Reconstructing spore evolution recalibrates the timescale of land plant diversification.

The origin and evolution of land plants precipitated fundamental changes to the Earth system and enabled the diversification of life on land. Their early evolution is documented by a rich fossil record of sporopollenin-walled spores that is difficult to interpret because the evolution of plant spore characteristics is unresolved. We employ a suite of ancestral state estimation methods to elucidate the pattern of spore character evolution among land plants. We find that the ancestral embryophyte underwent monoplastidic meiosis and produced unenveloped spores with a three-layered sporoderm. We infer that trilete spores (that developed in tetrahedral tetrads), traditionally considered a feature of vascular plants, were produced by the crown-embryophyte. We recalibrate the molecular timescale of early land plant evolution on the basis of a reinterpretation of the fossil record in this light. We find that crown-embryophytes originated in the Late Ordovician (457-450 Ma) and that both crown-tracheophytes (432-428 Ma) and crown-bryophytes (445-429 Ma) diverged in the early Silurian.