I am plant ecologist who investigates the response of Paleozoic plants and plant communities to environmental change during periods of mass extinction and deglaciation, and the possible evolutionary consequences. My primary research is focused on several aspects of the end-Permian biotic crisis and its aftermath, and the transition from a glacial-dominated world to an ice-free one during the Late Carboniferous to the Middle Permian. My studies strongly rely on an interdisciplinary approach combining quantitative palynological and paleobotanical data with organic geochemistry, isotope analysis, marine paleontology, biostratigraphy, ecology and plant physiology. Please find a selection of these projects below.
The end-Permian biotic crisis (~252 Ma ago), is long known for its profound reorganization of marine ecosystems. The extinction of marine species is generally correlated with a negative shift in δ13C in carbon isotopes of marine carbonates, reflecting a dramatic disturbance in the global carbon cycle. There is still animated discussion on the ultimate cause of the end-Permian crisis. Explanations differ with respect to source and rate. My dissertation research aimed to contribute to a botanical perspective of the end-Permian ecologic crisis and its aftermath. The results revealed some unanticipated patterns. There was a significant time-lag (>200 Ky) between the initiation of terrestrial collapse (coinciding with the marine extinctions) and the selective extinction among characteristic late Permian plant species. In Europe, a 4-5 million year period of lycopsid and seedfern domination followed. A gradual replacement by conifer woodland did not take place until shortly before the transition between Early and Middle Triassic. Currently I am studying the P-Tr transition within the Karoo Basin, South Africa (with Marion Bamford [Witswatersrand Univ], Bob Gastaldo [Colby Col.], Conrad Labandeira [NMNH], and Rose Prevec [Rhodes Univ.]), as well as Texas and New Mexico (with John Geissman [UNM], Paul Renne [BGC/UCB], Neil Tabor [SMU] and Renske Kirchholtes).
Palynological records worldwide reflect the terrestrial ecosystem collapse by a rapid decline of pollen produced by woody gymnosperms, and an increase of lycopsid microspores, often retained in tetrads. Ultrastructural analysis of the spore walls showed that the tetrads were produced by isoetalean lycopsids. The plants that produced these spores, the isoetalean genus Pleuromeia, can be found as dense populations in Early Triassic sediments all over the world. During a fellowship at the National Museum of Natural History Naturalis in the Netherlands, the morphology of Pleuromeia and extant Isoetaleans were compared (with Lea Grauvogel-Stamm [Univ. Louis Pasteur] and Han van Konijnenburg-van Cittert [Leiden Univ./NMNH Naturalis]). The earliest results suggest that an exceptional suite of physiological properties within this clade may explain their resilience during the end-Permian biotic crisis.
Following their demise, organisms leave behind molecular fossils. Some of these biomarkers reflect particular environmental conditions and their appearance and disappearance through time indicates how the prevailing conditions changed. Mark Sephton [Imperial Col.], Henk Visscher [Utrecht Univ.] and I reconstruct environments by comparing the biogeochemical signals of macro- and microfossils with responses from Permian and Triassic sedimentary rocks. This type of work involves merging morphological techniques with advanced analytical and spectroscopic methods. One of the most controversial biological proxies of environmental crisis at the close of the Permian is the organic microfossil Reduviasporonites. Originally Reduviasporonites was assigned to fungi, but subsequent geochemical data have been used to suggest an algal origin. We used high-sensitivity equipment, partly designed to detect interstellar grains in meteorites, to reexamine the geochemical signature of this microfossil and investigate its affinity.
For comparative purposes with today, the most recent cool-to-warm change in climatic state took place in the Early to Middle Permian. The collapse of Southern Hemisphere ice-sheets resulted in significant changes in the tropics, where the rainfall regime changed from wet to dry. Vegetation rich in conifers replaced spore-producing plants and seed ferns, which dominated the Late Carboniferous. Together with Bill DiMichele [NMNH] I am examining the relationship between landscape position, vegetational dynamics, and evolution. The results of this study could advance our understanding of the relationship between environmental conditions and aspects of terrestrial plant evolution, particularly body-plan innovation, and has the potential to serve as a model for the way in which plant communities respond to major climatically-driven environmental change.
Early conifers played a prominent role in the composition of Late Paleozoic plant communities in equatorial Euramerican floras. Phylogenetic analysis suggests that the conifer families Majonicaceae and Ullmanniaceae (voltzian Voltziales), known from equatorial Late Permian Euramerica, arose from a member of the paraphyletic group of walchian Voltziales. The voltzian conifers are more derived than their walchian counterparts in that they have fused female reproduction organs and a reproduction strategy that involves a pollen tube. Until recently, it has not been possible to establish with accuracy when they originated or when they started their rise to dominance over the walchian conifers. Our earliest findings from late Early Permian to early Middle Permian localities in north-central Texas prove that radiation in these conifer lineages occurred significantly earlier than previously thought. Evolution and migration of conifers in the Late Paleozoic tropical regions seem to have been strongly influenced by the increasingly drier climate. Studies on more taxonomically defining structures, such as cuticles and reproductive organs of these conifers are ongoing.
Several morphological features in leaves of fossil trees have been traditionally interpreted as sun/shade characteristics or evidence for drought stress. Todd Dawson, Lenny Kouwenberg and I would like to test if these features are indeed related to environmental factors. Climatic and physiological data collected within the canopy of California's giant redwoods by the Dawson Lab show a wide range of microclimate variation. This provides a unique opportunity to study the variation in leaf shape and particularly epidermal features in Coastal Redwood and Giant Sequoia for the same genotype in very different levels of drought stress and light in their natural environment. This study is important to improve the reconstruction of paleoenvironments based on plant fossils. We are currently looking for two undergraduate students to assist in this project through the URAP program.
During the Permian the more derived voltzian conifers (voltzian Voltziales) gain dominance over the walchian conifers (walchian Voltziales), in concurrence with an equatorial drier climate. Voltzian and walchian conifers are very different, not just in reproductive structures but also in vegetative organs. Lenny Kouwenberg and I plan to study both conifer types on a whole-plant basis (but special emphasis on leaves and cuticle) and attempt to draw inferences on physiological and ecological capabilities of the groups from morphological features associated with physiological performance. This may elucidate why the voltzian conifers became so successful during this particular time period.
In addition to the work described above, I am involved as a collaborator in several other projects. These include testing climate-controlled endemism of vertebrates in equatorial Pangea (with Chris Sidor [UW] and Neil Tabor [SMU]). Since 2002, I have been a member of the Paleobiology Database Paleobotany Working Group). In this group we attempt to quantify changes in the diversity and composition of terrestrial floras throughout the Phanerozoic.