Abstract
O'Leary et al. (O'Leary et al. 2013 Science 339, 662–667. (doi:10.1126/science.1229237)) performed a fossil-only dating analysis of mammals, concluding that the ancestor of placentals post-dated the Cretaceous–Palaeogene boundary, contradicting previous palaeontological and molecular studies that placed the ancestor in the Cretaceous. They incorrectly used fossil ages as species divergence times for crown groups, while in fact the former should merely form minimum-age bounds for the latter. Statistical analyses of the fossil record have shown that crown groups are significantly older than the oldest ingroup fossil, so that fossils do not directly reflect the true ages of clades. Here, we analyse a 20 million nucleotide genome-scale alignment in conjunction with a probabilistic interpretation of the fossil ages from O'Leary et al. Our combined analysis of fossils and molecules demonstrates that Placentalia originated in the Cretaceous.
Highlights
Placental mammals appear in the fossil record after the Cretaceous–Palaeogene (K–Pg) mass extinction event 66 Ma ago, when an estimated 76% of all species became extinct [1]
Molecular and palaeontological studies have supported a Cretaceous origin of Placentalia, but the age of placental mammal ordinal level crown groups relative to the K–Pg event has been the subject of protracted debate [2,3,4,5,6,7]
O’Leary et al [8] analysed a data matrix of 4541 morphological characters from 46 extant and 40 fossil mammal species to reconstruct and date the last common ancestor of placentals. They incorrectly estimate the age of living clades by the age of their oldest fossil representatives
Summary
Placental mammals (crown Eutheria) appear in the fossil record after the Cretaceous–Palaeogene (K–Pg) mass extinction event 66 Ma ago, when an estimated 76% of all species became extinct [1]. An explosive model of placental radiation, in which the last common ancestor of placentals post-dated the K–Pg event, has been rejected by molecular and palaeontological studies [2,3,4,5,6]. O’Leary et al [8] (see [9,10]) analysed a data matrix of 4541 morphological characters from 46 extant and 40 fossil mammal species to reconstruct and date the last common ancestor of placentals. They incorrectly estimate the age of living clades by the age of their oldest fossil representatives. O’Leary et al seek to reignite a controversy over the age of the placental ancestor that has otherwise been settled [2,3]
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