The taxonomy of the oryzomyine genus Neacomys currently is in a state of flux: systematic studies in the last 20 years increased its diversity from four to 16 species, with an additional several undescribed phylogenetic lineages. Despite this progress, morphological variation and species limits remain poorly known for several species groups and complexes within the genus, such as N. tenuipes, N. musseri, and N. dubosti. Here we analyze the variation of morphological characters and morphometric patterns of two new candidate species of Neacomys for northern Venezuela and Pará state, Brazil, that are characterized by the presence of derived carotid circulation, a rare character state for the genus. Analyzed material included holotypes, type series, and/or topotypes of almost all described species of Neacomys, including type series of N. musseri, the holotype of N. tenuipes, and sequenced specimens of N. xingu. Qualitative comparison showed that 25 characters are informative for the distinction of the two new forms, and morphometric analyses corroborated the morphological separation of the new species. Results also point that N. tenuipes might represent a species complex. The new forms are sympatric with other species, including N. tenuipes and N. xingu, and can be identified by a suite of morphological characters, including the presence of a derived carotid pattern and, for the Pará form, by a unique morphology of the first upper molar. We review the occurrence of carotid circulation patterns within Sigmodontinae and discuss its use for the systematics of the subfamily. Further studies involving the new Neacomys species will contribute to understanding the biogeographic patterns and evolutionary trends within this distinct and diverse genus.

中文翻译：

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啮齿动物属的系统学 Neacomys Thomas (Cricetidae: Sigmodontinae): 两个新物种和颈动脉模式的讨论

稻瘟病菌属 Neacomys 的分类学目前处于不断变化的状态：过去 20 年的系统研究将其多样性从 4 种增加到 16 种，另外还有几个未描述的系统发育谱系。尽管取得了这一进展，但对于该属内的几个物种群和复合体，例如 N. tenuipes、N. musseri 和 N. dubosti，形态变异和物种限制仍然知之甚少。在这里，我们分析了委内瑞拉北部和巴西帕拉州的两种新的 Neacomys 候选物种的形态特征和形态模式的变化，其特征是存在衍生的颈动脉循环，这是该属罕见的特征状态。分析的材料包括几乎所有描述的 Neacomys 物种的正型模型、类型系列和/或拓扑型，包括 N. musseri 的类型系列，N. tenuipes 的正型标本和 N. xingu 的测序标本。定性比较表明，25个性状对区分两种新形态具有参考意义，形态计量学分析证实了新种的形态分离。结果还指出 N. tenuipes 可能代表一个物种复合体。新的形式与其他物种，包括 N. tenuipes 和 N. xingu 同域，并且可以通过一套形态特征来识别，包括存在衍生的颈动脉模式，对于 Pará 形式，可以通过独特的形态特征来识别。第一上磨牙。我们回顾了 Sigmodontinae 内颈动脉循环模式的发生，并讨论了它在亚科系统学中的用途。