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Eutherians Experienced Elevated Evolutionary Rates in the Immediate Aftermath of the Cretaceous-Palaeogene Mass Extinction

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Eutherians Experienced Elevated Evolutionary Rates in the Immediate Aftermath of the Cretaceous-Palaeogene Mass Extinction

Thomas John Dixon Halliday et al. Proc Biol Sci.

Abstract

The effect of the Cretaceous-Palaeogene (K-Pg) mass extinction on the evolution of many groups, including placental mammals, has been hotly debated. The fossil record suggests a sudden adaptive radiation of placentals immediately after the event, but several recent quantitative analyses have reconstructed no significant increase in either clade origination rates or rates of character evolution in the Palaeocene. Here we use stochastic methods to date a recent phylogenetic analysis of Cretaceous and Palaeocene mammals and show that Placentalia likely originated in the Late Cretaceous, but that most intraordinal diversification occurred during the earliest Palaeocene. This analysis reconstructs fewer than 10 placental mammal lineages crossing the K-Pg boundary. Moreover, we show that rates of morphological evolution in the 5 Myr interval immediately after the K-Pg mass extinction are three times higher than background rates during the Cretaceous. These results suggest that the K-Pg mass extinction had a marked impact on placental mammal diversification, supporting the view that an evolutionary radiation occurred as placental lineages invaded new ecological niches during the Early Palaeocene.

Keywords: Cretaceous; Palaeogene; evolutionary rate; extinction; mammals; radiation.

Figures

Figure 1.
Figure 1.
Randomly selected exemplar time-scaled phylogeny with branches coloured according to whether they have significantly lower (blue) or higher (red) evolutionary rates than would be expected given an equal rate model. The grey box indicates Placentalia, with the four placental superorders identified, while nodes and branches outwith the grey box are non-placental eutherians.
Figure 2.
Figure 2.
Randomly selected exemplar time-scaled phylogeny with nodes coloured according to whether the clade for which they are the last common ancestor has significantly lower (blue) or higher (red) evolutionary rates than the remainder of the tree. The grey box indicates Placentalia, with the four placental superorders identified, while nodes and branches outwith the grey box are non-placental eutherians.
Figure 3.
Figure 3.
Two graphs depicting evolutionary rate through time calculated from the consensus of optimal topologies only from the six different original phylogenetic analyses. The Cretaceous–Palaeogene boundary falls between the Maastrichtian (M) stage and the Puercan (P) North American Land Mammal Age. (a) A sampling rate of 85%, which implies that the fossil record is relatively good throughout the tree. (b) A conservatively poor rate of 0.5%. Both show low Cretaceous rates with a significant two- to fourfold increase at the end-Cretaceous mass extinction, although (b) shows greater variance.

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