A new Toxodontidae (Mammalia, Notoungulata) from the upper Pliocene–lower Pleistocene of Uruguay

A specimen of Toxodontidae from the upper Pliocene–lower Pleistocene of Uruguay is studied in this paper. The material consists of associated skull and mandibular fragments previously identified as Dinotoxodon paranensis. The full description of the specimen, its comparison with different notoungulates, and a phylogenetic analysis are performed. According to the obtained results, Charruatoxodon uruguayensis, gen. et sp. nov. is recognized as a new taxon with unique features such as nasals compressed laterally, somewhat convex in transversal and longitudinal planes, incisive foramina deep and rounded, and other characteristics shared with several taxa of Toxodontidae. The mandibular expansion, present in the studied material, formerly used as a diagnostic character of D. paranensis (also present in Gyrinodon quassus, Pericotoxodon platignathus, and Hoffstetterius imperator), does not constitute a synapomorphy for these taxa as it appears to have evolved independently in different lineages according to our phylogeny. The new taxon is a medium-sized toxodontid more closely related phylogenetically to Toxodon platensis and D. paranensis than to other toxodontids. The family Toxodontidae constitutes a natural group. The clades within this group do not correspond to the subfamilies classically considered within the family, agreeing with later results. The new late Pliocene–early Pleistocene taxon from the Raigón Formation adds to the endemic fauna from Uruguay.

本文研究了一件产自乌拉圭上新统上部-更新统下部的弓齿兽科（Toxodontidae）标本。该标本材料为关联保存的头骨与下颌骨碎片，此前曾被鉴定为帕拉尼恐齿兽（Dinotoxodon paranensis）。本文对该标本进行了全面描述，将其与不同的南方有蹄类开展对比研究，并完成了系统发育分析。根据所得研究结果，我们建立新属新种查鲁弓齿兽（Charruatoxodon uruguayensis, gen. et sp. nov.），该新分类单元兼具独特特征与弓齿兽科多个类群共有的性状：其鼻腔侧扁，在横向与纵向平面上略呈凸状，切齿孔深且呈圆形。研究标本所保存的下颌扩张结构，此前曾被作为帕拉尼恐齿兽的鉴别特征（该特征也见于旋齿弓齿兽Gyrinodon quassus、扁腭周弓齿兽Pericotoxodon platignathus以及霍夫施泰捷尔兽Hoffstetterius imperator），但根据本次系统发育分析结果，该特征并非这些类群的共源性状，而是在不同演化支系中独立演化而来。该新分类单元为中型弓齿兽类，系统发育亲缘关系与扁齿弓齿兽（Toxodon platensis）、帕拉尼恐齿兽更为接近，而与其他弓齿兽类的亲缘关系相对较远。弓齿兽科（Toxodontidae）为一个自然类群，该类群内部的演化支与该科经典分类体系中的亚科并不相符，这与后续相关研究结果一致。这件产自莱贡组（Raigón Formation）的上新世晚期-更新世早期新分类单元，进一步丰富了乌拉圭的特有动物群。