Phylogenetic relationships of the European trilophosaurids <i>Tricuspisaurus thomasi</i> and <i>Variodens inopinatus</i>

Trilophosauridae are an extinct group of archosauromorphs from the Upper Triassic of western North America and Nova Scotia. Possible trilophosaurids Tricuspisaurus thomasi and Variodens inopinatus have also been reported from Upper Triassic fissure fills in the southwest of the United Kingdom, based on their multi-cuspid dentitions. Here, we provide an updated description of V. inopinatus and add to the description of T. thomasi, including re-examining CT scans of its holotype dentary. We confirm that the apparent gap on the dentary of T. thomasi is pathological, with the dentine of the teeth on either side of the gap asymmetrically thick, and that heavy vascularization of the anterior part of the dentary confirms it likely had a keratinous rhamphotheca. Our cladistic analyses, to which we add nine new characters, confirm that T. thomasi and V. inopinatus are closely related taxa and the youngest known trilophosaurids. Further, we support a previous suggestion that the relatively small size of these two European trilophosaurids results from insular dwarfism.三尖齿龙科（Trilophosauridae）是一类已灭绝的主龙形类（archosauromorphs）类群，其化石发现于北美西部与新斯科舍省的晚三叠世地层中。基于多齿尖的齿列特征，疑似隶属于三尖齿龙科的三锥齿龙托马斯种（Tricuspisaurus thomasi）与奇异多齿龙（Variodens inopinatus）也被报道发现于英国西南部的晚三叠世裂隙堆积物中。本文对奇异多齿龙进行了更新版描述，并补充了三锥齿龙托马斯种的相关描述内容，包括重新检视其正模齿骨的CT扫描（CT scan）数据。研究证实，三锥齿龙托马斯种齿骨上的疑似齿列间隙为病理所致，间隙两侧牙齿的牙本质（dentine）厚度不对称；同时齿骨前部的高度血管化特征表明，该类群大概率具有角质喙鞘（keratinous rhamphotheca）。本次分支系统学分析（cladistic analyses）新增了9个形态特征，分析结果确认三锥齿龙托马斯种与奇异多齿龙为亲缘关系密切的类群，且是目前已知最年轻的三尖齿龙科成员。此外，本研究支持此前的假说：这两种欧洲三尖齿龙科类群体型相对偏小，是岛屿矮化（insular dwarfism）的结果。