Modelling a rapid radiation of crown-group placentals

The timing of the origins of the crown-group placental mammals has provided one of the classic battlefields in the long-running debate over when clades arise. Undoubted fossil crown-group placentals appear only in the Paleogene, but even so most molecular analyses, and many palaeontologists, have suggested their true origin is somewhere between 70-100 Ma. However, apart from the fact of the fossil record itself, there are several reasons to believe that the true origin is indeed post-Cretaceous, including consideration of the dynamics of stem and crown groups, which strongly favour crown-group origins to lie just after, and not just before, mass extinctions. Here we consider this "hard explosive" model in the light of the newly-developed "Covariant Evolutionary Tempo (CET)" model which allows diversification and molecular evolution rates to covary. It predicts "early bursts" in both lineage creation and molecular evolution at the base of major radiations which lead to highly unequally-sized clades; and an inheritance of rapid rates from this initial event by extant rapidly-evolving clades. We show that when the placentals are constrained to emerge after the K-Pg boundary, they indeed show elevated rates of both diversification and molecular evolution, which rapidly decline. Nevertheless, although elevated, these rates are comparable to the fastest rates seen in extant clades such as the rodents. In addition, the contiguous lineages leading from the origin to the rodents and other fast evolving clades also show elevated rates. These patterns suggest that not only is a Paleogene origin for the placental crown-group plausible, as fossil evidence suggests, but they also provide support for the CET model, which should be considered in other cases of pronounced fossil record/molecular clock mismatch.