In this study, geometric morphometrics (GM) methods are used to assess shape variation among museum collections of Rhinichthys obtusus, the Western Blacknose Dace. Using fixed landmarks and equidistant semilandmarks placed along the outline of each fish's body, operculum, and orbit, this approach provided precise shape data for powerful analyses and visualizations.

GM methods were first used to identify developmental allometry—how the body shape of R. obtusus changes with growth. Results found non-linear allometric trends across stages of growth, prompting a size-binning procedure based on standard length that divided fish into 'small— and —large— bins.

Once allometric growth was considered, GM methods were then used to investigate how body shapes varied across drainage basins and a latitudinal gradient, within the context of post-glacial recolonization and environmentally driven variation. Analyses on separate size bins were run alongside an analysis of all fish together, and results were compared.

Results demonstrated that morphologies are significantly different at the collection and drainage basin levels, and that a large degree of variation occurs even within individual collections. These findings highlight the importance of considering both large-scale patterns, such as historical recolonization pathways, and finer-scale environmental influences when examining morphological variability. Results from this study suggest that while the morphology of R. obtusus may be conserved within genetic lineages, it also exhibits a large degree of sensitivity to local environmental conditions.

This study demonstrated the impressive morphological variability within a single species whose distribution is heavily influenced by Pleistocene glaciation and emphasizes the utility of GM methods in detecting subtle but significant variations in shape related to environmental influences and historical biogeography.