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谱系之一。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。蜥脚类恐龙的主要辐射发生在它们起源后不久的侏罗纪，到这一时期结束时，大多数主要的谱系都建立起来了。侏罗纪蜥脚类演化中最重要的事件之一是新蜥脚类的起源和初始辐射，新蜥脚类是蜥脚类中的主要分支，从晚侏罗世到最晚白垩纪的绝大多数已知类群都属于这一分支。然而，虽然新蜥脚类的主要分支之一，梁龙总科，有相当好的特点，有几个分类群的不确定的参考这个分支（如单棘龙），包容性的另一个主要分支，Macronaria，其早期分支成员的相互关系是更具争议性。这方面的一个问题是，我们对早期新蜥脚类动物演化的认识几乎完全基于北方半球的化石记录，坦桑尼亚Tendaguru组的晚侏罗世蜥脚类动物群基本上是冈瓦纳大陆唯一的信息来源。其中一个标本是一个部分骨骼，与南美洲晚侏罗世的另一个已知蜥脚类动物分类群，早期分支巨型特韦勒切龙（英语：Tehuelchesaurus）有相似之处，但也有一些差异，来自同一地层。因此，目前项目的目标是对这种新标本进行详细研究，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，我们还打算使用比较支序方法的先验和后验，以更好地了解周围的macronarians早期进化的问题，并确定可能的方法来提高分辨率和稳定性，在这部分蜥脚类动物的进化。因此，我们希望这个项目不仅会导致更多的信息“基础”macronarians从冈瓦纳，但也在更好地了解早期新蜥脚类动物的进化和解释这一进化事件的问题和挑战。

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.