Previous estimates for the age of the angiosperms derived from molecular clock models, although in conflict with one another, have generally implied that angiosperms originated long before their first appearance in the stratigraphic record (Early Cretaceous). We estimate the age of the angiosperms by integrating new data and new methods of analysis. Taxa were sampled from all major clades of extant land plants. Sequences of two highly conserved chloroplast genes, psaA and psbB, were used as primary data. A data set of combined psaA and psbB sequences for 54 genera of land plants was used to obtain hypotheses of relationships through parsimony analysis. Gnetales are resolved as the sister taxon of all other seed plants if codon positions are weighted equally (Tree 1), or alternatively, as the sister taxon to Pinaceae within a monophyletic gymnosperm clade if 1st and 2nd codon positions are given greater weight (Tree 2), in accordance with their much lower substitution rates. Substitution models for synonymous and non-synonymous sites were obtained through a likelihood approach in which a molecular clock was enforced. Temporal calibration was achieved by fixing the time of divergence between Marchantia and all other land plants at 450 Ma (Late Ordovician). Based on data from non-synonymous sites, the angiosperm crown group is dated as 144 Ma, according to Tree 1, and as 133 Ma, according to Tree 2. These preliminary estimates are closer to ages derived from the fossil record than published estimates, but confidence intervals are broad enough to include some previous dates. Subsequent analyses will incorporate methods for estimating divergence times assuming heterogeneous rates of molecular evolution, and temporal calibrations that include minimum ages for several nodes, obtained from reliably identified fossil taxa. These analytical approaches are expected to provide a narrower estimate for the age of the angiosperm crown group.

Key words: age, angiosperm origin, evolutionary rates, fossil record, seed plant phylogeny, temporal calibration