PATTERNS OF MORPHOLOGICAL EVOLUTION IN THE LEAVES OF FERNS AND SEED PLANTS

During the Devonian and Carboniferous, laminate leaves evolved independently in ferns, seed plants, and two exclusively Paleozoic lineages. In previous work, parallel patterns of morphological evolution were documented using a principal coordinates analysis of leaf species drawn from the Paleozoic floras of North America and Europe: within each lineage leaf morphologies diversified during the Late Devonian and Early Carboniferous, following which further diversification was limited. The convergent evolution of developmental mechanisms was inferred from details of lamina venation in each lineage. These patterns are now corroborated with a global analysis of the morphological characteristics of Paleozoic and Mesozoic leaf genera, as well as extant non-angiospermous plants. Two additional patterns are seen with inclusion of the post-Paleozoic record. First, by the Permian seed plants had evolved nearly the full range of morphologies ever found in non-angiospermous plants, but much of this morphological diversity was progressively lost through the Mesozoic. In contrast, the ferns have maintained the full range of Paleozoic morphologies through to the present, with the exception of the limited range of morphologies found in the living gymnosperms. This partitioning of morphological space likely reflects the partitioning of ecological niches between the two groups. Second, many of the complex characteristics of angiosperm leaf venation have evolved independently in a variety of extant and extinct fern and seed plant lineages. This suggests that, though these characteristics have been used in the past to argue angiosperm affinity for a variety of seed plant groups, they may provide a better indication of the physiologies of the plants and the climates in which they evolved.