Gnathostome dental pattern and the evolution of chondrichthyan dentitions

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One of the most significant vertebrate innovations was the evolution of jaws, and on these jaws, teeth arranged into functional dentitions. These dentitions allowed vertebrates to radiate into a number of major groups, including sharks (Chondrichthyes) and bony fishes(Osteichthyes). Along with these are a variety of fossil groups whose dentitions have been difficult to understand and interpret. Because these fossil groups are closer to the origin of jaws and teeth, it is crucial that we try to better understand how their dentitions evolved.One way to do this is to study how the dentitions of living groups developed and then apply these observations, as far as possible, to the development of dentitions in fossil taxa. The group Osteichthyes includes well-known fish such as the tuna, salmon, seahorse, coelacanth, lungfish and also tetrapods. Osteichthyan dentitions have been intensely studied in recent years, and we even understand what genes are involved in producing these dentitions. Despite this, even within this group we know nothing about tooth development of more primitive osteichthyans, such as the paddlefish and gar. As well, and perhaps somewhat surprisingly, we know very little about how genes controldevelopment of shark and ray dentitions. The goal of our project is to study development in a broad range of chondrichthyans to find common developmental characters of these dentitions that can be compared to osteichthyan dentitions and in the future, to the more primitive, problematic fossil taxa. Shark teeth are plentiful in the fossil record and features they show are important in identifying different species. Chondrichthyans show a wide variety of dentitions, ranging from large numbers of almost microscopic teeth along the jaw (in filter-feeding sharks such as Rhincodon, the Whale shark), to the pavement-like dentition in rays, to a dentition in which teeth change along the jaw from biting to crushing (Heterodontus, the Port Jackson shark). This diversity has made it difficult to find common patterns of dentition developmentthat could be used to compare to the problematic fossil vertebrates mentioned above. Sharks and rays also possess other 'tooth-like' structures such as the gill rakers in the filter-feeding basking shark Cetorhinus and 'teeth' along the rostrum of sawfish and sawsharks. Our project will study specimens of a wide range of modern sharks and rays of different growth stages in order to investigate the patterns of tooth development within the jaws and changes in the number and shape of teeth during growth. A number of exceptionally well-preserved fossils will also be studied in order to place the modern forms within a wider context within the Chondrichthyes. Additionally, we will study embryos of a shark, ray and a basal bony fish and will record the genes controlling their tooth development that will allow us to compare underlying mechanisms of the formation of teeth in all vertebrates.