A Molecular Temporal Evolutionary Framework of Land Plants and the Age of Angiosperms

A series of Bayesian molecular clock analyses with varying model combinations, parameters, and tree topologies were conducted to establish a temporal evolutionary framework of land plants and to estimate the age of angiosperms. The data consisted of five organellar and nuclear genes, 633 taxa, and 23 fossil calibration points (FCPs). The analyses were evaluated using an internal search thoroughness measure, effective sample sizes, and two external criteria: the consensus land plant phylogeny and a newly developed method that used the Hennigian reciprocal illumination principle to compare molecular age estimates of 21 nodes representing major land plant clades and the root split in green plants with those of their fossils in an a posteriori fashion. The results show that an analysis using a general time reversible (GTR) DNA substitution model, log normal branch-rate distribution, and Yule tree prior represented the best model combination, with the Birth-Death tree prior being equally optimal. This analysis estimated the age of the land plant crown group as 496.95 million years (MY), with a 95% highest posterior density (HPD) of 465.66–531.43 MY, and the age of the angiosperm crown group as 219.93 MY, with a 95% HPD of 184.09–256.82 MY. The former agrees with the fossil evidence, but the latter is significantly older than the fossil ages. A thorough examination of fossil evidence of angiosperms, as well as their evolutionary history in light of recent knowledge of morphology, physiology, and atmospheric CO2 concentration in the Mesozoic era, suggests that angiosperm radiation might have been delayed after their origin and that the current fossil record probably reflects only the products of that radiation, not the origin of angiosperms. Overall, the study shows that both molecular clock analyses and fossil studies, while agreeing on some aspects but disagreeing on others, can help develop a complete understanding of the temporal evolutionary history of land plants.