SEED PREDATORS, SEED DISPERSERS, AND THE DIVERSIFICATION OF CONIFER REPRODUCTIVE STRUCTURES IN THE MESOZOIC

Reproductive structures play a central role in understanding evolutionary relationships among seed plants because they provide most of the characters used in morphological phylogenetic analyses. Exploring evolutionary patterns in reproductive structures in terms of the functions they perform offers a framework in which to evaluate patterns of character evolution and may also provide important insights into the factors driving reproductive evolution. Conifers are a particularly useful group for this type of study because they have a long geologic history and separate reproductive structures (pollen-producing cones and seed-producing cones) that each have discrete functions. A multivariate analysis of morphological data from the pollen cones and seed cones of Paleozoic, Mesozoic, and extant conifer taxa shows that seed cones underwent a dramatic expansion of morphological diversity beginning in the Early-Middle Jurassic while pollen cones experienced little change in either morphology or morphological diversity. The increase in seed cone diversity in the Jurassic and Cretaceous is primarily the result of two novel features of cone structure and organization that evolved independently within extant conifer families: robust, tightly packed cones in members of Araucariaceae, Cupressaceae, and Pinaceae, and highly reduced, fleshy cones in members of Cupressaceae, Podocarpaceae, and Taxaceae. In extant conifers, these morphologies are associated with strong interactions between conifers and vertebrate seed predators and seed dispersers, particularly mammals and birds. This suggests that increases in the strength and complexity of biotic interactions over the Mesozoic have been the primary drivers of conifer reproductive evolution, and that patterns of character change specifically relate to increases in the importance of seed protection and seed dispersal. In contrast, reproductive structures with largely abiotic functional roles, like conifer pollen cones, are likely to experience little morphological change through time.