Subengius mengi Smith, Van Itterbeeck, and Missiaen, 2004

Missiaen, Pieter & Smith, Thierry, 2008, The Gashatan (late Paleocene) mammal fauna from Subeng, Inner Mongolia, China, Acta Palaeontologica Polonica 53 (3), pp. 357-378 : 370-372

Subengius mengi Smith, Van Itterbeeck, and Missiaen, 2004

Figs. 12 , 13 .

Referred material: IMM 2001 −SB−001, partial right p4 ; IMM 2001 − SB−002, left m1 ; IMM 2001 −SB−003, right m2 ; IMM 2001 −SB−004, right m3 ; IMM 2001 −SB−005, right I1 ; IMM 2001 −SB−006 left P4 (holotype) ; IMM 2001 −SB−007, right M2 ; IMM 2001 −SB−008, left M3 ; IMM−2001 −SB−059, left P3 ; IMM−2004 −SB−051, left P3 ; IMM−2004 − SB−052, left P3 ; IMM−2004 −SB−053, left jaw fragment with p4−m 3 in place ; IMM−2004 −SB−054, partial right p4 .

Description.—Since the description of Subengius mengi (see Smith et al. 2004), additional specimens have been collected at the type locality. The previously unknown P3 ( Fig. 12A, B ) of S. mengi seems to be slightly smaller than P4, but the available specimens have suffered wear and breakage, obscuring their original dimensions. Three cusps are present labially. The paraconule is prominent, forming part of a single median crest. The lingual side of the crown is moderately developed, with a marked talon basin and a posterolingual hypocone. A crest runs from the hypocone to the labial side, forming the posterior border of the crown; a swelling is developed halfway between the hypocone and the median crest.

IMM−2004−SB−053 ( Fig. 12C ) is a left jaw fragment containing p4–m3, and four anterior alveoli. The first of these is large and anteriorly aligned, the other three are of similar size and placed in a single row. These four alveoli probably correspond to those for the enlarged medial incisor, the reduced lateral incisor, the canine and p3. Therefore, the dental formula of the Subengius mengi lower jaw is 2.1.2.3. A mental foramen is present below the alveolus of p3. IMM−2004−SB−053 also contains a complete p4, showing that the talonid portion of p 4 in S. mengi bears a single small cusp aligned with the four apical cusps.

Discussion.—In their original description, Smith et al. (2004) suggested that S. mengi had a strong mosaic pattern of autapomorphic, primitive and derived characters, and based on this they created the new carpolestid genus Subengius , but placed it at an evolutionary stage slightly before the transition between the primitive genus Elphidotarsius and the more advanced genus Carpodaptes ( Smith et al. 2004) . The new specimens presented here support this hypothesis. The small size of P3 and the presence of only three labial cusps are features seen in the most primitive Elphidotarsius species. The p4 with only four apical cusps is typical of Elphidotarsius ( Rose 1975) . The absence of p2, the alignment of the apical cusps on p4 and the limited development of the lingual border of P3 is seen both in more advanced species of Elphidotarsius and primitive species of Carpodaptes ( Rose 1975; Silcox et al. 2001).

To test the original hypothesis of Smith et al. (2004) on the phylogenetic position of Subengius , we performed a cladistic analysis by adding Subengius to the analysis of carpolestids published by Bloch et al. (2001), to specifically resolve the relations of taxa within the family Carpolestidae . To this, we also added the new morphological data on Elphidotarsius russelli presented by Silcox et al. (2001) (see Table 4 for codelines). We do not consider the Asian Eocene Chronolestes to be a member of the Carpolestidae ( Bloch et al. 2001; Silcox et al. 2001), and follow Fox (2002) in restricting the genus Carpocristes to its Asian Eocene type species C. oriens .

Our analysis yielded a single most parsimonious tree of 67 steps, with CI 0.90 and RI 0.92, and places Subengius between E. shotgunensis and E. russelli ( Fig. 13 ). Because E. russelli is so close to Carpodaptes that it in fact obscures the generic distinction between Elphidotarsius and Carpodaptes ( Silcox et al. 2001) , we consider that the results of this analysis support the initial hypothesis relatively well. Detailed analysis of the character matrix shows that the morphology of Subengius is in fact closest to E. russelli , and that its slightly more primitive position is due to the lower number of labial cusps on P3 and P4. However, because of the very strong mosaic pattern of primitive and advanced characters, and because of the unique presence of two isolated median spurs on P4, we continue to place S. mengi in a separate genus, apart from Elphidotarsius . As in previous studies ( Bloch et al. 2001; Silcox et al. 2001), our analysis shows that Elphidotarsius and Carpodaptes are not monophyletic genera, and even the alternative of attributing Subengius to the genus Elphidotarsius would not change this taxonomic problem. A complete study of all known carpolestids might help to resolve this situation, but is obviously beyond the scope of this paper.

Our analysis suggests that Subengius and Carpocristes evolved independently from their North American ancestors. The taxon that is morphologically closest to Subengius , Elphidotarsius russelli , is known from the North American Tiffanian 1–2, while Elphidotarsius shotgunensis and Carpodaptes hazelae , that are also morphologically close to Subengius , are known from Tiffanian 1–3 ( Bloch et al. 2001; Silcox et al. 2001). The ancestor of Subengius thus probably migrated into Asia during the early Tiffanian. The closest relatives to Carpocristes oriens , Carpodaptes hobackensis , and C. cygneus are known from the late Tiffanian (Ti5) and middle Tiffanian (Ti3–4), respectively ( Bloch et al. 2001). Asian carpolestids thus represent two independent dispersal events, although it is not impossible these occurred simultaneously. Chronolestes simul from the early Eocene of Wutu (Shandong Province, China) represents yet another dispersal of plesiadapiforms into Asia, but the timing of this is more problematic (see Silcox et al. 2001).

Stratigraphic and geographic range.— Subengius mengi is currently only known from the Gashatan (late Paleocene) Nomogen Formation at Subeng (Inner Mongolia, China).

Figures

Fig. 12. The carpolestid Subengius mengi Smith, Van Itterbeeck, and Missiaen, 2004, from the Gashatan (Paleocene) of Subeng, China. A. IMM−2004− SB−052, LP3 in occlusal view. B. IMM−2001−SB−059, LP3 in occlusal view C. IMM−2004−SB−053, left dentary with p4−m3, in lingual (C1), occlusal (C2), anterolabial (C3), and labial (C4) views. SEM micrographs.

Fig. 13. Single most parsimonious tree of carpolestid relationships (CI 0.90, RI 0.92) based on the matrix of Bloch et al. (2001) and modified as noted in the appendix, with indication of unambiguous synapomorphies, i.e., invariable with respect to optimisation criteria.