A morphometric study of crocodylian ecomorphology through ontogeny and phylogeny

Morphological convergence is observed throughout the tree of life. Convergent morphologies have been attributed to a variety of mechanisms including ecomorphological adaptation, and ontogenetic plasticity among others. Parallelism has been considered a specialized subset of convergence where common descent has contributed toward the independent evolution of similar morphologies. However, morphological convergence can mislead phylogenetic reconstruction when assumptions of character independence are violated by a misunderstanding of morphological variation making it difficult to distinguish processes of convergence from parallelism. Rigorous study of morphological variation and ontogeny provide a means to resolve evolutionary pattern and process in organismal groups that are complicated by the evolution of recurrent or convergent morphologies.;Crocodylians show a high degree of cranial variation and convergence throughout their 80 million-year fossil record that complicates their phylogenetic reconstruction. Conflicting phylogenetic results from different data partitions and character homoplasies typify crocodylian phylogeny, and differences between molecular and morphological phylogenetic hypotheses are believed to be associated with the slender-snout skull shape of Gavialis gangeticus and Tomistoma schlegelii. Slender-snout skulls are one of five identified eusuchian cranial ecomorph shape categories (ESCs) thought to reflect functional or ecological specialization. This research tests the effect of transitions among general, blunt, and slender ESCs on cranial character state distributions in phylogeny using the concentrated changes test, and character compatibility analysis of character independence. Results suggest cranial ESCs do affect cranial character-state gains in phylogeny. Concentrated changes identify a broad suite of character state changes that significantly correlate with transitions to slender, general, and blunt ESCs on morphological, molecular, and combined-data tree topologies, but numbers of correlated characters for each category differ according to topology. Character compatibility analysis results do not mirror the concentrated changes test results and reflect hierarchically distributed support throughout the data. Since cranial ESCs affect character state transitions, it is possible that non-phylogenetic variables could affect inferences of crocodylian phylogeny by affecting cranial morphology. However, cranial ESCs are qualitatively defined and require testing and possible revision to more accurately reflect the observed diversity of crocodylian cranial morphology.;The quantification of the morphology provides a rigorous framework within which to evaluate morphology and investigate organismal evolution. Landmark-based geometric morphometrics provides an efficient quantitative summary, presentation, and analysis of morphology based on configurations of coordinate data. Geometric morphometric methods are used to explore the 3D cranial shape distribution of 22 extant crocodylian taxa, and the patterns of cranial shape change through posthatchling ontogeny. Results of shape analysis find that crocodylian cranial shapes are not uniformly distributed in morphospace but cluster according to three main influences, namely phylogenetic relationship, cranial ESCs, and patterns of posthatchling shape ontogeny. Existing systems of cranial shape classification for the purpose of identifying evolutionary patterns in Crocodylia can be further refined to account for phylogenetic, ontogenetic, and functional variables determining the distribution of cranial shapes. Crocodylian cranial shape distributions are heavily influenced by phylogeny, but cranial shape convergence can break this phylogeny-shape mold for blunt and slender ESC taxa, and patterns and rates of ontogenetic growth may mediate their morphological diversification.;During ontogeny, organisms can display different phenotypes as a result of living under different environmental conditions, and environmentally induced phenotypic plasticity can promote evolutionary diversification among populations. The Alligator mississippiensis agroindustry provides a unique opportunity to investigate whether the absence of seasonal climate change through ontogeny can induce cranial shape variability and whether the magnitude and direction of environmentally induced shape plasticity correspond with Alligator evolution. In various species of Crocodylians, captive-raised populations have been casually recognized to have cranial morphologies different from wild populations. Geometric morphometric analysis shows the removal of seasonality to permit continual growth in A. mississippiensis significantly alters the cranial shape and ontogenetic shape change from the wild population. Comparison of this phenotypic plasticity in A. mississippiensis with the evolution of Alligator cranial shape variation is suggestive of a causal link between climate change, ontogenetic change, and the evolution of Alligator cranial shapes.