Journal of Paleontology, 91(1), 2017, p. 179–193 Copyright © 2016, The Paleontological Society 0022-3360/16/0088-0906 doi: 10.1017/jpa.2016.143


The first Neogene record of Zygolophodon (Mammalia, Proboscidea) in Thailand: implications for the mammutid evolution and dispersal in Southeast Asia


Jaroon Duangkrayom,1,2,3 Shi-Qi Wang,1 Tao Deng,1 and Pratueng Jintasakul3


1Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, China ⟨jakrub2008@hotmail.com⟩, ⟨jduangkrayom@gmail.com⟩,


⟨wangshiqi@ivpp.ac.cn⟩, ⟨dengtao@ivpp.ac.cn⟩ 2University of Chinese Academy of Sciences, Beijing, China 3Northeastern Research Institutes of Petrified Wood and Mineral Resources, Nakhon Ratchasima Rajabhat University, Nakhon Ratchasima, Thailand ⟨petrifiedwood_museum@hotmail.com⟩


Abstract.—Lower and upper third molars of a proboscidean from the Tha Chang sand pits, Nakhon Ratchasima, northeastern Thailand, show a zygodont pattern. The crescentoids are less well developed than those of the type specimen of Zygolophodon gobiensis but similar to those of late Miocene specimens from south China assigned to Z. lufengensis and Z. chinjiensis. On the other hand, the loph(id)s are less oblique and the zygodont crests are less well developed than in Z. lufengensis and Z. chinjiensis.However, itisdifficult to erect a new species for these specimens because their anterior loph(id)s are so deeply worn that the morphology of the conelets on these loph(id)s is unclear. Thus, we identify the specimens as Zygolophodon sp. Zygolophodon is known from lower and middle Miocene sediments in Africa while they are usually dated to the early–late Miocene in Europe, South Asia such as Pakistan, and Central and North China. Based on its apparent grade of dental evolution, the new material of Zygolophodon from Thailand is probably late Miocene in age. In addition, this discovery is the first record of a zygodont proboscidean in Southeast Asia.


Introduction


Zygodont proboscideans (Tobien, 1975) have been referred to the family Mammutidae Hay, 1922. Shoshani and Tassy (2005) have reported this family containing two subfamilies, namely Eozygodontinae Mckenna and Bell, 1997 (including only Eozygodon Tassy and Pickford, 1983, from the lower Miocene bed of Africa) and Mammutinae Hay, 1922 (including Zygolophodon Vacek, 1877 from the middle-upper Miocene and its descendant Mammut Blumenbach, 1799 from the Plio-Pleistocene). However, the fossils of palaeomastodont–like proboscideans that first expressed mammutid characters (Sanders et al., 2004) were discovered from the late Oligocene at Chilga, Ethiopia as well as Rasmussen and Gutierrez (2009) also reported a new taxon of Mammutidae from the upper Oligocene bed at the Losodok (Lothidok), Kenya and named it Losodokodon losodokius. In this case, the Losodok mammutid is more primitive and older than Eozygodon so that the subdivision of Eozygodontinae and Mammutinae need to be revised or expanded. In addition, the late Oligocene


palaeomastodont-like fossils from Chilga, which suggest the first expression of mammutid features, and possibly that Palaeomastodontidae is not monophyletic but instead is composed of ancestral gomphotheriids (Phiomia) and ancestral mammutids (Palaeomastodon) (Sanders et al., 2004). Hence,


the first mammutids occur in the late Oligocene of Africa and then they immigrated into Europe by the end of the early Mio- cene, and by the middle Miocene had spread throughout Eurasia and into the New World (Tassy, 1986; Mazo, 1996; Sanders, 1996; Tobien, 1996; Sanders et al., 2010). Zygodont proboscidean fossils have flourished at middle to


rather high northern latitudes, in the early Miocene to Pleisto- cene bed of North America, Europe, northern Africa, and Asia (China and Pakistan). However, there are some reported in low latitude regions such as in the late Oligocene to middle Miocene bed of Kenya, Uganda and Namibia (Pickford, 2007) that are important for the origin of mammutids. In particular, zygodonts have never been reported from Southeast Asia, although zygodont material referred to Zygolophodon is known from Lufeng and Kaiyuan in Yunnan Province, southern China (Chow et al., 1978; Zhang, 1982). Although zygodonts have not been reported from Thailand,


10 proboscidean genera ranging in age from the middle Miocene to Holocene have been discovered in that country over the past several years. They include Archaeobelodon sp., Prodeinotherium pentapotamiae, Gomphotherium sp., cf. Protanancus macinnesi, Tetralophodon cf. xiaolongtanensis, Tetralophodon sp., Stegolophodon nasaiensis, Sl. cf. latidens, Sl. praelatidens, Sl. cf. stegodontoides, Stegodon elephantoides, S. insignis, S. cf. insignis, S. cf. orientalis, Anancus sp.,


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