CARR, T. D.; University of Toronto: The limits of inferring sequence heterochrony in fossil species: integration of ontogeny and phylogeny in tyrannosaurs

In neontology, sequence heterochrony advanced the study of intrinsic evolutionary processes by recognizing that the order of growth events is unique to each species. The methods of sequence heterochrony can be applied to fossil species although the incomplete fossil record presents a limited data set: often only the extremes or segments of growth series are preserved. Fossil species represented by the extremes of growth can be used to infer the processes that underlie evolutionary transformations by pairwise comparison of the growth changes of phylogenetic characters between species. If the ontogeny of a derived species reflects the evolutionary change of a synapomorphy, then the general heterochronic process can be inferred. Species represented by segments of growth series can be compared using the neontological method �event pair cracking,� which identifies the magnitude and direction of shifts in the offset of events between plesiomorphic and derived ontogenies. These approaches can be combined to identify specific heterochronic processes. For the first approach, four tyrannosaurid species share six characters for which growth changes can be compared. The evolution of two characters show evidence of peramorphosis, three show paedomorphosis, and one displays isomorphosis. The second approach was used to compare the growth series of Albertosaurus libratus and Tyrannosaurus rex. Relative to A. libratus, the growth series of T. rex shows evidence of four events shifted early and two events shifted late. This study illustrates the value of employing neontological approaches to propose defensible hypotheses for how evolutionary novelties in fossil taxa were produced developmentally, when such data are available.