Patterns of segregation and convergence in the evolution of fern and seed plant leaf morphologies

Abstract

Global information on Paleozoic, Mesozoic, and extant non-angiosperm leaf morphol- ogies has been gathered to investigate morphological diversity in leaves consistent with marginal growth and to identify likely departures from such development. Two patterns emerge from the principal coordinates analysis of this data set: (1) the loss of morphological diversity associated with marginal leaf growth among seed plants after sharing the complete Paleozoic range of such morphologies with ferns and (2) the repeated evolution of more complex, angiosperm-like leaf traits among both ferns and seed plants. With regard to the first pattern, morphological divergence of fern and seed plant leaf morphologies, indirectly recognized as part of the Paleophytic-Meso- phytic transition, likely reflects reproductive and ecological divergence. The leaf-borne reproduc- tive structures that are common to the ferns and Paleozoic seed plants may promote leaf morpho- logical diversity, whereas the separation of vegetative and reproductive roles into distinct organs in later seed plant groups may have allowed greater functional specialization—and thereby mor- phological simplification—as the seed plants came to be dominated by groups originating in more arid environments. With regard to the second pattern, the environmental and ecological distri- bution of angiosperm-like leaf traits among fossil and extant plants suggests that these traits pref- erentially evolve in herbaceous to understory plants of warm, humid environments, thus sup- porting inferences concerning angiosperm origins based upon the ecophysiology of basal extant taxa.