On the existence of non-microbiotherian Australidelphian marsupials (Diprotodontia) in the Eocene of Patagonia

A diverse assemblage of extinct mammals of early–middle Eocene age (Ypresian–Lutetian boundary) come from the Patagonian localities of La Barda and Laguna Fría around Paso del Sapo in northwestern Chubut Province (Argentina). Metatherians are well represented, mostly by dental remains of ‘Didelphimorphia’, Paucituberculata, Sparassodonta, Microbiotheria, and Polydolopimorphia. Here we analyse three calcanea and one astragalus referable to the same, indeterminate taxon, from La Barda, showing the fusion of their ectal and sustentacular facets. This facet arrangement characterizes the Australidelphia, a marsupial clade represented by Microbiotheria from South America and Antarctica, plus all Australasian lineages. Other australidelphian features shown by these tarsals include: in the calcanea, a reduced peroneal process and a tripartite cuboid facet; in the astragalus, a round, very small astragalar head in relation to the body; a convex trochlea with a half for the tibia and half, in sli...一批类群多样的早-中始新世（伊普雷斯阶-鲁蒂亚阶界线）灭绝哺乳动物化石，产自阿根廷丘布特省西北部帕索德尔萨波周边的巴塔哥尼亚地区两处遗址：拉巴尔达（La Barda）与拉古纳弗里亚（Laguna Fría）。后兽类（Metatherians）化石记录较为丰富，其遗存多为负鼠目（Didelphimorphia）、少带负鼠目（Paucituberculata）、袋剑虎目（Sparassodonta）、微兽目（Microbiotheria）以及多齿负鼠目（Polydolopimorphia）的牙齿化石。本文对采自拉巴尔达遗址的3块跟骨（calcanea）与1块距骨（astragalus）展开分析，这些标本隶属于同一未定属种类群，其特征为外踝面与载距突关节面发生愈合。该关节面排列方式是澳洲有袋类（Australidelphia）的典型特征，这一有袋类支系涵盖南美与南极洲的微兽目（Microbiotheria）类群，以及所有澳大拉西亚支系的有袋类。这批跗骨还展现出其他澳洲有袋类特征：在跟骨上，腓骨肌突退化且具有三分骰骨关节面；在距骨上，相较于距骨体，其距骨头呈圆形且体型极小；距骨滑车呈凸状，一半区域与胫骨形成关节，另一半则呈鞋套状