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


181


Type species.—Mastodon turicensis Schinz, 1824 (for more details including the non-validity of “Mastodon tapiroides Desmarets, 1822”, sometimes used by earlier authors, see Tassy, 1985).


Figure 2. Dental nomenclature: (acr1, 2, 3), anterior pretrite crescentoids of the first, second and third loph(id)s; (cga), anterior cingulum; (cgp), posterior cingulum (talon(id)); (clts.pr), pretrite conelets; (clts.po), posttrite conelets; (efx), ectoflexus; (hy), hypocone (pretrite main cone of the second loph(id)); (mc), main cone (pretrite main cone of the fourth loph(id)); (me), metacone (posttrite main cone of the second loph(id)); (pa), paracone (posttrite main cone of the first loph(id)): (pc), third posttrite cusp (posttrite main cone of the third loph(id)); (pcr1, 2), posterior pretrite crescentoids of the first and second loph(id)s; (pen), postentoconule (pretrite main cone of the third loph(id)): (pmc), posttrite main cone of the fourth loph(id); (pr), protocone (pretrite main cone of the first loph(id)); (sm), median sulcus; (zc), zygodont crest.


THP-10031, THP-18908, THP-18903, YV031, and YV0132 (Table 1).


Repositories and institutional abbreviations.—AMNH, American Museum of Natural History, New York; DPC, Duke Primate Center, North Carolina; F:AM, Frick Collection, American Museum of Natural History, New York; IVPP-V, Institute of Vertebrate Paleontology and Paleoanthropology, vertebrate collection, Beijing; NRRU-TKK, Northeastern Research Institute of Petrified Wood and Mineral Resources, Nakhon Ratchasima Rajabhat University, Takut Khon Village collection, Nakhon Ratchasima; PIN, Paleontological Institute, Moscow; PRY, Sukhothai Airport Natural History Museum, Sukhothai, Mr. PiriyaVachajitpan collection; RAM, RaymondM. AlfMuseumof Paleontology, Claremont, California; THP, Natural HistoryMuseum, Tianjin (Museum of Huangho and Paiho).


Systematic paleontology


Dental nomenclature follows Tassy (1996). Description of occlusal features of zygodont cheek tooth M3 follows Tobien (1975) (Fig. 2) and measurements are in millimeters.


Order Proboscidea Illiger, 1811 Family Mammutidae Hay, 1922 Genus Zygolophodon Vacek, 1877


Diagnosis.—A mammutid with intermediate and third molars in which the pretrite and posttrite elements are of bunodont character and arranged in clearly yoke-like transverse crests. D4 and all molars are zygodont, having a transverse or slightly oblique loph. In lateral view, the main cusps are wide at the base but taper to an apical point. Transverse valleys have few and small accessory cusps. Worn loph(id)s become sharpened at their summits and show a more zygodont character; zygodont crests show a sharp angle between the lingual and posterior surfaces of the principal lingual cuspids in the lower molars, and between the corresponding surfaces of the buccal cusps in the upper molars; the accessory conules are not developed but rather have crescentoids descending into the transverse valley between loph(id)s on the pretrite side at both upper and lower molars; lophs of upper molars are normal to the long axis of the crown; lophids of lower molars are more or less oblique (i.e., postero-external to antero-internal); upper third molar bears three lophs plus a strong talon or four lophs plus a weak cingulum; and lower third molar bears four lophids plus a more or less strong cingulid (Tobien in Shoshani and Tassy, 1996; Mazo and van de Made, 2012).


Zygolophodon sp. Figures 3 and 4


Occurrence.—Both specimens were found in sand pit no.10 of Tha Chang sand pits (14°59'50.27"N, 102°14'47.50"E) belonging to Takut Khon Village, Tha Chang Subdistrict, Chaloem Prakiat District, Nakhon Ratchasima Province (Fig. 1). Howard et al. (2003) and Haines et al. (2004) subdivided the sediments in the sand pits along the Mun River into a lower reduced unit (Unit A) and an upper oxidized unit (Unit B) (Fig. 1). The lower reduced unit is composed of reduced gray sediment in the form of cross–bedded, stratified to massive sand and gravel, with mud lenses. Abundant Neogene faunal and floral fossils were discovered in this unit (Suteethorn et al., 1997; Sato, 2002; Nakaya et al., 2002; Howard et al., 2003). The materials described in this study were found in the lower reducing unit. However, they were discovered by sand pit workers, and their detailed lithological context is unclear because the water jets used during the sand excavation process caused extensive damage to the strata and fossils collapsed.


Description.—m3, NRRU-TKK001 (Fig. 3). This right lower third molar is missing the first lophid. There are (×1)3× lophids plus a weak talonid (posterior cingulid). A crescentoid is present and the two half lophids are arranged as yoke-like transverse crests. The second lophid is oblique to the long axis of the crown, and the third lophid is slightly oblique. The lophids become more anteroposteriorly constricted at their apices. The first lophid is broken out. The second lophid is severely worn. The third lophid is also worn, bearing a narrow, transversely elongated wear facet.


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