The skull of the polydolopimorphian marsupialiform Epidolops ameghinoi is described in detail for the first time, based on a single well-preserved cranium and associated left and right dentaries plus additional craniodental fragments, all from the early Eocene (53–50 million year old) Itaboraí fauna in southeastern Brazil. Notable craniodental features of E. ameghinoi include absence of a masseteric process, very small maxillopalatine fenestrae, a prominent pterygoid fossa enclosed laterally by a prominent ectopterygoid crest, an absent or tiny transverse canal foramen, a simple, planar glenoid fossa, and a postglenoid foramen that is immediately posterior to the postglenoid process. Most strikingly, the floor of the hypotympanic sinus was apparently unossified, a feature found in several stem marsupials but absent in all known crown marsupials. “Type II” marsupialiform petrosals previously described from Itaboraí plausibly belong to E. ameghinoi; in published phylogenetic analyses, these petrosals fell outside (crown-clade) Marsupialia. “IMG VII” tarsals previously referred to E. ameghinoi do not share obvious synapomorphies with any crown marsupial clade, nor do they resemble those of the only other putative polydolopimorphians represented by tarsal remains, namely the argyrolagids. Most studies have placed Polydolopimorphia within Marsupialia, related to either Paucituberculata, or to Microbiotheria and Diprotodontia. However, diprotodonty almost certainly evolved independently in polydolopimorphians, paucituberculatans and diprotodontians, and Epidolops does not share obvious synapomorphies with any marsupial order. Epidolops is dentally specialized, but several morphological features appear to be more plesiomorphic than any crown marsupial. It seems likely Epidolops that falls outside Marsupialia, as do morphologically similar forms such as Bonapartherium and polydolopids. Argyrolagids differ markedly in their known morphology from Epidolops but share some potential apomorphies with paucituberculatans. It is proposed that Polydolopimorphia as currently recognised is polyphyletic, and that argyrolagids (and possibly other taxa currently included in Argyrolagoidea, such as groeberiids and patagoniids) are members of Paucituberculata. This hypothesis is supported by Bayesian non-clock phylogenetic analyses of a total evidence matrix comprising DNA sequence data from five nuclear protein-coding genes, indels, retroposon insertions, and morphological characters: Epidolops falls outside Marsupialia, whereas argyrolagids form a clade with the paucituberculatans Caenolestes and Palaeothentes, regardless of whether the Type II petrosals and IMG VII tarsals are used to score characters for Epidolops or not. There is no clear evidence for the presence of crown marsupials at Itaboraí, and it is possible that the origin and early evolution of Marsupialia was restricted to the “Austral Kingdom” (southern South America, Antarctica, and Australia).

中文翻译：

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来自巴西东南部早始新世 Itaboraí 动物群的 Epidolops ameghinoi 的头骨，以及已灭绝的有袋动物目 Polydolopimorphia 的亲缘关系

首次详细描述了多头形有袋动物 Epidolops ameghinoi 的头骨，基于一个保存完好的头盖骨和相关的左右牙齿以及额外的颅牙碎片，所有这些碎片都来自始新世早期（53-5000 万年）Itaboraí巴西东南部的动物群。E. ameghinoi 的显着颅牙特征包括没有咬肌、非常小的上颚骨窗、一个突出的翼状窝被突出的外翼骨嵴横向包围、缺少或很小的横管孔、一个简单的平面关节窝和一个关节盂后孔紧接在关节盂后突的后面。最引人注目的是，下鼓窦底部显然没有骨化，这一特征在几种有茎有袋动物中都有发现，但在所有已知的冠有袋动物中都没有。先前从 Itaboraí 描述的“II 型”有袋动物岩体似乎属于 E. ameghinoi；在已发表的系统发育分析中，这些岩层落在（冠枝）有袋类动物之外。之前提到的 E. ameghinoi 的“IMG VII”跗骨与任何冠有袋动物进化枝没有明显的突触，它们也不类似于唯一以跗骨残骸为代表的其他假定的多足形类动物，即 argyrolagids。大多数研究都将 Polydolopimorphia 置于有袋动物内，与 Paucituberculata 或 Microbiotheria 和 Diprotodontia 有关。然而，双原齿兽几乎可以肯定地在多足形目动物、少瘤齿兽类和双原齿兽类中独立进化，而 Epidolops 与任何有袋动物目都没有明显的突触。Epidolops 是牙科专业的，但一些形态特征似乎比任何冠有袋动物都更拟形。Epidolops 似乎很可能属于有袋动物，形态相似的形式如 Bonapartherium 和 polydolopids 也是如此。Argyrolagids 在它们已知的形态上与 Epidolops 显着不同，但与 paucituberculatans 有一些潜在的异形。建议目前公认的 Polydolopimorphia 是多系的，并且 argyrolagids（以及目前包括在 Argyrolagoidea 中的可能其他分类群，例如 groeberiids 和 patagoniids）是 Paucituberculata 的成员。这一假设得到了对总证据矩阵的贝叶斯非时钟系统发育分析的支持，该矩阵包含来自五个核蛋白编码基因、插入缺失、反转录子插入和形态特征的 DNA 序列数据：Epidolops 属于有袋动物，而 argyrolagids 与 paucituberculatans Caenolestes 和 Palaeothentes 形成一个进化枝，无论是否使用 II 型岩层和 IMG VII 跗骨为 Epidolops 的角色评分。没有明确的证据表明在 Itaboraí 存在冠有袋动物，有袋动物的起源和早期进化可能仅限于“澳大利亚王国”（南美洲南部、南极洲和澳大利亚）。