A re-assessment of the osteology and phylogenetic relationships of the enigmatic, large-headed reptile <i>Sphodrosaurus pennsylvanicus</i> (Late Triassic, Pennsylvania, USA) indicates archosauriform affinities

Most Triassic terrestrial diapsids belong to two clades, Lepidosauromorpha or (the more diverse) Archosauromorpha. Nevertheless, the phylogenetic relationships of some Triassic diapsids have remained uncertain for decades because of the lack of preservation of phylogenetically relevant anatomical regions or because of unusual combinations of features. One of these enigmatic forms is the small-sized <i>Sphodrosaurus pennsylvanicus</i> from the Upper Triassic Hammer Creek Formation of the Newark Supergroup in Pennsylvania (USA). It was first identified as a procolophonid parareptile, later as a probable rhynchosaur archosauromorph, and more recently as an indeterminate neodiapsid. Here we revise the anatomy of <i>Sphodrosaurus pennsylvanicus</i> in order to include it for the first time in a quantitative phylogenetic analysis, which is focused on Permo–Triassic neodiapsids. <i>Sphodrosaurus pennsylvanicus</i> is recovered in this analysis as a doswelliid proterochampsian within Archosauromorpha. As a result, this taxon is added to the list of doswelliids known from the Carnian–Norian of the eastern and south-western USA. Previous authors recognized that the most unusual feature of <i>Sphodrosaurus pennsylvanicus</i> is its proportionally very large skull. Phylogenetic generalized least squares regressions confirmed that <i>Sphodrosaurus pennsylvanicus</i> has a larger skull than the vast majority of Permo–Triassic diapsids. Optimization in the phylogeny of the skull width to presacral length ratio shows the most likely scenario is that the extremely broad skull of <i>Sphodrosaurus pennsylvanicus</i> is autapomorphic, but it is not unique among archosauromorphs, being paralleled by hyperodapedontine rhynchosaurs and the proterochampsian <i>Proterochampsa barrionuevoi</i>. Exploration of a morphospace of linear measurements shows that <i>Sphodrosaurus pennsylvanicus</i> shares strong similarities with the probably semi-aquatic <i>Proterochampsa barrionuevoi</i>, suggesting that the former species may have had a similar mode of life. A linear discriminant analysis of ungual functional categories found that the only preserved ungual of <i>Sphodrosaurus pennsylvanicus</i> was suitable for digging or some other way of substrate processing.绝大多数三叠纪陆生双孔类隶属于两个演化支：鳞龙形类（Lepidosauromorpha）或（种类更为丰富的）主龙形类（Archosauromorpha）。然而数十年来，部分三叠纪双孔类的系统发育关系始终悬而未决，其原因或在于与系统发育分析相关的解剖区域保存不佳，或在于其兼具异常独特的特征组合。这类谜一般的类群之一，便是产自美国宾夕法尼亚州纽瓦克超群上三叠统哈默克里克组的小型物种宾夕法尼亚斯福德拉龙（Sphodrosaurus pennsylvanicus）。该物种最初被鉴定为前棱蜥科副爬行动物，后续被认为可能属于喙龙类主龙形类，而近年来则被归入未定新双孔类。本文首次对宾夕法尼亚斯福德拉龙的解剖结构进行系统修订，并将其纳入针对二叠纪-三叠纪新双孔类的定量系统发育分析。分析结果显示，该物种隶属于主龙形类下的多斯韦尔龙科前弓形类。据此，本类群被加入美国东部及西南部卡尼期-诺利期产出的多斯韦尔龙科类群名录之中。此前的研究者已指出，宾夕法尼亚斯福德拉龙最与众不同的特征是其比例极不协调的宽大头骨。系统发育广义最小二乘回归分析证实，相较于绝大多数二叠纪-三叠纪双孔类，该物种的头骨尺寸显著更大。对头骨宽度与荐前椎长度比值的系统发育性状优化分析表明，最可能的演化情景为：宾夕法尼亚斯福德拉龙极度宽阔的头骨属于自有衍征，但这一特征在主龙形类中并非独有——超齿龙亚科喙龙类以及前弓形类的巴尔武埃诺前弓龙（Proterochampsa barrionuevoi）也演化出了类似特征。对线性测量数据构成的形态空间的探索显示，宾夕法尼亚斯福德拉龙与大概率半水生的巴尔武埃诺前弓龙具有高度相似的形态特征，这暗示前者可能拥有与之相仿的生活方式。对爪类功能类别的线性判别分析结果表明，宾夕法尼亚斯福德拉龙仅存的爪化石适用于挖掘或其他形式的底质处理活动。