KEY MORPHOLOGICAL ALTERATIONS IN THE EVOLUTION OF LEAVES

Evolution of plant form proceeds through sequential alterations in the development of plant organs. Leaves (or fronds) are organs that have diagnostic characteristics, including definite arrangement on a stem, bilateral symmetry (abaxial/adaxial identity), and determinate growth. Evolution of those diagnostic characteristics represents a series of critical steps in plant evolution that resulted from specific developmental alterations. The fossil record reveals a transformational series in leaf evolution that highlights steps that have occurred in parallel but independently in both leptosporangiate ferns and seed plants, resulting in superficially similar frond morphologies. In this study, the fronds of the most ancient fossil fern, Psalixochlaena antiqua, and the most ancient reconstructed seed plant, Elkinsia polymorpha, are characterized and compared with leaves of modern plants in order to identify the sequence in which features of leaves in two distinct clades of euphyllophytes arose. While both fronds show a combination of characters attributable to ancestral vegetative axes and characters attributable to leaves, each plant displays different combinations of those characters. These data document dissimilar sequences of character originations and, therefore, the independent evolution of developmental mechanisms in seed plants and ferns.