Origin of the avian respiratory system: a CT-study of postcranial pneumaticity in basal archosaurs

活着的鸟类(9000种)取得了令人难以置信的成功，部分原因是它们独特的呼吸系统，这是高度新陈代谢和飞行的关键进化创新的基础。这个系统包括肺和一组复杂的相互连接的气囊。气囊允许空气通过肺部进行单向流动，从而实现异常高效的气体交换。气囊的延伸穿透附近的骨骼并使其充气，包括脊椎和肢体元素，从而产生减少骨骼质量的相关效果。相比之下，鳄鱼是鸟类的近亲，没有气囊和相应的气动特征。现在有压倒性的证据表明，鸟类是兽脚亚目恐龙的直系后代。许多以前被认为是鸟类特有的特征首先出现在恐龙中(例如羽毛、孵化行为)。鸟类气囊系统是另一个这样的特征：它存在于兽脚类和蜥脚类恐龙(以及翼龙)中，是根据脊椎的气动特征推断出来的，这些特征与现代鸟类中看到的几乎相同。然而，人们对气囊系统的起源知之甚少：关于气囊和充气性是否存在于兽脚类、蜥脚类和翼龙的共同祖先中，或者它们是否在这三个类群中独立进化，目前还没有达成共识。此外，最近在距离更远的三叠纪始祖龙中发现了充气性的可能证据，这引发了一种有争议的假设，即充气性(由此推断，气囊和一些类似鸟类的呼吸能力)可能存在于鸟类和鳄鱼的最后一个共同祖先中，随后在鳄鱼中消失了。如果这是真的，这将需要我们彻底改变对鸟类和鳄鱼非凡生物学及其进化方式的理解。了解鸟类呼吸系统的起源显然是解释各种始祖龙谱系的成功和多样性的基础。然而，主要的替代假设尚未得到检验。我们建议进行一项初步研究，以检验解释鸟类呼吸起源的其他假说。考虑到最近对恐龙和鸟类呼吸系统的浓厚兴趣、研究小组的可用性以及自然历史博物馆新的微型X射线计算机断层扫描(CT)设施，这项工作是及时的。我们将确定选定的与鳄鱼和鸟类的共同祖先相近的三叠纪始龙的椎骨中是否存在气动结构。充气性的识别将基于外部和内部椎体解剖：后者的数据以前无法获得，但将使用CT扫描获得-这是解决这一问题的一种全新方法。将记录单个骨骼和整个骨骼内充气的程度，并通过将这些特征的存在/不存在映射到当前的始祖龙进化树来确定气动结构的分布。这将使我们能够确定：始祖龙何时出现气性；气性的获得(或丧失)是单个事件还是发生在多个独立的场合；以及气囊系统的不同组成部分出现的进化顺序。证明基础始祖龙没有充气性，将证明祖先始祖龙存在类似鸟的呼吸系统的假设是错误的，并支持另一种假设，即气囊的起源较晚。然而，如果在原始始祖龙中发现了充气性，这个项目将证明气囊系统的进化比目前假设的更复杂，并将有助于未来对鸟类和鳄鱼呼吸系统起源的研究。这项工作将对古生物学家、动物学家和生理学家具有重要意义。

The incredible success of living birds (>9000 species) results, in part, from their unique respiratory system, which underpins the key evolutionary innovations of high metabolism and flight. This system comprises the lungs and a complex array of interconnected air sacs. The air sacs allow a unidirectional flow of air through the lungs, permitting exceptionally efficient gas-exchange. Extensions from the air sacs penetrate and pneumatize nearby bones, including vertebrae and limb elements, with the associated effect of reducing skeletal mass. In contrast, the closest living relatives of birds, crocodiles, lack air sacs and corresponding pneumatic features. There is now overwhelming evidence that birds are direct descendents of theropod dinosaurs. Many features previously regarded as uniquely avian appeared first among dinosaurs (e.g. feathers, brooding behaviour). The avian air sac system is another such feature: its presence in theropod and sauropod dinosaurs (and pterosaurs) has been inferred on the basis of pneumatic features in vertebrae that are almost identical to those seen in living birds. However, the origin of the air sac system is poorly understood: there is no consensus on whether air sacs and pneumaticity were present in the common ancestor of theropods, sauropods and pterosaurs, or whether they evolved independently in these three groups. Furthermore, possible evidence of pneumaticity has recently been identified in more distantly related Triassic archosaurs, prompting the controversial hypothesis that pneumaticity (and, by inference, air sacs and some bird-like respiratory capabilities) may have been present in the last common ancestor of birds and crocodiles, and subsequently lost in crocodilians. If true, this would require radical alteration of our understanding of the remarkable biology of birds and crocodiles and how they evolved. Understanding the origin of the avian respiratory system is clearly fundamental to explaining the success and diversity of the various archosaur lineages. However, the main alternative hypotheses have not yet been tested. We propose a pilot study to test alternative hypotheses explaining the origin of bird-like respiration. This work is timely, given recent intensive interest in dinosaur and bird respiratory systems, the availability of the research team and a new micro X-ray Computed Tomography (CT) facility at the Natural History Museum. We will determine the presence/absence of pneumatic structures in the vertebrae of selected Triassic archosaurs that lie close to the common ancestry of crocodiles and birds. The identification of pneumaticity will be based on external and internal vertebral anatomy: the latter data were previously unavailable, but will be obtained using CT scans - an entirely novel approach to this problem. The extent of pneumaticity, both within individual bones and throughout the skeleton, will be documented and the distribution of pneumatic structures will be determined by mapping the presence/absence of these features onto current archosaur evolutionary trees. This will permit us to establish: when pneumaticity appeared in archosaurs; whether the acquisition (or loss) of pneumaticity was a single event or occurred on multiple independent occasions; and the evolutionary sequence in which the different components of the air sac system appeared. Demonstrating the absence of pneumaticity in basal archosaurs would falsify hypotheses that a bird-like respiratory system was present in the ancestral archosaur, and support alternative hypotheses suggesting a later origin of air sacs. However, if pneumaticity is identified in primitive archosaurs this project will demonstrate that evolution of the air sac system is more complex than currently assumed and will facilitate future investigations into the origins of avian and crocodilian respiratory systems. This work will be of fundamental importance to palaeontologists, zoologists and physiologists.