Bahinolophus birmanicus ( Pilgrim, 1925 ) Tsubamoto & Egi & Takai & Sein & Maung, 2005

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 : 124-128

Bahinolophus birmanicus ( Pilgrim, 1925) comb. nov.

Figs. 4 , 5 .

Chasmotherium (?) birmanicum Pilgrim, 1925: 25–28 , pl. 2: 9. Chasmotherium birmanicum ; Matthew 1929: 514–515, fig. 38. Deperetella (?) birmanicum ; Colbert 1938: 348–350, fig. 40 [sic]. Deperetella birmanicum ; Radinsky 1965: 227; Tsubamoto, Egi, et al. 2000: 60, pl. 93 [sic].

Diplolophodon birmanicum ; Ding et al. 1977: 44–45.

Deperetella birmanica ; Tsubamoto, Holroyd, et al. 2000 (in part):

185–187, figs. 3, 4C, D.

Holotype: GSI C348 (a left mandibular corpus with heavily worn p4–m3) and BMNH M12756 (a right mandibular corpus with heavily worn p4–m3), which belong to the same single individual ( Fig. 5C, D ; Pilgrim 1925).

Type locality: 2.4 km southwest of Thadut Village (= at or near the Pk5 locality), Myaing Township, central Myanmar ( Fig. 1 ; Pilgrim 1925).

Referred material.—NMMP−KU 0005 and 0006 (Tsubamoto, Egi, et al. 2000; Tsubamoto, Holroyd, et al. 2000).

New material.—NMMP−KU 1046, a left maxillary fragment with complete P1, somewhat broken P2–P4, and complete M1–M2; NMMP−KU 1199, broken left mandibular fragments including symphysis part with p2 and other broken postcanine teeth of a single individual (NMMP−KU 1199 probably belongs to the same individual as NMMP−KU 1046 does); NMMP−KU 1558, a right P3; NMMP−KU 1662, a distal part of a right upper cheek tooth; NMMP−KU 1795, a talonid of a right?p3.

Locality of the new material.— NMMP−KU 1046 and 1199 are from the Bh 4 locality (21°43´39˝N; 94°38´30˝E) , NMMP−KU 1558 is from the Pk 5 locality (21°45´20˝N; 94°38´33˝E) , NMMP−KU 1662 is from the PGN2 locality (21°42´32˝N; 94°48´46˝E) , and NMMP−KU 1795 is from the Pk 12 locality (21°44´56˝N; 94°39´14˝E) , all of which are located in Myaing Township , western part of central Myanmar ( Fig. 1 ; Tsubamoto, Egi, et al. 2000) .

Diagnosis.—As for genus.

Dental measurements of the new material.—NMMP−KU 1046: P1 length = 10.4 mm, P1 width = 9.7 mm, P2 length = 11.6 mm, P2 width (estimate) = 13.9 mm, P3 length = 12.8 mm, P3 width (estimate) = 16.3 mm, P4 length (estimate) = 14.3 mm, P4 width (estimate) = 16.9 mm, M1 length = 15.1 mm, M1 width = 17.0 mm, M2 length = 17.4 mm, M2 width = 18.4 mm; NMMP−KU 1199: p2 length = 12.1 mm, p2 trigonid width = 7.6 mm, p2 talonid width = 8.1 mm; NMMP−KU 1558: P3 length = 11.1 mm, P3 width = 15.0 mm; NMMP−KU 1662: maximum width of the preserved part = 15.4 mm; NMMP−KU 1795: talonid width of?p3 = 8.4 mm.

Description.—The upper dentition of the new material ( Fig. 4 ) shows a strong bilophodont structure with a relatively high crown, mesial and distal cingula, and no or very weak buccal and lingual cingula. The cingulum is much more weakly developed than that in Deperetella and Teleolophus . In NMMP−KU 1046 ( Fig. 4A ), P1 and M1 are moderately worn, P2–P4 and M2 are almost unworn, and M3 is probably not erupted or in eruption, indicating that P1 and M1 erupt earlier than the other adult postcanine teeth in Bahinolophus and that this individual is a subadult.

P1 is somewhat mesiodistally elongated, longer than it is wide, and triangular−shaped from the occlusal view. There are a tall and large paracone, a very low protoloph, and a very low metaloph. The protoloph and metaloph are not parallel to each other, but lingually converge. The metaloph is stronger than the protoloph.

P2–P4 are wider than they are long. The protoloph and metaloph are parallel to each other, extending buccolingually. The two lophs are lingually separated by a deep transverse groove. Slight dental crenulations are observed at the middle part of the mesial face of the metaloph. The P2 protoloph is lower and less lingually extended than the P2 metaloph, making the crown of P2 trapezoidal rather than rectangular from the occlusal view. The P3 crown is higher than the P2 crown. The P3 protoloph is nearly as high and lingually extended as the P3 metaloph. P2 <P3. On NMMP−KU 1558 (P3), a distinct parastyle and lingually and buccally ridged paracone are observed ( Fig. 4B ). The P4 crown is higher than the P3 crown. The mesial part of P 4 in NMMP−KU 1046 is broken.

M1–M2 also have parallel protoloph and metaloph. The two lophs are slightly diagonal to the tooth row and slightly convex mesially, being joined buccally by the U−shaped and buccally convex ectoloph. The ectoloph is less buccally projected than it is in Deperetella and Teleolophus . The paracone is identified with slightly conical aspects, though the metacone is difficult to identify. The parastyle is located mesial to the paracone. There is neither mesostyle nor metastyle. The postmetacrista extends mesiodistally, being located as buccal as the paracone, and is less developed than in other Eocene tapiroids. The crown in occlusal view is less squared and proportionally less wide than that in Deperetella and Teleolophus . M1<M2.

NMMP−KU 1662, a distal half of an upper postcanine tooth ( Fig. 4C ), is so fragmentary that its tooth class cannot be identified.

We judge that NMMP−KU 1199 ( Fig. 5A ), a heavily broken new lower dental specimen, is from the same individual as NMMP−KU 1046 . This lower dental specimen was discovered at exactly the same locality as the upper dentition, NMMP−KU 1046 . Although the teeth are badly broken, they are very similar in morphology and size to the lower dental material (type specimen) of Deperetella birmanica previously described by Pilgrim (1925). A very small fragment of the most mesial part of m3 is observed in the mandible of this specimen, indicating that m3 is unerupted or in eruption. Therefore, this specimen, like NMMP−KU 1046 , is from a subadult .

In NMMP−KU 1199 , the mandibular symphysis extends below the mesial root of p2. There is a long diastema between the canine (not preserved) and p1 (only root is preserved). The first lower premolar (p1) is single−rooted, and the p1 root is slightly longer than it is wide. The second premolar (p2) ( Fig. 5A ) is mesiodistally elongated and has mesiodistally elongated lophids that form shearing blades, a weak and low hypolophid, and neither lingual nor buccal cingulids. The p2 talonid is slightly wider than the p2 trigonid. The length of the lower premolar series of NMMP−KU 1199 is estimated to be about 43 mm. It is nearly as long as the length of the lower molar series of the type specimen. Therefore, the premolar series is estimated to be nearly as long as the molar series in Bahinolophus .

NMMP−KU 1795 ( Fig. 5B ) is a talonid of a right lower molariform tooth. It has a well−developed and buccolingually oriented hypolophid like that seen in p4–m3 of this species, so it is distinct from p2. It is smaller than p4–m3 and is nearly as wide as the distal part of p2. We tentatively identified this tooth as a right p3.

Comparison and discussion.—The morphology and size of the upper premolars and lower postcanine dentition of the new materials are very similar to those of the previously described dentition of the deperetellid species, Deperetella birmanica ( Pilgrim 1925; Tsubamoto, Egi, et al. 2000; Tsubamoto, Holroyd, et al. 2000) from the Pondaung Formation, indicating that the new materials belong to this species. However, the upper molar morphology of the present materials is distinct from that of Deperetella cristata and Deperetella similis (= Diplolophodon similis ) from China, indicating that it is generically not referable to Deperetella . The upper molar morphology of the new materials is also distinct from that of the other known deperetellid, Teleolophus , and from that of other tapiroids. Therefore, we establish a new genus, Bahinolophus , for the Pondaung species.

Bahinolophus is assigned to the family Deperetellidae based on its bilophodont dentition, a relatively high crown, parallel molar protoloph and metaloph joined buccally by U−shaped and buccally convex ectoloph, a weak molar postmetacrista, a molar metaloph not interrupted by the postmetacrista, no distinct molar metacone, no distinct posthypocrista on the upper postcanine dentition, and a m3 hypoconulid reduced to a cingular bulge ( Figs. 4 , 5 ).

Bahinolophus differs from other Eocene tapiroids including the other deperetellids in lacking distinct cingulum at the distobuccal corner of the crown on the upper postcanine dentition, and in having a more buccally located molar postmetacrista. Other Eocene tapiroids have at least a small distobuccal cingulum connecting to the postmetacrista on the upper molars. In addition, on the upper molars of other tapiroids, the metacone is much more lingually located than the paracone. In contrast, on the upper molars of Bahinolophus , the estimated metacone region is not so lingually located, and is, instead, nearly as buccal as the paracone.

Bahinolophus is further distinct from the other deperetellids ( Teleolophus and Deperetella ) in having a slightly less straight molar protoloph and metaloph, a less sharp molar postmetacrista, a less buccally prominent molar ectoloph, and less squared and proportionally less wide crown aspect of molars in the occlusal view. It further differs from Teleolophus in having a relatively longer premolar series, a more developed bilophodonty and higher crown on the premolars, much weaker buccal and lingual cingula, a weaker molar parastyle, and a weak but distinct p2 hypolophid. It further differs from Deperetella cristata from northern China and Deperetella khaitchinulensis from Mongolia in having a relatively shorter premolar series, much weaker buccal and lingual cingula, and a less mesiodistally elongated p2, and in being smaller. It further differs from D. cristata in having a parallel protoloph and metaloph on P2.

Bahinolophus is also distinct from Deperetella similis from central and southern China. D. similis has been considered to be phyletically closely related to Deperetella birmanica (= Bahinolophus birmanicus ) from the Pondaung Formation ( Radinsky 1965; Ding et al. 1977; Dashzeveg and Hooker 1997; Tsubamoto, Holroyd, et al. 2000). Tsubamoto, Holroyd, et al. (2000) synonymized D. similis with D. birmanica on the basis of upper premolar and lower dental morphology. However, as mentioned above, the Pondaung form is distinct from Deperetella , including D. similis , in its upper molar morphology. Although the upper premolar morphology of Bahinolophus is very similar to that of D. similis , as mentioned by Tsubamoto, Holroyd, et al. (2000), the P2 protoloph in D. similis is much more lingually extended (as lingually extended as the P2 metaloph) than that in Bahinolophus . This difference implies that p2 of D. similis is proportionally wider than that of Bahinolophus , although the p2 morphology of D. similis is not yet known. Furthermore, the upper molars of D. similis have a more lingually located postmetacrista, more developed distobuccal cingulum, and more buccally projected ectoloph, all of which are characteristic of the genus Deperetella . Therefore, D. similis is a distinct species of Deperetella , and is distinguishable from D. birmanica (= B. birmanicus ).

Among the Deperetellidae , Bahinolophus has both derived and primitive characteristics. The derived characteristics are the very slight but distinct P1 bilophodonty with protoloph and metaloph, and strongly developed P2–P4 bilophodonty with a high crown. The primitive characteristics are a less buccally projected ectoloph and a less elongated anterior premolar dentition.

The anterior premolar and upper molar morphologies suggest that Deperetella and Teleolophus are closer to each other than to Bahinolophus . Deperetella and Teleolophus have a double−rooted p1, whereas Bahinolophus has a single−rooted p1. Deperetella and Teleolophus have a straighter molar protoloph and metaloph, a more lingually located postmetacrista, and a more distinct and stronger distobuccal molar cingulum than does Bahinolophus .

However, posterior premolar morphology suggests that Deperetella and Bahinolophus are closer to each other than to Teleolophus . Deperetella and Bahinolophus have more developed bilophodonty and a higher crown on the posterior premolars than does Teleolophus .

On the other hand, Bahinolophus has intermediate characteristics between those of Deperetella and Teleolophus in terms of the relative length of the premolar series. As mentioned above, the length of the premolar series of Bahinolophus is estimated to be nearly as long as the molar series. In contrast, the length of the premolar series of Deperetella is longer than the molar series, and that of Teleolophus is shorter than the molar series ( Radinsky 1965).

Dashzeveg and Hooker (1997) described a deperetellid left mandibular fragment with p3–p4 (PSS.27−31) from the Sevkhul Member of the upper Eocene Ergilin Formation ( Mongolia) as Deperetella sp. cf. D. birmanica . Although p4 of this specimen (PSS.27−31) is similar in morphology and size to that of the type specimen of Deperetella birmanica (= Bahinolophus birmanicus ) as suggested by Dashzeveg and Hooker (1997), the former is slightly larger in size than the latter: p4 of PSS.27−31 is 13.8 mm long by 11.9 mm wide ( Dashzeveg and Hooker 1997), and that of the type of D. birmanica is 11.7 mm long by 9.7 mm wide ( Pilgrim 1925). The phyletic relationships of this Mongolian specimen (PSS.27−31) to Bahinolophus are unclear because Bahinolophus is characterized mainly by its upper dentition and because the upper dentition of the species of PSS.27−31 is as yet unknown.

Figures

Fig. 4. New specimens of the upper dentition of Bahinolophus birmanicus (Pilgrim, 1925) comb. nov. A. NMMP−KU 1046, a maxillary fragment with P1–M2; A1, occlusal view (stereo pair); A2, schematic drawing of the occlusal view; A3, lingual view; A4, buccal view. B. NMMP−KU 1558, a right P3; B1, occlusal view; B2, schematic drawing of the occlusal view. C. NMMP−KU 1662, a distal part of a right upper cheek tooth; C1, occlusal view; C2, schematic drawing of the occlusal view.

Fig. 5. Lower dentition of Bahinolophus birmanicus (Pilgrim, 1925) comb. nov. A. NMMP−KU 1199, a left mandibular corpus with p2 and heavily broken other postcanine teeth (new specimen); A1, occlusal view of the mandible; A2, lingual view of p2; A3, occlusal view of p2; A4, buccal view of p2; A5, schematic drawing of the occlusal view. B. NMMP−KU 1795, a talonid of a right?p3 (new specimen); B1, occlusal view; B2, schematic drawing of the occlusal view. C, D. The type specimens, left and right lower mandibular fragments with left and right p4–m3. C. GSI C348, left p4–m3; C1, occlusal view; C2, schematic drawing of the occlusal view. D. BMNH M12756, right p4–m3; D1, occlusal view; D2, schematic drawing of the occlusal view.

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