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.
Highlights
The K– Pg mass extinction occurred 66 Ma and was the second largest extinction event in the history of life, exterminating 75% of terrestrial species [1]
The diversification of Placentalia likely began in the latest Cretaceous, the majority of intraordinal diversification of placental mammals was in the Palaeogene
The enigmatic genus Protungulatum has been considered important in dating the placental origin, it is unclear whether it represents a placental [16] or a more basal eutherian [44]. Where it is supported as a placental, a Cretaceous origin for Placentalia is assured, as Protungulatum is known from the Maastrichtian
Summary
The K– Pg mass extinction occurred 66 Ma and was the second largest extinction event in the history of life, exterminating 75% of terrestrial species [1]. We use the most recent stochastic techniques [29] to date phylogenetic trees [13] generated from a dataset of mostly Cretaceous and Palaeogene eutherians (electronic supplementary material, file S7), and reconstruct evolutionary rates in cladistic characters to assess change across a broad suite of morphological traits. Combined, these analyses answer two major questions in placental mammal evolution: when did Placentalia originate, and did the K–Pg mass extinction result in an Early Palaeogene adaptive radiation of placental mammals?
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