The study by Maureen A. O'Leary et al. investigates the interordinal relationships of living and fossil placental mammals and the timing of the origin of placentals relative to the Cretaceous-Paleogene (K-Pg) extinction event. The researchers scored 4541 phenomic characters for 86 fossil and living species, combining these data with molecular sequences to construct a phylogenetic tree. The tree, when calibrated with fossils, indicates that the crown clade Placentalia and placental orders originated after the K-Pg boundary. Key findings include the monophyly of most traditional orders, the sister taxon relationship between Sundatheria (Dermaptera + Scandentia) and Primates, a close link between Proboscidea (elephants) and Sirenia (sea cows), and the monophyly of echolocating Chiroptera (bats). The study suggests that Placentalia first split into Xenarthra and Epitheria, with extinct New World species being the oldest members of Afrotheria. The results also indicate that the total clade Eutheria is younger than previously estimated, and the oldest age of Eutheria is constrained by taxa such as Maelestes, at 91 Ma. The study provides insights into the early diversification of placental mammals and challenges previous molecular clock-based estimates.The study by Maureen A. O'Leary et al. investigates the interordinal relationships of living and fossil placental mammals and the timing of the origin of placentals relative to the Cretaceous-Paleogene (K-Pg) extinction event. The researchers scored 4541 phenomic characters for 86 fossil and living species, combining these data with molecular sequences to construct a phylogenetic tree. The tree, when calibrated with fossils, indicates that the crown clade Placentalia and placental orders originated after the K-Pg boundary. Key findings include the monophyly of most traditional orders, the sister taxon relationship between Sundatheria (Dermaptera + Scandentia) and Primates, a close link between Proboscidea (elephants) and Sirenia (sea cows), and the monophyly of echolocating Chiroptera (bats). The study suggests that Placentalia first split into Xenarthra and Epitheria, with extinct New World species being the oldest members of Afrotheria. The results also indicate that the total clade Eutheria is younger than previously estimated, and the oldest age of Eutheria is constrained by taxa such as Maelestes, at 91 Ma. The study provides insights into the early diversification of placental mammals and challenges previous molecular clock-based estimates.