A dating success story: genomes and fossils converge on placental mammal origins

As George Gaylord Simpson [1] detailed in his ground-breaking volume, one of the main contributions of the fossil record to the modern synthesis is primary data on the tempo of evolution. In recent decades, this aspect of fossil data has been leveraged for calibrating molecular estimates of clade divergence times. Concurrently, there has been expanding use of molecular data in palaeobiological studies (for example, [2]), and linking of fossils and embryos in studies of evolutionary development (for example, [3]). Yet, these examples of integration between fields of evolutionary science are still rare, and fossils are often excluded from macroevolutionary analyses beyond palaeobiology because of the complexities associated with including extinct taxa of uncertain phylogenetic affinity or with incorporating incomplete data. It has also been suggested that fossil data will likely have little impact on reconstructions of evolution in extant lineages (for example, [4,5]), despite extensive evidence to the contrary (for example, [6,7]). One of the most persistent question marks concerning the quality of fossil data has come from molecular dating studies themselves, which can differ from fossil-based divergence time estimates by tens to hundreds of millions of years and are regarded by many as showing the poor quality of the fossil record [8].For this reason, many palaeobiologists were pleased when a recent study [9] using a vast genomic dataset to reconstruct divergence dates for major placental mammal clades, found extensive congruence with the fossil record. Indeed it appears that most placental mammal orders originated and diversified after the demise of the non-avian dinosaurs during the Cretaceous-Paleogene (K-Pg) mass extinction 65.5 million years ago (mya), as palaeomammalogists have long maintained. The timing of this radiation has been a contentious issue since the first published molecular clock studies of placentals pushed the divergence times for many clades d