甘肃秦安安湾的托氏原互棱齿象(Protanancus tobieni)化石及其剖面

Protanancus tobieni from the Anwan Section, Qinan County, Gansu Province

摘要: 详细记述了1976年甘肃秦安安湾剖面第三系地层中发现的一件托氏原互棱齿象(Protanancus tobieni)标本。该材料保存有完整的下颌并带有m3和下门齿，以及完整的左上门齿，与宁夏同心丁家二沟中中新世的托氏原互棱齿象具有相同的特征，可以归为同一种。托氏原互棱齿象的下颌联合部特别伸长；上门齿粗壮，向外分，具宽的釉质带；下门齿扁平，截面为双凹的同心层结构；m3副齿柱具三叶雏形，主副齿柱有初步的伪互棱齿(pseudo-anancody)特征。以上特征可以将其归为原互棱齿象属，而不是此前所认为的变齿象属。并且，其副齿柱三叶和主副齿柱伪互棱齿特征的发育程度比该属的已知各种都弱，又具有下门齿横截面双凹形这一自近裔特征，因而与已知的原互棱齿象各种均不相同，可以确定为一有效种。通过对秦安安湾剖面的考察，推测其可能发育有中新统的连续沉积。剖面以棕红色泥质粉砂岩和泥岩为主，中下段夹有砂砾岩、细砂岩、以及灰绿色泥灰岩透镜体，表明在新近纪的早期，该地区的沉积环境以洪泛平原的河漫滩与河道沉积为主，推测该地区在这一时期的环境较为湿润，含有大量水体。剖面的上段出现较多的钙质结核层，说明在新近纪的中期之后，该地区经历了多次干旱化的过程。

Abstract: We report a specimen of Protanancus tobieni discovered in 1976 from the Anwan Section, Qinan County, Gansu Province, including a complete mandible with lower incisors and a complete left upper incisor. The material can be referred to the same species as P. tobieni from the middle Miocene Dingjiaergou locality, Ningxia Hui Autonomous Region, because they show almost identical characters. The mandibular symphysis is extremely elongated; the upper incisor is robust with a broad enamel band; the lower incisor is flattened with a biconcave, concentrated-laminated cross-section; and the m3 shows posttrite ornamentation and pseudo-anancody. The above features indicate that the specimen is referable to Protanancus rather than Amebelodon, as it has been previously assigned. Furthermore, P. tobieni shows relatively poor posttrite ornamentation and pseudo-anancody; the lower incisor has a biconcave cross-section —an autapomorphy. These features distinguish it from other known species of Protanancus, and thus support the validity of P. tobieni as a distinct species. Based on the field survey, we infer that the Anwan Section records continuously from the lower to the upper Miocene. The lithology is primarily composed of brownish-red sandy-mudstones and mudstones. The middle and lower members of the section are intercalated with glutenite, sandstone, and grayish-green marl lenses, which represent channel deposits developed on the flood plain; this reflects a fairly humid environment with a large aquatic environmental component, which existed during the early period of the Neogene. The upper member of the section contains numerous layers of calcareous muddy granules, demonstrating a multi-acidification process of environmental evolution that occurred after the middle Neogene.