A new macronarian (DInosauria, Sauropoda) from the Late Jurassic of Argentina: Implications for our understanding of early macronarian evolution

蜥脚类是恐龙的主要分支之一。它们不仅包括已知最大的陆生动物，而且是中生代陆地生态系统的重要组成部分，是许多已知组合中最大和最丰富的食草动物。蜥脚类动物的主要辐射发生在它们起源于侏罗纪之后不久，到这一时期结束时，大多数主要的谱系已经建立起来。侏罗纪蜥脚类动物进化中最重要的事件之一是新蜥脚类动物的起源和最初的辐射，这是蜥脚类动物的主要分支，从晚侏罗世到晚白垩纪的绝大多数分类群都属于它。然而，尽管新蜥脚类动物的一个主要亚分支梁龙科已经有了很好的特征，并且很少有分类群与这个分支有不确定的关系（例如单棘龙），但另一个主要分支的包容性及其早期分支成员的相互关系更具争议性。这方面的一个问题是，我们对早期新蜥脚类动物进化的认识几乎完全基于北半球的化石记录，而坦桑尼亚腾达古鲁地层的晚侏罗世蜥脚类动物群基本上是冈瓦纳的唯一信息来源。最近在阿根廷晚侏罗世Cañadón Calcáreo组的野外工作导致了几个新的蜥脚类动物标本的恢复。其中一个标本是部分骨架，与南美洲晚侏罗世已知的唯一一种蜥脚类动物类群，即来自同一地层的早期分支巨龙Tehuelchesaurus，有相似之处，但也有一些不同。因此，当前项目的目标是对这个新标本进行详细的研究，包括大规模的系统发育分析，以确定其系统发育位置。在系统发育分析中，我们打算进一步采用先验和后知后觉的比较枝源学方法，以更好地理解围绕巨爪龙早期进化的问题，并确定可能的方法来提高这部分蜥脚类系统发育的分辨率和稳定性。因此，我们期望这个项目不仅能提供更多关于冈瓦纳“基础”巨龙动物的信息，还能更好地理解早期新蜥脚类动物的进化，以及解释这一进化事件所面临的问题和挑战。

The Sauropoda are one of the main lineages of dinosaurs. They do not only include the largest terrestrial animals known, but also represent important components of Mesozoic terrestrial ecosystems, being the largest and most abundant herbivores in many known assemblages. The main radiation of sauropods happened shortly after their origin in the Jurassic, and by the end of this period, most major lineages were established. One of the most important events in Jurassic sauropod evolution was the origin and initial radiation of Neosauropoda, the major clade within sauropods to which the vast majority of taxa known from the Late Jurassic to the latest Cretaceous belong to. However, whereas one of the main subclades of Neosauropoda, the Diplodocoidea, are rather well characterized and there are few taxa of uncertain referral to this clade (e.g. Haplocanthosaurus), the inclusivity of the other major clade, Macronaria, and the interrelationships of its early branching members is much more contentious. One problem in this respect is that our knowledge of early neosauropod evolution is almost entirely based on the fossil record of the northern hemisphere, with the Late Jurassic sauropod fauna from the Tendaguru Formation of Tanzania being basically the only source of information from Gondwana.Recent fieldwork in the Late Jurassic Cañadón Calcáreo Formation of Argentina has resulted in the recovery of several new sauropod specimens. One of these specimens is a partial skeleton with similarities, but also some differences, to the only other known sauropod taxon from the Late Jurassic of South America, the early branching macronarian Tehuelchesaurus, from the same formation. The objective of the current project is thus a detailed study of this new specimen, including a large scale phylogenetic analysis to establish its phylogenetic position. In the phylogenetic analysis, we furthermore intend to use comparative cladistic methods both a priori and a posteriori, to gain a better understanding of the problems surrounding the early evolution of macronarians and to identify possible ways to improve resolution and stability in this part of sauropod phylogeny. Thus, we expect this project to not only result in additional information on 'basal' macronarians from Gondwana, but also in a better understanding of early neosauropod evolution and the problems and challenges in interpreting this evolutionary event.