THE PLATYPUS IS NOT A RODENT - DNA HYBRIDIZATION, AMNIOTE PHYLOGENY AND THE PALIMPSEST THEORY

We present DNA-hybridization data on 21 amniotes and two anurans showi ng that discrimination is obtained among most of these at the class an d lower levels. Trees generated from these data largely agree with con ventional views, for example in not associating birds and mammals. How ever, the sister relationships found here of the monotremes to marsupi als, and of turtles to the alligator, are surprising results which are nonetheless consistent with the results of some other studies. The Ma rsupionta hypothesis of Gregory is reviewed, as are opinions about the placement of chelonians. Anatomical and reproductive data considered by Gregory do not unequivocally preclude a marsupial-monotreme special relationship, and there is other recent evidence for placing turtles within the Diapsida. We conclude that the evidential meaning of the mo lecular data is as shown in the trees, but that the topologies may be influenced by a base-compositional bias producing a seemingly slow evo lutionary rate in monotremes, or by algorithmic artefacts (in the case of turtles as well).