Rougier G W, Wible J R, Novacek M J
Department of Anatomical Sciences and Neurobiology, School of Medicine, University of Louisville, Kentucky 40292, USA.
Nature. 1998 Dec 3;396(6710):459-63. doi: 10.1038/24856.
We describe here two new specimens of the mammal Deltatheridium pretrituberculare from the Late Cretaceous period of Mongolia. These specimens provide information on tooth replacement in basal therian mammals and on lower jaw and basicranial morphology. Deltatheroidans, known previously from isolated teeth, partial rostra and jaws from the late Cretaceous of Asia and possibly North America, have been identified variously as eutherians, as basal metatherians (the stem-based clade formed by marsupials and their extinct relatives), or as an outgroup to both eutherians and metatherians. Resolution of these conflicting hypotheses and understanding of the early evolution of the therian lineage have been hampered by a sparse fossil record for basal therians. The new evidence supports metatherian affinities for deltatheroidans and allows a comprehensive phylogenetic analysis of basal metatherians and marsupials. The presence of specialized marsupial patterns of tooth replacement and cranial vascularization in Deltatheridium and the basal phylogenetic position of this taxon indicate that these features are characteristic of Metatheria as a whole. Other morphological transformations recognized here secure the previously elusive diagnosis of Metatheria. The new specimens of Deltatheridium illustrate the effectiveness of fairly complete fossil specimens in determining the nature of early evolutionary events.
我们在此描述了两件来自蒙古晚白垩世的哺乳动物三角齿兽（Deltatheridium pretrituberculare）的新标本。这些标本提供了关于基干兽亚纲哺乳动物牙齿替换以及下颌和颅底形态的信息。三角齿兽类此前仅在亚洲晚白垩世以及可能在北美洲发现过孤立的牙齿、部分吻部和颌骨，它们曾被分别鉴定为真兽类、基干后兽类（由有袋类及其已灭绝的近亲组成的基于分支的类群），或者是真兽类和后兽类的外类群。由于基干兽亚纲的化石记录稀少，这些相互矛盾的假说的解决以及对兽亚纲谱系早期演化的理解都受到了阻碍。新证据支持三角齿兽类与后兽类具有亲缘关系，并使得对基干后兽类和有袋类进行全面的系统发育分析成为可能。三角齿兽中存在特化的有袋类牙齿替换模式和颅骨血管分布，以及该分类单元的基干系统发育位置，表明这些特征是整个后兽亚纲的特征。这里识别出的其他形态转变确定了此前难以捉摸的后兽亚纲的诊断特征。三角齿兽的新标本说明了相当完整的化石标本在确定早期进化事件性质方面的有效性。
