Abstract: An almost complete Ursavus skull in association with its mandible is described. The skull was recently found from upper part of Liushu Formation in Linxia Basin. Its stratigraphic level and geologic age are correlated to the late Bahean ALMA/S, ~8 Ma. It represents a new species, here named Ursavus tedfordi. Cladistic analysis is conducted using the TNT software, based on a matrix of 11 taxa and 37 characters. The tree 5 of the 8 most parsimonious trees is chosen as the most reliable to reflect the phylogenetic history of the ursid clade. As the tree 5 shows, after divergence from the Oligocene-Early Miocene hemicyonids (Cephalogale), the ursid clade first yielded two stem-taxa: Ballusia elmensis and B. orientalis, the latter of which being an aberrant branch. Then, two major subclades emerged: one comprising Kretzoiarctos, Agriarctos and Ailurarctos, the other containing all Ursavus species and their descendants including all living bears (excluding Ailuropoda). Kretzoiarctos may not be the direct ancestral form of the giant panda as Abella and colleagues (2012) suggested, but the ancestral form of the lineage of Indarctos(+Agriotherium ?). Among the numerous Ursavus species, U. tedfordiis the most advanced and closest related to the living bears (excludingAiluropoda) in morphology, however, might belong to a side-branch, judging by the autapomorphies possessed by it.

摘要： 记述了一件与下颌咬合在一起的完整的祖熊头骨化石。化石发现于甘肃临夏州广河县槐沟村柳树组地层中上部，其层位与时代为新近纪灞河阶/期的晚期，距今约8 Ma。对比研究表明，该头骨与目前所有已知熊类祖先类型都不同，应为一新种，定名为Ursavus tedfordi (戴氏祖熊)。采用TNT软件包对11种祖先熊类和37个特征组成的数据进行不同条件(non-additive, additive等)下全搜索(Implicit Enumeration), 并采用自引导法(bootstrapping)进行可信度评价。我们从基于部分特征加积(partially additive)条件下所得出8个最简约树中选取树5作为最可信简约树。该树表明，熊科从渐新世－早中新世的豺熊类(hemicyonids)分出后产生了两类基干类型，Ballusia elmensis和B. orientalis。后者稍更进步，可能为一特异支。此后产生了两大次支：一支向印度熊(可能还有郊熊) +大熊猫演化；另一支则由中－晚中新世的祖熊(Ursavus)和此后由某类祖熊所产生的现生熊类(不包括大熊猫)组成。从现有的化石判断，新建戴氏祖熊是与现生熊类在形态上最为接近的祖先类型，但戴氏祖熊具有某些自近裔性状，可能为一早期旁支。