FOSSIL EVIDENCE FOR THE ORIGIN OF THE MARSUPIAL PATTERN OF TOOTH REPLACEMENT

EXTANT marsupials are distinctive in their pattern of dental developme nt(1), in that only one tooth is replaced postnatally in each jaw. Int erpretation 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 eviden ce of dental development and replacement in a Mesozoic marsupial. In t he known pattern of tooth replacement and development, Alphadon is ide ntical to living marsupials, a derived similarity suggesting that this pattern is ancestral to Marsupialia, and that it was established by t he Late Cretaceous, at least, This pattern has been correlated with so me specialized aspects of marsupial lactation(1,6). Hence the presence of a marsupial pattern of tooth replacement in Alphadon provides indi rect evidence that at least some specialized features of marsupial rep roductive processes arose during the Mesozoic.