Abstract

The Cretaceous-Paleogene (KPg) boundary, one of Earth’s five major extinction events, occurred just prior to the appearance of Placentalia in the fossil record. The Gobi Desert, Mongolia, and the Western Interior of North America, however, have important fossil mammals occurring just before and after the KPg boundary (e.g., Prodiacodon, Deltatheridium) that have yet to be phylogenetically tested in a character-rich context with molecular data. We present here a parsimony analysis of over 6,000 newly scored anatomical observations drawn from six untested fossils and added to the largest existing morphological matrix for mammals. These data are combined with sequence data from 27 nuclear genes. Results show the existence of a new eutherian sister clade to Placentalia, which we name and characterize. The extinct clade Leptictidae is part of this placental sister clade, indicating the sister clade survived the KPg event to co-exist in ancient ecosystems during the Paleogene radiation of placentals. Analyzing the Cretaceous metatherian Deltatheridium in this character-rich context reveals it is a member of Marsupialia, a finding that extends the minimum age of Marsupialia prior to the KPg boundary. Numerous shared-derived features from multiple anatomical systems support the assignment of Deltatheridium to Marsupialia. CT scans of exquisite new specimens better document the marsupial-like dental replacement pattern of Deltatheridium. The new placental sister clade has both Asian-North American species and is ancestrally characterized by shared derived features such as a hindlimb modified for saltatorial locomotion.