Eomoropus sp.

Tsubamoto, Takehisa, Egi, Naoko, Takai, Masanaru, Sein, Chit & Maung, Maung, 2005, Middle Eocene ungulate mammals from Myanmar: A review with description of new specimens, Acta Palaeontologica Polonica 50 (1), pp. 117-138 : 121-123

Eomoropus sp. cf. E. minimus Zdansky, 1930

Fig. 3A .

Material.—NMMP−KU 0708, a right maxillary fragment with M3.

Locality.— PA1 locality (21°46´24˝N; 94°36´04˝E), Myaing Township , western part of central Myanmar ( Fig. 1 ) .

Dental measurements.—Shown in Table 1.

Description.—The preserved M3 is brachyodont and wider than long, and shows typical small “eomoropid” M3 morphology. The paracone is the largest and tallest cusp. The metacone cannot be identified, but it appears to be located right at the junction of the metaloph and postparacrista as in other basal chalicotheres. The parastyle is large and isolated. The mesostyle is enlarged and is located at the distobuccal corner of the crown. The mesostyle is proportionally larger than that in E. amarorum and E. quadridentatus . The metacone−metastylar region is smaller than the mesostyle and is located just distal to the junction of the postparacrista, mesostyle, and metaloph. The ectoloph is incompletely W−shaped. The protoloph is incomplete with a relatively large paraconule. The metaloph is complete without a metaconule. The mesial cingulum originates from the tip of the parastyle and disappears at the mesiolingual base of the protocone. There are no distinct lingual, distal, and buccal cingula.

“ Eomoropidae ” gen. et sp. indet.

Fig. 3B .

Material.—NMMP−KU 1270, a left M3.

Locality.— The Kd1 locality (21°49´25˝N; 94°35´24˝E) in Myaing Township , western part of central Myanmar ( Fig. 1 ) .

Dental measurements.—Shown in Table 1.

Description and comments.—This M3 is very similar to M3 of NMMP−KU 0708 in morphology and size. The former differs from the latter in that it has a more mesiobuccally prominent parastyle, slightly stronger metacone−metastylar region, and slight lingual and distal cingula.

Comparison and discussion of the Pondaung “eomoropids”.— To date, seven genera of “eomoropids” have been reported ( McKenna and Bell 1997). Among these genera, however, two genera, Paleomoropus Radinsky, 1964 and Lophiaspis Depéret, 1910 , have been placed not in this family but in the Lophiodontidae by several authors ( Fischer 1977; Lucas and Schoch 1989; Prothero and Schoch 1989; Coombs 1998); Danjiangia Wang, 1995 was considered a primitive brontothere by Beard (1998: 27) and Hooker and Dashzeveg (2003: 491); and Lunania Chow, 1957 is represented only by mandibular fragments with lower molars ( Chow 1957; Huang 2002) and might be a phenacolophid condylarth ( Lucas and Schoch, 1989). Therefore, only the three genera ( Eomoropus Osborn, 1913 ; Grangeria Zdansky, 1930 ; and Litolophus Radinsky, 1964 ) have been recognized with confidence in the “ Eomoropidae ” by most researchers ( Lucas and Schoch 1989; Coombs 1998).

The present M 3 specimens show typical “eomoropid” M3 morphology in having an incomplete protoloph with retained paraconule, complete metaloph with no metaconule, isolated and large parastyle, developed mesostyle, and incompletely W−shaped ectoloph. They are morphologically similar to M3 of Eomoropus and Grangeria and are distinct from those of Litolophus in that they lack the distobuccal rotation of the M3 metaloph and the distal end of the ectoloph, have a much larger M3 mesostyle, and are proportionally shorter and wider ( Radinsky 1964; Chow et al. 1974; Lucas and Schoch 1989).

Eomoropus and Grangeria are relatively similar in morphology to each other. They are distinguished from each other mainly by their mandibular and anterior dental characteristics and also by the following features: Eomoropus is smaller than Grangeria and has less mesiobuccally prominent upper molar parastyles ( Radinsky 1964; Lucas and Schoch 1989). Lucas and Schoch (1989: 424, left column, line 2 from the bottom) mentioned that Eomoropus is distinguished from Grangeria by the less prominent upper molar “metastyles,” but we judged that the word “metastyles” was used mistakenly in this context and should be “parastyles”.

The present M3s are similar in size to each other. They can be assigned to Eomoropus rather than to Grangeria because of their size ( Table 1; Lucas and Schoch 1989: table 23.1). M3 of NMMP−KU 0708 has a less mesiobuccally prominent parastyle, which is one of the diagnostic characters of Eomoropus . Therefore, NMMP−KU 0708 is assigned to Eomoropus . On the other hand, NMMP−KU 1270 has a more prominent parastyle, which is one of the diagnostic characters of Grangeria . Therefore, we describe NMMP−KU 1270 as an indeterminate “eomoropid” in this paper.

However, the possibility that the present two specimens can be assigned to a single species of Eomoropus cannot be eliminated because of their similar size and morphology, their occurrence in the same formation, and the poor fossil record of the “ Eomoropidae ” in the Pondaung Formation. If these specimens truly belong to the same species, the diagnoses of Eomoropus and Grangeria concerning parastylar development by Lucas and Schoch (1989) would need to be reconsidered.

On the basis of molar size ( Table 1), M3 of the Pondaung Eomoropus is referable to that of E. minimus (IVPP V2403.1 and V2403.2), the smallest species of Eomoropus , which was discovered in the middle Eocene Rencun Member of the Heti Formation (central China) and was described and figured by Hu (1959: pl. 1: 2a, b), though the Pondaung form is slightly larger than the Heti form. The Pondaung form is much smaller than the other two species of Eomoropus , E. amarorum and E. quadridentatus ( Table 1). On the other hand, Radinsky (1964) and Lucas and Schoch (1989) considered that the holotype of E. minimus from the Heti Formation described by Zdansky (1930) as M1 is DP4 and that E. minimus is synonymous with E. quadridentatus . However, the upper molar material described as E. minimus by Hu (1959) and that of the Pondaung Eomoropus are much smaller than molars of E. quadridentatus , suggesting that they can be distinguished from E. quadridentatus . We identify the Pondaung Eomoropus specimen as Eomoropus sp. cf. E. minimus in order to avoid confusion between specific names in this paper.

Zong et al. (1996) described a left maxillary fragment with M1–M3 (IVPP V9911) discovered in the middle Eocene Xiangshan Formation of the Lijiang basin (southern China), and identified it as E. minimus . However, M 3 in V9911 has a much smaller mesostyle than that in Eomoropus and Grangeria , and is proportionally longer and narrower than M3 of the latter. These characteristics of M 3 in V9911 suggest that V9911 is referable not to Eomoropus but to Litolophus .

Figures

Fig. 3. A. Eomoropus sp. cf. E. minimus Zdansky, 1930, NMMP−KU 0708, a right maxillary fragment with M3, occlusal view; A1, stereo pair; A2, schematic drawing of M3. B. “Eomoropidae” gen. et sp. indet., NMMP−KU 1270, a left M3, occlusal view; B1, stereo pair; B2, schematic drawing.

Fig. 1. Geographical map of the Pondaung area of central Myanmar showing several vertebrate fossil localities in the Pondaung Formation.