Duangkrayom et al.—The first Neogene record of Zygolophodon in Thailand


suggesting this taxon may be closer to Z. gobiensis than to Z. turicensis. If Tobien et al. (1988) are right; the oldest name for Z. gobiensis should be Z. metachinjiensis Osborn, 1929. All Zygolophodon species that were synonymized by Tobien et al. (1988) should also be changed. In the Tha Chang m3 NRRU- TKK-001, the transverse lophids are so severely worn that the morphology of the pretrite conelets is generally unclear, except the fourth lophid. In the fourth lophid; however, very small conelets are clearly present on both the pretrite and posttrite sides. The third molars of Z. metachinjiensis are much larger, particularly with regard to the length of both the upper and lower molars, than their counterparts from Tha Chang (Tables 1–3; Fig. 5.1, 5.2).


Discussion


In the course of the “proboscidean datum event” (Madden and van Couvering, 1976), which actually comprised multiple proboscidean dispersals from Africa to Asia and Europe, mammutids initially immigrated, in the end of early Miocene, into Europe, and thereafter into Asia and South Asia (Sanders et al., 2010). In Europe, Tassy (1989) identified Zygolophodon turicensis in MN3b sediments in France. In Eurasia and Asia, the records of these taxa in Pakistan, Kazakhstan, and China occur in sediments that date to, or are slightly younger than, MN4 (Tassy, 1996b; Lucas and Bendukidze, 1997). However, material from the Loh Formation in Central Mongolia appears to be late early to early middle Miocene in age, which would make the specimens in question, slightly younger than the first gomphothere and mammutid immigrants (Göhlich, 2007). However, Mein (1990) thinks that Zygolophodon arrived at Europe coincident with Gomphotherium during MN4. If the Loh Formation indicates late early to early middle Miocene in age and considering the opinion of Mein (1990), the initially migration of mammutids is still unresolved. The family Mammutidae is characterized by a zygodont


pattern (yoke–like transverse crests) of the intermediate and third molars. The oldest, most primitive mammutid is Losodokodon losodokius, from Kenya dated to the late Oligocene (Rasmussen and Gutierrez, 2009). In Eurasia the Mammutidae is only represented by two genera, Zygolophodon and its descendant Mammut (Konidaris and Koufos, 2009). The oldest Zygolophodon, Z. aegyptensis, is from the lower Miocene of Wadi Moghara, Egypt (Sanders and Miller, 2002 in Konidaris and Koufos, 2009). The only European Zygolophodon representative is the abundant Z. turicensis, whose long stratigraphic range in Europe extends from themiddle to the upper Miocene (Konidaris and Koufos, 2009). The “Zygolophodon turicensis group” includes four Eurasian species: the type species Z. turicensis Schinz, 1824 from Europe, Z. atavus from Kazakhstan, Z. metachinjiensis from Pakistan and Z. gobiensis fromMongolia and China (Tassy, 1996). Zygolophodon has not previously been reported from


Southeast Asia. The known Zygolophodon specimens closest to the region are from Lufeng, Yunnan, South China, and were identified as Z. lufengensis Zhang, 1982 and Z. chinjiensis (Chow et al., 1978; Zhang, 1982). The new specimens from the Tha Chang sand pits are the first record in Southeast Asia. Z. chinjiensis and Z. lufengensis from Yunnan were assigned to


189


Z. gobiensis by Tobien et al. (1988) on the basis of dental morphology. Tassy (1985) and Tobien (1996) noted two dental


morphotypes within Mammutidae, robust and gracile, however, the dental morphology in this family is indeed high variable (Tables 2, 3). The zygodont specimens from the Tha Chang sand pits are close in size to the equivalent teeth of Zygolophodon gobiensis (Miomastodon tongxinensis Chen, 1978, IVPP- V5584; Z. gobiensis, AMNH 26467, Tobien et al., 1988) (both lower and upper molars), Z. lufengensis (lower molar) and Z. turicensis (both lower and upper molars), but they are smaller than the holotype of Z. gobiensis, Z. metachijiensis, and Mammut borsoni (Fig. 5.1, 5.2). Tha Chang specimens have small adaxial conelets on both the pretrite and posttrite sides, a feature seen in Zygolophodon specimens from both China and Pakistan. As mentioned above, Tobien et al. (1988) synony- mized many species of Zygolophodon with Z. gobiensis, implying high variation in the dental morphology of this species. In addition, they suggested that Z. gobiensis was more closely related to Pakistan, Z. metachinjiensis, which share the small pretrite conelets seen in Z. gobiensis, than to Z. turicensis. Nevertheless, the essential diagnostic characters of Z. gobiensis are the small anterior pretrite conelets on the upper and lower second and third molars, the fact that the lophids are only slightly oblique. Tha Chang specimens share some of these characters, but lack others. Although the Tha Chang specimens are deeply worn, it is


still evident that their crescentoids are less inflated than in the type specimen of Z. gobiensis, Miomastodon tongxinensis, and Z. jiningensis but similar in their degree of inflation to late Miocene Z. lufengensis and Z. chinjiensis. However, the lophids are less oblique and the zygodont crests less well developed than in Z. lufengensis and Z. chinjiensis, as in the type specimen and Miomastodon tongxinensis. The strongly oblique loph(id)s and zygodont crests are more remarkable in the derived zygodont proboscideans such as Mammut (Osborn, 1936; Tobien et al., 1988). The broken specimen of Z. gromovae has a loph–like rather than conule–like fourth loph, a point of resemblance to the Tha Chang specimen. In contrast, the fourth loph of the upper M3 of Tha Chang specimen is better developed than in Z. gromovae. South Chinese species have small and strongly divided posttrite adaxial conelets, making them more similar to species from the Pakistan (Z. metachinjiensis) than those from North China (Z. gobiensis, Miomastodon tongxinensis, Z. nemonguensis, and Z. jiningensis). The Tha Chang specimens share specific features with the zygodonts from the Pakistan and Yunnan such as small adaxial conelets, however, they are dee- ply worn and broken in the anterior loph(id)s and so it is difficult to establish their precise affinities though it is reasonable to designate them as Zygolophodon sp. The faunal assemblage in the Tha Chang sand pits no. 1 to 9


includes proboscideans, anthracotheres, pigs, rhinos, bovids, giraffids, horses, apes, crocodiles, and tortoises (Nakaya et al., 2002; Chimanee et al., 2004) (Fig. 1), and indicates a middle Miocene to early Pleistocene age. The middle Miocene mammalian fauna consists of amebelodontine gomphotheres (Protanancus), Gomphotherium,and Prodeinotherium. The late Miocene to early Pliocene fauna includes a new orangutan species Khoratpithecus piriyai Chimanee et al., 2004, a new


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