Earliest record of megaphylls and leafy structures, and their initial diversification

Evolutionary changes in the structure of leaves have had far-reaching effects on the anatomy and physiology of vascular plants, resulting in morphological diversity and species expansion. People have long been interested in the question of the nature of the morphology of early leaves and how they were attained. At least five lineages of euphyllophytes can be recognized among the Early Devonian fossil plants （Pragian age, ca. 410 Ma ago） of South China. Their different leaf precursors or ＂branch-leaf complexes＂ are believed to foreshadow true megaphylls with different venation patterns and configurations, indicating that multiple origins of megaphylls had occurred by the Early Devonian, much earlier than has previously been recognized. In addition to megaphylls in euphyllophytes, the laminate leaf-like appendages （sporophylls or bracts） occurred independently in several distantly related Early Devonian plant lineages, probably as a response to ecological factors such as high atmospheric CO2 concentrations. This is a typical example of convergent evolution in early plants.