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Congrats to Dr. Caleb Marshall Brown

October 8, 2013
Dr. Caleb Brown in the Gobi Desert, Mongolia in 2009. Courtesy of M. J. Ryan.

Dr. Caleb Brown in the Gobi Desert, Mongolia in 2009. Courtesy of M. J. Ryan.

Caleb Brown successfully defended his Ph.D. in the Department of Ecology and Evolutionary Biology at the University of Toronto last week. His thesis is entitled “Advances in Quantitative Methods in Vertebrate Palaeobiology: A Case Study in Horned Dinosaur Evolution”. His thesis explores the limits of several quantitative methods in paleobiological research and applies best practices to a pioneering study on centrosaurine ceratopsid evolution based on specimens collected in Dinosaur Provincial Park, Alberta. This study is the first to quantify cranial morphological variation at essentially the population-level (based on huge collections from successive mass-death bonebeds), and employ sophisticated model-fitting methods to assess evolutionary modes in dinosaurs. In the analysis, he finds strong support for morphological stasis in Centrosaurus and Styracosaurus, and finds no evidence for directional evolution in these lineages.

ABSTRACT: Discerning modes and rates of biological evolution and speciation are some of the primary objectives of evolutionary biology. Much palaeobiological work has focused on developing robust methods for testing and fitting evolutionary models to samples of fossils across a stratigraphic or temporal axis, with most analyses centering on marine invertebrates. Recent extensive sampling of dinosaur deposits now allows for testing of evolutionary modes in this clade, a first for large-bodied terrestrial vertebrates. Within dinosaur palaeobiology, the relative roles of anagenesis and cladogenesis in diversification, particularly for horned dinosaurs, are hotly debated. Due to their large sample sizes, well-documented stratigraphic positions, highly diagnostic ornamentation, and monodominant bonebeds (representing populations), centrosaurine dinosaurs from the Belly River Group of Alberta make an ideal model system for testing the predictions of these two divergent evolutionary modes. Despite this unparalleled fossil record, it (as well as most fossil records) is limited by missing data, small sample size, taphonomic biases, and stratigraphic error. In this thesis, I present case studies that attempt to quantify and better understand these limitations, and inform best practices for overcoming them. The first four chapters, utilizing data sets for crocodilians (extant archosaurs) and a model geological system (upper Belly River Group), allow for a better- constrained quantitative evolutionary analysis of the Belly River Group centrosaurines in chapter five. Correlations and time-series analyses of morphology and stratigraphic position of Centrosaurus apertus and Styracosaurus albertensis are used to test for directional trends and evolutionary model fitting. Evolutionary results are robust to multiple simulations of stratigraphic uncertainty, and overlap between the taxa depends on a single locality. Results find no support for anagenesis, and rather are consistent with taxonomic turnover due to punctuated evolutionary events or, more likely, ecological replacement due to habitat tracking.”

Congrats to Caleb on completion of his degree, and I also want to thank him for all of his help in the lab and in the field over the last four years. Caleb is moving on to the position of Elizabeth Nichols Postdoctoral Fellow  at the Royal Tyrrell Museum of Paleontology starting in December.

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