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


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(Pickford, 2007, p. 31, fig. 3). Moreover, the crescentoids of Tha Chang specimens are blunter than those of Z. turicensis (see Tobien, 1975, p. 196, fig. 1; Tassy, 1977, fig. 3, p. 659; Tables 2, 3). A closer inspection of the molars of the zygodont


proboscidean “Mastodon” atavus Borissiak, 1936, fromthe lower MioceneDjilancik beds of Kazakhstan, will be needed in order to determine whether this specimen is assignable to Z. gobiensis, Z. turicensis, or neither.However, the symphysis is longer, and the incisors more strongly developed, than in Z. gobiensis (Tassy, 1985, fig. 201). These differences seem consistent with the somewhat earlier age (Shanwangian/Orleanian) of the Djilancik site. The shortening of the symphysis and incisors seen in Z. gobiensis represents an evolutionary step towards Mammut,in which the symphysis and incisors are reduced (Tobien et al., 1988). Them3 of Z. atavus is larger andmuchwider than the Tha Chang m3 (Tables 1, 2; Fig. 5.1). Previously described Chinese zygolophodonts show varying


degrees of similarity to the Tha Chang specimens. The type specimen of Zygolophodon gobiensis Osborn and Granger, 1932 (AMNH 26461) is a hemimandible with a complete m2 and m3 from the Tunggur locality, NeiMongol. The m3 of this specimen resembles the Tha Chang m3 NRRU-TKK001 in having small conelets on both pretrite and posttrite sides, a thick talonid on the pretrite, lophids with a low degree of obliquity, weakly developed zygodont crests, a reduced lateral cingulum, and valleys that lack cementum. The m3 of the type specimen of Z. gobiensis is larger than the Tha Chang specimen; however, the specimen AMNH 26467 is nearly equal in size to Tha Changm3. In addition, molar has a narrower crown and themain cone and conelets of the fourth lophid are blunter in Z. gobiensis (Tables1,2;Fig.5.1). Zygolophodon gromovae Dubrovo, 1970 is represented by


Figure 5. Scatterplot of third molar proportions for various zygodont proboscideans, including Zygolophodon sp. from Tha Chang sand pit no. 10: (1) lower molar, (2) upper molar.


The furrow-like posttrite conelets of the fourth loph(id)s of


both Tha Chang specimens resemble the equivalent structures in the middle Miocene European zygolophodont Zygolophodon turicensis Schinz, 1824. The Tha Chang specimens are only slightly narrower than the equivalent molars of Z. turicensis but slightly longer in upper molar and slightly shorter in lower molar (in the lowermolar case, the length is probably a bit longer than it estimated) (Table 1; Fig. 5.1, 5.2). However, the zygodont crests and cingula of the Tha Chang specimens are reduced, whereas very strong cingula are present in Z. turicensis. In addition, awell- developed lingual cingulum is also present in the upper third molar of Z. turicensis from the middle Miocene of Jebel Cherichera, Tunisia (Tassy, 1985, pp. 509–510, fig. 213; Pickford, 2007, p. 30, fig. 2A) and the upper second molars from the Grildain, Ngorora Formation, Member A, Tugen Hills, Kenya


an upper M2 and M3 (PIN. no. 2002-5, 2002-6 respectively), from the middle Miocene in age of the Tunggur locality, Nei Mongol. In these specimens, small pretrite conelets are present. Upper M3 has three conelets on the posttrite side. The fourth loph is crest-like rather than conule-like (Tobien et al., 1988). In these characters, the M3 of Z. gromovae resembles the Tha Chang M3, PRY200. However, the Tha Chang M3 is smaller than the M3 of Z. gromovae and has a better-developed fourth loph (Tobien et al., 1988) (Tables 1, 3; Fig. 5.2). However, the type specimen of Z. gromovae is so badly damaged that itsM3is difficult to compare to the Tha Chang M3 in other respects. The only representative of Zygolophodon nemonguensis


Chow and Chang, 1961, an M3 (IVPP-V2487) from Nei Mongol, has four lophs. The fourth loph is as well developed as that of the Tha Chang M3, PRY200. However, the fourth loph is equal in width to the third loph in IVPP-V2487, but clearly narrower than the third loph in PRY200. The talon of IVPP- V2487 is stronger than that of the Tha Chang M3. Nevertheless, the zygodont crest of the first and second posttrite half lophs, the lingual beaded antero-external cingula, and the crescentoids are more prominent in IVPP-V2487 than in PRY200. The pretrite main cone of the fourth loph of PRY200 is larger and more transversely elongate than in IVPP-V2487, and the posttrite half lophs are divided into more conelets. Furthermore, PRY200 is smaller than IVPP-V2487 (Tables 1, 3; Fig. 5.2). The Zygolophodon jiningensis Chow and Chang, 1974 is an unnumbered M3 from Jining, NeiMongol. This tooth is unworn,


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