Investigating Heterochrony in the Fossil Record: A Geochemical, Morphometric, and Phylogenetic Study of Thecideidina (Brachiopoda), Triassic-Recent.

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9902984CarlsonHeterochrony, the evolutionary consequences of changes in the rate and timing of development, is considered to be one of the most important means by which evolution occurs. Documenting heterochrony fully requires knowledge of growth rates, phylogenetic relationships, and ontogenetic size and shape change for putative ancestors and descendants. The goal of this study is to understand the role of heterochrony in the origin and evolution of thecideidine brachiopods, a poorly understood group of small, benthic, cemented, pan-tropical, cryptic invertebrates with a long fossil record (Triassic - Recent) and novel morphologies. We will use parsimony-based, morphologic phylogenetic methods to develop robust hypotheses of relationship among thecideidine species and genera, and of thecideidines to other articulated brachiopods. We will document thecideidine growth rates by analyzing the oxygen isotopic composition of microsamples of successive shell growth layers, to quantify size-age-growth relationships throughout ontogeny. Finally, we will utilize multivariate morphometric methods to chronicle shape change that accompanies ontogenetic size increase and characterizes the range of observed adult morphologies. Phylogenetic, geochemical, and morphometric results will be combined to test whether, and in what ways, heterochrony is responsible for the origin and evolution of thecideidines. The results of this study will provide a rare, rigorous test of heterochrony in the fossil record by accounting for all three primary components: phylogenetic relationship, growth rate, and shape and size change.