New perspectives on a classic macroevolutionary trend: Complexity, extinction selectivity, and the evolution of Paleozoic ammonoid suture morphology

The evolutionary history of the Paleozoic ammonoids was extremely volatile and provides an important source of insight into the relative impact of extinction and recovery on morphological evolution. Here, I focus on the evolution of ammonoid suture morphology, a problem of particular interest as complexity of sutures persistently increased over time despite selective loss of more complex morphotypes during mass extinctions.;I explore a new method for the morphometric analysis of biological features defined by complex curves and apply that method to a comprehensive sample of Paleozoic and lower Triassic ammonoid sutures. Data derived from the morphometric analysis successfully discriminates morphotypes confused by traditional complexity indices, and preliminary analysis reveals significant changes in suture morphology after mass extinctions; the structure of lower Triassic ammonoid sutures was unique when compared to that of the Paleozoic taxa.;More detailed analyses further emphasize the impact of extinctions. Despite having similar complexities, Devonian and Carboniferous sutures occupied disparate regions of morphospace, and the novel morphotypes generated after the end-Devonian event established a mode of suture morphology that persisted until the Permo-Triassic extinction. Severe extinctions (>50% genera lost) were highly selective for suture morphology; however, the mode of selectivity and pattern of 1 recovery varied significantly during the evolutionary history of the ammonoids. Extinctions during the Devonian consistent removed dominant subclades. Many of the lost suture forms were regenerated during gradual rebounds. Extinctions in the Carboniferous selected against outlying forms, while those in the Permian removed taxa possessing modal suture morphologies. Recovery from the late Paleozoic extinctions was very rapid and characterized by significant innovation in suture morphology.;Previous work on suture evolution has focused on measures of relative complexity. Thus, the application of the short-time Fourier analysis marks the first true quantification of suture morphologies. Here, I evaluate the effectiveness of complexity metrics as proxies for suture morphology and re-examine hypothesized evolutionary mechanisms underlying the complexity trend. Metrics quantifying the number of suture lobes did the best job of approximating suture morphology. Results for all metrics generally supported previously published arguments for an active mechanism underlying the trend for increasing suture complexity.*.;*This dissertation is a compound document (contains both a paper copy and a CD as part of the dissertation).