Let There Be Mammals The timing of the evolution and radiation of placental mammals and their most recent common ancestor has long been debated, with many questions surrounding the relationships of groups that pre- and postdate the Cretaceous-Paleogene boundary (66 million years ago). While the fossil record suggests that placental mammals radiated after the Cretaceous, molecular clocks have consistently placed the ancestors of mammalian lineages earlier. O'Leary et al. (p. 662 ; see the Perspective by Yoder ) examined the morphology of fossil and extant taxa and conclude that living placentals originated and radiated after the Cretaceous and reconstruct the phenotype of the ancestral placental mammal.