ZSC071.fm Page 231 Thursday, September 13, 2001 6:12 PM New0Blackwell Science, Ltd remains of primitive ruminants from Thailand: evidence of the early evolution of the Ruminantia in Asia GRÉGOIRE MÉTAIS, YAOWALAK CHAIMANEE, JEAN-JACQUES JAEGER & STÉPHANE DUCROCQ Accepted: 23 June 2001 Métais, G., Chaimanee, Y., Jaeger, J.-J. & Ducrocq S. (2001). New remains of primitive rumi- nants from Thailand: evidence of the early evolution of the Ruminantia in Asia. — Zoologica Scripta, 30, 231–248. A new tragulid, Archaeotragulus krabiensis, gen. n. et sp. n., is described from the late Eocene Krabi Basin (south Thailand). It represents the oldest occurrence of the family which was pre- viously unknown prior to the Miocene. Archaeotragulus displays a mixture of primitive and derived characters, together with the M structure on the trigonid, which appears to be the main dental autapomorphy of the family. We also report the occurrence at Krabi of a new Lophiomerycid, Krabimeryx primitivus, gen. n. et sp. n., which displays affinities with Chinese representatives of the family, particularly Lophiomeryx. The familial status of Iberomeryx is dis- cussed and a set of characters is proposed to define both Tragulidae and Lophiomerycidae. Results of phylogenetic analysis show that tragulids are monophyletic and appear nested within the lophiomerycids. The occurrence of Tragulidae and Lophiomerycidae in the upper Eocene of south-east Asia enhances the hypothesis that ruminants originated in Asia, but it also challenges the taxonomic status of traguloids within the Ruminantia. Grégoire Métais, Institut des Sciences de l’Evolution, UMR 5554 CNRS, Case 064, Université de Montpellier II, 34095 Montpellier cedex 5, France. E-mail: [email protected] Yaowalak Chaimanee, Department of Mineral Resources, Geological Survey Division, Rama VI Road, Bangkok 10400, Thailand. Jean-Jacques Jaeger, Institut des Sciences de l’Evolution, UMR 5554 CNRS, Case 064, Université de Montpellier II, 34095 Montpellier cedex 5, France. E-mail: [email protected] Stéphane Ducrocq, Institut des Sciences de l’Evolution, UMR 5554 CNRS, Case 064, Université de Montpellier II, 34095 Montpellier cedex 5, France. E-mail: [email protected] Introduction considered as the oldest of the Pecora group (Matthew & Ruminant artiodactyls are the most geographically and Granger 1925a; Colbert 1941). ecologically successful living group of large mammals. The Tragulina are believed to be of primitive grade among suborder Ruminantia is traditionally defined as artiodactyls ruminants (e.g. Janis 1987). This group includes both North possessing a cuboid fused with the navicular and incisiform American and Eurasian forms, and may be considered as a lower canines. The former character is ambiguous because paraphyletic group of primitive non-pecoran ruminants of its occurrence in the late Eocene ruminant-like genus (Scott & Janis 1993), although no consensus is established Amphimeryx from Western Europe (Sudre 1977), whereas concerning the first unambiguous Pecora. During the late the second feature is rarely preserved in fossils of primitive Palaeogene, tragulines was diversified and one classically ruminants. The latter are classically divided into two main comprised by Eocene North American traguloids and some groups whose content is discussed. Pecora (horned ruminants) ambiguous Eurasian forms whose affinities are discussed and Tragulina-Pecora are nowadays the most diversified below. Among North American Palaeogene traguloids, ruminants, and fossils are well known during the Neogene in hypertragulids are the most primitive forms. This is mainly Eurasia, North America and Africa. Globally, the phylogenetic based on their cranial and postcranial characters. Recently, relationships between early ruminants are misunderstood, however, Vislobokova (1998) pointed out the presence mainly because of the scarcity of remains of Eocene Asian of a new hypertragulid ruminant from the late Eocene of ruminants. Archaeomeryx, the earliest known ruminant from Khoer-Dzan (Mongolia), extending the geographical range of Shara Murun (late–middle Eocene, Mongolia), is sometimes hypertragulids to Asia during the Palaeogene. Leptomerycids ZSC071.fm Page 232 Thursday, September 13, 2001 6:12 PM • are mainly known from the late Eocene of North America the Dorcatherium fold refers to the fold occurring on the onwards, but some unclear and poorly documented Asian posterior side of the metaconid on the lower molars of some forms are related to leptomerycids (e.g. McKenna & Bell primitive ruminants and extant tragulids (see ‘Discussion’ 1997). Their cranial and postcranial features indicate a more section). The ‘entoconidian groove’ refers to the two parallel derived state in comparison with hypertragulids (Webb & folds (thus forming a groove) occurring on the mesial side of Taylor 1980; Janis 1987), and Geraads et al. (1987) considered the entoconid. This structure was first pointed out by Pilgrim leptomerycids as being close to Old World tragulids. (1928) in Indomeryx arenae, and may be considered as equiv- Tragulids are traditionally considered as the most primi- alent to the Zhailimeryx fold (Guo et al. 2000). Finally, the tive living ruminants (e.g. Janis 1984), and they still survive as Tragulus fold (Geraads et al. 1987) more specifically design- tropical relicts: the water chevrotains (Hyemoschus) of Africa ates the fold situated on the posterior side of the protoconid and the mouse deer or Asiatic chevrotain (Tragulus) of south- and basally linked to the prehypocristid (Fig. 3, see later). east Asia. They are characterized by their skeletal and dental We used the following abbreviations: TF, Thai fossil at features which are primitive within ruminants, and their gen- the Department of Mineral Resources, Bangkok, Thailand; eral shape, digestive system and ethology which are reminis- WIF/A 415, specimen housed in the Museum Repository of cent to those of pigs (Dubost 1965). The family Tragulidae Wadia, Institute of Himalayan Geology, Dehra Dun, India. is the only surviving family from the assumed paraphyletic Tragulina (Janis 1984). Therefore, the evolutionary history Systematics of tragulids remains unknown before the Eurasian and African Miocene Dorcatherium, although the potential Eocene Order ARTIODACTYLA Owen, 1848 occurrence of Tragulidae has been previously expected by Suborder RUMINANTIA Scopoli, 1777 both morphological (e.g. Webb & Taylor 1980) and molecu- lar (e.g. Miyamoto et al. 1993) data. Sudre (1984) considered Family TRAGULIDAE Milne-Edwards, 1864 the Oligocene genus Cryptomeryx (since put in synonymy with Iberomeryx by Bouvrain et al. 1986) from the Oligocene Genus Archaeotragulus, gen. n. of Quercy (France), Benara (Georgia) and north India (Gabounia 1964; Sudre 1984; Nanda & Sahni 1990) as the Type species. Archaeotragulus krabiensis, sp. n. (Fig. 2A–D) oldest Tragulidae. However, Janis (1987) suggested that this only known species of genus. problematic genus should be included in the new family Locality. Wai Lek lignite pit, Krabi Basin, southern Lophiomerycidae, mainly because of the presence of an Thailand. anterior cingulid and of the figure eight shape trigonid on Horizon and age. Upper level of the main lignite seam of lower molars. While the first version of this paper was in Wai Lek pit (Formation B2, see Bristow 1991), late Eocene review, Guo et al. (2000) published the description of the (see Ducrocq et al. 1995). lophiomerycid Zhailimeryx from the late–middle Eocene of Etymology. From the Greek ‘archaeos’ that refers to the central China, thus confirming the antiquity of the Lophio- archaic dental morphology of the Thai specimen; ‘meryx’, merycidae in Asia. Greek for ruminant. The species name refers to the Krabi The Tertiary Krabi Basin, located in Peninsular Thailand Basin where the material was found. (Fig. 1), has yielded more than 30 mammalian species, most Holotype. Fragmentary mandible with P2–M2 (TF 2997). of which are typical of swampy environments (Ducrocq Paratype. Fragmentary lower jaw with M1–M3 (TF 2989). 1994). The study of the Krabi fauna has led to a late Eocene All specimens are housed in the Department of Mineral age being proposed for this community, on the basis of faunal Resources (DMR) of Bangkok, Thailand. comparisons (Ducrocq et al. 1997; Ducrocq 1999) and mag- netostratigraphic investigations (Benammi et al. 2001). The Diagnosis. Small primitive ruminant with lower molars purpose of this paper is to describe new tragulid and lophi- resembling those of Dorcatherium by the presence of a typical omerycid dental remains from the late Eocene of Thailand, tragulid M structure at the rear of the trigonid (Fig. 4) and and to discuss the origin and early diversification of the rumi- the derived pattern of its lower premolars, including the lack nants in south-east Asia. of a metaconid and the presence of a longitudinal groove on In this work, we followed the nomenclature from Gentry the posterior half of P4. Differs from Dorcatherium by its & Hooker (1988) and Moya-Sola (1988) for the upper and smaller size, its cusps more labio-lingually compressed, the lower teeth. The term ‘M structure’, first used by Mottl lack of an ectostylid, the presence of a well-marked ‘entoco- (1961), is the same as the ‘Σ structure’, sometimes used by nidian groove’ opening forwards, the presence of a rudimen- later authors (e.g. Qiu & Gu 1991), and the cristid obliqua is tary hypoconulid on M2–3 and by the transversely compressed equivalent to the prehypocristid. According