“Sparassocynids” are small, carnivorously-adapted marsupials known from the late Miocene and Pliocene of South America, thought to be relatives of living didelphid opossums but of otherwise uncertain phylogenetic relationships. Here, we describe a nearly complete juvenile skull of the “sparassocynid” Sparassocynus derivatus, from the Pliocene (~5–3 million years old) Chapadmalal Formation, Argentina. It provides new information on the morphology of Sparassocynus, including the deciduous dentition, and (together with previously collected specimens) allows reinterpretation of the derived auditory region of “sparassocynids.” The new specimen also exhibits several distinctive apomorphies characteristic of Didelphidae and of subclades within the family. Undated Bayesian phylogenetic analysis of a total evidence dataset (132 craniodental characters, 7.3 kb of DNA sequence data from five nuclear genes) places “sparassocynids” within the didelphid genus Monodelphis, whereas “tip-and-node” dating analysis of the same dataset with an Independent Gamma Rates (IGR) clock model places them as sister to Monodelphis, showing that temporal information influenced the resultant topology. We conclude that “sparassocynids” warrant tribal separation only, as Sparassocynini, new rank. Based on our dated phylogeny, we also provide a revised scenario for didelphid diversification. Crown-clade didelphids probably originated close to the Oligocene-Miocene boundary. We agree with previous proposals that the appearance of carnivorously-adapted didelphids in South America during the late Miocene, including sparassocynins, is likely related to a decline in diversity of the sparassodonts at this time, and that the disappearance of these carnivorously-adapted didelphids at the end of the Pliocene may have been due to the arrival of placental carnivorans, such as mustelids, from North America.

中文翻译：

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阿根廷上新世有袋动物 Sparssocynus derivatus 的近乎完整的幼年头骨、“Sparassocynids”的亲缘关系以及负鼠的多样化（有袋类；Didelphimorphia；Didelphidae）

“Sparassocynids”是南美洲中新世晚期和上新世已知的小型食肉有袋动物，被认为是活的双豚负鼠的亲戚，但在其他方面尚不确定系统发育关系。在这里，我们描述了来自阿根廷上新世（约 5-300 万年）Chapadmalal 组的“sparassocynid”Sparssocynus derivatus 的几乎完整的幼年头骨。它提供了关于 Sparassocynus 形态的新信息，包括乳牙列，并且（连同以前收集的标本）允许重新解释“sparassocynids”的衍生听觉区域。新标本还展示了双翅目和科内亚进化枝的几个独特的异形性特征。总证据数据集的未注明日期的贝叶斯系统发育分析（132 个颅牙特征，7. 来自五个核基因的 3 kb DNA 序列数据）将“sparassocynids”置于双海豚属中，而使用独立伽马率 (IGR) 时钟模型对同一数据集进行“尖端和节点”测年分析将它们作为姐妹Monodelphis，表明时间信息影响了结果拓扑。我们得出的结论是，“sparassocynids”仅保证部落分离，如 Sparassocynini，新等级。根据我们过时的系统发育，我们还提供了双海豚多样化的修订方案。冠枝双海豚可能起源于渐新世-中新世边界附近。我们同意先前的提议，即在中新世晚期南美洲出现食肉性双鱼科动物，包括鱼尾藻，可能与此时鱼齿龙的多样性下降有关，