Four vascular plant lineages, the ferns, sphenopsids, progymnosperms, and seed plants, each independently evolved laminated leaves in the Paleozoic. A principal coordinates analysis of 634 leaf species from North American and European floras ranging in age from Middle Devonian through the end of the Permian shows that the clades followed parallel trajectories of evolution: each clade exhibits rapid radiation of leaf morphologies from simple (and similar) forms in the Late Devonian/Early Carboniferous to diverse, differentiated leaf forms, with strong constraint on further diversification beginning in the Mid Carboniferous. Similar morphospace trajectories have been documented in studies of morphological evolution in animals; however, plant fossils present unique opportunities for understanding the developmental processes that underlie such patterns. Comparison of the details of venation of Paleozoic leaves with modern leaves for which developmental mechanisms are known suggests developmental interpretations for the origination and early evolution of leaves. The parallel evolution of a marginal meristem by modification of the only developmental mechanisms available in the common ancestor of all the groups resulted in the pattern of early evolution repeated by each clade. The early steps of leaf evolution are followed by constraint on further diversification after the exhaustion of the possible elaborations of marginal growth. Hypotheses of the development of Paleozoic leaves can be tested by the study of living plants with analogous leaf morphologies.

Key words: development, leaf, Paleobotany, Paleozoic, Principal Coordinates Analysis