First record of an Anthracobunidae (Mammalia, ?Tethytheria) from the Eocene of the Pondaung Formation, Myanmar

C. R. Acad. Sci. Paris, Sciences de la Terre et des planètes / Earth and Planetary Sciences 330 (2000) 725–730 © 2000 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS. Tous droits réservés S1251805000001877/FLA

Palaeontology / Paléontologie (Vertebrate Palaeontology / Paléontologie des Vertébrés)

First record of an Anthracobunidae (Mammalia, ?Tethytheria) from the Eocene of the Pondaung Formation, Myanmar Stéphane Ducrocq*a, Aung Naing Soeb, Bo Boc, Mouloud Benammia, Yaowalak Chaimaneed, Than Tunc, Tin Theine, Jean-Jacques Jaegera a

Institut des sciences de l’Évolution, UMR 5554 CNRS, case courrier 064, université Montpellier-2, place Eugène-Bataillon, 34095 Montpellier cedex 5, France b Department of Geology, University of Yangon, Myanmar c Office of Strategic Studies, Ministry of Defence, Yangon, Myanmar d Department of Mineral Resources, Geological Survey Division, Rama VI Road, Bangkok 10400, Thailand e Department of Geology, University of Pathein, Myanmar Received 21 January 2000; accepted 17 April 2000 Communicated by Philippe Taquet

Abstract – A new genus and species of Anthracobunidae, Hsanotherium parvum, is described from the Middle Eocene Pondaung Formation in Myanmar. This form is the smallest and the most primitive known in the family, and it suggests that the Eocene South-Asian radiation of Anthracobunidae occurred in a diachronous way on the Indian Subcontinent and eastward. © 2000 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS Mammals / Anthracobunidae / Middle Eocene / Pondaung / Myanmar

Résumé – Première découverte d’un Anthracobunidae (Mammalia, ?Tethytheria) dans l’Éocène de la formation de Pondaung, Myanmar. Un nouvel Anthracobunidae, Hsanotherium parvum gen. et sp. nov. de l’Eocène moyen de la Formation de Pondaung au Myanmar est décrit. Cette forme birmane est la plus petite et la plus primitive connue, et suggère que la radiation éocène des Anthracobunidae en Asie du Sud s’est déroulée de manière diachrone sur le sous-continent indien et plus à l’est. © 2000 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS Mammifères / Anthracobunidae / Eocène moyen / Pondaung / Myanmar

Version abrégée L’Asie du Sud se caractérise par sa richesse en restes de mammifères primitifs d’âge Éocène inférieur et moyen, témoignant d’un centre d’origine de plusieurs groupes actuels. Les investigations récentes dans la localité de Yarshe Kyitchaung au Myanmar [10, 11, 17] (figure 1) ont livré des restes dentaires pouvant être attribués à un nouvel Anthracobunidae. Cette famille, connue jusqu’à présent uniquement dans l’Éocène d’Inde et du Pakistan [13, 25], a vu son statut controversé [9, 22, 27],

mais il est communément admis qu’elle se place au sein des Téthythères [9]. Ordre Tethytheria Mc Kenna, 1975 Famille Anthracobunidae Wells et Gingerich, 1983 Genre Hsanotherium gen. nov. Derivatio nominis. De hsan, mot birman signifiant étrange, et du grec, θgqi’om, animal sauvage. Espèce Type. Hsanotherium parvum sp.nov. Diagnose. Celle de l’espèce. Hsanotherium parvum gen. et sp. nov. Derivatio nominis. En référence à la très petite taille des spécimens.

* Correspondence and reprints: [email protected]

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Holotype. Maxillaire droit avec M1–M3 (Bhn 10), collections du National Museum of Myanmar, Yangon (figure 2A). Matériel supplémentaire. Maxillaire droit avec M2–M3 (Bhn 11), collections du National Museum of Myanmar, Yangon (figure 2B). Horizon et localité. Fin de l’Éocène moyen, Pondaung Formation, localité de Yarshe Kyitchaung (Primate Resort), district de Myaing, Myanmar (figure 1). Diagnose. Plus petit Anthracobunidae connu. Diffère de Pilgrimella, Anthracobune, Ishatherium et Nakusia par son protolophe et son métalophe à peine développés, son hypocône et ses conules plus petits, et par son fort mésostyle (ses dimensions sont reportées dans le tableau). La morphologie des molaires supérieures de Hsanotherium permet de les distinguer de celles des Raoellidae [25], caractérisées par la présence de crêtes transversales et l’absence d’hypocône, ce dernier caractère permettant également de différencier la forme birmane des Diacodexeidae. De même, les Dichobunidae ne présentent jamais l’association de six tubercules distincts et d’un mésostyle développé aux molaires supérieures. Seul Aumelasia, de l’Éocène moyen d’Europe, présente ces caractères [24], mais sa structure dentaire est différente de celle de Hsanotherium. Parmi les formes asiatiques paléogènes, les Anthracobunidae sont les seuls à posséder des molaires supérieures à six tubercules, sans ectolophe et augmentant de taille de la M1 à la M3. Cette famille, incluant les genres Anthracobune, Ishatherium, Jozaria, Lammidhania, Pilgrimella et Nakusia de l’Éocène inférieur et moyen du sous-continent indien [9, 22] a vu son statut changer très souvent [3, 4, 7, 8, 13, 15, 18, 19, 20, 21, 27, 28, 29, 30] ; cependant, la plupart des auteurs considèrent actuellement ces formes comme probablement apparentées aux téthythères [6]. Les Anthracobunidae se singularisent, entre autres, par leurs molaires supérieures larges, à six tubercules distincts, dont les antérieurs sont alignés selon une courbe légèrement convexe, à l’échancrure buccale marquée, et relativement plus larges antérieurement que postérieurement [13, 27]. Ces caractères se retrouvent également chez Hsanotherium. Ce dernier se distingue de Pilgrimella, Ishatherium, Anthracobune et Nakusia (seuls gen-

1. Introduction Southern Asia has long been known for having yielded numerous remains of primitive mammals in Middle Eocene formations, mainly in Pakistan and India. These fossils generally testify to a centre of origin for several higher taxa in that part of the world. New fossil collecting in the Paleogene deposits of the Union of Myanmar has led to the discovery of several mammal remains in the Middle Eocene Yarshe Kyitchaung locality (figure 1), among which two fragmentary upper jaws that can be

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res pour lesquels les molaires supérieures sont connues) par ses plus petites dimensions, le faible développement de ses lophes transverses, de ses conules, de son hypocône, et de son cingulum distal et buccal. En revanche, la forme birmane et les Anthracobunidae partagent la présence d’un parastyle marqué (légèrement plus développé chez Anthracobune et Pilgrimella), la position de l’hypocône légèrement plus buccal que le protocône (Anthracobune) et l’expansion distale de la crête postérieure du métacône de la M3 (Anthracobune). En résumé, Hsanotherium peut être attribué aux Anthracobunidae par l’association aux molaires supérieures de six tubercules distincts (dont un hypocône), d’un mésostyle, d’une muraille buccale échancrée et d’une augmentation de taille de M1 à M3. En revanche, Hsanotherium diffère de tous les anthracobunidés connus par sa très petite taille et sa morphologie plus primitive, cette dernière annonçant toutefois la structure typique de la famille. Les condylarthres phénacolophidés (en particulier Minchenella du Paléocène supérieur de Chine) sont considérés comme le groupe ancestral probable des Anthracobunidae [5, 27], mais ce dernier n’étant connu que par sa dentition inférieure, aucune comparaison n’est possible avec la forme birmane. Les genres Phenacolophus, Tienshanilophus et Radinskya sont également attribués aux Phenacolophidae [2, 14, 15, 16, 26, 27, 31], mais leur morphologie dentaire se distingue clairement de celle de Hsanotherium par leurs tubercules arrangés en forme de p, formant ainsi une bilophodontie marquée, et par leur M3 généralement réduite par rapport aux dents précédentes. Les reconstitutions paléogéographiques récentes [23] semblent indiquer que le sous-continent indien et les régions plus orientales comme le Myanmar et la Thaïlande devaient être en communication durant l’Éocène inférieur et moyen [1, 12], la présence de Hsanotherium au Myanmar confirmant cette hypothèse, en élargissant la répartition géographique de la famille plus à l’est qu’elle n’était connue jusqu’à présent. Enfin, les Anthracobunidae semblent avoir été caractérisés par deux radiations distinctes, celle ayant eu lieu au Myanmar conduisant à des formes plus primitives que celle d’Inde et du Pakistan.

referred to a new anthracobunid. These remains are associated with insectivores, rodents, primates [10, 11], anthracothere and ruminant artiodactyls, and titanothere, amynodont, and helaletid perissodactyls [17]. Anthracobunidae are a peculiar group of large mammals known in the Early–Middle Eocene of Pakistan and India and they are characterized by their primitive bunodont and moderately lophodont teeth. This family was previously known only in the Eocene of India and Pakistan (for example [13, 25], and their status is widely discussed [9, 22, 27], but most authors admit that they

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Figure 2. Hsanotherium parvum gen. et sp. nov. Occlusal views of A: Bhn 10 (holotype), right M1–M3, and B: Bhn 11, right M2–M3. Scale bar: 5 mm. Figure 2. Hsanotherium parvum gen. et sp. nov. Vues occlusales de A : Bhn 10 (holotype), M1–M3 droites, et B: Bhn 11, M2–M3 droites. Échelle: 5 mm.

Figure 1. Location map of Yarshe Kyitchaung locality, central Myanmar. Star indicates the position of Yarshe Kyitchaung locality. Figure 1. Carte de localisation du site de Yarshe Kyitchaung, Myanmar. L’étoile indique la localisation du site de Yarshe Kyitchaung.

belong to the Tethytheria (see Ginsburg et al. [9] for discussion).

2. Systematics Order Tethytheria McKenna, 1975 Family Anthracobunidae Wells and Gingerich, 1983 Genus Hsanotherium gen. nov. Derivation of name. From hsan, Myanmar for strange, and from θηρι’oν, Greek for beast. Type species. Hsanotherium parvum sp. nov. Diagnosis. Same as for species. Hsanotherium parvum gen. et sp. nov. Derivation of name. Species name in reference to the very small size of the specimens. Holotype. A fragmentary maxillary preserving the right M1–M3 (Bhn 10), collections of the National Museum of Myanmar, Yangon (figure 2A). Additional material. A fragmentary maxillary preserving the right M2–M3 (Bhn 11), collections of the National Museum of Myanmar, Yangon (figure 2B).

Horizon and locality. Late Middle Eocene of Pondaung Formation, Yarshe Kyitchaung locality (Primate Resort), Bahin village, Myaing Township, Central Myanmar (figure 1). Diagnosis. Smallest known Anthracobunidae. Differs from Pilgrimella, Anthracobune, Ishatherium and Nakusia by its incipient protoloph and metaloph, its smaller hypocone and conules, and by its strong mesostyle (measurements in the table). Description. Bhn 10 (right M1–M3): the upper molars are subquadrangular in shape with three main pyramidal cusps (paracone, metacone, and protocone) and three other smaller cusps (hypocone, paraconule, and metaconule). The paracone is larger than the metacone, and it is about the same size as the protocone, but higher than it. The paracone and the protocone are in line. The

Table. Measurements (in mm) of the dental material referred to Hsanotherium parvum gen. et sp. nov. Tableau. Dimensions (en mm) du matériel dentaire de Hsanotherium parvum gen. et sp. nov.

Length Bhn 10

Bhn 11

rM1 rM2 rM3 rM2 rM3

5.6 6.6 7.5 7.0 8.0

anterior width posterior width 5.1 6.6 7.8 6.6 7.8

5.8 6.7 6.8 6.5 7.5

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centrocrista is straight and rather low, and mesially ends in a well developed parastyle which forms a broad mesiobuccally process on the tooth. A very tiny metastyle occupies the distobuccal corner of the crown. The strong preprotocrista is mesiobuccally directed and stops just in front of the paracone, while the postprotocrista is much weaker and joins a very small metaconule lingual to the metacone. A small paraconule occurs in the middle of the preprotocrista, but it is less distinct than the metaconule. A very faint ridge runs down from the apex of the paracone lingually and joins the paraconule. The cingulum is well developed all around the crown, and a small hypocone arises from its distolingual part. The hypocone has no connection with other cusps or crest on the crown. The mesiolingual part of the cingulum also bears a small cuspule about the same size as the hypocone. A distinct mesostyle occurs on all molars; it is situated on the buccal wall of the crown between the metacone and the end of the central valley, and it decreases in size from M1 to M3. Also, the teeth exhibit a small but distinct protoconule that rises in front of the protocone on the mesial cingulum. The upper molars increase in size from M1 to M3, and M3 is somewhat wider than the preceding teeth. The paracone and the metacone are about the same size only on M1, but the cusp and crest structure is similar on all molars. Bhn 11 (right M2–M3): Bhn 11 has much more corrugated enamel, but the structure of the crowns is identical to that of Bhn 10. M1 is lacking, but remains of roots are visible, and it is therefore possible to observe that this tooth, like the others, had three roots, two buccal and one lingual.

3. Discussion The overall structure of the upper molars Bhn 10 and Bhn 11 can be distinguished from that of the teeth of Raoellidae (a group of artiodactyls known in the Early to Middle Eocene of India and Pakistan, and including Khirtaria, Bunodentus, Metkatius, and possibly Haqueina according to Thewissen et al. [25]) which displays characteristic transverse crests on upper molars. In addition, Hsanotherium is clearly smaller than all known raoellids. Several other Asian Eocene artiodactyls display morphological similarities with Hsanotherium. However, the Diacodexeidae do not exhibit a hypocone on the distolingual corner of their upper molars and therefore an attribution of Hsanotherium to that family cannot be justified. Similarly, several features observed on Bhn 10 and 11 are unknown in dichobunid artiodactyls. These are the association of six distinct cusps on the crown of upper molars with the occurrence of a large mesostyle in the middle of the buccal cingulum. The latter character can be observed only in Aumelasia from the Middle Eocene of Europe [24], but the overall structure of the Burmese teeth is different from that of the European genus.

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Among other Asian Eocene artiodactyls, the Anthracobunidae display broad upper molars that grow larger from M1 to M3 and with six distinct cusps and no ectoloph. These characteristic features are also recognized in Bhn 10 and 11, and might reflect phylogenetical relationships. No other Asian Paleogene artiodactyl family exhibits six cusps on upper molars except Anthracobunidae. This family includes the genera Anthracobune, Ishatherium, Jozaria, Lammidhania, and Pilgrimella according to Shoshani et al. [22]. Recently a new form (Nakusia shahrigensis) has been described from the Lower Eocene of Pakistan [9]. The taxonomic status of Anthracobunidae has continuously changed, since they have been included within dichobunids [4], anthracotheriids [7, 18, 28], perissodactyls [3], sirenians [13, 19], phenacolophid condylarths [15], or moeritheriid proboscideans [8, 13, 20, 21, 27, 29, 30]. Most authors now regard anthracobunids as related to proboscideans, or at least to Tethytheria [6]. Only upper molars are known for the new Burmese form, and comparisons with Anthracobunidae are therefore limited to the genera Anthracobune, Pilgrimella, Ishatherium, and Nakusia, although some of these are known only from few scarce remains of upper cheekteeth. According to Kumar [13], Anthracobunidae are medium-sized animals with transverse upper molars lacking an ectoloph, and increasing in size posteriorly. Wells and Gingerich [27] also noted that known anthracobunid upper molars are characterized by six cusps arranged in two transverse rows. Bhn 10 and 11 display all features listed by Kumar, and Wells and Gingerich, with the exception of the last character which is discussed below. Hsanotherium differs from Pilgrimella and Anthracobune in its much smaller size, and in the poorly developed or incipient transverse lophs of upper molars. The roughly triangular occlusal outline of the Burmese upper molars is reminiscent of that of the P4 in the Pakistani genera, whereas the upper molars of the latter are more squared. Also, the conules and hypocone in Bhn 10 and 11 are less developed than in Pilgrimella and Anthracobune, and their buccal and distal cingulae are weaker (especially compared to those of Pilgrimella). However, Hsanotherium and Pilgrimella share the median notch on the buccal face of the molars. A small but well developed mesostyle occurs on upper molars of Hsanotherium which is absent in those of Pilgrimella, Nakusia, and Anthracobune, with the exception of the M2 of the latter (see description of LUVP 15006 in West [29]). Also, Anthracobune, like Hsanotherium, displays a distinct parastyle on all molars. The mesiobuccal parastyle, the metaconule anteriorly situated, the discontinuous lingual and distal cingulae around the hypocone, the hypocone in a more buccal situation with respect to the protocone can be observed in both Pakistani and Burmese forms. On the other hand, the M3, and to a lesser degree the M1 and M2, of Anthracobune displays a small expan-

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sion of the posterior crest of the metacone that gives the tooth a triangular outline backwards, as in Hsanotherium. Ishatherium was first described by Sahni and Kumar in 1980 from the Subathu Formation, and its upper dentition is so far known only from a partial upper molar [27]. However, it can be noted that the specimen referred to Ishatherium is much larger than upper molars of Hsanotherium, and that its hypocone is more developed than in the Burmese form, with a longer and more quadrangular lingual part of the molar. Finally, Nakusia mainly differs from Hsanotherium in its somewhat larger size, its well-developed mesial and distal transverse rows, its more squared occlusal outline, and in its more developed hypocone and conules. In summary, Bhn 10 and 11 cannot be referred to any of the known anthracobunid genera, but the general structure of their upper molar seems to herald the common dental structure of Anthracobunidae. The upper molars of Hsanotherium increase in size from M1 to M3, they are roughly wider anteriorly than posteriorly, they display six distinct cusps including small (or incipient) but distinct conules and hypocone, their paraconule and metaconule are somewhat anteriorly situated with respect to the laterally adjacent cusps and bound to them by sharp crests, they lack an ectoloph, they display a buccal notch like in Pilgrimella, Anthracobune, and Nakusia, and a well developed cingulum that surrounds the crown, especially on the mesial, distal and lingual faces. The main characters that distinguish the upper molars of Hsanotherium from those of other anthracobunids are their incomplete or only incipient transverse lophs, their small hypocone, their poorly developed conules, and their roughly triangular outline. However, it is noteworthy that the triangular outline of upper molars in Bhn 10 and 11 is reminiscent of the occlusal shape of the P4 of the other representatives of the family. Hsanotherium clearly differs from Dichobunidae in the occurrence of bunodont and rounded cusps and of a well developed mesostyle. The Burmese specimens are distinctly more primitive than all known anthracobunids, but they might be related to the latter because of the association on their upper molars of six distinct cusps including a hypocone, of a mesostyle, a notched buccal wall, an anterior width larger than the posterior width, and of an increase of size from M1 to M3. As no other Eocene ungulates in South Asia display this set of characters, Hsanotherium can reasonably be included into the Anthracobunidae. The dental similarities shared by the Burmese molars and those of genera assigned to anthracobunids are more impressive than any resemblance to other ungulates known in the Early Paleogene of South Asia. Given these similarities and the chronological and palaeobiogeographical context, Hsanotherium probably represents a primitive, small form which might be related to Anthracobunidae.

According to Wells and Gingerich [27], the Chinese Late Paleocene phenacolophid genus Minchenella is the most plausible ancestor of anthracobunids, an opinion reiterated by Domning et al. ([5], p. 44). Unfortunately, that genus is known only by its lower dentition, and comparisons with Hsanotherium are thus impossible. The Phenacolophidae are presently included within the order Condylarthra [27, 31], although other authors have previously attributed the family to the Embrithopoda [15] or to the Pantodonta [2]. Several taxa are referred to the Phenacolophidae, among which Phenacolophus fallax from the Late Paleocene or Early Eocene of Mongolia (previously described as a condylarth by Matthew and Granger [14], and then redescribed by McKenna and Manning [15], who referred it to the Embrithopoda), Tienshanilophus lianmuqinensis [26], and Radinskya yupingae from the Late Paleocene of China [16]. All these taxa are known by their upper molars which are distinctly bilophodont with six well developed cusps arranged in a π-pattern. Upper molars of Hsanotherium do not exhibit that structure, but they only display a slight crest joining the protocone, the paraconule, and the paracone, and ending in front of the paracone, that might correspond to an incipient protoloph, similar to that of anthracobunid molars. The enlargement of the paraconule and especially the metaconule on upper molars, the strong metaloph formed by a conjoined metaconule and hypocone, the loss of the postprotocrista, the strong but short crests of the protoloph and metaloph are characters of phenacolophids and of perissodactyls [16] that are barely expressed or even unknown on Bhn 10 and 11. In addition, condylarths generally have a M3 rather reduced by comparison with preceding molars, which is not the case in Hsanotherium. The latter also differs from Phenacolophus and Tienshanilophus in having the conules somewhat anterior to the main cusps, whereas they are more in line with the main cusps in the Chinese and Mongolian genera. This feature is thus in favour of a closer relationship with anthracobunids rather than with phenacolophid condylarths. This might suggest that Hsanotherium had already engaged in the way leading to Anthracobunidae, but that it had evolved locally farther east than the forms known on the Indian Subcontinent (Pakistan and India).

4. Conclusions In conclusion, Hsanotherium shares with anthracobunids (at least those for which upper teeth are known) the increase in size from M1 to M3, the buccal notch of the upper molars, the occurrence of six distinct cusps, the anterior ones being arranged in a somewhat convex curve, the lack of an ectoloph, and the relatively great anterior breadth of the molars. This form therefore cannot be referred to other known Asian artiodactyls. According to recent palaeogeographic reconstructions of South Asia during the Early and Middle Eocene (see

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for example [23]), Myanmar seems to have been part of continental Asia, like Thailand and the Indian Subcontinent [1, 12]. The occurrence of anthracobunids during the first part of the Eocene in India, Pakistan and Myanmar is therefore compatible with palaeogeographic data. Pakistani and Indian Eocene mammal faunas are usually regarded as endemic because they are distinct from a systematic point of view from all other Eocene faunas from Asia. On the other hand, anthracobunids are thought to have possibly evolved from Asian phenacolophid

condylarths (see for example [27]), a family restricted to the Paleocene of Asia [16]. The occurrence of Hsanotherium in the Middle Eocene of Myanmar not only increases the geographical range of the family, but it also indicates that anthracobunids have undergone two distinct radiations. The radiation of anthracobunids in Myanmar seems to have been distinct from that in the Indian Subcontinent, and might have led to more primitive forms than those known in Pakistan.

Acknowledgements. We thank P.D. Gingerich, J. Sudre, and D.E. Russell for their helpful comments and the Foundation Singer-Polignac for financial support. The drawings are by L. Meslin. Publication ISEM 2000-052.

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