconid, and entoconid are partially broken. The mesostylid is preserved. The median valley is partially closed by a small tubercle. A thick cingulid is preserved at the base of the postprotocristid. The roots are well preserved.

PUPC 16/209 is mandibular fragment with lm3 (Fig. 2(4)). The preserved portion of the corpus is extremely weathered, fragile and cracked at various places. The anterior and posterior valleys show small tubercles. A thick cingulum is present at the base of the preprotocristid. The hypoconulid is massive and has a tubercle at its lingual side. A small cingulum is covered base of the hypoconulid.

The characteristics of this material is also the presence of an additional cusp on the mesiolingual apex of the premolars, the lack of premetacristid in the molar, access of prehypocristid and preentocristid to the lingual border of the molar and the position of the hypoconulid, in line with buccal cusps. All these characters belong to Merycopotamus (Lihoreau et al., 2007). The Siwalik Merycopotamus is represented by three species: M. nanus, M. medioximus and M. dissimilis (Falconer, 1868; Falconer and Cautley, 1836, 1847; Lihoreau et al., 2004). Merycopotamus nanus is different from M. medioximus by the smaller size of the lower molars (Fig. 3). It differs from the teeth of M. dissimilis by lacking the entoconid fold and by having loop-like hypoconulid in the m3 (Lihoreau et al., 2007).

The size of the described material clearly documents a species smaller than M. medioximus and M. dissimilis, their size falling within the range of M. nanus (Fig. 2, 3; Table 1). Furthermore, also the described lower molars present the occlusal pattern of M. nanus (Lihoreau et al., 2007).

Table 1. The teeth measurements (in mm) of Merycopotamus. * represents the specimens described in the present study. Comparative material from Lihoreau et al. (2007), and Khan et al. (2013).

Taxa###Inventory No.###Nature and Position###Length###Width###W/L

###PUPC17/232*###rp3###14.52###10.10###0.70

###PUPC 18/251*###lp3###16.5###12.0###0.73

###PUPC 15/440*###lm1###15.00###11.24###0.75

###PUPC16/209*###lm3###30.0###15.5###0.52

M. nanus

###p3###15.7###10.1###0.68

###Lihoreau et al., 2007 (average values)###m1###19.7###12.4###0.63

###m3###33.2###15.9###0.48

###PUPC 13/5###rm3###32.0###16.0###0.50

###p3###18.0###10.0###0.56

M. dissimilis###Lihoreau et al., 2007 (average values)###m1###20.7###14.0###0.68

###m3###39.2###20.1###0.51

###CCZ234###p3###21.2###13.5###0.64

M. medioximus###CCZ234###m1###21.7###14.6###0.67

###CCZ233###m3###40.7###19.5###0.48

DISCUSSION

Merycopotamus was documented from the Siwalik Hills of the Indian Subcontinent (Falconer and Cautley, 1836, 1847; Falconer, 1868; Colbert, 1938, 1943), as well as from Iraq (Brunet and Heintz, 1983), Nepal (Munthe et al., 1983), Indonesia (Konigswald, 1933) and Thailand (Hanta et al., 2005, 2008). In the Siwaliks, this genus was first described and discussed in detail by Falconer and Cautley (1847). Merycopotamus was recorded from the Chinji to Dhok Pathan formations of the Siwalik succession in the Potwar Plateau, Pakistan (Pilbeam et al., 1977, 1979). The chronologically youngest Merycopotamus in this region was found from Azad Kashmir (2.6-2.4 Ma; Steensma and Hussain, 1992). Temporally, the genus ranges from the middle Miocene to early Pleistocene of the Siwalik Group (Steensma and Hussain, 1992; Barry et al., 2002; Lihoreau et al., 2007).

The Tatrot Formation yielded two species of Merycopotamus i.e. M. minor and M. major, both species being renamed as M. nanus by Falconer (1868) and M. dissimilis by Falconer and Cautley (1847) respectively. Later, Colbert (1935) synonymized M. nanus with M. dissimilis. Lihoreau (2003) studied the Merycopotamus specimens collected by the team of the Geological Survey of Pakistan and the Peabody Museum of Yale University, then he established a new additional species, M. medioximus (Lihoreau et al., 2004). M. nanus (basal species) was found in the Chinji Formation (13.9-11.3 Ma, middle Miocene) of the Siwalik Group (Pilbeam et al., 1977, 1979; Barry et al., 2002, Lihoreau, 2003; Hanta et al., 2008). M. medioximus (slightly derived), described by Lihoreau et al., (2004), is an intermediate form.

It was found in the Nagri and lower Dhok Pathan formations (10.4-8.6 Ma, early Late Miocene) of the Siwalik Group (Lihoreau et al., 2004). M. dissimilis (derived form) is the youngest species and present in the upper Dhok Pathan, Tatrot, and lower Pinjor formations (7.8-2.4 Ma, late Miocene to early Pleistocene) of the Siwalik Group (Barry et al., 2002; Lihoreau et al., 2007). M. medioximus and M. dissimilis share a synapomorphic character (the mesostyle division) which clearly reflects that M. nanus is a stem group of the genus. M. nanus is confined to the Indian subcontinent and is the oldest representative in the middle Miocene of Lower Siwalik Subgroup (Chinji Formation) (Fig. 4).

If the morphology (division) of the mesostyle is regarded as the important character in Merycopotamus evolution, it is considered that M. medioximus (early Late Miocene) is the first slightly more derived species emigrated from Indian Subcontinent to Thailand and developed into a new form, Tha Chang Merycopotamus (Lihoreau et al., 2007; Hanta et al., 2008).

Conclusions: A rare species, M. nanus is reported from the outcrops of Potwar Plateau, Pakistan, based on additional specimens discovered recently. This material adds to the lower dentition of middle Miocene age of the Siwalik Group referred to M. nanus. The localities Chabbar Syedan and Dhok Bun Ameer Khatoon correspond to the upper part of the middle Miocene of the Siwalik Group, thus confirming the first appearance of M. nanus in the subcontinent.

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Author:M. Asim, K. Aftab, M. K. Nawaz, M. A. Khan, M. A. Babar, S. G. Abbas, M. Hussain, M. Akhtar and A. M
Publication:Journal of Animal and Plant Sciences
Geographic Code:9THAI
Date:Dec 31, 2020
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