June 21 , 2016

Mammalian faunas in North America experienced dramatic change during the Cretaceous, with earlier faunas characterized by eutriconodontans, symmetrodontans, and unspecialized therians giving way to a major diversification of therian lineages by the Campanian– Maastrichtian. The Aquilan North American Land Mammal Age (NALMA), originally based on the well-studied fauna of the Milk River Formation (Santonian) of southern Alberta, records the start of this transition. Notable are first appearances of pediomyoid marsupialiforms and the eutherian Paranyctoides, and last occurrences of eutriconodontans and symmetrodontans. The Campanian Wahweap Formation has yielded a similar fauna, but until now the John Henry Member of the Straight Cliffs Formation was the only other unit of known Santonian age from which fossil mammals have been recovered, leaving this transitional interval represented by limited sampling. The Eagle Formation in central and northern Montana is considered to be laterally equivalent to the Milk River Formation, with northernmost exposures correlated to the upper Santonian, based on palynomorphs and magnetostratigraphy. Here, we describe the first fossil mammals known from the Eagle Formation. A relatively small rock sample yielded a rich, diverse assemblage including two genera of spalacotheriid symmetrodonts, several “alphadontid” marsupialiforms and the large pediomyoid Aquiladelphis, and at least two eutherians including Paranyctoides. Multituberculates, to be described separately, are also abundant and diverse. The Eagle Formation assemblage is broadly similar in composition to that from the Milk River Formation, but shares the spalacotheriid Spalacotheridium with older units, including the Straight Cliffs Formation from southern Utah. These initial results provide another biostratigraphic data point linking Santonian faunas across a broad latitudinal range, and encourage reevaluation of the Aquilan NALMA.

Davis, B.M., Cifelli, R.L., and Cohen, J.E. (2016)
First fossil mammals from the Upper Cretaceous Eagle Formation (Santonian, northern Montana, USA), and mammal diversity during the Aquilan North American Land Mammal Age.
Palaeontologia Polonica 67: 101–126.
doi: 10.4202/pp.2016.67_101