Ancient cave deposits across South Wales and southwest England have yielded Late Triassic to Early Jurassic tetrapod faunas that occupied a sub-tropical island archipelago. Here we present a hitherto little studied fissure at Ruthin Quarry in South Wales. A tooth of the primitive neoselachian shark Rhomphaiodon minor indicates the age of the fissure as early Rhaetian, some 205 Ma, possibly equivalent to the first bone-bed horizons in the Westbury Formation. We identify 11 taxa, including chondrichthyans, procolophonids including a new genus and species named here Smilodonterpeton ruthinensis, rhynchocephalians, the trilophosaurid Tricuspisaurus thomasi and archosaurs, including a small crocodylomorph like Terrestrisuchus. Ruthin is dominated by procolophonids and secondarily by archosaurs. Rhynchocephalians are rare at Ruthin, unlike at penecontemporaneous fissure localities. Many of these faunas are reminiscent of those found in Carnian-Norian deposits from North America, some of them 25 Myr older, suggesting the Ruthin faunas are relictual. In exploring the palaeogeography of Ruthin and other British Late Triassic fissure deposits, we find that factors in addition to island size may affect the recorded species richness of these islands. Further, most of the taxa appear to be ‘primitive’ (i.e. possess closest relatives that are much older stratigraphically), and their body sizes are small, so this could be the oldest record of insular dwarfism.

横跨南威尔士和英格兰西南部的古代洞穴沉积物已产生了三叠纪至侏罗纪早期的四足动物群，这些动物群占据了亚热带岛屿群岛。在这里，我们介绍了迄今为止在南威尔士的Ruthin Quarry进行的很少研究的裂缝。原始的新南海鲨鲨Rhomphaiodon小齿的齿表明裂隙的年龄为早Rhaetian，约205 Ma，可能相当于Westbury组的第一个骨床层。我们确定11类群，其中包括chondrichthyans，procolophonids包括一个新的属种这里命名Smilodonterpeton ruthinensis，rhynchocephalians的trilophosaurid Tricuspisaurus thomasi和祖龙，包括像小鳄形超目陆鳄。芦丁主要由原生动物构成，其次是弓龙类。在露辛市，鼻畸形人很少见，而在Penecontemporaneous裂隙地区不常见。这些动物中的许多都让人联想到来自北美的Carnian-Norian矿床中的动物，其中一些年龄比25 Myr大，这表明Ruthin动物区系属于遗迹。在研究Ruthin和其他英国晚期三叠纪裂隙沉积物的古地理时，我们发现除岛屿大小以外的其他因素也可能影响这些岛屿记录的物种丰富度。此外，大多数分类单元似乎是“原始的”（即拥有地层上较近的近亲），而且它们的体型很小，因此这可能是岛上矮化的最早记录。