Abstract: New material of Pararhizomys from the Late Miocene Baogeda Ula Formation of central Nei Mongol are described, which is referred to P. hipparionumTeilhard de Chardin & Young, 1931 based on the size, morphology and dentine tract height of teeth. Pararhizomys are scarcely known from the area around the Mongolian Plateau chronologically from the early Late Miocene throughout Middle Pliocene. The genus is similar to Brachyrhizomys, Rhizomys and Tachyoryctoides in having a simple dental pattern, but can be distinguished from them by its absence of mesoloph(id)s and having only 2−3 reentrants on the molars. For expediently judging and comparing of this genus with others, an “H” index has been set for measuring the height of dentine tract of molars, and the lower incisor of P. hipparionum from Baogeda Ula was sectioned to observe the enamel microstructure. The dentine tract of Pararhizomys seems to have the gradually heightening and undulating tendency in its evolution, as the same in siphneids and arvicolids. The enamel microstructure of lower incisor of P. hipparionum is remarkably different from those of the rhizomyines Brachyrhizomys and Rhizomys and tachyoryctoididTachyoryctoides by the very thick inner portion of enamel and arrangement of prisms and interprismatic matrix. According to the morphology of cheek teeth and enamel microstructure of incisor, it seems that Pararhizomys could not be included in Rhizomyidae, and possibly represents a separate lineage. Pararhizomys andTachyoryctoides, restricted biogeographically within the Palaearctic Region appears to be adapted for temperate, arid and steppe environment, whereas Brachyrhizomysand Rhizomys are the typical factors of the Oriental Realm, living in a warmer, more humid and forested environment.

摘要： 描述了1996－2007年间在内蒙古阿巴嘎旗晚中新世宝格达乌拉组采集的副竹鼠化石。材料包括下颌骨1件、牙齿14枚和肢骨4件，分别采自该组的2个地点。根据标本较大的尺寸、形态特征，以及颊齿的釉质曲线高度，宝格达乌拉材料被归入三趾马层副竹鼠Pararhizomys hipparionumTeilhard de Chardin & Young, 1931。Pararhizomys是一类具高冠、脊形和冠面构造简单颊齿的啮齿动物。该属目前发现的化石地点少，材料也不多，主要集中于中国北方和蒙古高原周缘，其地史分布为晚中新世早期至上新世中期。与鼢鼠类(siphneids)和鼠平类(arvicolids)一样，Pararhizomys属的颊齿有由低冠到高冠发展的趋势，具体表现为牙齿侧面釉质曲线(dentine tract)由平直、远离内外两侧谷底到起伏、接近甚至超过内外两侧谷底。为方便比较，本文为釉质曲线的高度设立“H”指数，即从每侧最深谷的谷底和釉质曲线的最高处取平行于冠面的平行线，此两平行线之间的垂直距离为“H”。H值越大，表明齿冠越低，反之则齿冠越高。尽管目前发现的副竹鼠标本不多，但似乎可以看到从早期到晚期，其颊齿的个体有从小变大，釉质曲线高度H值逐渐减小，即齿冠逐渐增高的趋势。由于Pararhizomys的牙齿形态多少与竹鼠类的低冠竹鼠Brachyrhizomys和竹鼠属Rhizomys及拟速掘鼠属Tachyoryctoides有相似之处，故常被与Brachyrhizomys和Rhizomys一起归入竹鼠科(Rhizomyidae), 但该属的牙齿不具中脊和下中脊，褶沟少，一般为2－3个，与上述三属有明显的不同。本文对下门齿所作的切片观察也表明，Pararhizomys的釉质层结构与Brachyrhizomys, Rhizomys和Tachyoryctoides者相差甚远，其内层(PI)明显增厚，釉柱和釉柱间质(IPM)的排列方式存在明显差别。颊齿的冠高、冠面形态，以及下门齿的釉质层结构，似乎都表明了Pararhizomys属不大可能归入竹鼠科或速掘鼠科(Tachyorictoididae)。而下门齿釉质层切片显示了Brachyrhizomys与Rhizomys有着高度相似的釉质结构，进一步证明了两者有较接近的亲缘关系。地理分布上，Pararhizomys与Tachyoryctoides属只发现于古北区，伴生的哺乳动物显示了其可能适应偏冷、干旱的草原环境，而Brachyrhizomys和Rhizomys主要局限于东洋区，共生的哺乳动物多能适应温暖、湿润的树林环境。