THE ORIGINS AND EARLY EVOLUTION OF THE MEGAPHYLLOUS LEAF

Paleobotanical data, with emphasis on anatomical characters in ferns and seed plants, confirm that the megaphyllous leaf evolved independently several times from the Middle Devonian to the Early Carboniferous. Fernlike cladoxylaleans and aneurophytalean progymnosperms possessed only megaphyll precursors homologous with the small megaphylls of archaeopteridalean progymnosperms. Different trends toward developing a large megaphyll, from the modification of lateral branch systems of some basal euphyllophytes, are shown by the fernlike rhacophytaleans and by the zygopterid and tedelean ferns. Zygopterids, with their tridimensional frond, petiole of the phyllophore type, and annulate sporangia, must be considered as the earliest undisputable ferns while the probably derived tedeleans have evolved toward a convergent filicalean-type of frond. The leaf of other early ferns certainly derived through a minimal change in anatomy from other Devonian euphyllophytes. Early seed plant leaves probably evolved through the modification of an entire lateral branch system of aneurophytalean or stenokolean ancestors. In contrast to ferns, the seed plants early developed broad laminate pinnules with dichotomous nervation. The leaves of ferns and seed plants are not homologous, and fundamental differences in their petiole anatomy support the early divergence of the two groups.