<i>Alphadon halleyi</i>(Didelphidae, Marsupialia) from the Two Medicine Formation (Late Cretaceous, Judithian) of Montana

Abstract

ABSTRACT Two lower jaws of the Judithian didelphid Alphadon halleyi were recovered from the Egg Mountain locality, Two Medicine Formation, near Chouteau, Montana. These are the first mammalian fossils reported from this formation and they document for the first time the association of M, through M4 in A. halleyi. Of the small, Judithian species of Alphadon, A. halleyi Sahni, A. sahnii Lillegraven and McKenna, and A. attaragos Lillegraven and McKenna were established on isolated lower teeth. Only A. russelli Fox from the Oldman Formation was established on associated upper and associated lower molars. These species were distinguished by differences in size and subtle morphological features. The new material from the Two Medicine Formation documents the changes in size of the teeth in a single lower molar dentition and provides the basis for a test of the validity of the small Judithian species of Alphadon. A. praesagus, A. russelli, and A. halleyi are recognized as members of the Judithian faunas. A. sahnii may be a valid species or might be based on a sample of larger teeth of A. halleyi.