Mimomys chandolensis, Tiunov & Golenishchev & Voyta, 2016

Tiunov, Mikhail P., Golenishchev, Fedor N. & Voyta, Leonid L., 2016, The first finding of Mimomys in the Russian Far East, Acta Palaeontologica Polonica 61 (1), pp. 205-210 : 206-209

Mimomys chandolensis sp. nov.

Figs. 3 , 4 .

Etymology: After type region of Lozovyi Ridge (formerly Chandolaz).

Holotype: Left m1 ( ZIN 101600 ) the anterior enamel edging of both of the outward lingual angles is slightly damaged.

Type locality: Cave of Medvezhyi Klyk , Mountain Chain of Lozovyi (465 m above sea level), the southern spur of Sikhote-Alin , the Partizanski Region of the Primorski Krai, Primorye, Russia .

Type horizon: MIS 3, Late Pleistocene.

Material.— Holotype only. Diagnosis.—Hypsodont, with little cement, without enamel isle or prismatic fold, tangential and lamellar layers of enamel ultrastructure—poorly expressed.

The crown height/occlusive length ratio is 1.83 and the extent of cementation is mild. Unlike in M. gansunicus , which is characterized by a high extent of cementation, in our specimen, the buccal inward angles of the occlusive surface is one third filled with cement, while the lingual angles are halfway filled. There is some cement in the paraconid inward fold as well. There is no enamel isle or a Mimomys -type prismatic fold. The specimen is smaller than all of the other molars found in the East Palearctic. Our molar has a m1 length and width of 2.42 mm and 1.05 mm, respectively, while for the holotype of M. gansunicus , those measurements are 2.97 mm and 1.25 mm, respectively ( Erbajeva 2005). On the ultrastructural level, the layer of radial enamel prevails; the tops of the inward and outward angles are covered with only radial enamel. The lamellar and tangential layers are no more than one third of the total enamel width.

Measurements (in mm).—L, 2.42; Lant, 1.03; W, 1.05; Want, 0.94; Lbas, 2.30; ASD, 3.95; HSLD, 4.23; R, 0.34; SDQ, 128.

Description.—The level of preservation of this Mimomys molar is consistent with other arvicoline remains found in the same layer. The tooth was weakly mineralized, with light yellow enamel and slightly darker yellowish dentine. The basic measurements and morphology of this molar are presented in Table 1 and Fig. 3 .

The occlusive surfaces include the posterior loop, the 3 alternating triangles and the anteroconid area ( Fig. 3 ). The second and third triangles are the most widely fused. Thus, there are 4 isolated dentine fields: the posterior loop, the first triangle, the second and the third triangles, and the anteroconid. On the buccal side there are 2 deep inward folds filled with cement and a slightly developed anteroconid fold. On the lingual side there are 4 inward angles filled with cement. The anteroconid includes widely fused plates of the fourth and fifth triangular prisms and the plate of the anterior loop. The enamel islet is absent and there is no rudimentary evidence of it. On the buccal side of the tooth there is a slightly developed Mimomys - type ridge. The enamel edge of the m1 occlusive surface possesses two disjunctions at the posterior and one disjunction at the buccal anteroconid loop. The enamel is of a differentiated thickness. The thinnest enamel is in the deepest sites of the inward angles, but the enamel edge of the posterior loop and posterior sites of the prisms is comparatively thicker; a finding that is typical for Mimomys . Additionally, the front enamel in some prisms is also thickened. The enamel differentiation quotient, the rear/ front enamel thickness ratio in the main triangles SDQ (enamel differentiation index) is 128. The SDQ was calculated as:

SDQ = [Σ (teet × 100/leet)]/N

where N refers to the number of dentine fields of the studied tooth; teet (trailing edge enamel thickness) refers to the maximum thickness of the posterior enamel loop; and leet (leading edge enamel thickness) refers to the maximum thickness of the anterior enamel loop (Heinrich 1978; Lozano-Fernandez et al. 2013).

The molar is hypsodont and the root development is in the initial (merorhiz) stage because the inward prismatic folds do not penetrate the basal surface of the tooth (indicating an underdeveloped molar neck). The tracks are stretched up to the occlusal surface, and the track on the buccal side of the anteroconid is narrow and without the Mimomys - type barb.

The enamel ultrastructure of the Mimomys sp. nov. m1 ( Fig. 4 ) is somewhat different from that of the other known representatives of the genus ( Koenigswald 1980). The tangential and lamellar layers are underdeveloped. On the buccal side of the molar, the tangential layer appears only at the limited sites of the posterior edges of the triangles, whereas the lamellar layer is observed only at the anterior edge of the hypoconid and forms a mere third of the total enamel thickness. On the lingual side, both pattern types are present within the enamel edging of all the outward angles and tapers to complete absence towards the tops of the inward angles.

Remarks.—Among the known Eurasian and North American hypsodont Mimomys , M. gansunicus and M. haplodentatus – M. tornensis – M. tigliensis appears to be the most similar to the molar we found ( Zheng and Li 1986; Tesakov 1998, 2004; Zhang et al. 2010; Tesakov and Kolfschoten 2011); the comparison with the latest representatives of Mimomys – M. savini ( Fejfar et al. 1998; Lozano-Fernández et al. 2013) was also carried out ( Table 1). The latter occurred to be considerably larger, than our specimen. Because our specimen differed from all the other representatives of the genus in most of the m1 characteristics measured, we describe this as coming from a new species. The differential diagnosis was made in comparison with M. gansunicus , as the closest one both geographically and morphologically.

Stratigraphic and geographic range.— Type locality and horizon only.

Figures

Fig. 3. Left m1 of a voleMimomys chandolensis sp. nov. from Primorsky Krai, Russia, Late Pleistocene (MIS 3); ZIN 101600 in root (A), occlusal (B), buccal (C), lingual (D), and anterior (E) views. T1–3, the triangles of the occlusal suface; dentine tracks and fields of lateral sides are hatched; arrows mark the anterior direction. Terminology and measurements modified from van der Meulen (1973), Rabeder (1981), Tesakov (2004), Lozano-Fernández et al. (2013).

Fig. 4. Vole Mimomys chandolensis sp. nov. (ZIN 101600) from Primorsky Krai, Russia, Late Pleistocene (MIS 3). Left m1 in occlusal (A), buccal (B), and lingual (C) views, and photographs of the enamel ultrastructures (D). Positions of D1–D6 indicated on the drawing. Abbreviations: d, dentine; L, lammelar enamel layer; r, radial enamel; t, tangential enamel.