Department of Biology

Iridium abundance anomaly at the Cretaeous-Tertiary boundary, one of many lines of evidence for a bolide impact

A non-marine Cretaceous-Tertiary boundary clay, which represents the fallout layer from a bolide impact.

Primary succession of ferns in Hawaii. The anomalously high abundance of ferns immediately above the Cretaceous-Tertiary boundary clay is evidence for recolonization of the landscape following mass mortality at the Cretaceous-Tertiary boundary.

• B.S. Botany, 1975, University of Nebraska
• M.S. Botany, 1978, University of Michigan
• Ph.D. Botany, 1981, University of Michigan
• Postdoctoral research, 1981-1991

• Smithsonian Institution
• Indiana University
• U.S. Geological Survey
• University of Colorado
• National Center for Atmospheric Research

Courses Taught

BIO 1320, Modern Biology I (non-majors)

BIO 1421,Modern Biology II (non-majors)

BIO 4411/5411, Morphology of Vascular Plants

BIO 4412/5412, Plant Anatomy

BIO 5110E, Mass Extinction Seminar

BIO 5308/7308, History of Vegetation and Climate

My formal academic training is in Botany with extensive postdoctoral experience in Earth Science.  My research is interdisciplinary and draws from Botany, Geology, and Atmospheric Science.  I focus on the history of vegetation and climate, and feedbacks between vegetation and the atmosphere.  My long-term goal is to develop a comprehensive history of floras and their environment during the rise of angiosperms and the end-Cretaceous mass extinction, and to identify plant-atmosphere feedbacks that helped shape this history.

I believe that paleobotany has much untapped potential for deciphering the causes of past global change.  Plants provide forensic evidence for climatic change and mass mortality because they are sensitive indicators of environment.  Plants help regulate climate because they strongly affect the fluxes of energy, water vapor, and carbon dioxide between the land surface and the atmosphere.  Plants serve as both environmental indicators and regulators of environmental change.  This means that paleobotany can provide important data for climatic reconstruction and important input parameters for Earth System Models.

My empirical studies focus on fossil angiosperm leaves, and in particular leaf megafossils and dispersed leaf cuticles.  Leaf megafossils provide a large suite of characters for systematics, yet another suite of characters for environmental reconstruction.  Dispersed plant cuticles are the most abundant component of the plant fossil record next to pollen and spores, which means that dispersed cuticles can provide the high sampling intensity needed for studies of paleoecology and floral turnover.  Dispersed cuticles form an important complement to palynomorphs because they provide direct evidence on leaf physiognomy and atmospheric CO₂ and have different taphonomic histories and sampling biases.

Current and Future Research Projects

Mass extinction

The Cretaceous-Tertiary (K-T) boundary serves as the archetypal example of extinction by a bolide impact.  Paleobotany provides important forensic evidence for mass mortality associated with impact, rapid extinction of taxa at the K-T boundary, and possible long-term changes in temperature, precipitation, and CO₂.  Poorly understood are the exact pattern of long-term environmental change and its link to floral turnover, and the three-dimensional response of the climate system to a bolide impact.

A major long-term goal is to elucidate the pattern of long-term floral and environmental change across the K-T boundary in megathermal assemblages from the southern Western Interior and adjacent regions, building on earlier work with Jack Wolfe, Robert Tschudy, and others.  This work will integrate the record of leaf megafossils, dispersed cuticles, and palynomorphs with environmental information derived from foliar physiognomy, stomatal indices, and stable isotopes. 

A second long-term goal is to model climatic change associated with an end-Cretaceous bolide impact, building on earlier work with Bette Otto-Bliesner and Chris Scotese.  The objective is to predict geographic variation in environmental change at the K-T boundary and to use this variation to evaluate patterns of extinction in the marine and terrestrial realms.

Vegetation-atmosphere interactions during the Cretaceous and Cenozoic

Climate strongly influences plant physiognomy and distribution, while plants influence climate through their effects on land-surface physics and the carbon cycle.  Modeling studies of Cretaceous and Cenozoic climate indicate that feedbacks between high-latitude forests, the atmosphere, and oceans helped maintain warm polar climates of the geologic past.  They also indicate that the shift to grassland and tundra vegetation during the Late Cenozoic amplified cooling and drying trends.  Not well understood are mechanistic links between key biological and environmental events, especially in light of new data that contradict earlier ideas on the timing of mountain uplift and levels of atmospheric CO₂ during the Cenozoic.

Chris Scotese, Arne Micheels, Jack Wolfe, and I are beginning a project on global paleogeography and vegetation for the Late Miocene (Tortonian) and the feedbacks between mountain uplift, vegetation, and Late Cenozoic cooling and drying.  Previous climate modeling studies of the Late Cenozoic, while demonstrating important linkages between uplift, cooling, and drying, could not address timing of key events because they used unrealistic paleogeography.   Our global-scale study will produce realistic data sets for modeling the effects of climatic uplift and vegetation change and is unique in its use of paleobotany as a major source of data on paleoelevation, sea ice, and glacial ice.

Angiosperm diversification? The Cretaceous rise of angiosperms was a major event in the history of terrestrial ecosystems.  Angiosperm families and orders appeared rapidly during the Cretaceous and caused major changes in species diversity.  Changes in ecological dominance also occurred, but their extent and timing are debated.  The systematics of Cretaceous angiosperm leaves and their stratigraphic record appear to be consistent with published molecular phylogenies based on multiple genes.  However, revision of previous systematic work on fossil angiosperm leaves is needed to evaluate older identifications and document biogeographic history and the minimum age of clades.

I am currently reviewing the Cretaceous and Early Cenozoic leaf record from North America for the four orders of primitive eudicots? Ranunculales, Proteales, Trochodendrales, and Buxales.  My preliminary results confirm earlier evidence for the rapid appearance of these four orders during the mid-Cretaceous, and support molecular evidence for a close relationship between Platanaceae (sycamores) and Proteaceae.  Another interesting finding is that herbaceous representatives of these orders are present as low-abundance elements in leaf floras preserved in volcanic ash.  Geographic and stratigraphic patterns of diversity correlate well with climate.

My students, collaborators, and I are testing competing hypotheses about the role of angiosperms in regional vegetation during the Cretaceous with the analysis of leaf litter and in situ trees.  Our results indicate that Late Cretaceous angiosperms were a major component of biomass in megathermal vegetation and comprised trees up to 20 m or more in height.  The angiosperm rise to dominance was directly influenced by climate and showed a strong equator to pole gradient.  

Dispersed plant cuticles as environmental indicators Plant cuticle-the decay-resistant outer layer of leaves and young stems is the most abundant component of the plant fossil record next to pollen and spores.  Plant cuticle has many features diagnostic of taxonomy, yet others diagnostic of climate and atmospheric chemistry.  Analysis of plant cuticle has focused on megafossils, but a more palynological approach based on bulk samples shows good potential for paleoenvironmental reconstruction.

Two in-progress studies are exploring new uses of dispersed cuticle in environmental analysis.  The first is an analysis of stable carbon isotopes in modern and fossil plant cuticle, which provide a tool for tracking the global carbon cycle.  My results indicate that cuticle and whole tissue show a consistent isotopic offset, which means that cuticle can be used to reconstruct the isotopic composition of whole tissue.  My results also indicate that the analysis of bulk organic carbon is potentially problematic, because bulk carbon is a mixture of isotopically distinct compounds whose ratios can vary with decay.

The second in-progress study is the use of plant cuticle in the reconstruction of vegetation physiognomy.  Thickness of plant cuticle shows a significant correlation with evergreen and deciduous growth habit, which should allow determination of temperature trends and the mapping of freeze lines in the fossil record.