Beck Robin M D
School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW 2052, Australia.
Naturwissenschaften. 2012 Sep;99(9):715-29. doi: 10.1007/s00114-012-0953-x. Epub 2012 Aug 5.
Recent molecular data strongly support the monophyly of all extant Australian and New Guinean marsupials (Eomarsupialia) to the exclusion of extant South American marsupials. This, together with available geological and fossil evidence, has been used to argue that the presence of marsupials in Australia is simply the result of a single dispersal event from South America during the latest Cretaceous or Palaeocene, without subsequent dispersals between the two continents. Here, I describe an isolated ankle bone (calcaneus) of a metatherian from the early Eocene Tingamarra Local Fauna in northeastern Australia. Strikingly, this specimen, QM F30060, lacks the 'continuous lower ankle joint pattern' (CLAJP), presence of which is a highly distinctive apomorphy of the marsupial clade Australidelphia, which includes Eomarsupialia, the living South American microbiotherian Dromiciops and the Tingamarran fossil marsupial Djarthia. Comparisons with a range of marsupials and stem-metatherians strongly suggest that the absence of the CLAJP in QM F30060 is plesiomorphic, and that this specimen represents the first unequivocal non-australidelphian ('ameridelphian') metatherian known from Australia. This interpretation is confirmed by phylogenetic analyses that place QM F30060 within (crown-group) Marsupialia, but outside Australidelphia. Based on these results, the distribution of marsupials within Gondwana cannot be explained by simply a single dispersal event from South America and Australia. Either there were multiple dispersals by marsupials (and possibly also stem-metatherians) between South America and Australia, in one or both directions, or, alternatively, there was a broadly similar metatherian fauna stretching across southern South America, Antarctica and Australia during the Late Cretaceous-early Palaeogene.
最近的分子数据有力地支持了所有现存澳大利亚和新几内亚有袋类动物（始有袋目）构成单系群，而现存南美有袋类动物不包含在内。这一点，连同现有的地质和化石证据，被用来论证澳大利亚有袋类动物的存在仅仅是在白垩纪晚期或古新世期间从南美洲单次扩散事件的结果，此后两大洲之间没有后续的扩散。在此，我描述了一块来自澳大利亚东北部始新世早期廷加马拉当地动物群的有袋类踝关节骨头（跟骨）。引人注目的是，这个标本，即昆士兰博物馆F30060，缺乏“连续的下踝关节模式”（CLAJP），而该模式的存在是有袋类支系澳洲有袋亚目的一个高度独特的衍征，澳洲有袋亚目包括始有袋目、现存南美有袋类的南猊以及廷加马拉化石有袋类的雅氏袋兽。与一系列有袋类动物和有袋类干群的比较强烈表明，昆士兰博物馆F30060中CLAJP的缺失是祖征，并且这个标本代表了从澳大利亚已知的首个明确的非澳洲有袋亚目（“美洲有袋亚目”）有袋类动物。系统发育分析将昆士兰博物馆F30060置于（冠群）有袋目之内，但在澳洲有袋亚目之外，这证实了这一解释。基于这些结果，冈瓦纳大陆内有袋类动物的分布不能简单地用从南美洲到澳大利亚的单次扩散事件来解释。要么有袋类动物（可能还有有袋类干群）在南美洲和澳大利亚之间有多次双向扩散，要么在晚白垩世 - 早古近纪期间，南美洲南部、南极洲和澳大利亚有广泛相似的有袋类动物群。
