| Summary: | The mustelids (weasels), as a group, are excellent candidates to examine the evolution of morphological form. The mustelid scapula is a highly integrated structure with several musculoskeletal and ligamentous attachments holding it securely in place as the most proximal segment of the tetrapod forelimb. Multiple ossification centers on the scapula undergo endochondral ossification, and fuse with the previously ossified scapula at different time points throughout development. With such a complex developmental background allowing for modifications and alterations to arise, it is not uncommon for members of the same family to exhibit marked differences in shape and size. Several studies have shown that shoulder morphology varies with locomotor style and environmental preferences of each respective taxon. This study seeks to better the understanding of how the form of a given structure, in this case, the mustelid scapula, is influenced by evolutionary pressures, such as gene regulatory networks, phylogenetic and environmental constraints, abiotic factors and organismal performance abilities. This study examines dimensions of the scapula of six mustelids: Northern river otter (Lutra canadensis), American mink (Neovison vison ), Long-tailed weasel (Mustela frenata), Ermine (Mustela erminea), American marten (Martes americana ) and fisher (Martes pennanti). Twenty linear dimensions representing the spine, blade and glenoid of the scapula were measured. To test these claims, I took raw dimensions of the lateral and inferior aspects of the scapula for use in an intraspecific analysis of sexual dimorphism. Data were assessed using multivariate analysis of variance (MANOVA) and univariate analysis of variance (ANOVA). Interspecific differences were examined by taking geomean converted interlandmark distances and assessing them via MANOVA and ANOVA. The test of multivariate allometry tested for an interrelationship of shape and size variation. Group membership among taxa was tested by using discriminate function analysis (DFA) based on scapular traits. The results from the DFA, in the form of Mahalanobis (D2) distances, were assessed using an unweighted pair group method with arithmetic mean (UPGMA) to generate a phenogram that was used to assess the similarity among taxa on a morphological level. The patterns generated by the phenogram were examined to see if they were associated with particular locomotor adaptations. A Mantel test was used to test the level of overall similarity between taxa, as well as levels of regional integration within the scapula of each mustelid. The results of the intraspecific MANOVA and ANOVA revealed varying degrees of sexual dimorphism within the mustelid taxa. Sexual dimorphism was observed in roughly half the taxa, and showed no association to overall body size. The interspecific MANOVA and ANOVA tests revealed opposing levels of variation across each taxon when the scapula was assessed as a whole, and within each scapular region. Taxa that share similar habitats contain less interspecific variation than those who do not. The multivariate allometry results indicated that all mustelids studied exhibited varying rates of positive allometry across the scapula. The results of the DFA displayed that each mustelid scapula was morphologically distinct with the exception of M. erminea and M. frenata. The phenogram depicts a grouping of taxa in line with their respective morphological adaptations regardless of evolutionary relation. The semi-aquatic taxon L. canadensis was the most morphologically distinct species, and was followed by increasing levels of similarity by the terrestrial taxa M. erminea, M. frenata and N. vison followed by the arboreal taxa M. americana and M. pennanti. The Mantel test reported that all the overall comparisons have a statistically significant level of morphological correlation across taxa, with all p-values <0.05. The regional integration showed higher levels of modularity within the glenoid than was observed in either the blade or spine and metacromion.
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