CANCELLED --- CANCELLED - CANCELLED ---Despite significant recent progress in understanding mammalian evolution, the relationships among the various orders of placental mammals and the timing of their diversification in the geological past are still debated. We use a large dataset, containing genome-scale DNA sequences from representatives of all placental orders, to construct a well-supported mammalian phylogeny based on techniques that circumvent key biases rooted in properties of genomic data. Our results provide a resolution to some controversial questions about mammalian phylogeny, for example by showing that flying lemurs are closely related to primates. By utilizing our new phylogenetic tree in combination with 21 well-dated fossils that allow calibration of evolutionary rates, we infer that the radiation of placental orders occurred in a rapid episode spanning the Cretaceous-Paleogene (KPg) boundary. We therefore propose a new hypothesis, the trans-KPg burst model of placental diversification, which matches the mammalian fossil record more closely than previous “molecular clock” reconstructions.