This study investigates the origin and diversification of placental mammals, focusing on their relationship to the Cretaceous-Paleogene (K-Pg) boundary. By analyzing 4541 phenomic characters from 86 species, including 40 fossil species, the researchers constructed a phylogenetic tree that, when calibrated with fossils, shows that the crown clade Placentalia and placental orders originated after the K-Pg boundary. The results suggest that placental mammals diversified rapidly after the K-Pg event, with the first members of modern placental orders appearing 2 to 3 million years after the event. The study challenges previous molecular clock estimates, which suggested that placental mammals originated earlier, and indicates that the fossil record provides a more accurate timeline. The research also reveals that the placental ancestor had a unique set of traits, including a small size, insectivorous diet, and specific dental characteristics. The study highlights the importance of integrating phenotypic and genomic data to understand placental mammal evolution. It also shows that the placental ancestor is distinct from other mammal groups, such as marsupials and monotremes, and that the diversification of placental mammals occurred in a radically transformed terrestrial ecosystem following the K-Pg event. The findings support the "explosive model" of placental diversification, suggesting that placental mammals evolved rapidly after the K-Pg event, rather than before or during it. The study also provides insights into the biogeography and paleoenvironments of placental mammals, indicating that they diversified in a world without non-avian dinosaurs. The results challenge previous hypotheses about the origin of placental mammals and provide a more accurate timeline for their evolution.This study investigates the origin and diversification of placental mammals, focusing on their relationship to the Cretaceous-Paleogene (K-Pg) boundary. By analyzing 4541 phenomic characters from 86 species, including 40 fossil species, the researchers constructed a phylogenetic tree that, when calibrated with fossils, shows that the crown clade Placentalia and placental orders originated after the K-Pg boundary. The results suggest that placental mammals diversified rapidly after the K-Pg event, with the first members of modern placental orders appearing 2 to 3 million years after the event. The study challenges previous molecular clock estimates, which suggested that placental mammals originated earlier, and indicates that the fossil record provides a more accurate timeline. The research also reveals that the placental ancestor had a unique set of traits, including a small size, insectivorous diet, and specific dental characteristics. The study highlights the importance of integrating phenotypic and genomic data to understand placental mammal evolution. It also shows that the placental ancestor is distinct from other mammal groups, such as marsupials and monotremes, and that the diversification of placental mammals occurred in a radically transformed terrestrial ecosystem following the K-Pg event. The findings support the "explosive model" of placental diversification, suggesting that placental mammals evolved rapidly after the K-Pg event, rather than before or during it. The study also provides insights into the biogeography and paleoenvironments of placental mammals, indicating that they diversified in a world without non-avian dinosaurs. The results challenge previous hypotheses about the origin of placental mammals and provide a more accurate timeline for their evolution.