Palaeostylops iturus 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 : 360-362

Palaeostylops iturus Matthew and Granger, 1925

Figs. 3 , 4 ; Table 2.

Referred material: 204 jaw fragments, isolated teeth and identifiable fragmentary teeth.

Discussion.— Palaeostylops is a typical taxon at Gashatan fossil mammal sites, and is by far the most abundant taxon in the Subeng fauna. Originally, two species of Palaeostylops were described, P. iturus and P. macrodon , with the latter differing from the former by the larger size of the cheek teeth, and the comparatively enlarged second molars ( Matthew et al. 1929). Cifelli et al. (1989) reported that P. macrodon differed further from P. iturus by having cuspules on the lingual cingula of the upper molars, by the weakness or absence of a sulcus separating the lingual cusps on M1, and by the number of upper incisors and the curvature of the snout; these authors placed P. macrodon in a new monotypic genus “ Gashatostylops ”. However, the cuspules on the lingual cingulum are variably developed in both P. iturus and P. macrodon , and a strong sulcus separating the lingual cusps is known in only one specimen of P. iturus (AMNH 20415). Moreover, the alleged differences in the number of upper incisors and the curvature of the snout cannot be evaluated because well−preserved complete rostra are unavailable for both species, and thus their value as a generic difference is limited ( Kondrashov and Lucas 2004a). Several authors have since suggested that the validity of “ Gashatostylops ” is weakly supported and considered it a junior subjective synonym (Meng et al. 1998; Kondrashov and Lucas 2004a).

At the Subeng site, both morphology and size indicate that only a single form is present. Comparisons with the type specimens of both species clearly support referral of all Subeng arctostylopid specimens to P. iturus , and none of the available specimens shows the relatively enlarged second molar typical of P. macrodon . Table 2 and Fig. 4 show the measurements of the Palaeostylops cheek teeth from Subeng, and compare these to the extensive measurements modified after Kondrashov and Lucas (2004; personal communication with Peter Kondrashov, 27/07/2007). The mean values and the size ranges of the Subeng Palaeostylops teeth are very close to those reported for P. iturus , and are clearly different from those reported for P. macrodon . The low variation of measurements within the Subeng sample indicates the presence of only one species. Nevertheless, we note the presence of variably developed lingual cuspules on M1 and M2, and the absence or weak development of a sulcus separating the lingual cusps on M1. The Subeng specimens therefore weaken morphological distinctions between Palaeostylops iturus and P. macrodon suggested by Cifelli et al. (1989) to justify generic distinctions, and we continue to consider both species member of one genus Palaeostylops .

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

Figures

Fig. 3. The gliriform Palaeostylops iturus Matthew and Granger, 1925, from the Gashatan (Paleocene) of Subeng, China, in labial (A1–F1), occlusal (A2–F2), and lingual (A3–F3) views. A. IMM−2004−SB−021, left maxillary fragment with P3−4. B. IMM−2001−SB−024, LM1. C. IMM−2004−SB−022, LM2. D. IMM−2004−SB−023, LM3. E. IMM−2004−SB−024, left incomplete dentary with p2−4. F. IMM−2004−SB−025, left incomplete dentary with p4–m3. SEM micrographs.

Fig. 4. Dental measurements of Palaeostylops iturus Matthew and Granger, 1929, from the Gashatan of Subeng (black circles) compared to those of P. iturus (light grey circles) and Palaeostylops macrodon Matthew, Granger, and Simpson, 1929 (dark grey diamonds), modified after Kondrashov and Lucas (2004a). The graph illustrates the similar sizes of both P. iturus samples, and their size difference from P. macrodon. L, anteroposterior length. W, transverse width. Error bars on the Subeng measurements are 1 standard deviation.