A new proteid salamander from the early Oligocene of Romania with notes on the paleobiogeography of Eurasian proteids

The Proteidae represent a small group of perennibranchiate salamanders with a disjunct distribution in North America and southeastern Europe. Herein, we describe a new species of proteid salamander from the early Oligocene of Romania representing the geologically oldest member reported from the Cenozoic of Eurasia. Mioproteus gardneri, sp. nov., based on features of the vomers, dentaries, atlas, and vertebrae, appears to be closely allied with M. caucasicus and M. wezei, standing altogether as the sister group of Proteus, the only living European proteid. The apomorphies of the European clade, shared with the American members Necturus and Paranecturus, include the presence of dorsoventrally flattened anterior cotyles and alar-like processes, whereas a few other characters appear to be more derived in the European forms, such as the diverging anterior cotyles of the atlas enclosing an angle of 225°, the trunk vertebrae with strongly bifurcated neural spines, and unicipitate transverse processes. Peculiarly, the proteid remains from the early Oligocene locality of Suceag 1, Romania, were accompanied by the palaeobatrachid frog Albionbatrachus, a form closely linked to aquatic environments and having a limited fossil record in the Paleogene (upper Eocene of southern England, lower Oligocene of Romania, and upper Oligocene of Germany), suggesting that these fully aquatic groups probably used similar freshwater corridors to reach more southerly European territories.

洞螈科（Proteidae）是一类小型终生具外鳃的蝾螈类群，在北美洲与欧洲东南部呈现间断分布格局。本文记述了一件产自罗马尼亚渐新世早期的洞螈科蝾螈新种，该物种是目前欧亚大陆新生代地层中已报道的地质年代最古老的洞螈科类群成员。Gardner氏拟洞螈（Mioproteus gardneri，sp. nov.）基于犁骨（vomers）、齿骨（dentaries）、寰椎（atlas）及椎骨（vertebrae）的形态特征，与高加索拟洞螈（M. caucasicus）和韦氏拟洞螈（M. wezei）亲缘关系密切，三者共同构成洞螈属（Proteus，欧洲现存唯一的洞螈科类群）的姊妹群。欧洲洞螈支系的特有衍征与美洲类群泥螈属（Necturus）和副泥螈属（Paranecturus）共有，包括背腹扁平的前关节窝与翼状突起；而欧洲类群还演化出若干更为衍化的特征，例如寰椎的前关节窝呈发散状且夹角达225°、躯干椎的神经棘强烈分叉以及单头式横突。值得注意的是，产自罗马尼亚Suceag 1化石点渐新世早期的洞螈科化石，与古锄足蟾科（Palaeobatrachidae）蛙类阿尔比恩蟾属（Albionbatrachus）伴生。该类群严格依赖水生环境，在古近纪的化石记录较为有限（仅见于英格兰南部始新世上部、罗马尼亚渐新世下部以及德国渐新世上部地层），这表明这两类完全水生的类群可能通过相似的淡水通道迁徙至欧洲南部区域。