Hyracolestes ermineus Matthew and Granger, 1925

Missiaen, Pieter & Smith, Thierry, 2008, The Gashatan (late Paleocene) mammal fauna from Subeng, Inner Mongolia, China, Acta Palaeontologica Polonica 53 (3), pp. 357-378 : 368

Hyracolestes ermineus Matthew and Granger, 1925

Fig. 10 .

Referred material: IMM−2001−SB−051, right m2; IMM−2004−SB−045, right m1; IMM−2004−SB−046, right m2 fragment; IMM−2004−SB−047 left m2.

Description.— Hyracolestes is a poorly known taxon, based on very sparse material. The lower jaw has only six postcanine tooth loci, usually interpreted as p4 to m2, but only the supposed p3–m1 positions have been published ( Szalay and McKenna 1971; Meng et al. 1998; Lopatin and Kondrashov 2004; Lopatin 2006). Collectively, the specimens from Subeng document the m1 position of Hyracolestes ermineus ( Fig. 10A ), and the previously unpublished ultimate tooth.

The m2 of H. ermineus ( Fig. 10B ) is markedly larger than m1, as would be expected based on the size of the alveoli for this position in the holotype. It resembles m 1 in having a high trigonid, an anteriorly projecting paraconid with a strong paracristid, and a trigonid that is wide open lingually. The anterolabial cingulum is poorly developed. Although the protoconid is broken, it was clearly taller than the metaconid, and in this respect the morphology of m2 is closer to that of p4 than to that of m1. The m2 metaconid is positioned slightly more anteriorly than it is in m1. The talonid is low, shorter and narrower than the trigonid, and the oblique crest is very low. The entoconid is the smallest talonid cusp, the hypoconid and hypoconulid have about the same width but the hypoconulid is notably taller. The hypoconulid is set off posteriorly, but does not form a separate lobe. The talonid basin is open lingually.

Discussion.— Hyracolestes has had a confusing taxonomic history, but the genus was recently tentatively placed in an endemic Asian subfamily, the Sarcodontinae , based on its similarities to Sarcodon ( Lopatin and Kondrashov 2004) . The m2 of Hyracolestes is similar to that of S. pygmaeus and thus supports this phylogenetic placement. As is the case for m1, the m2 of Hyracolestes differs from that of S. pygmaeus by the more anteroposteriorly expanded trigonid and the lingually open talonid, and from other Sarcodontinae also by the very weak anterolabial cingulum.

Stratigraphic and geographic range.— Hyracolestes ermineus is typical for the Gashatan (late Paleocene), and is known from Member I of the Gashato Formation and from the Zhigden Member of the Naran Bulak Formation ( Mongolia); and from the Nomogen Formation at Bayan Ulan and Subeng (Inner Mongolia, China).

Figures

Fig. 10. The sarcodontid Hyracolestes ermineus Matthew and Granger, 1925, from the Gashatan (Paleocene) of Subeng, China, in labial (A1, B1), occlusal (A2, B2), and lingual (A3, B3) views. A. IMM−2004−SB−032, Rm1. B. IMM− 2001−SB−051, right dentary fragment with m2. SEM micrographs.