More debate on the origin of placentals

The first placental mammal
From O'Leary et al. Science 2013; 339: 662-7
Reprinted with permission from AAAS

Controversy over the timing of the mammalian tree (previous post) continues. The latest contribution, by Mario dos Reis et al. (here), boldly states, "O'Leary et al. seek to reignite a controversy over the age of the placental ancestor that otherwise has been settled." The italics are mine. The references that support this statement are a paper by the same authors and one by Meredith et al. No false modesty here.



Alternative calibrations of the same tree
From dos Reis et al. Biol Lett 2014; 10: 20131003
(c) The authors (Creative Commons)

The problem arises because of different approaches to calibration of the molecular clock. The figure shows trees with identical topology but dated with different sets of fossils. In the first three, many extant orders of mammal can trace their origin to the Late Cretaceous. The problem is that all known fossils of placentals date from after the mass extinction event at the Cretaceous-Paleogene boundary. A literal interpretation of the fossil record, on the other hand, has all orders and even placentals themselves postdating the boundary.
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?