A palaeontological solution to the origin of the vertebrate pectoral girdle

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.