Tsaganius Russell and Dashzeveg, 1986

Missiaen, Pieter & Smith, Thierry, 2008, The Gashatan (late Paleocene) mammal fauna from Subeng, Inner Mongolia, China, Acta Palaeontologica Polonica 53 (3), pp. 357-378 : 368-369

Genus Tsaganius Russell and Dashzeveg, 1986

Fig. 11 .

Type species: Tsaganius ambiguus Russell and Dashzeveg, 1986 ; Bumbanian (early Eocene), Tsagan Khushu ( Mongolia) .

Tsaganius ambiguus Russell and Dashzeveg, 1986 Referred material: IMM−2001−SB−052, partial right M1 or M2; IMM−2001−SB−053, right P4; IMM−2001−SB−054, partial right M1 or M2; IMM−2001−SB−055, partial left P4; IMM−2001−SB−056, partial right M1 or M2; IMM−2001−SB−057, right M1 or M2; IMM−2001−SB−058, left p4; IMM−2004−SB−048, right dp4; IMM−2004−SB−049, left jaw fragment with partial m2 and complete m3; IMM−2004−SB−050, right m3.

Description.—The P4 ( Fig. 11A ) of Tsaganius was previously unknown. The crown is semimolariform, with a slen− der metacone that is connate with the paracone. The metacone is lower than the paracone, but taller than the protocone. The stylar shelf is narrower than that of the molars, but has a well−differentiated parastyle and metacrista. The P4 shows no conules or associated conule wings. The precingulum is narrow, whereas the postcingulum is somewhat wider but lacks a hypocone.

The upper molars ( Fig. 11B, C ) assigned here to Tsaganius are fragmentary and/or heavily worn, but together provide an overall idea of the upper molar morphology. The crowns are strongly elongated transversely, with a narrow trigon basin. The paracone is slightly taller than the metacone, and the cusps are closely approximated. The stylar shelf is moderately developed, with a small parastyle and a strong metacrista. The paraconule is prominent, with robust pre− and postparaconule cristae, but the metaconule is weaker and the premetaconule crista is reduced. The pre− and postcingulum are transversely long and relatively wide, and a small hypocone is developed on the postcingulum, lingual of the level of the protocone.

IMM−2004−SB−048 ( Fig. 11D ) is here identified as a dp4 of T. ambiguus : it differs from the p 4 in the stronger, more anteriorly projecting paraconid and anterolabial cingulum. The protoconid and metaconid are more gracile than in p4, with the metaconid slightly higher than in p4, but still lower than the protoconid. The oblique crest runs from the base of the trigonid wall towards the single talonid cusp, probably equivalent to the hypoconid, and a small talonid basin is present lingually of this cusp.

Discussion.—Although several of these specimens were preliminarily identified as belonging to “ Naranius ” ( Missiaen and Smith 2004) and “ Tsaganius sp. nov. ” ( Van Itterbeeck et al. 2007), additional specimens and comparison with topotypic material clearly indicate that the referred specimens are best identified as pertaining to T. ambiguus .

Among Cimolestidae , the upper molar morphology reported here for Tsaganius is close to that of Naranius . Morphological similarities include the transversely expanded crown with broad stylar shelf, the reduced premetaconule crista, and the wide pre− and postcingulum. Differences include the shallower ectoflexus and the unreduced postparaconule crista in Tsaganius . The semimolariform P4 of Tsaganius is clearly different from the premolariform P4 of Naranius reported by Lopatin (2006); this is not surprising, considering that the p4 is also semimolariform in Tsaganius but premolariform in Naranius ( Russell and Dashzeveg 1986) . Moreover, a semimolariform P4/p4 is also seen in other Paleocene Cimolestidae such as Procerberus and Aboletylestes . However, the overall similarity of these taxa to Tsaganius is low and the semimolariform P4/p 4 in Tsaganius does not indicate a special relationship to the latter taxa.

Stratigraphic and geographic range.— Tsaganius ambiguus is known from the Gashatan (late Paleocene) of the Nomogen Formation at Subeng (Inner Mongolia, China) and from the Bumbanian (early Eocene) of the Bumban Member of the Naran Bulak Formation at Tsagan Khushu ( Mongolia).

Figures

Fig. 11. The cimolestid Tsaganius ambiguus Russell and Dashzeveg, 1986, from the Gashatan (Paleocene) of Subeng, China, in labial (A1–G1), occlusal (A2–G2), and lingual (A3–G3) views. A. IMM−2001−SB−053, RP4. B. IMM−2001−SB−052, RM1 or M2. C. IMM−2001−SB−054, RM1 or M2. D. IMM− 2004−SB−048, Rdp4. E. IMM−2001−SB−058, Lp4. F. IMM−2004−SB−049, left dentary fragment with m3 and incomplete m2. G. IMM−2004−SB−050, Rm3. SEM micrographs.