I am ultimately interested in understanding the causes of morphological diversity in organisms, and I primarily approach this wide topic by asking how relationships between form and function generate and influence evolutionary patterns over million-year time scales. To answer these questions, I incorporate techniques from paleontology, functional morphology, and phylogenetics, with an emphasis on linking results from studies of modern organisms with data from the fossil record. My work focuses primarily on seed plants, because they are highly diverse and important in structuring terrestrial ecosystems, they have an extensive fossil record, and the relationship between morphology and function can be directly tested in living as well as in extinct members. Currently, I have several major research areas exploring relationships between form and function at a variety of evolutionary scales.
Cones and Animals
One major area of my research focuses on the functional interactions between pollen grains and seed cones, in conifers as well as in several extinct groups of plants. Many species of conifers possess air bladders on their pollen grains (called sacci) that are often thought to aid in aerial pollen dispersal. A number of studies suggest that these structures primarily function in pollen capture, however, because species with saccate pollen all possess a suite of seed cone features that enables buoyant saccate pollen to float inside ovules for fertilization. Through pollination experiments and in vitro laboratory studies, I have demonstrated that flotation dramatically concentrates saccate pollen grains relative to non-buoyant grains, and also functions to efficiently transport pollen grains inside the ovule. Multivariate character analyses further showed that ancient saccate pollen is morphologically similar to modern saccate pollen. Likewise, seed cone morphology in most extinct groups that produced saccate pollen is consistent with the downwards-facing orientation of living plants that use flotation in pollination. This work suggests that pollen flotation has a deep and widespread history in seed plant reproduction, and illustrates how studies of living plants can inform our understanding of evolutionary patterns in deep time.
Diversification and Landscape History
The gradual appearance of more diverse plant-animal interactions from the Paleozoic to the recent must have increased the relative importance of functions such as seed protection and animal-mediated seed dispersal, and this should be reflected in reproductive structures through time. Based on analyses of morphological data from living and extinct species, I have suggested that evolutionary patterns in conifer cones are in fact largely consistent with their functional roles, and that shifts in function through time can lead to morphological change. For example, pollen-producing cones, whose basic functions in aerial pollen release have remained similar throughout conifer history, show little structural evolution. In contrast, the morphological diversity of seed-producing cones increases significantly in the Jurassic and Cretaceous due to the appearance of both fleshy cone morphologies and more heavily armored morphologies. In living conifers, such cone types are associated with strong interactions between conifers and animals, particularly vertebrates. Furthermore, conifer seed cones produced proportionally more protective tissue from the Jurassic onward, which suggests a fundamental increase in the strength and complexity of interactions between conifers and seed predators during the middle Mesozoic. This work suggests that changes in the importance of seed protection and dispersal through time have been major drivers of morphological diversification in conifers, and by extension, in other seed plant groups as well.
These types of functional interactions take place within a larger geological and climatological context, and recent work has explored how these can shape evolutionary patterns in plants. Working with colleagues at Yale and Harvard Universities, we have developed a time-calibrated phylogeny for living conifers based on four nuclear and chloroplast genes that samples 80% of living diversity. This work has revealed novel and unexpected differences in global diversification patterns among different conifer groups. Specifically, Northern Hemisphere clades contain an abundance of extremely recent species divergences (i.e., within the past 5 million years) while Southern Hemisphere clades contain older species divergences. Although work on understanding the specific mechanism underlying this pattern is just beginning, shifts in climate over the past 10 million years combined with differences in the distribution of landmasses between the hemispheres may ultimately drive these differences. For example, Northern Hemisphere conifers include a number of subgroups that appear to have diversified along with these environmental changes, while such diversification is largely absent in Southern Hemisphere groups. This work suggests that continental-scale geographic differences can leave a signature on the evolutionary history of major clades that is independent of specific environments, climatic regimes, or latitudinal gradients.
I also maintain a strong component of field-based paleobotany, which includes past work on the paleoecology of Mississippian floras from Arctic Canada and the systematics of charcoalified fossils from Late Cretaceous deposits in Georgia and Long Island. More recently, I have been working with colleagues from Yale, Niigata University in Japan, and the Mongolian Academy of Sciences to collect and describe extremely well preserved Early Cretaceous plant fossils from central Mongolia. I have also been working with collaborators from the Missouri Botanical Garden, the Royal Botanic Gardens Melbourne, and the Institut de Recherche pour le Développement to collect and describe Late Cretaceous and Miocene floras from New Caledonia, an island with an important and unique flora whose history is poorly understood.