Deltatheridium and Marsupials

Abstract

THE evidence for the radical transfer of the famous genus Deltatheridium to the Marsupialia seems to be partly different from that already presented¹. The cheek tooth formula, a key character separating marsupials and placentals, is questionable for Deltatheridium. There are seven cheek teeth, as usual in both groups, and the fourth is molariform, as in marsupials. This is evidence on phyletic affinity only if the primitive state, that of the latest common ancestor, was otherwise. In a manuscript that has circulated privately since 1963 I have argued from diverse evidence that the seven cheek teeth of each group may well be directly homologous with those of the other, with an ambiguity as to the permanent or deciduous premolars. In other words, P₄⁴ or DP₄⁴ of placentals may well be homologous to M₁¹ of marsupials. If so, a more or less molariform state of the fourth cheek tooth is primitive to both groups and the often nonmolariform state in more or less primitive placentals is secondary. This was why I suggested² that Deltatheridium might have one more molar than previously thought; the suggestion is now confirmed. Relative wear of the teeth is a useful but unreliable criterion (ref. 3, footnote on page 86).