Mesodmops tenuis, Missiaen & Smith, 2008

Missiaen, Pieter & Smith, Thierry, 2008, The Gashatan (late Paleocene) mammal fauna from Subeng, Inner Mongolia, China, Acta Palaeontologica Polonica 53 (3), pp. 357-378 : 358-359

Mesodmops tenuis sp. nov.

Fig. 1A–E ; Table 1.

Etymology: From Latin tenuis = thin, fine, slender, for the overall more slender, anteroposteriorly elongate shape of the molars.

Holotype: IMM−2004 −SB−013, right m1.

Type locality: Subeng, Inner Mongolia, China.

Type horizon: Upper part of the Nomogen Formation, Gashatan (late Paleocene).

Referred material: IMM−2001−SB−016, left M1; IMM−2001−SB−017, right DP3?; IMM−2001−SB−018, left m1; IMM−2004−SB−014, left M2; IMM−2004−SB−015, left m1; IMM−2004−SB−016, left m2; IMM−2004− SB−017, left m1; IMM−2004−SB−018, left M1.

Diagnosis.—Neoplagiaulacid multituberculate similar in size to Mesodmops dawsonae . Differs from M. dawsonae in having less inflated, less rounded lower molars with a more irregular outline, and in the 6:5 and 3:2 cusp formulae of m1 and m2, respectively. The M1 differs from the M1 of M. dawsonae in having smaller anterior−most cusps and square cusps in the middle cusp row, instead of rectangular cusps as in M. dawsonae . M. tenuis is further generally characterized by slightly longer but narrower, less inflated molars.

Description.—Five tooth loci are known for M. tenuis . The cusp formula of M1 ( Fig. 1A ) is 8:10:5, with the cusps of M1 become gradually larger toward the posterior end of the tooth. The middle cusp row has very small, subpyramidal cusps anteriorly that become larger and more crescentic posteriorly. The cusp formula of M2 ( Fig. 1B ) is 1:3:3, and M2 has a trapezoidal shape, tapering toward the posterior end. The anterior−most cusp of the middle row has an anteroposteriorly compressed shape due to the presence of the preceding M1.

The m1 ( Fig. 1D ) is rectangular in shape with the lingual and labial rows roughly parallel. The labial and lingual sides of the crown are, however, somewhat irregular and undulating. The cusp formula of m1 is 6:5, with cusps bearing grooves on their valley−facing sides and with the anterior cusps of both rows notably smaller. The anterior cusps of the labial row are subpyramidal, becoming crescentic posteriorly. The posterior cusp of the lingual row forms a large crest. The m2 ( Fig. 1E ) has a simple, slender shape with a cusp formula of 3:2. The cusps bear grooves on their valley−facing sides and become slightly less crescentic posteriorly.

IMM−2001−SB−017 ( Fig. 1C ) is tentatively identified here as a right DP3. The cusp formula of 2:3 matches that of DP3 shown in M. dawsonae ( Tong and Wang 1994) and the general shape is similar, although it is notably smaller and relatively more slender.

Discussion.—The teeth of Mesodmops tenuis from Subeng closely resemble those of the type−species M. dawsonae from the Bumbanian of Wutu (Shandong Province, China) in size and general morphology. The lower molars of M. dawsonae differ in having a more rounded outline in occlusal view with smooth labial and lingual sides. The m1 of M. dawsonae is not rectangular, as in M. tenuis , but the cusp rows in M. dawsonae curve toward the midline of the tooth anteriorly and posteriorly, and the greatest transverse length is situated at the height of the fifth labial cusp. The cusp formulae of m1 and m 2 in M. tenuis are 6:5 and 3:2, respectively, but 7:5 and 4:2 respectively for M. dawsonae . On M1, the anterior−most cusps are relatively smaller compared to the posterior cusps in M. tenuis than in M. dawsonae . The cusps of the middle cusp row of M1 are rectangular and wider than long in M. dawsonae , not square as in M. tenuis .

M. tenuis further differs in having generally more slender teeth, with a higher length−width ratio. Although these differences may seem limited or based on few specimens, they are consistently present in all specimens, allowing to distinguish them from M. dawsonae , and we therefore to allocate them to the new, closely related species M. tenuis .

Tong and Wang (1994) suggested that Mesodmops , then known only from the Eocene, could have been present already in the Paleocene of Asia; the discovery of M. tenuis confirms this idea. They also suggested that M. dawsonae from the Eocene of China could be derived from the North American Mesodma , noting similarity in the P4 cusp formula similar to that of Mesodma thompsoni ( Tong and Wang 1994) . Neoplagiaulacid phylogeny is mainly based on the highly diagnostic P4 and p4, and because both loci are unknown for M. tenuis , the possibilities for phylogenetic analysis are limited. Nevertheless, the lower molar cusp formulae of Mesodmops tenuis (6:5 for m1, 3:2 for m2) are intermediate between Mesodma thompsoni (6:4 and 3:2) ( Sloan 1987) and Mesodmops dawsonae (7:5 and 4:2), and therefore seem to corroborate the ideas of Tong and Wang (1994).

Figures

Fig. 1. Multituberculates from the Gashatan (Paleocene) of Subeng, China, in occlusal view (SEM micrographs). A–E. Mesodmops tenuis sp. nov. A. IMM−2001−SB−016, LM1. B. IMM−2004−SB−014, LM2. C. IMM−2001− SB−017, RDP3. D. IMM−2004−SB−013, Rm1 (holotype). E. IMM−2004− SB−016, Lm2. F–I. Lambdopsalis bulla Chow and Qi, 1978. F. IMM− 2001−SB−019, RM1. G. IMM−2001−SB−020, RM2. H. IMM−2001−SB−021, Rm1. I. IMM−2001−SB−022, Rm2.