Abstract:

The zygodont proboscideans from the Miocene strata of China are widely distributed. However, the materials are scarce, and their classification has experienced a longtime controversy, from the chaotic state of multiple Zygolophodon species to the only one species, Zygolophodon gobiensis. The combined species Z. gobiensis comprises both the gracile type with a high degree of zygodonty and the robust type that is between the typical bunodont and zygodont morphology. Recently, as the robust type has been re-allocated to another genus Miomastodon and new fossil remains were discovered, it is necessary to further evaluate and classify the zygodont proboscideans from the Miocene of China. In the present paper, we restudied the previously published zygodont specimens of the gracile type, as well as several unpublished Mammutidae specimens. The former including Z. nemonguensis, Z. gromovae, Z. jiningensis, Z. chinjiensis and two specimens of Gomphotherium xiaolongtanensis, represents Zygolophodon in the original sense in China. In these specimens, the tip of the loph(ids) are sharp. The anterior and posterior pretrite central conules are absent or very weak, and the anterior and posterior crescentoids are sharp and slender. The posttrite mesoconelets are well subdivided and the zygodont crests are developed. In buccal view, the loph(id)s are “Ʌ-shaped” and the interloph(id)s are “V-shaped”. Their molar morphology resembles that of Z. turicensis, and hereby, they were identified as Zygolophodon cf. Z. turicensis. Several unpublished specimens from Hezheng, Gansu, Tunggur, Nei Mongol, Tongxin, Ningxia and Junggar, Xinjiang exhibit a lower degree of zygodonty, corresponding to the “robust type of Zygolophodon” in which the molar morphology is between the typical bunodonts and zygodonts. The pretrite crescentoids are thicker than Zygolophodon cf. Z. turicensis, and the pretrite central conules usually present on the first and second interloph(id)s. According to the stratigraphic age and characteristics, two species, Miomastodon gobiensis and Mio. tongxinensis were identified. The anterior and posterior pretrite crescentoids of Mio. tongxinensis are weaker and the pretrite central conules are larger than Mio. gobiensis. Geographical distribution indicates that Miomastodon is the predominant member of zygolophodonts in the Early and Middle Miocene in northern China. The discovery of new materials and the reclassification of zygolophodonts provide further evidence for dispersal of Mammutidae from Eurasia to North America and the evolutionary relationships among the species of the family Mammutidae in China.

Key words: China, Miocene, Mammutidae, Zygolophodon, Miomastodon

摘要：

中国中新世的轭型齿长鼻类化石虽然分布广泛，但材料稀少，分类上经历了从多个轭齿象(Zygolophodon)种到全部归为戈壁轭齿象(Zygolophodon gobiensis)一个种的状态。戈壁轭齿象既包含轭型化程度较高的纤细型也包含介于轭型齿和丘型齿中间的粗壮型。最近，随着粗壮型被归入中新乳齿象属(Miomastodon)以及新材料的发现，有必要对中国中新世的轭齿象类进行重新厘定。结合已知的纤细型轭齿象颊齿材料和新发现的玛姆象科(Mammutidae)材料，对中国中新世轭齿象类材料进行研究，认为轭型化程度较高的纤细型种类包括内蒙古轭齿象(Z. nemonguensis)、格氏轭齿象(Z. gromovae)、集宁轭齿象(Z. jiningensis)、庆义轭齿象(Z. chinjiensis)以及归入小龙潭嵌齿象(Gomphotherium xiaolongtanensis)的两件标本，都应该归入传统意义上的轭齿象属，因为它们臼齿齿脊尖锐，主齿柱前后新月嵴成细嵴状，中心小尖消失或发育很弱，副齿柱中附锥分裂，轭齿嵴发育，侧视齿柱呈Ʌ形，齿谷V形，这些零散的颊齿形态与欧洲苏黎世轭齿象(Z. turicensis)正型标本相似，由于材料比较缺乏，暂将其归为苏黎世轭齿象相似种(Zygolophodon cf. Z. turicensis)。来自甘肃和政、内蒙古通古尔、宁夏同心和新疆准噶尔的新材料，轭型化程度较低，对应于介于轭型齿和丘型齿之间的粗壮型，将其分别归入中新乳齿象属的戈壁种(Miomastodon gobiensis)和同心种(M. tongxinensis)。与戈壁种相比，同心种的臼齿主齿柱前后新月嵴发育更弱，前后中心小尖更大。地质时代及地理分布表明，早中新世至中中新世中国北方的轭齿象类主要以Miomastodon为主。新材料的发现和对整个轭齿象类的重新厘定，为玛姆象类从欧亚大陆向北美的扩散以及在中国本土的演化提供了证据。

关键词: 中国, 中新世, 玛姆象类, 轭齿象属, 中新乳齿象属