云南元谋小河组脊棱齿象（Stegolophodon）化石新材料

NEW MATERIAL OF STEGOLOPHDON FROM THE UPPER MIOCENE XIAOHE FORMATION, YUANMOU BASIN, YUNNAN PROVINCE

文章报道了云南元谋盆地上中新统小河组芝麻大村小夹巷巷地点的脊棱齿象属（Stegolophodon）的新化石材料：包括一完整的、轻度磨耗的左上第二臼齿,和一不完整的、重度磨耗的右上第三臼齿.第二臼齿和第三臼齿主齿柱中附锥和前中心小尖愈合;后中心小尖很弱小;前二脊发育中沟,且几乎排成一条直线,仅主齿柱的中附锥比主尖略微靠前;第三臼齿的后三脊略向前弯.这些特征与西瓦利克道克派珊组的剑齿象型脊棱齿象（Stegolophodon stegodontoides）形态上十分相似,可归为同一种.元谋盆地小河组此前发现的所谓“Stegolophodon banguoensis”和“Stegolophodon aff.banguoensis”经比较均可以归为此种.小河组的磁性地层年代约为8.3～ 7.2Ma,而相距不远的昭通盆地出产最原始的剑齿象Stegodon zhaotongensis的层位的磁性地层年代约为6.5～6.OMa,因此,剑齿象型脊棱齿象代表了脊棱齿象演化到晚中新世中期的进步类型,此后,脊棱齿象便迅速演化为剑齿象.小河组的剑齿象型脊棱齿象的新材料,能为这一幕演化历史提供生物年代学和形态学的双重证据.

We report on newly discovered material of Stegolophodon from the Xiaojiaxiangxiang Quarry（25°58＇19.18＂N, 101°47＇24.31＂E, elevation 1248.8m）, Zhima Big Village, Zhima Subbasin of the Yuanmou Basin, Yunnan Province, belonging to the Upper Miocene Xiaohe Formation.The Xiaohe Formation yields a typical Late Miocene fauna including an ape, Lufengpithecus hudienensis.Stegolophodon is the most important proboscidean in the Xiaohe Fauna and in most Late Miocene faunas of southern China.However, this genus has not been sufficiently studied. Stegolophodon is an important genus representing an intermediate evolutionary stage between primitive gomphotheres and derived Stegodon. It is also a problematic taxon with numerous established species.Most of these species are based on very little material and should be further revised.Our new material includes a complete, slightly worn left M2 and an incomplete, deeply worn right M3.Important morphological features of the two teeth are as follows： pretrite mesoconelets and corresponding anterior central conules are fused, posterior pretrite central conules are very weak, medium sulcus are developed on the first two lophs which are almost arranged in line, and the posterior three lophs of the M3 are slightly curving anteriorly.These features are very similar to the type specimen of Sl.stegodontoides from the Dhok Pathan Formation of Siwalik, and thus they are suitable to be attributed into the same species.We also attributed the previous reported stegodontids from the Xiaohe Formation-Sl.banguoensis and Sl. aff.banguoensis- to Sl.stegodontoides based on similarities in morphology. Furthermore, we compare the new material with other known species of Stegolophodon. The new material differs from Sl.hueiheensis in possessing more lophs in M3 and in lacking enamel furrows and enamel conules, differs from members of Sl.pseudolatidens group（i.e.Sl.pseudolatidens, Sl.tsudai, and Sl.miyakoae）in slightly larger posterior pretrite central conules and in slightly less straight posterior lophs in M3, and differs from members of Sl.latidens group（e.g. Sl.nasaiensis, Sl.praelatidens, Sl.latidens, and Sl.cautleyi）in possessing more lophs in M3, and in less development of pretrite central conules.All of the comparisons confirm that Sl.stegodontoides is closer to primitive Stegodon than any other species.It represents the most derived species of Stegolophodon in the middle Late Miocene age. The paleomagnetic age of the Xiaohe Formation is about 8.3~7.2Ma.Whereas, the horizon yielding the most primitive Stegodon, Stegodon zhaotongensis, is dated at about 6.5~6.0Ma paleomagnetically, about 1~2Ma younger than the Xiaohe Formation.Thus during 7.2~6.5Ma, Stegolophodon evolved to Stegodon rapidly.The new material of Sl.stegodontoides from the Xiaohe Formation provides both morphological and biostratigraphic evidence for this scenario.