MEREDITH SMITH, Moya*; FRASER, gareth; King’s College London: The origin of vertebrate dentitions each of their own design

It has been proposed that amongst Gnathostomata, in both stem and crown gnathostomes, several different strategies unique to each taxon have evolved to pattern the dentition. Variation occurs in the spatial-temporal pattern of tooth addition, either in statodont, or shedding and lyodont replacement modes. Assumptions regarding homology of vertebrate dentitions are questioned, and it is suggested from current phylogenies that dentitions of chondrichthyans, acanthodians and osteichthyans may each have evolved separately. Recent studies on extinct placoderms, phylogenetically the most basal jawed vertebrates, suggest teeth originated at least twice as they only occur in more derived taxa. Alternate theories on the origin of teeth will be discussed, such as controversially, from pharyngeal denticles rather than those of the skin. Within the Osteichthyes, crown gnathostomes, data on genes deployed in tooth development in the rainbow trout <(Oncorhynchus mykiss)> show that some signalling genes are the same as in the mouse and choreograph this evolutionary stable event since the osteichthyan divergence around 420 mya. Teeth as modular units of the dentition have conserved a phenotypic stability at this level of gene action, in tooth initiation and histogenesis, but regulation of pattern is still unknown. In comparing tooth initiation in the oral and pharyngeal dentition of recent osteichthyan fish, significant differences can be inferred from new gene expression data. These studies also demonstrate that teeth develop superficially in the basal layer of the oral epithelium (odontogenic band) and replace from the basal epithelium of functional teeth and not from a deep epithelial fold (dental lamina) proposed as a synapomorphy for all jawed vertebrates.