Mammutidae from the Yuanmou Basin (southern China) reveal the earlier origin of Sinomastodon
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Abstract:
This paper reports fossil mammutid proboscideans from the late Miocene of the Xiaohe Formation, Yuanmou Basin, Yunnan Province. They were recovered from three localities: Zhupeng, Huiwoqing, and Leilao, and belong to two taxa: Pliomastodon? zhupengensis comb. nov. and Mammut lufengense. SYSTEMATIC PALEONTOLOGYOrder Proboscidea Illiger, 1811 Family Mammutidae Hay, 1922 Genus Pliomastodon Osborn, 1926 Pliomastodon? zhupengensis (Zhang et al., 1991 in Ji and Zhang, 1997, comb. nov.) Type specimen YICRA YV0787, left M3, see Ji and Zhang (pl. 28, fig. 1). Type locality Zhupeng. Diagnosis Pliomastodon? with moderately elongated symphysis and lower tusks, lacking the posterior mental foramen. Upper tusks straight, dorsally bent and enamel-less. A low degree of cheek teeth zygodonty: relative to Mammut, subdivision of lophs/lophids not pronounced, thick pretrite crescentoids with the possibility of presence of pretrite central conules, interlophs/interlophids half stuffed by pretrite crescentoids and/or central conules. Referred material MCY C1134ZA94, mandible, with associated palate and right upper tusk, from Huwoqing; YMM 297, incomplete mandible, from Leilao; YICRA YV768 and YV784, two left M3s, as well as YMM 3121X081, right m3, from Zhupeng (Figs. 3, 4). Age: Late Miocene, ~8.2?7.2 Ma (Dong and Qi, 2013). Description and comparison The mandibular possesses a straight ventral border. The mandibular symphysis is narrow and moderately elongated with a deep trough. It tips slightly downward. The corpus is strong and the ramus is low. The upper tusk is straight and long. It is slightly dorsally bent without an enamel-band. The lower tusks are short rod-like. They are closely apposed with a nearly round cross-section. The above features are very similar to those of Mammut obliquelophus(Tobien, 1976). The M3 is tetralophodont. The lophs are blunter than those of Mammut. The interlophs are narrow. The pretrite half lophs possess thicker anterior and posterior crescentoids than those of Mammut. Occasionally the pretrite central conules are present. In lateral view from the pretrite side, the crescentoids and/or central conules reach at least half of the interlophs. The posttrite half lophs are less subdivided than those of Mammut. The posttrite main cusp and mesoconelets are generally well-separated. The zygodont crests are blunt. The m2 is trilophodont, and the m3 is tetra- or pentalophodont. The interlophids are wider than the M3, and the posterior pretrite central conules are usually present. Lophids are obliquely arranged with a somewhat chevron in the distal lophids. In some cases, the second pretrite and posttrite half lophids are alternatively positioned. The other features are similar to those of the M3, and differs from the m3 of Mammut in the same way as those of its M3. Pliomastodon? zhupengensis succeeded the low degree of zygodonty from Miomastodon gobiensis of the Middle Miocene of northern China (Wang et al., 2020). Like the other Late Miocene mammutids with high zygodonty, i.e., Mammut and Pliomatodon (Tobien, 1976, Osborn, 1936), Pliomastodon? zhupengensis curved the upper tusks upward and lost the enamel band. The molar morphology of Pliomastodon? zhupengensis displays few differences from Sinamstodon praeintermedius (Figs. 6-A–6-D), except the lower tooth crown and fewer lophids of the m3. It is the potential direct ancestor of the latter, which was recovered from the Shuitangba locality, Zhaotong Basin in the vicinity, ~6.5?6.0 Ma (Wang et al., 2016). Tobien et al. (1986) believed that Sinomastodon is morphologically and phylogenetically close to the American brevirostrinegomphotheres, especially the Notiomastodon, and migrated back from American. This opinion is questioned here: 1, the earliest Notiomastodon occurred at ~2.5 Ma in South America (Mothé et al., 2016), which is far later than the occurrence of Sinomastodon; 2, Sinomastodon sensu Tobien et al. actually comprises two morphological types, a more zygodont-like type (Mastodon intermedius, originally attributed to Mammutidae) from the Pliocene, and a more bunodont-like type from the Pleistocene (originally attributed to Gomphotherium); 3, collagen sequence of Notiomastodon are closer to that of Mammut than of the true elephantids (derived from the gomphotheres) (Buckley et al., 2019), which reveals that the evolution of Gomphotheriidae and Mammutidae was deeply involved rather than simply detached. Sinomastodon is more likely a mammutid. Genus Mammut Blumenbach, 1799 Mammut lufengense (Zhang, 1982) Type specimen YICRA YV0131, left m3, see Zhang (pl. 1, fig. 1). Type locality Shihuiba, Lufeng, Yunnan Province. Referred material YICRA XDYV001, right M2, YICRA XDYV002, anterior two lophs of right M3, YICRA XDYV003, left m2, YICRA XDYV004, left m3, unearthed at the same time from Leilao, Yuanmou, Yunnan Province (Fig. 5). Age: Late Miocene, 6.9?6.2 Ma (Dong and Qi, 2013). Description and comparison The deeply worn M2 and m2 are trilophodont. The M3 (keeping the anterior two lophs) possesses sharp lophs. The posttrite half lophs are subdivided into small conelets with an obscure separation between the main and mesoconelets. The pretrite mesoconelets are short crest-like, and the pretrite crescentoids are thin and sharp, reaching the bottom of the interlophs in side views. The zygodont crest are thin and clear. The m3 is tetralophodont. The lophids are highly mesiodistally compressed with wide interlophids. The posttrite half lophids are moderately to highly subdivided, and the separation between the main and mesoconelets is clearer than that of the M3. The pretrite crescentoids are somewhat thicker than those of the M3, and reach the 1/3 of the interlophid height. Chevron is developed on the posterior two lophids. Generally, the tooth morphology of Mammut lufenense shows little difference from the contemporary Eurasian species, Mammut obliquelophus (= M. preatypicum). Unfortunately, the condition of the upper and lower tusks, and mandibular symphysis is unknown. Here we temporarily keep the species name, ‘lufengensis’, and transferred it to Mammut, as a highly zygodont mammutid representative of southern China during this age. Besides from Yuanmou, Mammut lufenense was also recovered from the adjacent Lufeng (6.9?6.2 Ma) (Dong and Qi, 2013) and Zhaotong (6.5?6.0 Ma) (Ji et al., 2013), Yunnan Province (Figs. 6-E?6-H). It can be inferred that two types of Mammutidae, the less zygodont Pliomastodon?, and the high zygodont Mammut, coexisted in a long period during the late Miocene of southern China.
