MORPHOLOGICAL EVOLUTION OF THE SCAPULA IN TREE SQUIRRELS, CHIPMUNKS, AND GROUND SQUIRRELS (SCIURIDAE): AN ANALYSIS USING THIN‐PLATE SPLINES

The mammalian scapula, like many bones, is a single structural element that serves as an attachment site for several muscles. The goal of this study was to determine whether the scapula evolves as an integrated unit, or as a collection of distinct parts. Shape differences among the scapulae of tree squirrels, chipmunks, and ground squirrels were described using thin‐plate spline analysis. This technique produces a geometric description of shape differences that can be decomposed into a series of components ranging in scale from features that span the entire form to features that are highly localized. Shape differences among tree squirrel scapulae were found only in large‐scale features, indicating spatially integrated shape change. Chipmunks and ground squirrels differ from tree squirrels in several features, but shared differences reflecting divergence of their common ancestor were found only in the small‐scale features. Divergence of ground squirrels from the common ancestor involved some large‐scale changes but was dominated by small‐scale changes. Divergence of chipmunks was dominated by large‐scale changes. Thus, the scapula evolved as an integrated unit during some transitions but as a collection of distinct parts during others. These results suggest that evolutionary patterns of the postcranial skeleton may be as complex as the patterns that have been described for skulls and feeding mechanisms.