Anatomy of the dinosaur Pampadromaeus barberenai (Saurischia—Sauropodomorpha) from the Late Triassic Santa Maria Formation of southern Brazil

Sauropodomorphs are the most abundant and diverse clade of Triassic dinosaurs, but the taxonomy of their earliest (Carnian) representatives is still poorly understood. One such taxon is Pampadromaeus barberenai, represented by a nearly complete disarticulated skeleton recovered from the upper part of the Santa Maria Formation of Rio Grande do Sul, Brazil. Here, the osteology of Pam. barberenai is fully described for the first time. Detailed comparisons with other Carnian sauropodomorphs reveal a unique anatomy, corroborating its status as a valid species. Potential autapomorphies of Pam. barberenai can be seen in the articulation of the sacral zygapophyses, the length of the pectoral epipodium, the shape of the distal articulation of the femur and the proximal articulation of metatarsal 1. A novel phylogenetic study shows that relationships among the Carnian sauropodomorphs are poorly constrained, possibly because they belong to a “zone of variability”, where homoplasy abounds. Yet, there is some evidence that Pam. barberenai may nest within Saturnaliidae, along with Saturnalia tupiniquim and Chromogisaurus novasi, which represents the sister group to the larger sauropodomorphs, i.e. Bagualosauria.蜥脚形亚目（Sauropodomorphs）是三叠纪恐龙中丰度最高、多样性最强的演化支，但其最早的卡尼期（Carnian）代表类群的分类学认知仍不甚明晰。其中一类为潘帕德罗龙（Pampadromaeus barberenai），其化石为一件未关节连接的近乎完整骨架，发现于巴西南里奥格兰德州（Rio Grande do Sul）圣玛丽亚组（Santa Maria Formation）上部地层。本文首次对潘帕德罗龙的骨骼解剖学特征进行了全面系统的描述。通过与其他卡尼期蜥脚形亚目类群的详细对比分析，发现其具备独特的解剖结构，证实了其作为有效物种的分类地位。潘帕德罗龙的独有衍征（autapomorphies）可观察于以下特征：荐椎椎弓突（sacral zygapophyses）的关节连接方式、胸肢远肢段（pectoral epipodium）的长度、股骨远端关节面的形态以及第一跖骨近端关节面的形态。一项全新的系统发育研究表明，卡尼期蜥脚形亚目类群间的系统发育关系约束性较弱，这可能是因为它们处于"变异区"中，该区域内趋同演化（homoplasy）现象频发。不过，现有证据显示潘帕德罗龙可能与图皮奎姆土星龙（Saturnalia tupiniquim）和诺瓦斯色龙（Chromogisaurus novasi）一同归入土星龙科（Saturnaliidae），而土星龙科是更大的蜥脚形亚目类群巴瓜龙类（Bagualosauria）的姊妹群。