Inside-out turned upside-down

Abstract

Sophisticated microscopy analysis of conodont elements suggests that these mysterious fossil structures are not, as has been previously suggested, evolutionary precursors to vertebrate teeth. See Letter p.546 Sedimentary rocks often contain small tooth-like microfossils called conodont elements in such abundance that they are commonly used to date strata. They are components of the pharynx, in the throat of soft-bodied eel-like animals classified as Conodonta. Their histology strongly resembles that of vertebrate teeth, suggesting an 'inside out' evolutionary model in which teeth originated in the mouth. A new study using synchrotron radiation X-ray tomographic microscopy to compare the microstructure of morphologically similar euconodont and paraconodont elements raises doubts over this interpretation. The data suggest that the last common ancestor of Conodonta and jawed vertebrates lacked mineralized skeletal tissues. Teeth seem to have evolved through the extension of odontogenic competence from the external dermis to internal epithelium soon after the origin of jaws. On this model, similarities between conodont elements and teeth are a classic example of parallelism in evolution.