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

Abstract

The timing of the placental mammal radiation has been a source of contention for decades. The fossil record of mammals extends over 200 million years, but no confirmed placental mammal fossils are known prior to 64 million years ago, which is approximately 1.5 million years after the Cretaceous-Paleogene (K-Pg) mass extinction that saw the end of non-avian dinosaurs. Thus, it came as a great surprise when the first published molecular clock studies suggested that placental mammals originated instead far back in the Cretaceous, in some cases doubling divergence estimates based on fossils. In the last few decades, more than a hundred new genera of Mesozoic mammals have been discovered, and molecular divergence studies have grown from simple clock-like models applied to a few genes to sophisticated analyses of entire genomes. Yet, molecular and fossil-based divergence estimates for placental mammal origins have remained remote, with knock-on effects for macro-scale reconstructions of mammal evolution. A few recent molecular studies have begun to converge with fossil-based estimates, and a new phylogenomic study in particular shows that the palaeontological record was mostly correct; most placental mammal orders diversified after the K-Pg mass extinction. While a small gap still remains for Late Cretaceous supraordinal divergences, this study has significantly improved the congruence between molecular and palaeontological data and heralds a broader integration of these fields of evolutionary science.

Figure 1

Schematic comparison of placental mammal divergence estimates based on: (A) previous molecular divergence studies [4,10]; (B) the fossil record of eutherians, including placentals[14,15]; and (C) the recently published divergence estimates from the phylogenomic analysis of dos Reis et al. [9]. The shaded grey areas denote the period during which significant intraordinal divergences within placental mammals occur, which, for B and C, correspond with large increases in placental mammal taxonomic diversity recorded in the fossil record. Note that these representations are for general pattern only, and do not include estimates of error, which were particularly large in the early molecular clock analyses [20]. Many molecular divergence studies show a similar pattern as A, with several placental inter- and intraordinal divergences in the Cretaceous [4,11,13], while some others approach fossil-based estimates [9,17-19].