EXTANT marsupials are distinctive in their pattern of dental development(1), in that only one tooth is replaced postnatally in each jaw. Interpretation of this pattern for marsupials ancestrally is disputed(2-5), partly because ontogenetic data in fossils have been unobtainable. Here we present an ultra-high-resolution X-ray computed tomography (CT) study of the tiny fossil Alphadon, which represents the first evidence of dental development and replacement in a Mesozoic marsupial. In the known pattern of tooth replacement and development, Alphadon is identical to living marsupials, a derived similarity suggesting that this pattern is ancestral to Marsupialia, and that it was established by the Late Cretaceous, at least, This pattern has been correlated with some specialized aspects of marsupial lactation(1,6). Hence the presence of a marsupial pattern of tooth replacement in Alphadon provides indirect evidence that at least some specialized features of marsupial reproductive processes arose during the Mesozoic.