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I seek to understand the mechanisms behind the evolutionary origin of new anatomical features and faunas. The philosophy that underlies all of my empirical work is derived from the conviction that progress in the study of evolutionary biology results from linking research across diverse temporal, phylogenetic, and structural scales. The Origin of Novel Faunas and Anatomical Systems: Much of today's vertebrate diversity was defined by ecological and evolutionary shifts that happened during two critical intervals in the history of the Earth: the Devonian and the Triassic. These periods serve as the focal point for my research because they witness the origin of both new ecosystems and new anatomical designs. My expeditionary research supplies new fossils and a paleoenvironmental context to understand the origin of faunas, whereas our morphological, functional, and developmental studies yield hypotheses on anatomical transformations.
Over the past fifteen years, I have developed expeditionary research programs in Canada, Africa, the continental United States, Asia, and Greenland. These expeditions have led to new insights on the origin of major groups of vertebrates (mammals, frogs, crocodiles, tetrapods, and sarcopterygian fish). Future studies on the origins of pterosaurs, rhizodontid fish, dinosaurs, and salamanders will rely heavily on fossils discovered over the past five years. Examples include the newly discovered adult fin and juvenile skeleton of the fish, Sauripterus. These fossils are providing evidence on the ways that appendage function and skeletal development shifted during the evolutionary radiation of lobe-finned fish. Indeed, this evolutionary radiation is temporally linked to the origin of new freshwater environments. Consequently, the analysis of Sauripterus will place comparative studies of fin structure, development and function in a phylogenetic and paleoenvironmental context.
The goals of the paleontological research dovetail with those of my neontological studies. New fossils, such as Sauripterus, offer tests of hypotheses that derive from our comparative analysis of genetic and morphogenetic processes. For example, the comparison of developmental pathways common to the appendages of all animals suggests genetic mechanisms for parallel evolution and homology. Regularities of variation may reflect the fact that similar regulatory genes are used in the developmental patterning of diverse types of animals.
The Origin of Morphological Variation: The ~400 million year history of terrestrial animals reveals surprising patterns of anatomical stasis and parallel evolution: similar designs crop up in different species living in different environments. Salamanders, for example, arose over 150 million years ago, but have retained a very stable body plan in the face of environmental change and genetic variation. The study of these regularities transcends ecological and paleontological timescales because explanations of larger-scale patterns can be sought in the mechanisms that structure anatomical variation in populations today. Accordingly, my research has involved collecting data on intraspecific variation from diverse populations, developing predictive models of variation based on ontogeny, and comparing developmental processes in diverse salamanders that live in different environmental settings.
Salamander limbs are a model system to approach these issues because of the diversity of their developmental systems and life histories. In addition, the widespread occurrence of parallelism provides us with a window to develop predictive rules about the origin of variation in populations. Over the past seven years, colleagues and I have composed a database of limb variation and ontogeny in populations of diverse salamanders. Virtually all of the species analyzed to date possess variant conditions that both restore ancient features and anticipate more derived conditions seen in distantly-related species. Much of the observed intraspecific variation is predictable from a knowledge of phylogenetic history or development. Ultimately, if these historical and developmental effects resulted in long-term evolutionary patterns, they must have acted over geological timescales. Tests of this hypothesis will come from the study of the Chinese Cretaceous where, in collaboration with colleagues, I am studying variation of salamanders in a Cretaceous pond that were killed in a single mass-mortality event.
Phylogenetic analysis of ontogenetic trajectories in salamanders affords critical assessments of the role of historical, ecological, and structural factors in evolution. Analysis of development in salamanders with different life histories suggests certain aspects of early limb development are highly sensitive to variation in larval biology. I intend to explore this link between ontogenetic diversity and anatomical variation in the future by using experimental and comparative studies of ontogeny.