A Detailed description of <i>Rugarhynchos sixmilensis</i>, gen. et comb. nov. (Archosauriformes, Proterochampsia), and cranial convergence in snout elongation across stem and crown archosaurs

The Triassic Period (252–201.5 Ma) records a great expansion of saurian diversity and disparity, particularly in skull morphology. Stem archosaurs exhibit substantial cranial disparity, especially by taxa either shortening or elongating the skull. This disparity is exemplified in the North American Late Triassic proterochampsians by the ‘short-faced’ <i>Vancleavea</i> and the ‘long-faced’ doswelliids. To critically investigate skull elongation and character evolution in these proterochampsians, we evaluate ‘<i>Doswellia</i>’ <i>sixmilensis</i>, known from much of a skull, cervical centra, and osteoderms from the Bluewater Creek Member of the Chinle Formation of New Mexico. We redescribe the holotype based on extensive repreparation of the holotype material, resulting in the identification of the orbit and clarifying the extent of related bones. As such, the diagnosis of the taxon is substantially modified and a new genus, <i>Rugarhynchos</i>, gen. nov., is erected. <i>Rugarhynchos sixmilensis</i>, gen. et comb. nov., exhibits an elongate snout with characteristics known in stem and crown archosaurs, including a downturned premaxilla and fluted teeth. We included <i>R. sixmilensis</i> in a phylogenetic analysis of archosauromorphs consisting of 677 characters and 109 taxa under both parsimony and Bayesian models. We recover <i>R. sixmilensis</i> as a doswelliid, sister to <i>Doswellia kaltenbachi</i>. Our parsimony and Bayesian models differ in the placement of Doswelliidae, either as sister to or within Proterochampsidae, respectively. We use archosauromorph relationships from the Bayesian model to estimate cranial disparity between stem and crown archosaurs and find a narrow breadth of morphological disparity in the stem. Our results suggest that crown archosaurs evolved disparate crania from a low-disparate archosauriform condition.三叠纪（Triassic Period，252–201.5 Ma）记录了蜥形类（saurian）多样性与形态分异度（disparity）的大幅扩张，尤以头骨形态最为显著。基干主龙类（stem archosaurs）展现出显著的头骨形态分异，这体现为不同类群的头骨要么缩短、要么延长。这种分异在北美晚三叠世前弓龙类（proterochampsians）中尤为典型：包括“短头骨型”的范克莱夫龙（Vancleavea）与“长头骨型”的多斯韦尔龙类（doswelliids）。 为严谨探究此类前弓龙类的头骨延长演化与性状进化，我们对“多斯韦尔龙属（Doswellia）sixmilensis”展开研究——该物种的化石材料产自美国新墨西哥州钦利组（Chinle Formation）布鲁沃特溪段（Bluewater Creek Member），包含近乎完整的头骨、颈椎椎体（cervical centra）与皮内成骨（osteoderms）。 我们基于对正模标本（holotype）的大量重新修复工作对该物种进行重新描述，成功识别出眶孔（orbit）并厘清了相关骨骼的分布范围。据此，该类群的分类鉴别特征得到大幅修订，并建立新属——鲁格吻龙属（Rugarhynchos），gen. nov.。鲁格吻龙六米尔种（Rugarhynchos sixmilensis，gen. et comb. nov.）拥有延长的吻部，兼具基干主龙类与冠群主龙类（crown archosaurs）的特征，包括向下弯曲的前上颌骨（premaxilla）与具棱脊的牙齿（fluted teeth）。 我们将R. sixmilensis纳入包含677个性状、109个类群的主龙形类（archosauromorphs）系统发育分析（phylogenetic analysis），同时采用简约法（parsimony）与贝叶斯模型（Bayesian models）进行推演。分析结果显示，R. sixmilensis属于多斯韦尔龙类，为卡尔滕巴赫多斯韦尔龙（Doswellia kaltenbachi）的姊妹群。我们的简约法与贝叶斯模型在多斯韦尔龙科（Doswelliidae）的系统发育位置上存在差异：前者将其置于前弓龙科（Proterochampsidae）的姊妹群位置，后者则将其归入前弓龙科内部。 我们基于贝叶斯模型得到的主龙形类系统发育关系，估算了基干主龙类与冠群主龙类的头骨形态分异度，发现基干类群的形态分异度范围较窄。研究结果表明，冠群主龙类的头骨形态分异是从低分异度的主龙形类（archosauriform）祖先状态演化而来的。