Anatomy and systematics of Notohippus toxodontoides Ameghino, 1891 (Mammalia, Notoungulata), from the Miocene of Santa Cruz Province, Argentina

The specimens described herein come from the Río Bote locality, Santa Cruz Formation (early Miocene), Santa Cruz Province, Argentina. They include the most complete cranial material of the ‘Notohippidae’ Notohippus toxodontoides and other remains that increase knowledge of the mandible and dentition. The new remains allow extension of the original description proposed by Ameghino in the 19th century and provide data on ontogenetic variation in the dental traits of this species. Notohippus differs from the Deseadan–Colhuehuapian genus Argyrohippus in the following features: discontinuous dentition; C and p1 absent; shorter P2–M3/p2–m3 series; crown of I1 highly curved; upper molars with less developed parastyle and paracone fold, less undulating ectoloph, deeper distolingual groove of M3 directed more lingually and presence of a very shallow distolabial groove; incisiform lower canine; and lower molars with posterolabial angle of talonid more rounded and narrower trigonid valley. As stated in previous works, our phylogenetic analysis indicates that the ‘Notohippidae’ do not constitute a monophyletic group, because Argyrohippus, Notohippus, and Toxodontidae are recovered as a clade. Internal relationships within this clade are not resolved in any of the phylogenetic analyses performed here. The presence of N. toxodontoides in the faunal assemblage found at the Río Bote locality adds to its previous records in Santa Cruz (Kar-Aiken and Cerro Centinela localities) Province, Argentina, and Sierra Baguales locality in Chile.本次描述的标本采自阿根廷圣克鲁斯省圣克鲁斯组（中新世早期）的里奥博特（Río Bote）化石点。这批标本包含了弓齿兽科（Notohippidae）下属箭齿弓兽（Notohippus toxodontoides）最为完整的头骨材料，以及其他可增进学界对该类群下颌骨与齿列认知的化石遗存。本次发现的新标本补充了19世纪阿梅吉诺（Ameghino）最初提出的物种描述，并为该物种齿部特征的个体发育变异提供了数据支撑。 箭齿弓属（Notohippus）与德塞达南期（Deseadan）-科尔韦瓦皮亚南期（Colhuehuapian）的阿吉罗弓兽属（Argyrohippus）存在如下形态差异：齿列不连续；犬齿（C）与第一前臼齿（p1）缺失；P2-M3/p2-m3齿段更短；第一门齿（I1）齿冠高度弯曲；上臼齿的前尖柱（parastyle）与前尖褶（paracone fold）发育程度更低，外脊（ectoloph）起伏更平缓，M3的远中舌沟更深且更偏向舌侧，同时存在极浅的远中唇沟；下颌犬齿呈门齿形态；下臼齿的跟座（talonid）后唇角更为圆润，三角座谷（trigonid valley）更为狭窄。 正如此前研究成果所言，本次系统发育分析表明，弓齿兽科（Notohippidae）并非单系类群，因为阿吉罗弓兽属、箭齿弓属与箭齿兽科（Toxodontidae）被复原为一个单系支。本次开展的所有系统发育分析均未能解析该单系支内部的系统发育关系。 里奥博特（Río Bote）产地的动物群组合中发现箭齿弓兽（N. toxodontoides），进一步补充了该物种在阿根廷圣克鲁斯省（此前发现于Kar-Aiken与Cerro Centinela产地）以及智利巴瓜莱斯山（Sierra Baguales）产地的化石记录。