A palaeontological solution to the origin of the vertebrate pectoral girdle

成对的鳍是脊椎动物最关键的解剖学创新之一。这些结构起源于4.3亿多年前有颌骨脊椎动物的祖先，并形成了包括我们自己的手臂和腿在内的所有脊椎动物肢体的进化基础。150多年来，进化生物学家一直对这些结构的起源感到困惑，这些结构产生了一系列令人困惑的运动适应能力，如游泳、行走、跑步、攀登、挖掘和飞行。发育生物学家和古生物学家一直致力于解开成对鳍的神秘起源。然而，在无颌鱼类(祖先没有对鳍)和有颌脊椎动物(拥有两组鳍)之间存在着一个巨大的形态鸿沟。尽管如此，这个问题仍然是形态新颖性研究的基本案例之一-进化如何产生全新的结构。最近的发育遗传学研究突出表明，鳍和腰带可能是从远古祖先的一块改良的支持鳃的骨骼进化而来的。这重新唤起了19世纪的一种理论，该理论因缺乏化石证据而失宠。在这个项目中，我们将使用化石记录来记录4.3亿-4亿年前脊椎动物肩鳍和成对胸鳍起源的新证据。这个项目背后的想法是，事实上，最关键的化石并不是没有的。相反，古生物学家忽略了隐藏在已知化石群解剖中的关键中间阶段。这些化石表明，喉弓和颧弓是肩带起源不可或缺的部分。通过高分辨率的3D x射线技术，我们将展示无颌鱼的头骨靠近成对鳍的起点，揭示了头部进化为独特的头骨和肩膀的先驱阶段。接下来，我们将调查一项新的发现，该发现表明，最古老的有颌鱼的头和肩之间有一个活动的连接，最初是由一个改良的颧弓形成的。我们将使用额外的保存完好的化石的3D技术来描述这种移动联系是如何作为鳃弓开始的。然后我们将揭示它是如何被改造成头部和肩部之间支持呼吸和进食的各种骨骼关节的。我们的项目将为胸鳍是如何进化的提供关键的见解，使比较发育生物学家能够识别导致重大进化变化的基因。此外，它还将肩带的起源与颌骨起源期间用于进食和呼吸的结构的深刻变化联系在一起。这将允许对重大进化转变中如何需要功能和解剖变化的关键的、整体的理解。

Paired fins were among the most critical anatomical innovations in vertebrate animals. These structures originated in the ancestors of jawed vertebrates more than 430 million years ago and formed the evolutionary foundation for all vertebrate limbs, including our own arms and legs. For over 150 years, evolutionary biologists have puzzled over the origins of these structures, which gave rise to a bewildering array of adaptations for movement, such as swimming, walking, running, climbing, digging, and flying. Developmental biologists and palaeontologists have worked vigorously to solve the mysterious origin of paired fins. However, a gaping morphological chasm lies between jawless fishes (which ancestrally lack paired fins) and jawed vertebrates (which possess two sets). Despite this, the question remains one of the foundational cases in the study of morphological novelty---how evolution generates completely new structures. Recent studies in developmental genetics have highlighted evidence that fins and girdles could have formed evolutionarily from a modified gill-supporting bone of an ancient ancestor. This revives a theory from the 19th century which fell out of favour because of a lack of fossil evidence. In this project, we will use the fossil record to document new evidence for the origin of the shoulder and paired pectoral fins of vertebrates from 430-400 million years ago. Underlying this project is the idea that the most critical fossils are not, in fact, absent. Instead, palaeontologists have overlooked crucial intermediate stages hidden in the anatomy of known fossil groups. These fossils show that the throat and gill arches were integral to the origin of the shoulder girdle. Through high-resolution 3D x-ray techniques, we will show that the skulls of jawless fishes near the origin of paired fins reveal precursors stages in the evolution of the head into a distinct skull and shoulder. Next, we will investigate a new finding showing that the oldest jawed fishes possessed a mobile linkage between the head and shoulder initially formed from a modified gill arch. We will use additional 3D techniques of exceptionally preserved fossils to describe how this mobile linkage began as a gill arch. We will then reveal how it became modified into a diverse array of bony joints between the head and shoulder that supported breathing and feeding. Our project will provide crucial insights into how pectoral fins evolved, allowing comparative developmental biologists to identify the genes that led to major evolutionary changes. Furthermore, it ties the origin of the shoulder girdle to profound transformations in the structures used for feeding and respiration during the origin of jaws. This will allow a critical, holistic understanding of how functional and anatomical changes are required across major evolutionary transitions.