Data from: Eocene Pachynolophinae (Perissodactyla, Palaeotheriidae) from China and their paleobiogeographic implications

The Eocene Palaeotheriidae was traditionally considered a nearly endemic European group within Equoidea, but a few palaeotheres have been reported from Asia. Here I reanalyze a maxilla containing M1-3 from the Lunan Basin, Yunnan Province, China. This element was initially assigned to a new tapiromorph species, Lophialetes yunnanensis, but I erect a new genus Lophiohippus within Pachynolophinae for the Yunnan specimen based mainly on the absence of mesostyles, the strongly inclined metalophs, the strong development of lophodonty, parastyles overlapping metastyles of preceding teeth and situated mesial to the paracone, and the fact that M3 is longer than wide and has a large and buccally deflected metastyle. L. yunnanensis differs from European Anchilophus and Paranchilophus in that the parastyles are situated mesial or even slightly lingual, rather than mesiobuccal, to the paracones, and M3 is relatively pronouncedly larger than M1. I further reanalyze Qianohippus magicus from the Shinao Basin of Guizhou Province, China, in which the complete dentition is known. Qianohippus is characterized by a molariform P2 and non-molariform P3-4; a relatively high degree of lophodonty; the absence of mesostyles; an angular bending in the protoloph on P3-M3 and the metaloph on M1-3 at the paraconule and metaconule, respectively; and weakly developed “metastylid” on the lower cheek teeth. A cladistic analysis supports a close relationship between Lophiohippus yunnanensis and Paranchilophus, and suggests that Qianohippus is closely related to some derived pachynolophs. The appearance of the pachynolophins Lophiohippus and Qianohippus in China supports the existence of a biogeographic connection between Europe and Asia in the Middle-Late Eocene, and the dispersal route was probably along the Tethyan micro-continents in the south.

始新世古兽科（Eocene Palaeotheriidae）传统上被视为马总科（Equoidea）中一个近乎特有于欧洲的类群，但此前仅有少量古兽类化石在亚洲被报道。本文重新研究了产自中国云南省路南盆地的一件包含M1-3的上颌骨标本。该标本最初被归入一个新的貘形类物种——云南脊貘（Lophialetes yunnanensis），但本文基于以下多项特征，将该云南标本归入厚兽亚科（Pachynolophinae）下的新属脊齿马属（Lophiohippus）：无中尖（mesostyles）、后脊（metalophs）强烈倾斜、脊齿型（lophodonty）发育程度高、前尖（parastyles）覆盖前一枚牙齿的后尖且位于原尖（paracone）近中位置，以及M3长大于宽且后尖（metastyle）硕大并向颊侧偏转。

云南脊齿马（Lophiohippus yunnanensis）与欧洲的锚兽属（Anchilophus）及副锚兽属（Paranchilophus）的区别在于：其前尖位于原尖近中甚至略偏舌侧，而非近中颊侧，且M3相对M1显著更大。

本文还重新研究了产自中国贵州省石垴盆地的神奇黔马（Qianohippus magicus），该物种的完整齿列已有报道。黔马属（Qianohippus）的鉴别特征包括：P2为臼齿化前臼齿，P3-4为非臼齿化前臼齿；脊齿型发育程度较高；无中尖；P3-M3的原脊（protoloph）与M1-3的后脊分别在前小尖（paraconule）和后小尖（metaconule）处呈角状弯折；下颊齿的下后尖（metastylid）发育较弱。

分支系统学分析结果显示，云南脊齿马与副锚兽属（Paranchilophus）亲缘关系密切，且黔马属与若干进步的厚兽类类群亲缘关系较近。脊齿马属与黔马属这两种厚兽类在中国的发现，支持了中晚始新世欧洲与亚洲之间存在生物地理联系的假说，其物种扩散路径可能沿南部的特提斯微大陆（Tethyan micro-continents）展开。