DNA-HYBRIDIZATION STUDIES OF MARSUPIALS AND THEIR IMPLICATIONS FOR METATHERIAN CLASSIFICATION

We review past DNA-hybridisation studies of marsupials and present a r eanalysis of the data, utilising results from our and additional studi es to formulate and rationalise a new classification of Marsupialia. I n the reanalysis, 13 individual DNA-hybridisation matrices, many lacki ng some pairwise comparisons, were sutured in stages to provide the ba sis for generating a tree of 101 marsupials plus an outgroup eutherian ; a fourteenth matrix provided data for a tree including eight additio nal eutherians and a monotreme. Validation was achieved by jackknifing on taxa for each matrix as well as on tables combining two or more ma trices generated during assembly of the 102-taxon data set. The result s are consistent with most conclusions from the individual studies and dramatise the unevenness of hierarchical levels in current classifica tions of marsupials. In particular, the affinities of the American mar supial Dromiciops gliroides with, and the distinctness of marsupial ba ndicoots from, Australasian metatherians are reaffirmed, while apossum s are shown to be as internally divergent as are most members of the o rder Diprotodontia. Calibration of the 102-taxon tree and dating of th e major dichotomies suggest that no extant marsupial lineage originate d before the latest Cretaceous, and that all of them together with mos t South American and all Australasian fossils should be recognised as a monophyletic group contrasting with a largely Laurasian (if possibly paraphyletic) taxon. These inferences, together with the derails of t he phylogeny, mandate that the misleading 'Australian' v. 'American' d istinction be abandoned, even as a geographic convenience.