The evolutionary origin of teeth

Teeth are a key innovation underpinning the adaptive radiation of jawed vertebrates. They are a synapomorphy of gnathostomes, but the evolutionary origin of teeth must lie deeper within the jawless vertebrates. Jawless vertebrates comprise the extant groups of lampreys and hagfish and the extinct intermediate stem-gnathostomes, which possess a variety of tooth-like structures and denticles. To test among different hypotheses of tooth evolution, we must study the development and function of these tooth-like structures in jawless vertebrates. For this I use a combination of computational methods including Computational Fluid Dynamics (CFD), Finite Element Analysis (FEA), micro-CT (computed tomography) and synchrotron-radiation X-ray tomographic microscopy (srXTM). I investigate the oral plates of heterostracans, one of the earliest skeletonising vertebrates. Using CFD and FEA I test functional hypotheses of suspension feeding and mechanical feeding and show that the oral plates performed a mechanical function. Using srXTM I study histology and morphogenesis of the oral plates and denticles and reveal that they are developmentally linked to the dermal skeleton, supporting the ‘outside-in’ hypothesis of tooth evolution. I show that the oral plate denticles are non-homologous to gnathostome teeth and that the capacity to form internal odontodes evolved several times independently in stem-gnathostomes. Adding to the fossil perspective I present foundational work on the development of lamprey toothlets. Using micro-CT and srXTM I describe toothlet replacement at tissue level and show that replacement is a conserved feature of the lamprey crown group. Comparing lamprey toothlet development and replacement to hagfish and gnathostomes, I lay the foundation for future investigations into the genetic regulation of tooth development across vertebrate lineages. Using a multifaceted computational approach this thesis sheds light on the function and development of tooth-like struct