A new ektopodontid possum (Diprotodontia, Ektopodontidae) from the Oligocene of central Australia, and its implications for phalangeroid interrelationships

The Ektopodontidae are an enigmatic group of phalangeroid marsupials known from the late Oligocene to the Early Pleistocene of Australia. Although represented to date only by isolated teeth and several partial dentaries and maxillae, their highly distinctive dental morphology has allowed three genera and nine species to be distinguished. Here, we describe possibly the geologically oldest ektopodontid, <i>Chunia pledgei</i> sp. nov., from the Oligocene Pwerte Marnte Marnte fossil locality of central Australia. Phylogenetic analyses of Phalangeroidea, using 80 primarily dental characters framed by a molecular scaffold, support placement of the new taxon in the genus <i>Chunia</i>. The analyses failed to recover species of the genus <i>Durudawiri</i> in a monophyletic Miralinidae, indicating that they require systematic review. We also transfer the purported basal phalangerid <i>Eocuscus sarastamppi</i> to Miralinidae (<i>Miralina sarastamppi</i> comb. nov.). Additionally, the M1 specimens used to describe the Early to Middle Miocene miralinid genus <i>Barguru</i>, and three species therein, are re-identified as deciduous third premolars from early macropodoids. These findings imply that the Miralinidae are known only from the late Oligocene, whereas the oldest named phalangerids are from the Early Miocene. From a functional consideration of ektopodontid dental morphology, we infer support for prior suggestions of a granivorous and/or frugivorous diet for them. The relative stage-of-evolution expressed by the new taxon is comparable to those in the lower faunal zones of the Namba and Etadunna formations, which supports a late Oligocene age for the Pwerte Marnte Marnte assemblage.

外齿兽科（Ektopodontidae）是一类隶属于袋貂超科（Phalangeroidea）的神秘有袋类，其化石记录分布于澳大利亚渐新世晚期至更新世早期。尽管目前仅通过孤立牙齿、数件局部齿骨与上颌骨化石为人所知，但该类群极具辨识度的牙齿形态已支持区分出3属9种。本文描述了产自澳大利亚中部渐新世Pwerte Marnte Marnte化石点的、可能为地质年代最古老的外齿兽科物种——新种*Chunia pledgei*（sp. nov.）。基于以分子支架为约束的80个以牙齿特征为主的系统发育分析，支持将该新分类群归入*Chunia*属。本次分析未将*Durudawiri*属物种归入单系的米拉林科（Miralinidae），表明该类群亟需开展系统性分类修订。本文同时将原被认为是基底袋貂科的*Eocuscus sarastamppi*归入米拉林科，新组合为*Miralina sarastamppi*（comb. nov.）。此外，原用于描述早-中中新世米拉林科*Barguru*属及其3个物种的M1（第一上臼齿）标本，经重新鉴定为早期袋鼠类的乳齿第三前臼齿。上述研究结果表明，米拉林科的化石记录仅见于渐新世晚期，而已知最古老的命名袋貂科物种则产自早中新世。通过对外齿兽科牙齿形态的功能学分析，本文支持此前关于该类群具有食谷性或食果性食性的推论。新分类群所体现的演化相对阶段，与Namba组与Etadunna组下部动物群带的类群相当，这一结果佐证了Pwerte Marnte Marnte化石组合的时代为渐新世晚期。