Abstract: A new myodont rodent is reported from the lower part of the Arshanto Formation in Nuhetingboerhe of Nei Mongol, which represents one of the earliest and most primitive myodont rodents inAsia. The new species is characterized by several primitive features, such as M1 and M2 subequal in size, anterior cingulum(id) pronounced and having weak or no connection with the protocone and protoconid, presence of a small p4 or dp4 in at least some individuals, the m1 anteroconid weak or absent, protoconid and metaconid isolated, anterolophulid absent, hypoconulid on m2−3 distinct, hypolophid short or extending to the hypoconulid on m2−3, ectolophid weak, mesoloph(id) absent or weak. The new species is most similar to the Asian Aksyiromys, Primisminthus yuenus and other species. The primitive features of the new species suggest that the common ancestor of Eocene myodonts is probably similar to Erlianomys and originated inAsia in Early Eocene.

摘要： 努和廷勃尔和剖面位于内蒙古二连市西南40 km的呼和勃尔和地区，依据沉积间断可以将50 m厚的地层分为脑木根组和阿山头组，地层时代从中古新世到中始新世。在阿山头组下部层位中发现大量的啮齿类化石，其中一类原始的鼠齿类在此被命名为一个新的属种：Erlianomys combinatus gen. et sp. nov. (综合二连鼠)。其主要牙齿特征为：齿冠低，主尖较为发育，连接各尖的脊简单、细弱。有P4, m1有前压痕也表明有一个小的p4或者dp4。M1和M2大小相当。臼齿前齿带(下前齿带)明显，与原尖(下原尖)之间没有连接或连接很弱。M1和M2原尖后臂、后脊和次尖前臂在中尖处相交。m1的下前尖很弱或缺失，下原尖与下后尖之间连接很弱，基本为孤立的两个尖；m2−3下次小尖明显，下次脊短，有时直接与下次小尖相连；下外脊低矮、不发育。上、下臼齿都没有中脊或很弱。新属种的发现，为进一步认识古近纪啮齿类的起源和演化提供了新的证据。Erlianomys与北美的Elymys和亚洲的Aksyiromys, Primisminthus, Allosminthus, Palasiomys都有很多相似的特征，预示着它们可能有共同的祖先。在牙齿形态上，Erlianomys比中始新世的其他鼠齿类更为原始，可表明其产出层位即阿山头组下部的时代要早于中始新世，可能属于早始新世；其形态更接近亚洲的Aksyiromys, Primisminthus和Allosminthus, 而与北美的Elymys相差较大。因此推测鼠齿类的共同祖先可能与Erlianomys更为相似，早始新世时在亚洲起源，向其他大陆的迁移扩散不会晚于早始新世晚期。