An acrocarpous moss in Cretaceous amber from Myanmar

• Plants have utterly transformed the planet, but testing hypotheses of causality requires a reliable time-scale for plant evolution. While clock methods have been extensively developed, less attention has been paid to the correct interpretation and appropriate implementation of fossil data. • We constructed 17 calibrations, consisting of minimum constraints and soft maximum constraints, for divergences between model representatives of the major land plant lineages. Using a data set of seven plastid genes, we performed a cross-validation analysis to determine the consistency of the calibrations. Six molecular clock analyses were then conducted, one with the original calibrations, and others exploring the impact on divergence estimates of changing maxima at basal nodes, and prior probability densities within calibrations. • Cross-validation highlighted Tracheophyta and Euphyllophyta calibrations as inconsistent, either because their soft maxima were overly conservative or because of undetected rate variation. Molecular clock analyses yielded estimates ranging from 568-815 million yr before present (Ma) for crown embryophytes and from 175-240 Ma for crown angiosperms. • We reject both a post-Jurassic origin of angiosperms and a post-Cambrian origin of land plants. Our analyses also suggest that the establishment of the major embryophyte lineages occurred at a much slower tempo than suggested in most previous studies. These conclusions are entirely compatible with current palaeobotanical data, although not necessarily with their interpretation by palaeobotanists. © 2011 The Authors. New Phytologist © 2011 New Phytologist Trust.

• Land plants play an essential role in the evolution of terrestrial life. Their time of origin and diversification is fundamental to understanding the evolution of life on land. We investigated the timing and the rate of molecular evolution of land plants, evaluating the effects of different types of molecular data, including temporal information from fossils, and using different molecular clock methods. • Ages and absolute rates were estimated independently with two substitutionally different data sets: a highly conserved 4-gene data set and matK, a fast-evolving gene. The vascular plant backbone and the crown nodes of all major lineages were calibrated with fossil-derived ages. Dates and absolute rates were estimated while including or excluding the calibrations and using two relaxed clocks that differ in their implementation of temporal autocorrelation. • Land plants diverged from streptophyte alga 912 (870-962) million years ago (Mya) but diversified into living lineages 475 (471-480) Mya. Ages estimated for all major land-plant lineages agree with their fossil record, except for angiosperms. Different genes estimated very similar ages and correlated absolute rates across the tree. Excluding calibrations resulted in the greatest age differences. Different relaxed clocks provided similar ages, but different and uncorrelated absolute rates. • Whole-genome rate accelerations or decelerations may underlie the similar ages and correlated absolute rates estimated with different genes. We suggest that pronounced substitution rate changes around the angiosperm crown node may represent a challenge for relaxed clocks to model adequately.