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


relative to the posttrite part. The zygodont crest of the first loph is stronger than those of the other lophs, and continuous with the anterior cingulum. Anterior and posterior pretrite crescentoids (acr1 and pcr1) are present. However, their surfaces are deeply worn. The acr1 is connected to the anterior cingulum. The second loph is aligned with the transverse axis of the


crecentiod of the second loph (acr2). A similar feature is also present in the valley between the second and the third lophs. However, the valley between the third and fourth lophs is open. No cementum remains in the valleys. The pretrite of the first loph is displaced slightly posteriorly


185


Moroto, Uganda (Pickford and Tassy, 1980; Tassy and Pickford, 1983; Pickford, 2007) that they have three loph(id)s in the third upper and lower molars. The fourth loph(id)s in m3/ M3 are absent or poor developed. On the contrary, the Tha Chang specimens have four loph(id)s with the posterior cingula and larger in molar size (Tables 1–3; Fig. 5). The zygodont crests of the Tha Chang specimens are not


crown, and is deeply worn. An anterior pretrite crescentoid (acr2) is present, and extends downward to the base of the adjacent valley, whereas the posterior pretrite crecentoid (pcr2) is not prominent. The pretrite of the third loph is slightly posteriorly, while the posttrite is straight. The wear surface of the third loph is moderately worn. The posttrite half loph has a rather large main cone and small adaxial conelets (clts.po), which appear furrow–like. However, the morphology of the conelets is not clear in the pretrite half loph because of surface wear. The anterior pretrite crescentoid (acr3) is slightly worn and narrow in shape. The posterior pretrite crescentoid (pcr3) is very weak in this loph. The unworn fourth loph shows two small adaxial conelets (clts.po) on the posttrite side, which appear furrow-like. The main cone is quite large. The pretrite half loph has a large and transversely elongate main cone, and a small conelet near the median sulcus. The anterior crescentoid (acr4) is clearly and extends down to the base of the nearby valley. The talon (posterior cingulum) is shelf-like, and better developed on the pretrite side than on the posttrite side (Fig. 4).


Materials.—The right lower third molar (m3) is stored at the Northeastern Research Institute of Petrified Wood and Mineral Resources, Nakhon Ratchasima Rajabhat University, as NRRU-TKK-001. The left upper third molar (M3) is stored at the Sukhothai Airport Natural History Museum, as PRY–200 (Figs. 2–4).


Remarks.—The lophids of the specimen NRRU-TKK-001 (right m3) are more oblique than the loph of PRY-200 (left M3). This is a generally different pattern between the lower and upper molar, especially in the zygodont proboscideans. The enamel wear surfaces are narrow, transversely elongated and yoke–like (Tobien, 1973b). Both of specimens from Tha Chang have clear zygodont crests. The conelets on the posttrite side (clts.po.) are better developed than those on the pretrite side (clts.pr.). The conelets and half loph(id)s are somewhat less regularly arranged than in the bunodont molars of probosci- deans such as Gomphotherium. In addition, the mesoconelets of Gomphotherium are blunter and more morphologically distinct from the zygodont proboscideans (Tobien, 1973b). The pretrite central conules of bunodonts are arranged in a conule–like pattern, rather than a yoke-like pattern as in the specimens from Tha Chang. These characters above indicate that the Tha Chang specimens are the zygodont proboscideans. Compared with the oldest zygodont proboscidean,


Losodokodon losodokius, from the late Oligocene of Losodok (Lothidok), Kenya (Rasmussen and Gutierrez, 2009), and the early to middle Miocene species, Eozygodon morotoensis, from


strongly developed, in contrast to the condition in Mammut.In addition, the lateral cingula of the Tha Chang specimens are less well developed than in Mammut, whereas the crescentoids of the Tha Chang specimen are more distinct than in Mammut. According to the differentiation between the Tha Chang specimens and the oldest and youngest zygodont proboscideans, Losodokodon losodokius, Eozygodon morotoensis,and Mammut, the dental features such as molar size and well-developed anterior and posterior ridges of Tha Chang specimens indicate that it belongs to Zygolophodon. Comparison between the Tha Chang specimens and the


Egyptian early Miocene zygolophodont Zygolophodon aegyptensis (see Sander and Miller, 2002, p. 398, fig. 9) reveals that the third and fourth lophids of m3 are strongly convex anteriorly in the Egyptian species but quite straight in the Tha Chang specimen. In lateral view, the lophids of Z. aegyptensis are more widely separated than those of the Tha Chang specimens. However, the anterior and posterior pretrite crescentoids of the m3 of Z. aegyptensis are clearly in contact with each other according to wear, which is also the case in the Tha Chang specimen. The complete M3 of Z. aegyptensis possesses three lophs or variably a fourth loph, a thin anterior cingulum composed of numerous tiny tubercles, and a moderate-sized heel; whereas the Tha Chang M3 possesses four lophs and a tiny posterior cingulum. In addition, the lower m3 from Tha Chang is wider but shorter than that of the Egyptian species, whereas the upper M3 from Tha Chang is both wider and longer than its Egyptian counterpart (Tables 1–3; Fig. 5.1, 5.2). The M3 of Zygolophodon cf. proavus, RAM 908 (see


Lofgren and Anand, 2011, p. 1393, fig. 4) from the middle Miocene of the Mud Hills, Mojave Desert, California, North America resembles the Tha Chang M3 in several respects. In Z. cf. proavus central conules are not present; however, the wear facet on the protocone extends onto thickened enamel that is present on both anterior and posterior slopes. The posterior and anterior extensions of this wear facet give a false impression that the central conules are only weakly developed. These characters are reminiscent of the Tha Chang M3, but in other important respects this tooth differs from its equivalent in Z.cf. proavus. The fourth loph is much less well developed in Z. cf. proavus than in the Tha Chang M3. In Z. cf. proavus, the first three lophs reduce their transverse dimension anteroposteriorly, unlike in the Tha Chang M3, and an ectoflexus is present on all three interlophs in Z. cf. proavus but only on the first interloph of the Tha Chang M3. Moreover, the labial cingulum is better–developed in Z. cf. proavus than in the Tha Chang M3. However, the upper molars of Z. cf. proavus described by Lofgren and Anand (2011) are smaller than other specimens of the Z. proavus. The Tha Chang M3 is longer and wider than RAM908, but narrower than other specimens of Z. proavus (Tables 1, 3; Fig. 5.2).


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