a genus of extinct mammals (mammal) found as fossils in rocks from Upper Cretaceous (Cretaceous Period) times (about 100–65.5 million years ago) of Asia and, questionably, North America. Deltatheridium was a small insectivorous mammal about the size of a small rat. It is now recognized to be a metatherian, a member of the group of mammals related to marsupials (marsupial). Deltatheridium has figured prominently in debates about mammalian evolution because it also has some features that are similar to early placental mammals (placental mammal).