Establishing a time-scale for plant evolution.

Area of Science:

• Evolutionary Biology
• Paleobotany
• Molecular Phylogenetics

Background:

• Accurate plant evolutionary timelines are essential for testing causal hypotheses about planetary transformation.
• Molecular clock methods require careful calibration using fossil data, an area needing more attention.
• Previous studies may have inaccuracies due to the interpretation and implementation of fossil constraints.

Purpose of the Study:

• To construct and validate molecular clock calibrations for major land plant lineages.
• To estimate divergence times for key plant groups, including embryophytes and angiosperms.
• To reassess the timing of major plant evolutionary events in light of new calibration data.

Main Methods:

• Constructed 17 calibrations using minimum and soft maximum constraints for major land plant divergences.
• Performed cross-validation of calibrations using a seven-plastid gene dataset.
• Conducted six molecular clock analyses, varying calibration parameters to assess impact on divergence estimates.

Main Results:

• Cross-validation identified inconsistencies in Tracheophyta and Euphyllophyta calibrations, likely due to conservative maxima or rate variation.
• Estimated crown embryophyte divergence at 568–815 million years ago (Ma).
• Estimated crown angiosperm divergence at 175–240 Ma.

Conclusions:

• Rejects a post-Jurassic origin for angiosperms and a post-Cambrian origin for land plants.
• Suggests a slower tempo for the establishment of major embryophyte lineages than previously proposed.
• Findings are consistent with palaeobotanical data, though potentially challenging existing interpretations.