Paleoecological impact of changing substrates during the Ordovician radiation
The Ordovician radiation is one of the three most significant events in the history of metazoan life along with the Cambrian radiation and the end-Permian mass extinction. Like the Cambrian radiation, the Ordovician radiation was facilitated by changes in the substrate. Hard substrates, such as the shell bed shown at right, became widely available in shallow marine settings during the Early-Middle Ordovician due to a combination of physical and biotic factors, such as a greenhouse climate and an abundance of bioclastic material. Earlier studies have suggested a link between the Ordovician proliferation of hard substrates and the diversification of echinoderms and other groups dominated by hard-substrate-attaching forms. In this project, I am exploring the range of possible connections between substrate change and metazoan radiation during the Ordovician. My field areas include south-central Pennsylvania, eastern Nevada, and western Utah.
Ecosystem engineering (habitat modification through the activities of organisms)
Ecosystem engineers are animals that modify their habitat, and the local distribution of resources, through their life activities. This concept had, until recently, been applied only to modern environments, in which the effects of animal behavior can be observed directly. Modern ecosystem engineers include beavers, which alter stream habitats through the construction of dams. Ancient examples of ecosystem engineering are harder to identify. Paleoecologists have recently begun to recognize examples from the fossil record. Archaeocyath sponges (upper right), the first skeletonized organisms to build reefs (such as the one shown at lower right, in western Nevada), were early ecosystem engineers. I am investigating other examples of ecosystem engineering in the fossil record, particularly early behavioral innovations that transformed marine ecosystems.
Cambrian bioturbation and substrate ecology
During the Ediacaran-Cambrian transition, bioturbation became more plentiful, complex, and physically disruptive as a direct consequence of the Cambrian radiation of marine invertebrates. This increase in biogenic sediment disturbance transformed substrates and dramatically expanded available ecospace. In particular, microbial mats, which were ubiquitous on Proterozoic seafloors, virtually disappeared from open marine environments by the end of the Cambrian Period. This early instance of ecosystem engineering has been termed the agronomic revolution, and the ecological and evolutionary effects are known as the Cambrian substrate revolution. I am interested in the record of behavioral evolution that is provided by Cambrian trace fossils, such as the looping Lower Cambrian trace Taphrhelminthopsis (upper right) and the tire-track-like Upper Cambrian trace Climactichnites (lower right). My field areas include eastern California, western Nevada, north-central Wisconsin, and southern Sweden.
Quantifying bioturbation in the fossil record
The quantity of bioturbation present within an outcrop can reflect a variety of ecological and environmental factors, including oxygenation, sediment accumulation rate, and substrate consistency. Thus, quantitative data can augment qualitative descriptions of bioturbation and facilitate the detailed characterization of facies. To date, few methods have been designed for precisely estimating quantities of bioturbation from outcrops. My colleague David Bottjer and I recently developed the intersection grid method for quantitatively assessing the intensity of bioturbation on bedding plane surfaces. An example bedding plane, analyzed using this method, is shown above. Intersection grid method data are both quantitative and unitless, making it possible to compare and manipulate these data in large datasets. An analysis using the intersection grid method results in a map of the spatial distribution of bioturbation within a sample area, which can be used for paleoecological reconstruction. In addition, the method employs a reproducible, uniform presence/absence technique for evaluating both distinct and indistinct bioturbation, which minimizes analytical subjectivity. Although less efficient than existing semiquantitative methods, the intersection grid method is considerably more accurate and precise and can be used to refine estimates made in the field and to augment descriptive ichnological data.