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How extrinsic (like functional constrains and ecological interactions) and intrinsic (like modularity and integration) interactions drive diversification is a formative area of evolutionary biology. In this dissertation, I explore the phenotypic diversification of the armored catfishes using geometric morphometrics, stable isotope analyses, and phylogenetic comparative methods. I found that the armored catfish body is highly modularized, with varying degrees of integration between each module, suggesting that interactions within and between modules influence morphological evolution. Additionally, slight changes in modularity and integration patterns in clades may have allowed for diversification along a specific trajectory. When focused on the oral jaw shape, I found that traditional and automated processes captured shape more effectively when all jaw components were combined. Although ecological traits do not play a role in jaw shape, there was a correlation between clades with diverse diets and fast evolutionary rates of shape. These results suggest that shape is not constrained to diet and that similarly shaped jaws coupled with different types of teeth could allow the fishes to feed on a wide range of materials. Finally, I built a vector-based analysis, baseline‐standardized isotopic vector analysis (BaSIVA) to visualize dietary variation while accounting for isotopic discrepancies between locations. Results from BaSIVA delineate trophic groups better than traditional trophic positioning methods while accounting for variation in basal resources, suggesting BaSIVA should be the standard for vector‐based stable isotope analysis in riverine environments with similar baseline resources.