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Discovery of new ruminant dental remains from the Middle Eocene Pondaung Formation (Myanmar): reassessment of the phylogenetic position of Indomeryx
C. R. Acad. Sci. Paris, Sciences de la Terre et des planètes / Earth and Planetary Sciences 330 (2000) 805–811 © 2000 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS. Tous droits réservés S1251805000001841/FLA
Palaeontology / Paléontologie (Vertebrate Palaeontology / Paléontologie des Vertébrés)
Discovery of new ruminant dental remains from the Middle Eocene Pondaung Formation (Myanmar): reassessment of the phylogenetic position of Indomeryx Grégoire Métaisa*, Mouloud Benammia, Yaowalak Chaimaneeb, Jean-Jacques Jaegera, Than Tunc, Tin Theind, Stéphane Ducrocqa a Institut des sciences de l’évolution, UMR 5554 CNRS, université Montpellier-2, place Eugène-Bataillon, 34095 Montpellier cedex 05, France b Department of Mineral Ressources, Geological Survey Division, Paleontological Section, Rama VI Road, Bangkok 10400, Thailand c Office of Strategic Studies, Ministry of Defence, Yangon, Myanmar d Department of Geology, University of Pathein, Myanmar
Received 20 February 2000; accepted 17 April 2000 Communicated by Yves Coppens
Abstract – Two new species of Indomeryx are described from the Late Middle Eocene Pondaung Formation, central Myanmar. These two species differ from I. cotteri by their more primitive dental features, thus challenging the inclusion of Indomeryx in the new Pecora family Prodremotheridae. Despite poor knowledge of this genus, we rather suggest a basal position within ruminantia, close to Archaeomeryx with which it displays several affinities. © 2000 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS Ruminants / Indomeryx / Middle Eocene / Myanmar
Résumé – Découverte de nouveaux restes dentaires de ruminants dans l’Eocène moyen de la formation de Pondaung (Myanmar) : réexamen de la position phylogénétique de Indomeryx. Deux nouvelles espèces, appartenant au genre Indomeryx et provenant de la formation de Pondaung (fin de l’Éocène moyen), sont décrites. Ces deux espèces se distinguent d’Indomeryx cotteri par leurs caractères primitifs, et remettent ainsi en cause l’inclusion de ce genre au sein des Prodremotheridae (Pecora). En dépit des connaissances fragmentaires concernant Indomeryx, nous suggérons une position basale de ce genre au sein des ruminants, proche d’Archaeomeryx, avec lequel il présente des caractères en communs. © 2000 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS Ruminants / Indomeryx / Éocène moyen / Myanmar
Version abrégée 1. Introduction La faune de Pondaung (Myanmar) est connue depuis le début du siècle par ses restes de Primates [1, 6, 11], artiodactyles et périssodactyles [9, 10]. La formation continentale de Pondaung est intercalée entre deux formations marines datées grâce aux microfossiles, les marnes lutétiennes de Tabyin et les argiles priabonniennes de
Yaw [5]. Une étude magnétostratigraphique est en cours pour préciser l’âge de cette formation. Des restes de ruminants ont été collectés dans quatre localités de la formation de Pondaung (figure 1), dans le même niveau d’argile de base que celui qui a livré les anthropoïdes Bahinia, Amphipithecus, et Pondaungia. Pilgrim [10] a reconnu deux espèces de ruminants (Indomeryx cotteri et I. arenae) dans la faune de Pondaung, sur la base d’un matériel très fragmentaire, mais Colbert [1] a considéré I. arenae comme synonyme de
* Correspondence and reprints: [email protected]
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I. cotteri. Qiu [12] étudia les ruminants des bassins de Baise et Yongle, et décrivit une nouvelle espèce, I. youjiangensis, mise en synonymie avec I. cotteri par Guo et al. [4]. Ces derniers, après un réexamen des ruminants paléogènes d’Asie, créèrent la nouvelle famille des Prodremotheridae (Pecora), incluant I. cotteri et résolvant le statut des problématiques Gelocidae [7]. Les deux nouvelles espèces I. pilgrimi nov. sp. (figure 2A) et I. minus nov. sp (figure 2B) décrites ici laissent entrevoir une radiation initiale des ruminants au cours de l’Éocène moyen en Asie.
par ses molaires inférieures au trigonide fermé, au pli Dorcatherium faible diminuant en importance de M1 à M3, au sillon sur la face mésiale de l’entoconide, et au faible cingulide labial. Diffère de I. cotteri par ses dimensions plus faibles, la présence d’un faible pli Dorcatherium, et d’un sillon sur la face mésiale de l’entoconide. Diffère de I. pilgrimi par l’absence de métaconide dupliqué par sa plus petite taille et par la présence d’un faible cingulide labial.
2. Systématique
Les molaires inférieures de I. pilgrimi nov. sp. (figures 2A–2B) diffèrent de celles de I. cotteri par leur plus grande taille, un reste de paraconide incomplètement fusionné avec le métaconide, la présence d’un sillon sur la face mésiale de l’entoconide, un pli Dorcatherium peu marqué parfois associé à un très faible métastylide, un fort post-cingulum sur M1 et M2 et par un émail légèrement plissé. Les molaires supérieures (figure 2C) possèdent un faible cingulum lingual entourant le protocône, mais non relié au post-cingulum. Un métastyle existe seulement sur M3. La dentition inférieure de I. minus nov. sp. (figures 2D–2E) se caractérise par le fort métaconide sur P4, situé presque au niveau du protoconide, avec lequel il est jumelé. Ses molaires inférieures possèdent un trigonide fermé, un faible pli Dorcatherium, un sillon sur la face mésiale de l’entoconide et un faible cingulide labial. I. minus diffère d’I. cotteri par sa plus petite taille, la présence d’un faible pli Dorcatherium et d’un sillon sur la face mésiale de l’entoconide. Il diffère, en outre, de I. pilgrimi par l’absence de métaconide dupliquée, par sa plus petite taille et par la présence d’un mince cingulide labial.
Ordre Artiodactyla Owen 1848 Infra-ordre Pecora Linnaeus 1777 Famille incertaine Indomeryx pilgrimi nov. sp. (figures 2A–2C) Holotype. Mandibule gauche fragmentaire avec M2–M3 (Bhn 911, National Museum of Myanmar, Yangon). Matériel additionnel. Maxillaire gauche avec M1–M3 (Mgg 2), maxillaire gauche avec M2–M3 (Mgg 14), mandibule droite avec M2–M3 (LK 6), M3 droite (Mgg 5). Tous les spécimens sont déposés au National Museum of Myanmar, Yangon. Horizon et localité. Yarshe Kyitchaung, base du niveau d’argile rouge, Bahin, formation de Pondaung. Âge. Fin de l’Éocène moyen. Étymologie. Espèce nommée en l’honneur de G.E. Pilgrim, pour son importante contribution paléontologique sur les mammifères fossiles du Myanmar. Diagnose. Ruminant primitif dont les molaires inférieures diffèrent de celles de I. cotteri par leur plus grande taille, la présence d’un reste de paraconide incomplètement fusionné au métaconide, d’un sillon sur la face mésiale de l’entoconide, d’un faible pli Dorcatherium parfois associé à un très faible métastylide, et un fort post-cingulide sur M1 et M2. L’émail est fortement ridulé. Les molaires supérieures montrent un faible cingulum entourant le protocône mais isolé du postcingulum, et un métastyle sur M3. Indomeryx minus nov.sp. (figures 2D–2F) Holotype. Mandibule gauche fragmentaire avec P4–M3 (Bhn 3, National Museum of Myanmar, Yangon). Matériel additionnel. Maxillaire droit avec M1–M3 (Bhn 1115), M3 gauche (Bhn 6), M3 droite (Bhn 4), maxillaire droit avec D3–D4 (LK 3). Tous les spécimens sont déposés au National Museum of Myanmar, Yangon. Horizon et localité. Yarshe Kyitchaung, base du niveau d’argile rouge, Bahin, formation de Pondaung. Âge. Fin de l’Éocène moyen. Étymologie. Espèce nommée en référence de sa petite taille comparée à celle des autres espèces d’Indomeryx. Diagnose. Petit ruminant primitif, caractérisé par sa P4 au fort métaconide lingual collé au protoconide, et
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3. Description et comparaison
4. Discussion et conclusions I. pilgrimi présente des caractères plus primitifs que I. cotteri, tels que le métaconide dupliqué, la présence d’un sillon sur la face mésiale de l’entoconide (« sillon entoconidien »). Le premier caractère est inconnu chez les ruminants éocènes, tandis que le second est signalé par Pilgrim [10] sur I. arenae, mis depuis en synonymie avec I. cotteri. Chez I. pilgrimi et I. minus, le sillon entoconidien diminue fortement avec l’usure, et sur leur M1 un faible entostylide se forme à la base de la crête mésio-linguale, partant de l’apex de l’entoconide. La question de l’importance phylogénétique d’un tel caractère est posée. Durant l’occlusion, la face mésiale de l’entoconide entre en contact avec la postprotocrista de la molaire supérieure correspondante. Comme chez la plupart des ruminants primitifs, Indomeryx possède une postprotocrista courte et orientée antéro-postérieurement. Or, ce sillon entoconidien, non signalé chez les autres ruminants éocènes présentant la même configuration de la postprotocrista, existe chez certains Prodremotheriidae et traguloïdes asiatiques (par exemple Lophiomerycidae, Métais et al., en préparation). Ce
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caractère disparaît chez les taxons oligocènes plus sélénodontes, bien qu’il soit absent à la fois chez Archaeomeryx et les taxons éocènes nord-américains (Hypertragulidae et Leptomerycidae). Le pli Dorcatherium est présent sur les deux nouvelles espèces d’Indomeryx, alors que Guo et al. [4] signalent plutôt un petit métastylide sur les spécimens chinois. De plus, Geraads et al. [3] suggèrent que le métastylide représente l’état dérivé du pli Dorcatherium, bien que ces deux structures semblent coexister sur certains Bachitherium [8]. Ce pli Dorcatherium, que l’on retrouve chez Iberomeryx et certains Lophiomeryx, n’est probablement pas homologue de celui des Tragulidae, à cause de la connection chez ces derniers de la protocristide linguale avec la préentocristide. De plus, chez les Tragulidae, cette structure est toujours associée avec une crête sur l’arrière du protoconide (pli Tragulus pour Geraads et al. [3]) formant avec le pli Dorcatherium un « M » sur l’arrière du trigonide. Le pli Dorcatherium est probablement un caractère plésiomorphique de ruminants qui disparaît avec l’accentuation de la sélénodontie [13]. Depuis sa découverte, la position phylogénétique d’Indomeryx est très débattue [10, 14, 15]. Récemment, Guo et al. [4] ont créé la nouvelle famille des Prodremotheridae, y incluant Indomeryx sur la base des caractères suivants : présence d’un métastylide, un double pli sur la face postérieure de l’entoconide et absence de pli Dorcatherium et de pli Palaeomeryx. Cependant, la présence d’un métastylide n’est pas claire sur Indomeryx. Guo et al. [4] ont d’ailleurs souligné que ce caractère variait d’un individu à l’autre. En outre, la P4 d’Indome-
ryx est morphologiquement moins dérivée que celle de Notomeryx et Prodremotherium (les prémolaires inférieures de Gobiomeryx ne sont pas connues), à cause de la présence des deux crêtes parallèles partant du protoconide et dirigées vers l’arrière chez Indomeryx. Cette morphologie rappelle plutôt celle de la P4 d’Archaeomeryx, le plus ancien ruminant connu de Shara Murun (Éocène moyen de Mongolie). Les autres Prodremotheridae (Prodremotherium, Notomeryx et Gobiomeryx) montrent une configuration plus avancée, avec un début de molarisation de la P4 avec de courtes crêtes d’émail partant du métaconide et de l’entoconide et orientées lingualement. Savage et al. (ms) ont signalé un métaconide jumelé sur certains spécimens de I. cotteri de Pondaung. I. pilgrimi possède un métaconide dupliqué que nous interprétons comme un reste de paraconide. Néanmoins, Archaeomeryx possède des molaires inférieures tétratuberculées comme la plupart des ruminants paléogènes, excepté Iberomeryx, qui montre un reste d’hypoconulide sur M1 et M2 [15]. Dans l’état actuel des connaissances concernant Indomeryx, nous préférons le considérer comme une forme basale parmi les ruminants (et non inclus dans les Prodremotheridae [Pecora]), plus primitif au niveau dentaire que Archaeomeryx. La grande diversité morphologique des ruminants non encore décrits à Pondaung semble indiquer une radiation très précoce de ce groupe en Asie du Sud-Est, au cours de l’Éocène moyen. Néanmoins, seule la récolte de matériel supplémentaire permettra de mieux comprendre l’évolution des premiers ruminants.
1. Introduction
mation. The type species has been collected near Sinzwe and Pangan. Pilgrim distinguished the two species among his very scanty material on the basis of their size and of their tooth structure. However, Colbert [1] considered these slight differences as intra-specific variation, and he regarded I. arenae as a synonym of I. cotteri. Qiu [12] studied ruminant material from Baise and Yongle Basin (China), and described a new species, I. youjiangensis. The recent reassesment of the specimens from Middle Eocene deposits of Baise and Yongle Basins led Guo et al. [4] to revise the diagnosis of Indomeryx in the light of new dental remains. All the specimens described until now and ascribed to Indomeryx were reported to the single species I. cotteri. Moreover, the reassessment of several Paleogene ruminants from Asia led them to create the new family Prodremotheridae including Indomeryx, resolving the problematic family of Gelocidae [7]. The validity of this new family is briefly discussed here.
The Pondaung fauna has long been known mainly for its primate [1, 6, 11], artiodactyl [10], and perissodactyl (Titanotheriidae and Amynodontidae) remains [9]. Pilgrim [10] suggested a Middle Eocene (Bartonian) age for this continental formation based on the numerous anthracothere remains. The Pondaung Formation is interbedded between two marine formations dated with microfossils, the Lutetian Tabyin Marl and the Priabonian(?) Yaw clay [5]. However, preliminary magnetostratigraphic investigations in different fossiliferous sections indicate a normal polarity that cannot be used yet to confirm the Bartonian stage. Further magnetostratigraphic investigations are in preparation by our team (unpublished data). Several ruminant remains have been collected from four localities in the Pondaung Formation (Yarshe Kyitchaung, Lema Kyichaung, Thandaung Kyitchaung, and Moggaung; figure 1). The fossils come from the base of the clay level, which also yielded the anthropoids Bahinia, Amphipithecus, and Pondaungia. Pilgrim [10] recognised two species of ruminants (Indomeryx cotteri and I. arenae) from the Pondaung For-
About a dozen of specimens have been collected in the Pondaung Formation by the Pondaung Fossil Expedition Team. The new remains include upper and lower teeth, and we describe here two new species of
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Figure 1. Location map of the different fossiliferous localities (Pondaung Formation) from where new ruminant dental remains have been collected. Figure 1. Carte des differentes localités fossilifères (Pondaung Formation) ayant livré les restes dentaires de ruminants.
Indomeryx. Moreover, the great diversity of dental remains from Pondaung seems to confirm that the initial radiation of ruminants very likely occurred in Asia during the Middle Eocene.
2. Systematic palaeontology Order Artiodactyla Owen 1848 Infra-Order Pecora Linnaeus 1777 Uncertain Family Indomeryx pilgrimi nov. sp. (figures 2A–2C)
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Holotype. Fragmentary left lower jaw preserving M2–M3 (Bhn 911, National Museum of Myanmar, Yangon). Referred material. Left maxilla with M1–M3 (Mgg 2), fragmentary left maxilla with M2-M3 (Mgg 14), right lower jaw with M2–M3 (LK 6), right M3 (Mgg 5). All specimens are housed in the National Museum of Myanmar, Yangon. Horizon and locality. Yarshe Kyitchaung, base of the reddish clay level, Bahin village, Pondaung Formation. Age. Late Middle Eocene. Etymology. Species named in honour of the late G.E. Pilgrim, for his great palaeontological contribution on fossil mammals from Myanmar.
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Figure 2. A. Indomeryx pilgrimi n. sp.: holotype Bhn 911, left M1–3. Occlusal view. B. Indomeryx pilgrimi n. sp.: holotype Bhn 911, left M1-3. Labial view. C. Indomeryx pilgrimi n. sp.: Mgg 2, left M1–M3. Occlusal view. D. Indomeryx minus n. sp.: holotype Bhn 3, left P4–M3. Occlusal view. E. Indomeryx minus n. sp.: holotype Bhn 3, left P4–M3. Labial view. F. Indomeryx minus n. sp.: Bhn 1115, right M1–M3. Occlusal view. Scale bar: 1 cm. Figure 2. A. Indomeryx pilgrimi n. sp. : holotype Bhn 911, M1–3 gauches. Vue occlusale. B. Indomeryx pilgrimi n. sp.: holotype Bhn 911, M1–3 gauches. Vue labiale. C. Indomeryx pilgrimi n. sp. : Mgg 2, Mgg 2, M1–M3 gauches. Vue occlusale. D. Indomeryx minus n. sp. : holotype Bhn 3, P4–M3 gauches.Vue occlusale. E. Indomeryx minus n. sp. : holotype Bhn 3, P4–M3 gauches. Vue labiale. F. Indomeryx minus n. sp. : Bhn 1115, M1–M3 gauches. Vue occlusale. Barre d’échelle : 1 cm.
Diagnosis. Primitive ruminant with lower molars differing from those of I. cotteri by their greater size, a remain of paraconid not completely fused with the metaconid, the presence of a groove on the mesial side of the entoconid, a weak Dorcatherium fold sometimes associated with a very weak metastylid, and by a strong postcingulum on M1 and M2. The enamel is slightly wrinkled. The upper molars display a weak cingulum surrounding the protocone but not connected to the postcingulum. A metastyle occurs only on M3. Description. The lower molars are bunoselenodont, rectangular in shape, and they increase in size from front to back. The molars have crescentic outer cusps and transversely compressed inner cusps. The metaconid and entoconid are slightly more mesially situated than the labial cusps. The anterior and posterior spurs of the molar protoconid connect to the metaconid which displays a weak Dorcatherium fold on its distal side, but no meta-
stylid on unworn teeth. On the mesiolabial side and just below the apex of the metaconid occurs the remain of a paraconid which is almost fully fused with the metaconid. A strong wear surface extends anteriorly from the apex of the paraconid to the base of the metaconid. This wear surface is well marked on M1 and M2, but apparently absent on the fresh M3, which is also broken in this area. The single cristid obliqua joins the base of the protoconid, whereas the posthypocristid reaches the postero-lingual corner of the tooth. The mesial side of the entoconid shows two sharp and parallel ridges which run from the apex to the base of this cusp, forming a marked and lingually open groove. The labial crest is stronger than the inner one which in advanced wear, displays an incipient entostylid on the mesio-lingual base of the entoconid. The posterior part of the entoconid is weakly depressed and is close to an incipient postentocristid. Two parallel crests run forward from the hypo-
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conulid of M3 and end in a single loop connected to the entoconid and hypoconid. A thick posterior cingulum occurs at the base of the crown, and the anterior cingulum is weakly developed except below the mesio-labial base of the protoconid. No labial cingulum can be observed (except between the outer cups). The enamel is wrinkled. The upper molars are brachyodont and quadritubercular and they increase in size from M1 to M3. The paracone displays a strong labial rib, whereas the metacone has a weak labial one on M1 and M2, but not on M3. M1 and M2 do not exhibit a metastyle, but a weak one occurs on M3. There is no trace of entostyle on the weak lingual cingulum surrounding the protocone. The enamel of all teeth is slightly wrinkled.
parallel crests that link to the entoconid and the hypoconid. A thin postcingulum extends between the hypoconid and hypoconulid. The precingulum and postcingulum are linked by a weak labial cingulum. There is no ectostylid between the protoconid and the hypoconid. The upper molars do not display a metastyle, except M3 which has a weak one. The paracone has a labial rib, whereas the labial side of the metacone is flattened. There is no lingual cingulum although an entostyle emerges between the protocone and the metaconule. The enamel is smooth on lower and upper molars.
Indomeryx minus nov. sp. (figures 2D–2F) Holotype. Fragmentary left lower jaw preserving P4-M3 (Bhn 3, National Museum of Myanmar, Yangon). Referred material. Right maxilla with M1–M3 (Bhn 1115), left M3 (Bhn 6), right M3 (Bhn 4), fragmentary right maxilla with D3–D4 (LK 3). All specimens are housed in the National Museum of Myanmar, Yangon. Horizon and locality. Yarshe Kyitchaung, base of the reddish clay level, Bahin village, Pondaung Formation. Age. Late Middle Eocene. Etymology. Species named in reference of its small size compared to other species of Indomeryx. Diagnosis. Primitive small ruminant characterized by its P4 with strong and lingually situated metaconid twinned with protoconid, and by its lower molars with closed trigonid, a weak Dorcatherium fold becoming weaker from M1 to M3, a groove on the mesial side of the entoconid, and a weak labial cingulum. Differs from I. cotteri by its smaller size, the presence of a weak Dorcatherium fold and of a groove on the mesial side of the entoconid. Differs from I. pilgrimi by the lack of duplicated metaconid, its smallest size, and by the presence of a thin labial cingulum. Description. On the type specimen, the lower molars are moderately worn. The P4 is nearly as long as M1 and it is characterized by its bulbous metaconid twinned with the largest protoconid, and slightly posterolingual to the apex of the protoconid. A wear surface extends forwardly from the protoconid to a strong parastylid, and two prominent crests run steeply down to the rear to form a basin-shaped heel. The lower molars increase in size from front to back. The labial cusps of the molars are crescentic, whereas the lingual ones are laterally slightly compressed. The protoconid does not display a Palaeomeryx fold, but its anterior and posterior crests join the metaconid. The metaconid exhibits a single premetacristd and a thin Dorcatherium fold, but no trace of metastylid. The entoconid is more mesial than the hypoconid, and a groove occurs on its mesial side. The prehypocristid joins the base of the protoconid, whereas the posthypocristid does not reach the postero-lingual corner of the tooth. The hypoconulid of M3 sends two
Our knowledge on the genus Indomeryx was until recently very poor, because the type of Indomeryx cotteri consists of a very fragmentary (probably juvenile) lower jaw with P4–M3, and that of Indomeryx arenae is only defined on the posterior half of a M3. Although I. pilgrimi displays affinities with I. cotteri (general morphology and similar size), it shares several primitive features which are absent on the material from Guangxi referred to I. cotteri [4] such as the duplicated metaconid or the groove on the mesial face of the entoconid. The former character is unknown among Eocene ruminants, whereas the second feature was pointed out by Pilgrim [10] for the specimen G.S.I B769 which was referred to I. arenae. The groove on the mesial side of the entoconid tends to become weaker when the wear of the tooth increases. Indeed, on Bhn 911 (figures 2A–2B), this structure is well marked on the fresh M3, whereas it is almost absent on M1, on which a weak entostylid occurs at the base of the lingual crest that runs down from the apex of the entoconid. Although this groove is less marked on M1, it is always present, and probably corresponds to a biomechanical chewing artefact. However the phylogenetical implication of such a character may be questioned. During occlusion, the mesial side of the entoconid occludes with the postprotocrista of the corresponding upper molar. In Indomeryx, the postprotocrista is short and anteroposteriorly oriented, as in most of primitive ruminants. On Mgg 2 (figure 2C), the protocone is strongly worn on M1 in comparison to other cusps. As the wear increases, both parallel crests extending on the mesial side of the entoconid tend to disappear to form a small and incipient entostylid. However, this ‘entoconidian groove’ is not present in all ruminants which exhibit the same postprotocrista configuration. Also, we may consider this character as a symplesiomorphy occurring both in Indomeryx and some Asian traguloids (e.g Lophiomerycidae, Métais et al. in preparation). This character seems to disappear in more selenodont Oligocene taxa, although it is absent in Archaeomeryx and both Eocene North American families (Hypertragulidae and Leptomerycidae). As noted above, both new species of Indomeryx display a weak Dorcatherium fold on the distal face of the
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metaconid. Guo et al. [4] did not observe this feature on the revised material of I. cotteri, but rather an incipient metastylid. Geraads et al. [3] suggested that the metastylid might be derived from the Dorcatherium fold. Both metastylid and Dorcatherium fold may coexist, as noted by Moya Sola [8], on some specimens of Bachitherium. The Dorcatherium fold occurs in primitive ruminants like some primitive form of Lophiomeryx [2] and Oligocene Iberomeryx, but it is probably not homologous with that of Tragulidae, because of the connection of the lingual protocristid with the preentocristid in the Tragulidae. Moreover, that structure is always associated in Tragulidae with a protoconid crest connected to the prehypocristid (Tragulus fold for Geraads et al. [3]), forming a ‘M’ on the posterior face of the trigonid. As suggested by Scott and Janis [13], the Dorcatherium fold is probably a plesiomorphic character of ruminants. Its loss might be, as for the ‘entoconidian groove’, a consequence of increased selenodonty, although it seems to be present on some individuals of the bovid genus Cephalophus [7]. The phylogenetic status of Indomeryx has long been debated, its affinities having been thought to lie with Tragulidae [10], Hypertragulidae [1, 12, 16], or Gelocidae [14, 15]. Recently, however, the discovery of new remains of Indomeryx led Guo et al. [4] to erect the new ruminant family Prodremotheridae based on presence of metastylid, a double fold at the rear of the entoconid, and neither Dorcatherium fold nor Palaeomeryx fold. However, the presence of a metastylid is not clear on Indomeryx. Guo et al. [4] claimed that the degree of development of this feature varies in different individuals. Besides, the P4 of Indomeryx is morphologically less
derived than that of Notomeryx and Prodremotherium (lower premolars of Gobiomeryx are unknown), because both parallel crests backwardly extending from the apex of protoconid occur only in Indomeryx. This morphology rather recalls that of the P4 of Archaeomeryx, the oldest known ruminant from the Middle Eocene of Shara Murun (Mongolia). The other genus included in Prodremotheridae (Prodremotherium, Notomeryx and Gobiomeryx) displays a more advanced configuration with the beginning of the molarization of P4 with short enamel folds present on the lingual side of the metaconid and entoconid. Moreover, in their unpublished work on Paleogene ruminants of China, Savage et al. pointed out a twinned metaconid in some material of I. cotteri from Pondaung. I. pilgrimi possesses a duplicated metaconid that we interpret as the remains of a paraconid. However, Archaeomeryx displays tetratuberculate lower molars like most of Paleogene ruminants, except Iberomeryx that exhibits a remnant of hypoconulid on M1–2 [15]. Although the scarcity of remains does not allow us to definitively conclude on the systematic position of Indomeryx, it seems inappropriate to include that genus in the Pecoran family Prodremotheridae. At the moment, we prefer to consider Indomeryx as a basal form among ruminants, more primitive in its dental morphology than the Middle Eocene genus Archaeomeryx. The apparent great diversity of undescribed ruminants remains in Pondaung seems to indicate an early radiation of ruminants in South East Asia during the Middle Eocene. Nevertheless, additional material is now urgently needed in order to better understand the origin and the early evolution of hornless ruminants.
Acknowledgements. We wish to thank Dr. J. Sudre for his helpful comments, Dr. D.E Russell for providing us with casts, and the Foundation Singer-Polignac for financial support. Photographs are from M. Pons. Publication ISEM No. 2000-054.
References [1] Colbert E.H., Fossil mammals from Burma in the American Museum of Natural History, Bull. Am. Mus. Nat. Hist. 74 (1938) 255–436. [2] Fejfar O., A Lower Oligocene mammalian fauna from Detan, northwest Bohemia, Czechoslovakia, in: Proc. International Paleogene Symposium, Mainz, 1987. [3] Geraads D., Bouvrain G., Sudre J., Relations phylétiques de Bachitherium Filhol 1882, ruminant de l’Oligocène d’Europe occidentale, Palaeovertebrata 17 (1987) 43–73. [4] Guo J.W., Qi T., Sheng H.J., A restudy of the Eocene ruminants from Baise and Yongle Basins, Guangxi, China, with a discussion of the systematic positions of Indomeryx, Notomeryx, Gobiomeryx and Prodremotherium, Vert. Pal. Asiat. 37 (1999) 18–39. [5] Holroyd P.A., Ciochon R.L., Relative ages of Eocene Primatesbearing deposits of Asia, in: Fleagle J.G., Kay R.F. (Eds.), Anthropoid Origins, Plenum Press, New York, 1994, pp. 123–141. [6] Jaeger J.J., Tin Thein, Benammi M., Chaimanee Y., Aung Naing Soe, Thit Lwin, Than Tun, San Wai, Ducrocq S., A new Primate from the Middle Eocene of Myanmar and the Asian Early Origin of Anthropoids, Science 286 (1999) 528–530. [7] Janis C.M., Grades and clades in hornless ruminant evolution:
the reality of the Gelocidae and the systematic position of Lophiomeryx and Bachitherium, J. Vert Paleontol. 7 (1987) 200–216. [8] Moya-Sola S., Morphology of lower molars of the ruminants (Artiodactyla, Mammalia): phylogenetic implications, Paleont. Evol. 22 (1988) 61–70. [9] Pilgrim G.E., The Perissodactyla of the Eocene of Burma, Palaeont. Indica 8 (1925) 1–28. [10] Pilgrim G.E., The Artiodactyla of the Eocene of Burma, Palaeont. Indica 134 (1928) 1–39. [11] Pilgrim G.E., Cotter G. de P., Some newly discovered Eocene mammals from Burma, Rec. Geol. Surv. India 47 (1915) 42–77. [12] Qiu Z., Late Eocene Hypertragulids of Baise Basin, Kwangsi. Vert. Pal. Asiat. 16 (1978) 7–12. [13] Scott K.M., Janis C.M., Relationships of the Ruminantia (Artiodactyla) and an analysis of the characters used in ruminant taxonomy, in: Szalay F.S., Novacek M.J., McKenna M.C. (Eds.), Mammal Phylogeny, Placentals, 2, Springer Verlag, New York, 1993, pp. 282–302. [14] Simpson G.G., The principles of classification and a classification of mammals, Bull. Am. Mus. Nat. Hist. 85 (1945) 1–350. [15] Sudre J., Cryptomeryx (Schlosser 1886), Tragulidé de l’Oligocène d’Europe ; relation du genre et considérations sur l’origine des ruminants, Palaeovertebrata (1984) 1–22. [16] Xu Q., Paleogene climate change and mammalian fauna in Bose Basin of Guangxi, Vert. Pal. Asiat. 20 (1982) 327–336.
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Palaeontology / Paléontologie (Vertebrate Palaeontology / Paléontologie des Vertébrés)
Discovery of new ruminant dental remains from the Middle Eocene Pondaung Formation (Myanmar): reassessment of the phylogenetic position of Indomeryx Grégoire Métaisa*, Mouloud Benammia, Yaowalak Chaimaneeb, Jean-Jacques Jaegera, Than Tunc, Tin Theind, Stéphane Ducrocqa a Institut des sciences de l’évolution, UMR 5554 CNRS, université Montpellier-2, place Eugène-Bataillon, 34095 Montpellier cedex 05, France b Department of Mineral Ressources, Geological Survey Division, Paleontological Section, Rama VI Road, Bangkok 10400, Thailand c Office of Strategic Studies, Ministry of Defence, Yangon, Myanmar d Department of Geology, University of Pathein, Myanmar
Received 20 February 2000; accepted 17 April 2000 Communicated by Yves Coppens
Abstract – Two new species of Indomeryx are described from the Late Middle Eocene Pondaung Formation, central Myanmar. These two species differ from I. cotteri by their more primitive dental features, thus challenging the inclusion of Indomeryx in the new Pecora family Prodremotheridae. Despite poor knowledge of this genus, we rather suggest a basal position within ruminantia, close to Archaeomeryx with which it displays several affinities. © 2000 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS Ruminants / Indomeryx / Middle Eocene / Myanmar
Résumé – Découverte de nouveaux restes dentaires de ruminants dans l’Eocène moyen de la formation de Pondaung (Myanmar) : réexamen de la position phylogénétique de Indomeryx. Deux nouvelles espèces, appartenant au genre Indomeryx et provenant de la formation de Pondaung (fin de l’Éocène moyen), sont décrites. Ces deux espèces se distinguent d’Indomeryx cotteri par leurs caractères primitifs, et remettent ainsi en cause l’inclusion de ce genre au sein des Prodremotheridae (Pecora). En dépit des connaissances fragmentaires concernant Indomeryx, nous suggérons une position basale de ce genre au sein des ruminants, proche d’Archaeomeryx, avec lequel il présente des caractères en communs. © 2000 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS Ruminants / Indomeryx / Éocène moyen / Myanmar
Version abrégée 1. Introduction La faune de Pondaung (Myanmar) est connue depuis le début du siècle par ses restes de Primates [1, 6, 11], artiodactyles et périssodactyles [9, 10]. La formation continentale de Pondaung est intercalée entre deux formations marines datées grâce aux microfossiles, les marnes lutétiennes de Tabyin et les argiles priabonniennes de
Yaw [5]. Une étude magnétostratigraphique est en cours pour préciser l’âge de cette formation. Des restes de ruminants ont été collectés dans quatre localités de la formation de Pondaung (figure 1), dans le même niveau d’argile de base que celui qui a livré les anthropoïdes Bahinia, Amphipithecus, et Pondaungia. Pilgrim [10] a reconnu deux espèces de ruminants (Indomeryx cotteri et I. arenae) dans la faune de Pondaung, sur la base d’un matériel très fragmentaire, mais Colbert [1] a considéré I. arenae comme synonyme de
* Correspondence and reprints: [email protected]
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I. cotteri. Qiu [12] étudia les ruminants des bassins de Baise et Yongle, et décrivit une nouvelle espèce, I. youjiangensis, mise en synonymie avec I. cotteri par Guo et al. [4]. Ces derniers, après un réexamen des ruminants paléogènes d’Asie, créèrent la nouvelle famille des Prodremotheridae (Pecora), incluant I. cotteri et résolvant le statut des problématiques Gelocidae [7]. Les deux nouvelles espèces I. pilgrimi nov. sp. (figure 2A) et I. minus nov. sp (figure 2B) décrites ici laissent entrevoir une radiation initiale des ruminants au cours de l’Éocène moyen en Asie.
par ses molaires inférieures au trigonide fermé, au pli Dorcatherium faible diminuant en importance de M1 à M3, au sillon sur la face mésiale de l’entoconide, et au faible cingulide labial. Diffère de I. cotteri par ses dimensions plus faibles, la présence d’un faible pli Dorcatherium, et d’un sillon sur la face mésiale de l’entoconide. Diffère de I. pilgrimi par l’absence de métaconide dupliqué par sa plus petite taille et par la présence d’un faible cingulide labial.
2. Systématique
Les molaires inférieures de I. pilgrimi nov. sp. (figures 2A–2B) diffèrent de celles de I. cotteri par leur plus grande taille, un reste de paraconide incomplètement fusionné avec le métaconide, la présence d’un sillon sur la face mésiale de l’entoconide, un pli Dorcatherium peu marqué parfois associé à un très faible métastylide, un fort post-cingulum sur M1 et M2 et par un émail légèrement plissé. Les molaires supérieures (figure 2C) possèdent un faible cingulum lingual entourant le protocône, mais non relié au post-cingulum. Un métastyle existe seulement sur M3. La dentition inférieure de I. minus nov. sp. (figures 2D–2E) se caractérise par le fort métaconide sur P4, situé presque au niveau du protoconide, avec lequel il est jumelé. Ses molaires inférieures possèdent un trigonide fermé, un faible pli Dorcatherium, un sillon sur la face mésiale de l’entoconide et un faible cingulide labial. I. minus diffère d’I. cotteri par sa plus petite taille, la présence d’un faible pli Dorcatherium et d’un sillon sur la face mésiale de l’entoconide. Il diffère, en outre, de I. pilgrimi par l’absence de métaconide dupliquée, par sa plus petite taille et par la présence d’un mince cingulide labial.
Ordre Artiodactyla Owen 1848 Infra-ordre Pecora Linnaeus 1777 Famille incertaine Indomeryx pilgrimi nov. sp. (figures 2A–2C) Holotype. Mandibule gauche fragmentaire avec M2–M3 (Bhn 911, National Museum of Myanmar, Yangon). Matériel additionnel. Maxillaire gauche avec M1–M3 (Mgg 2), maxillaire gauche avec M2–M3 (Mgg 14), mandibule droite avec M2–M3 (LK 6), M3 droite (Mgg 5). Tous les spécimens sont déposés au National Museum of Myanmar, Yangon. Horizon et localité. Yarshe Kyitchaung, base du niveau d’argile rouge, Bahin, formation de Pondaung. Âge. Fin de l’Éocène moyen. Étymologie. Espèce nommée en l’honneur de G.E. Pilgrim, pour son importante contribution paléontologique sur les mammifères fossiles du Myanmar. Diagnose. Ruminant primitif dont les molaires inférieures diffèrent de celles de I. cotteri par leur plus grande taille, la présence d’un reste de paraconide incomplètement fusionné au métaconide, d’un sillon sur la face mésiale de l’entoconide, d’un faible pli Dorcatherium parfois associé à un très faible métastylide, et un fort post-cingulide sur M1 et M2. L’émail est fortement ridulé. Les molaires supérieures montrent un faible cingulum entourant le protocône mais isolé du postcingulum, et un métastyle sur M3. Indomeryx minus nov.sp. (figures 2D–2F) Holotype. Mandibule gauche fragmentaire avec P4–M3 (Bhn 3, National Museum of Myanmar, Yangon). Matériel additionnel. Maxillaire droit avec M1–M3 (Bhn 1115), M3 gauche (Bhn 6), M3 droite (Bhn 4), maxillaire droit avec D3–D4 (LK 3). Tous les spécimens sont déposés au National Museum of Myanmar, Yangon. Horizon et localité. Yarshe Kyitchaung, base du niveau d’argile rouge, Bahin, formation de Pondaung. Âge. Fin de l’Éocène moyen. Étymologie. Espèce nommée en référence de sa petite taille comparée à celle des autres espèces d’Indomeryx. Diagnose. Petit ruminant primitif, caractérisé par sa P4 au fort métaconide lingual collé au protoconide, et
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3. Description et comparaison
4. Discussion et conclusions I. pilgrimi présente des caractères plus primitifs que I. cotteri, tels que le métaconide dupliqué, la présence d’un sillon sur la face mésiale de l’entoconide (« sillon entoconidien »). Le premier caractère est inconnu chez les ruminants éocènes, tandis que le second est signalé par Pilgrim [10] sur I. arenae, mis depuis en synonymie avec I. cotteri. Chez I. pilgrimi et I. minus, le sillon entoconidien diminue fortement avec l’usure, et sur leur M1 un faible entostylide se forme à la base de la crête mésio-linguale, partant de l’apex de l’entoconide. La question de l’importance phylogénétique d’un tel caractère est posée. Durant l’occlusion, la face mésiale de l’entoconide entre en contact avec la postprotocrista de la molaire supérieure correspondante. Comme chez la plupart des ruminants primitifs, Indomeryx possède une postprotocrista courte et orientée antéro-postérieurement. Or, ce sillon entoconidien, non signalé chez les autres ruminants éocènes présentant la même configuration de la postprotocrista, existe chez certains Prodremotheriidae et traguloïdes asiatiques (par exemple Lophiomerycidae, Métais et al., en préparation). Ce
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caractère disparaît chez les taxons oligocènes plus sélénodontes, bien qu’il soit absent à la fois chez Archaeomeryx et les taxons éocènes nord-américains (Hypertragulidae et Leptomerycidae). Le pli Dorcatherium est présent sur les deux nouvelles espèces d’Indomeryx, alors que Guo et al. [4] signalent plutôt un petit métastylide sur les spécimens chinois. De plus, Geraads et al. [3] suggèrent que le métastylide représente l’état dérivé du pli Dorcatherium, bien que ces deux structures semblent coexister sur certains Bachitherium [8]. Ce pli Dorcatherium, que l’on retrouve chez Iberomeryx et certains Lophiomeryx, n’est probablement pas homologue de celui des Tragulidae, à cause de la connection chez ces derniers de la protocristide linguale avec la préentocristide. De plus, chez les Tragulidae, cette structure est toujours associée avec une crête sur l’arrière du protoconide (pli Tragulus pour Geraads et al. [3]) formant avec le pli Dorcatherium un « M » sur l’arrière du trigonide. Le pli Dorcatherium est probablement un caractère plésiomorphique de ruminants qui disparaît avec l’accentuation de la sélénodontie [13]. Depuis sa découverte, la position phylogénétique d’Indomeryx est très débattue [10, 14, 15]. Récemment, Guo et al. [4] ont créé la nouvelle famille des Prodremotheridae, y incluant Indomeryx sur la base des caractères suivants : présence d’un métastylide, un double pli sur la face postérieure de l’entoconide et absence de pli Dorcatherium et de pli Palaeomeryx. Cependant, la présence d’un métastylide n’est pas claire sur Indomeryx. Guo et al. [4] ont d’ailleurs souligné que ce caractère variait d’un individu à l’autre. En outre, la P4 d’Indome-
ryx est morphologiquement moins dérivée que celle de Notomeryx et Prodremotherium (les prémolaires inférieures de Gobiomeryx ne sont pas connues), à cause de la présence des deux crêtes parallèles partant du protoconide et dirigées vers l’arrière chez Indomeryx. Cette morphologie rappelle plutôt celle de la P4 d’Archaeomeryx, le plus ancien ruminant connu de Shara Murun (Éocène moyen de Mongolie). Les autres Prodremotheridae (Prodremotherium, Notomeryx et Gobiomeryx) montrent une configuration plus avancée, avec un début de molarisation de la P4 avec de courtes crêtes d’émail partant du métaconide et de l’entoconide et orientées lingualement. Savage et al. (ms) ont signalé un métaconide jumelé sur certains spécimens de I. cotteri de Pondaung. I. pilgrimi possède un métaconide dupliqué que nous interprétons comme un reste de paraconide. Néanmoins, Archaeomeryx possède des molaires inférieures tétratuberculées comme la plupart des ruminants paléogènes, excepté Iberomeryx, qui montre un reste d’hypoconulide sur M1 et M2 [15]. Dans l’état actuel des connaissances concernant Indomeryx, nous préférons le considérer comme une forme basale parmi les ruminants (et non inclus dans les Prodremotheridae [Pecora]), plus primitif au niveau dentaire que Archaeomeryx. La grande diversité morphologique des ruminants non encore décrits à Pondaung semble indiquer une radiation très précoce de ce groupe en Asie du Sud-Est, au cours de l’Éocène moyen. Néanmoins, seule la récolte de matériel supplémentaire permettra de mieux comprendre l’évolution des premiers ruminants.
1. Introduction
mation. The type species has been collected near Sinzwe and Pangan. Pilgrim distinguished the two species among his very scanty material on the basis of their size and of their tooth structure. However, Colbert [1] considered these slight differences as intra-specific variation, and he regarded I. arenae as a synonym of I. cotteri. Qiu [12] studied ruminant material from Baise and Yongle Basin (China), and described a new species, I. youjiangensis. The recent reassesment of the specimens from Middle Eocene deposits of Baise and Yongle Basins led Guo et al. [4] to revise the diagnosis of Indomeryx in the light of new dental remains. All the specimens described until now and ascribed to Indomeryx were reported to the single species I. cotteri. Moreover, the reassessment of several Paleogene ruminants from Asia led them to create the new family Prodremotheridae including Indomeryx, resolving the problematic family of Gelocidae [7]. The validity of this new family is briefly discussed here.
The Pondaung fauna has long been known mainly for its primate [1, 6, 11], artiodactyl [10], and perissodactyl (Titanotheriidae and Amynodontidae) remains [9]. Pilgrim [10] suggested a Middle Eocene (Bartonian) age for this continental formation based on the numerous anthracothere remains. The Pondaung Formation is interbedded between two marine formations dated with microfossils, the Lutetian Tabyin Marl and the Priabonian(?) Yaw clay [5]. However, preliminary magnetostratigraphic investigations in different fossiliferous sections indicate a normal polarity that cannot be used yet to confirm the Bartonian stage. Further magnetostratigraphic investigations are in preparation by our team (unpublished data). Several ruminant remains have been collected from four localities in the Pondaung Formation (Yarshe Kyitchaung, Lema Kyichaung, Thandaung Kyitchaung, and Moggaung; figure 1). The fossils come from the base of the clay level, which also yielded the anthropoids Bahinia, Amphipithecus, and Pondaungia. Pilgrim [10] recognised two species of ruminants (Indomeryx cotteri and I. arenae) from the Pondaung For-
About a dozen of specimens have been collected in the Pondaung Formation by the Pondaung Fossil Expedition Team. The new remains include upper and lower teeth, and we describe here two new species of
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Figure 1. Location map of the different fossiliferous localities (Pondaung Formation) from where new ruminant dental remains have been collected. Figure 1. Carte des differentes localités fossilifères (Pondaung Formation) ayant livré les restes dentaires de ruminants.
Indomeryx. Moreover, the great diversity of dental remains from Pondaung seems to confirm that the initial radiation of ruminants very likely occurred in Asia during the Middle Eocene.
2. Systematic palaeontology Order Artiodactyla Owen 1848 Infra-Order Pecora Linnaeus 1777 Uncertain Family Indomeryx pilgrimi nov. sp. (figures 2A–2C)
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Holotype. Fragmentary left lower jaw preserving M2–M3 (Bhn 911, National Museum of Myanmar, Yangon). Referred material. Left maxilla with M1–M3 (Mgg 2), fragmentary left maxilla with M2-M3 (Mgg 14), right lower jaw with M2–M3 (LK 6), right M3 (Mgg 5). All specimens are housed in the National Museum of Myanmar, Yangon. Horizon and locality. Yarshe Kyitchaung, base of the reddish clay level, Bahin village, Pondaung Formation. Age. Late Middle Eocene. Etymology. Species named in honour of the late G.E. Pilgrim, for his great palaeontological contribution on fossil mammals from Myanmar.
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Figure 2. A. Indomeryx pilgrimi n. sp.: holotype Bhn 911, left M1–3. Occlusal view. B. Indomeryx pilgrimi n. sp.: holotype Bhn 911, left M1-3. Labial view. C. Indomeryx pilgrimi n. sp.: Mgg 2, left M1–M3. Occlusal view. D. Indomeryx minus n. sp.: holotype Bhn 3, left P4–M3. Occlusal view. E. Indomeryx minus n. sp.: holotype Bhn 3, left P4–M3. Labial view. F. Indomeryx minus n. sp.: Bhn 1115, right M1–M3. Occlusal view. Scale bar: 1 cm. Figure 2. A. Indomeryx pilgrimi n. sp. : holotype Bhn 911, M1–3 gauches. Vue occlusale. B. Indomeryx pilgrimi n. sp.: holotype Bhn 911, M1–3 gauches. Vue labiale. C. Indomeryx pilgrimi n. sp. : Mgg 2, Mgg 2, M1–M3 gauches. Vue occlusale. D. Indomeryx minus n. sp. : holotype Bhn 3, P4–M3 gauches.Vue occlusale. E. Indomeryx minus n. sp. : holotype Bhn 3, P4–M3 gauches. Vue labiale. F. Indomeryx minus n. sp. : Bhn 1115, M1–M3 gauches. Vue occlusale. Barre d’échelle : 1 cm.
Diagnosis. Primitive ruminant with lower molars differing from those of I. cotteri by their greater size, a remain of paraconid not completely fused with the metaconid, the presence of a groove on the mesial side of the entoconid, a weak Dorcatherium fold sometimes associated with a very weak metastylid, and by a strong postcingulum on M1 and M2. The enamel is slightly wrinkled. The upper molars display a weak cingulum surrounding the protocone but not connected to the postcingulum. A metastyle occurs only on M3. Description. The lower molars are bunoselenodont, rectangular in shape, and they increase in size from front to back. The molars have crescentic outer cusps and transversely compressed inner cusps. The metaconid and entoconid are slightly more mesially situated than the labial cusps. The anterior and posterior spurs of the molar protoconid connect to the metaconid which displays a weak Dorcatherium fold on its distal side, but no meta-
stylid on unworn teeth. On the mesiolabial side and just below the apex of the metaconid occurs the remain of a paraconid which is almost fully fused with the metaconid. A strong wear surface extends anteriorly from the apex of the paraconid to the base of the metaconid. This wear surface is well marked on M1 and M2, but apparently absent on the fresh M3, which is also broken in this area. The single cristid obliqua joins the base of the protoconid, whereas the posthypocristid reaches the postero-lingual corner of the tooth. The mesial side of the entoconid shows two sharp and parallel ridges which run from the apex to the base of this cusp, forming a marked and lingually open groove. The labial crest is stronger than the inner one which in advanced wear, displays an incipient entostylid on the mesio-lingual base of the entoconid. The posterior part of the entoconid is weakly depressed and is close to an incipient postentocristid. Two parallel crests run forward from the hypo-
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conulid of M3 and end in a single loop connected to the entoconid and hypoconid. A thick posterior cingulum occurs at the base of the crown, and the anterior cingulum is weakly developed except below the mesio-labial base of the protoconid. No labial cingulum can be observed (except between the outer cups). The enamel is wrinkled. The upper molars are brachyodont and quadritubercular and they increase in size from M1 to M3. The paracone displays a strong labial rib, whereas the metacone has a weak labial one on M1 and M2, but not on M3. M1 and M2 do not exhibit a metastyle, but a weak one occurs on M3. There is no trace of entostyle on the weak lingual cingulum surrounding the protocone. The enamel of all teeth is slightly wrinkled.
parallel crests that link to the entoconid and the hypoconid. A thin postcingulum extends between the hypoconid and hypoconulid. The precingulum and postcingulum are linked by a weak labial cingulum. There is no ectostylid between the protoconid and the hypoconid. The upper molars do not display a metastyle, except M3 which has a weak one. The paracone has a labial rib, whereas the labial side of the metacone is flattened. There is no lingual cingulum although an entostyle emerges between the protocone and the metaconule. The enamel is smooth on lower and upper molars.
Indomeryx minus nov. sp. (figures 2D–2F) Holotype. Fragmentary left lower jaw preserving P4-M3 (Bhn 3, National Museum of Myanmar, Yangon). Referred material. Right maxilla with M1–M3 (Bhn 1115), left M3 (Bhn 6), right M3 (Bhn 4), fragmentary right maxilla with D3–D4 (LK 3). All specimens are housed in the National Museum of Myanmar, Yangon. Horizon and locality. Yarshe Kyitchaung, base of the reddish clay level, Bahin village, Pondaung Formation. Age. Late Middle Eocene. Etymology. Species named in reference of its small size compared to other species of Indomeryx. Diagnosis. Primitive small ruminant characterized by its P4 with strong and lingually situated metaconid twinned with protoconid, and by its lower molars with closed trigonid, a weak Dorcatherium fold becoming weaker from M1 to M3, a groove on the mesial side of the entoconid, and a weak labial cingulum. Differs from I. cotteri by its smaller size, the presence of a weak Dorcatherium fold and of a groove on the mesial side of the entoconid. Differs from I. pilgrimi by the lack of duplicated metaconid, its smallest size, and by the presence of a thin labial cingulum. Description. On the type specimen, the lower molars are moderately worn. The P4 is nearly as long as M1 and it is characterized by its bulbous metaconid twinned with the largest protoconid, and slightly posterolingual to the apex of the protoconid. A wear surface extends forwardly from the protoconid to a strong parastylid, and two prominent crests run steeply down to the rear to form a basin-shaped heel. The lower molars increase in size from front to back. The labial cusps of the molars are crescentic, whereas the lingual ones are laterally slightly compressed. The protoconid does not display a Palaeomeryx fold, but its anterior and posterior crests join the metaconid. The metaconid exhibits a single premetacristd and a thin Dorcatherium fold, but no trace of metastylid. The entoconid is more mesial than the hypoconid, and a groove occurs on its mesial side. The prehypocristid joins the base of the protoconid, whereas the posthypocristid does not reach the postero-lingual corner of the tooth. The hypoconulid of M3 sends two
Our knowledge on the genus Indomeryx was until recently very poor, because the type of Indomeryx cotteri consists of a very fragmentary (probably juvenile) lower jaw with P4–M3, and that of Indomeryx arenae is only defined on the posterior half of a M3. Although I. pilgrimi displays affinities with I. cotteri (general morphology and similar size), it shares several primitive features which are absent on the material from Guangxi referred to I. cotteri [4] such as the duplicated metaconid or the groove on the mesial face of the entoconid. The former character is unknown among Eocene ruminants, whereas the second feature was pointed out by Pilgrim [10] for the specimen G.S.I B769 which was referred to I. arenae. The groove on the mesial side of the entoconid tends to become weaker when the wear of the tooth increases. Indeed, on Bhn 911 (figures 2A–2B), this structure is well marked on the fresh M3, whereas it is almost absent on M1, on which a weak entostylid occurs at the base of the lingual crest that runs down from the apex of the entoconid. Although this groove is less marked on M1, it is always present, and probably corresponds to a biomechanical chewing artefact. However the phylogenetical implication of such a character may be questioned. During occlusion, the mesial side of the entoconid occludes with the postprotocrista of the corresponding upper molar. In Indomeryx, the postprotocrista is short and anteroposteriorly oriented, as in most of primitive ruminants. On Mgg 2 (figure 2C), the protocone is strongly worn on M1 in comparison to other cusps. As the wear increases, both parallel crests extending on the mesial side of the entoconid tend to disappear to form a small and incipient entostylid. However, this ‘entoconidian groove’ is not present in all ruminants which exhibit the same postprotocrista configuration. Also, we may consider this character as a symplesiomorphy occurring both in Indomeryx and some Asian traguloids (e.g Lophiomerycidae, Métais et al. in preparation). This character seems to disappear in more selenodont Oligocene taxa, although it is absent in Archaeomeryx and both Eocene North American families (Hypertragulidae and Leptomerycidae). As noted above, both new species of Indomeryx display a weak Dorcatherium fold on the distal face of the
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3. Discussion and conclusions
G. Métais et al. / C. R. Acad. Sci. Paris, Sciences de la Terre et des planètes / Earth and Planetary Sciences 330 (2000) 805–811
metaconid. Guo et al. [4] did not observe this feature on the revised material of I. cotteri, but rather an incipient metastylid. Geraads et al. [3] suggested that the metastylid might be derived from the Dorcatherium fold. Both metastylid and Dorcatherium fold may coexist, as noted by Moya Sola [8], on some specimens of Bachitherium. The Dorcatherium fold occurs in primitive ruminants like some primitive form of Lophiomeryx [2] and Oligocene Iberomeryx, but it is probably not homologous with that of Tragulidae, because of the connection of the lingual protocristid with the preentocristid in the Tragulidae. Moreover, that structure is always associated in Tragulidae with a protoconid crest connected to the prehypocristid (Tragulus fold for Geraads et al. [3]), forming a ‘M’ on the posterior face of the trigonid. As suggested by Scott and Janis [13], the Dorcatherium fold is probably a plesiomorphic character of ruminants. Its loss might be, as for the ‘entoconidian groove’, a consequence of increased selenodonty, although it seems to be present on some individuals of the bovid genus Cephalophus [7]. The phylogenetic status of Indomeryx has long been debated, its affinities having been thought to lie with Tragulidae [10], Hypertragulidae [1, 12, 16], or Gelocidae [14, 15]. Recently, however, the discovery of new remains of Indomeryx led Guo et al. [4] to erect the new ruminant family Prodremotheridae based on presence of metastylid, a double fold at the rear of the entoconid, and neither Dorcatherium fold nor Palaeomeryx fold. However, the presence of a metastylid is not clear on Indomeryx. Guo et al. [4] claimed that the degree of development of this feature varies in different individuals. Besides, the P4 of Indomeryx is morphologically less
derived than that of Notomeryx and Prodremotherium (lower premolars of Gobiomeryx are unknown), because both parallel crests backwardly extending from the apex of protoconid occur only in Indomeryx. This morphology rather recalls that of the P4 of Archaeomeryx, the oldest known ruminant from the Middle Eocene of Shara Murun (Mongolia). The other genus included in Prodremotheridae (Prodremotherium, Notomeryx and Gobiomeryx) displays a more advanced configuration with the beginning of the molarization of P4 with short enamel folds present on the lingual side of the metaconid and entoconid. Moreover, in their unpublished work on Paleogene ruminants of China, Savage et al. pointed out a twinned metaconid in some material of I. cotteri from Pondaung. I. pilgrimi possesses a duplicated metaconid that we interpret as the remains of a paraconid. However, Archaeomeryx displays tetratuberculate lower molars like most of Paleogene ruminants, except Iberomeryx that exhibits a remnant of hypoconulid on M1–2 [15]. Although the scarcity of remains does not allow us to definitively conclude on the systematic position of Indomeryx, it seems inappropriate to include that genus in the Pecoran family Prodremotheridae. At the moment, we prefer to consider Indomeryx as a basal form among ruminants, more primitive in its dental morphology than the Middle Eocene genus Archaeomeryx. The apparent great diversity of undescribed ruminants remains in Pondaung seems to indicate an early radiation of ruminants in South East Asia during the Middle Eocene. Nevertheless, additional material is now urgently needed in order to better understand the origin and the early evolution of hornless ruminants.
Acknowledgements. We wish to thank Dr. J. Sudre for his helpful comments, Dr. D.E Russell for providing us with casts, and the Foundation Singer-Polignac for financial support. Photographs are from M. Pons. Publication ISEM No. 2000-054.
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