Dental shape variation of Neogene Pachyrukhinae (Mammalia, Notoungulata, Hegetotheriidae): systematics and evolutionary implications for the late Miocene Paedotherium species

Paedotherium was the last representative of the pachyrukhines (Notoungulata, Hegetotheriidae), a small rabbit- or caviomorph rodent-like clade, with hypsodont and simplified dentition. In contrast to the Pliocene Paedotherium species, the Miocene species are less known and mainly represented by teeth. Different systematic studies of Paedotherium based on the same set of subtle trait variations of the cheek teeth have arrived at different proposals. With the aim of testing the validity and analysing the morphological variation of Paedotherium species, and assessing the definition and diagnostic value of teeth traits, we performed a dual approach: a traditional qualitative study and exploratory geometric morphometric analyses of the upper and lower cheek teeth (both, partial and complete series). In addition, the configurations of cheek tooth shape were used as landmark characters, in combination with traditional characters, to perform a phylogenetic analysis. We propose the validity of the late Miocene P. borrelloi Zetti, and the assignment of north-western Argentinian representatives of Paedotherium to P. aff. minor. Paedotherium borrelloi is morphologically more similar and phylogenetically more closely related to the Pliocene P. bonaerense than to other Paedotherium species. Our results suggest that during the late Miocene the coexistence of two Paedotherium lineages was already established, whose main morphological differences persisted or even intensified in Pliocene representatives, supporting a more complex evolutionary scenario than previously believed.巴德尔兽属（Paedotherium）是厚突颌兽类（pachyrukhines）的最后一个代表类群，该类群隶属于后弓兽目（Notoungulata）赫格托兽科（Hegetotheriidae），是一类体型类似兔子或豚鼠型啮齿类的演化支，具有高冠齿且齿列简化的特征。与上新世的巴德尔兽属物种不同，中新世的该属物种研究较为匮乏，且主要仅以牙齿化石为研究材料。以往基于颊齿细微性状差异开展的多项系统学研究，对巴德尔兽属的分类提出了各不相同的假说。为验证巴德尔兽物种种级划分的有效性、分析其形态变异规律，并评估牙齿性状的定义与分类诊断价值，本研究采用了双重研究方案：一是传统的定性形态学研究，二是针对上下颌颊齿（包括部分齿列与完整齿列）的探索性几何形态测量学分析。此外，本研究将颊齿形态构型作为地标特征，结合传统形态性状开展了系统发育分析。本研究支持晚中新世的P. borrelloi Zetti的物种有效性，并将阿根廷西北部的巴德尔兽属标本归入P. aff. minor。巴德尔兽属的P. borrelloi在形态上与上新世的P. bonaerense更为相似，且系统发育关系也更接近后者，而非其他巴德尔兽属物种。本研究结果表明，晚中新世时期已有两支巴德尔兽谱系共存，二者的主要形态差异在随后的上新世类群中甚至进一步强化，这支持巴德尔兽属的演化场景比此前学界认知的更为复杂。