CHANGING FUNCTIONAL ROLES AND THE EVOLUTION OF CONIFER REPRODUCTIVE MORPHOLOGY

Conifers, a seed plant group with a long evolutionary history and a well-studied fossil record, are an ideal group in which to study how differences in the functions that seed plant reproductive structures perform may drive broad patterns of morphological evolution. Analyses of discrete morphological data from the reproductive structures of fossil and extant conifer species show that pollen-producing structures (which function only in pollen dispersal) exhibit morphologies similar to modern species earlier in conifer history than seed-producing structures (which function in pollen capture, seed protection, and seed dispersal) and that seed-producing structures show a more diverse range of morphologies through time. Additionally, an analysis of the size of seed-producing cones through time reveals that the cones produced by Jurassic, Cretaceous, and extant conifer species are much wider than those produced by the earliest conifers (Permian and Triassic species), reflecting a greater amount of tissue growth in the cones of later species. Because this tissue surrounds the developing seed, this change in growth probably reflects an increasing importance of seed protection through time. This study suggests that the greater morphological diversity of conifer seed-producing structures, as compared to pollen-producing structures, results from performing a greater number of basic functions combined with major changes in the role and importance of these functions over the history of the lineage. This study also highlights the role of biotic interactions, specifically seed predation, as potential drivers of morphological change in conifer seed-producing structures.