Research Goals

My goal in research is to understand the interaction between environmental change and biological evolution using fossils and the sedimentary rock record. How does environmental change influence evolutionary and ecological processes? And conversely, how do evolutionary and ecological changes affect the physical environment? I am focused primarily on finding answers to these questions on two timescales: 1) the timescale of catastrophic extinction events and their immediate aftermaths (up to a few million years); and 2) the timescale of geological periods and eras (tens to hundreds of millions of years). My research combines macro-scale, field-based work on the stratigraphy and paleontology of carbonate platforms with micro-scale, laboratory-based work on the petrography and geochemistry of individual limestone samples and mineral phases. In addition to field and laboratory study, I also compile literature-based data and use theoretical models to help constrain interpretation of field-based data and to determine the extent to which local biotic patterns reflect global processes.

[A note on the Stanford fossil collections: The holotypes from the 'Stanford Shell Collection' formerly at Stanford were transferred to the California Academy of Sciences and placed on permanent loan to the Academy. This move took place over several years and was finalized in a formal manner in March 1977 under direction of Prof. W.R. Evitt. The Stanford Micropaleontology Collection, including holotypes. was officially transferred for permanent curation to the Museum of Paleontology, University of California at Berkeley, in 2007 under direction of Prof. J. Ingle.]

I. Causes and Consequences of Mass Extinction

A. End-Permian mass extinction

One primary focus of current research in the Paleobiology Lab is field-based examination of biological evolution and environmental change associated with the end-Permian extinction and its aftermath. We have used a variety of approaches to attempt to better characterize the cause(s) of mass extinction, to quantify the pattern and timing of extinction and recovery, and to identify connections between biological and environmental change through this important interval of Earth history.

Much of our work on the Permian-Triassic transition uses carbonate platform sediments from China, Turkey, and Japan. We use the carbonate strata as biological, environmental, and geochemical archives. Recent work has been aimed at obtaining a high-resolution record of biotic recovery from foraminifers, understanding the physical and biological controls on changes in carbonate depositional style across the immediate end-Permian extinction horizon, documenting and interpreting the recovery of reef ecosystems during the Middle Triassic, and constraining environmental changes through high resolution stable isotope records of carbon, strontium, and calcium.

We have supplemented our field-based work with literature compilations of gastropod and foraminiferan occurrences so that we can test the extent to which local changes in the size and diversity fossils are likely to reflect global trends.

B. End-Triassic mass extinction

More recently, we have taken a similar approach to that described above regarding the end-Permian mass extinction to better constrain the biological and environmental circumstances of the end-Triassic mass extinction. In particular, we are interested in exploring the extent to which the end-Permian and end-Triassic mass extinction events may reflect similar forcing mechanisms. Chief among these is the hypothesis that each resulted from environmental changes driven by volatile release during flood basalt volcanism.

II. Extinction Selectivity

One of the most important unsolved questions in the fields of paleobiology, evolution, and conservation biology is why some species go extinct while others survive. Patterns of extinction selectivity in the fossil record can shed light on the causes of mass extinction events, reveal differences in process between background and mass extinction, quantify the importance of selection above the species level in driving evolutionary patterns, and help us to predict which living species are at greatest risk of extinction. Our current research focuses on quantifying extinction selectivity with respect to global and local parameters (e.g., geographic range, body size, local abundance) for both background and mass extinction intervals. Two important, but not exclusive, foci of the work are on quantifying differences in selectivity between background and mass extinction events and on using selectivity patterns to test among hypothesized causes of mass extinctions.

III. Evolution of Body Size

Research on the evolution of body size in the lab has grown out of observed shifts in maximum and mean size in gastropods and other higher taxa across the end-Permian mass extinction. Our goal is to use time-series of sizes in gastropods and other higher taxa to identify environmental and biological controls on body size evolution. Our current research efforts are focused on the Permian-Triassic and Triassic-Jurassic transitions. We are focused in particular on isolating the contributions of size-biased extinctions and originations and within-lineage size trends to overall shifts in the size distributions of higher taxa through geological time. Isolating these components allows us to better test hypothesized links between environmental change and evolutionary pattern.

IV. The Abundance of Animals through Geological Time