Although teeth are considered one of the most important steps in vertebrate evolution, details of their origins are obscure. Two prominent and opposing theories for the evolution of the vertebrate dentition are current: the ‘Outside-in’ hypothesis and the ‘inside-out’ hypothesis. One of the main arguments against the ‘outside-in’ hypothesis is that, although similarities between teeth and scales have been observed, there is little fossil evidence of transitional forms between the two structures. Specimens of ischnacanthid acanthodians from the Man On The Hill (MOTH) locality in the Mackenzie Mountains of Canada provide the first unequivocal example of such transitional forms in an Early Devonian (Lochkovian) vertebrate assemblage. The head scales of these specimens are modified with proximity to the mouth to be extremely tooth-like. Three distinct morphotypes of modified cheek and lip scales are described. Their detailed similarity to teeth suggests that they are a result of the same developmental processes, and also suggests the existence of a field of gene expression near the mouth margin in which scales could be transformed into teeth. These transitional forms remove one of the chief objections to the ‘outside-in’ hypothesis for the origins of teeth in vertebrates.
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