Leaves, lignin, and the evolution of novel organ and cell types in early land plants

The establishment of anatomically and morphologically complex land plants during the Paleozoic required the evolution of new developmental mechanisms and physiological functions. Two aspects of the radiation of land plants have been studied: the early evolution of tracheid cell wall chemistry and microstructure and the early evolution of leaves and leaf development.; The adaptation of methods of organic and inorganic chemistry to the study of fossils in collaboration with the Carnegie Geophysical Laboratories has allowed millimeter to micron-scale measurement of the elemental, isotopic, and organic chemistry of anatomically preserved fossils. Taphonomic studies document that chemistry is preserved in place within individual cell walls at this fine scale. Analysis of fossils from the Rhynie Chert shows a discordance between the anatomical and chemical evolution of water conducting cells. Some tracheid types show no indication of lignification; lignin may have been first deployed elsewhere in the plant with the wall chemistry and anatomy only later coupled to form true tracheids like those of living plants.; Investigations of fossil leaf morphologies have documented that four vascular plant lineages each independently evolved laminated leaves in the Paleozoic and followed a similar sequence of morphological evolution. Each group exhibited rapid radiation of leaf morphologies from simple (and similar) forms in the Late Devonian/Early Carboniferous to diverse, differentiated leaf forms, followed by strong constraint on further diversification beginning in the Mid Carboniferous. Comparison of the details of venation of Paleozoic leaves with modern leaves for which developmental mechanisms are known suggests that the pattern of early evolution repeated by each clade is the result of the parallel evolution of a marginal meristem by modification of developmental mechanisms present in the common ancestor of all groups. The early steps of leaf evolution were followed by constraint on further diversification after the exhaustion of the possible elaborations of marginal growth. By the Permian, leaf morphologies covered much of the range of morphologies that exist today, including forms with characteristics broadly similar to those of angiosperms. This complete range of morphologies has been maintained since the Paleozoic, but has been progressively partitioned among different groups beginning in the Early Mesozoic.