BOYCE, C. KEVIN. Harvard Botanical Museum, 26 Oxford St., Cambridge MA 02138. - The evolution of leaves and leaf development in the Paleozoic.
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