|The first placental mammal|
From O'Leary et al. Science 2013; 339: 662-7
Reprinted with permission from AAAS
|Alternative calibrations of the same tree|
From dos Reis et al. Biol Lett 2014; 10: 20131003
(c) The authors (Creative Commons)
It is not easy to choose appropriate fossils for calibration. Dos Reis and colleagues criticise O'Leary et al. for placing too much weight on Protungulatum donnae. They point out that the fossils of the genus may be found in the Cretaceous, yet the authors they cite (here) are unsure whether or not Protungulatum is a placental.
For a sober discussion of the difficulties in fossil calibration I highly recommend a recent paper by Bibi (here) on the evolution of ruminants. En passant Bibi observes, "The problem stems in part from the opacity of paleontological literature to non-specialists."
The O'Leary paper is co-authored by some of the most eminent specialists in fossil mammals. Arraigned gainst them we have some of the most competent molecular phylogeneticists. There seems little chance of compromise to judge from the acrimonious tone in this latest contribution. Surely the reviewers and editor of Biology Letters could asked the authors to damp things down?